About Biosecurity_Poultry

What is biosecurity?

Biosecurity at a poultry farm includes all measures taken to minimize the risk of introduction and spread of disease agents and thus, includes all actions for keeping poultry and the farm healthy. By taking these biosecurity measures and performing efficient management, on-farm animals are protected against both endemic and epidemic diseases (Dewulf and Van Immerseel, 2018).

A distinction is made between external and internal biosecurity. 

External biosecurity focuses on the contact points of the farm with the outside world and aims to prevent that pathogens can enter or leave the farm. This applies both to exotic diseases, which occur rarely in a country, as well as to endemic diseases, which are common in a country but do not occur at every farm (Ribbens et al., 2008). All measures taken to counteract the spread of pathogens within a farm are covered by internal biosecurity (Laanen et al., 2010; Anonymous, 2010).

Why biosecurity?

The poultry sector has evolved over the years from a small scale to an industrial animal husbandry. The aim is to have a thorough production through the optimization of components such as nutrition and genetics (Vaillancourt and Carver, 1998; Graham et al., 2008; Butcher and Miles, 2012). However, due to the high productivity, a disease outbreak will have a major impact on farm level, but also on the poultry sector in general. The most effective and economic way to control poultry diseases is the implementation of biosecurity measures (Butcher and Miles, 2012).

The main objective of biosecurity is to keep the infection pressure at the farm as low as possible. This way, the immune system of a certain animal will be less stressed, resulting in a lower risk of disease outbreak and, consequently, a better animal health and welfare.

By reducing the potential risks for a disease outbreak with the implementation of biosecurity measures, other favourable consequences may also be established for the farm. For example, in several studies with pigs, biosecurity showed a positive association with the production results (such as daily growth) and the profitability of the farm. This can certainly be extended to the poultry industry. Along with this, the use of antibiotics can be greatly reduced (Tablante, 2008; Laanen et al., 2011; Corrégé et al., 2012; Siekkinen et al., 2012; Laanen et al., 2013; Postma, 2016a; Postma et al., 2016b; Postma et al., 2016c; Postma et al., 2016d). In addition, decreasing the use of antibiotics within animal production will reduce the development of antibiotic resistance and this is good for both animal and human health (Angulo et al., 2004; Chantziaras et al., 2014).

Disease transmission routes

Regarding disease transmission, not all transmission routes are of equal importance. Therefore, it is not easy to rank the different routes according to their relevance. This is mainly due to the large variation among the infectious agents in their ability to infect a living creature, such as their survival chances in the environment. So it is clear that not all biosecurity measures will contribute in the same way to the prevention of different infectious diseases of poultry (Gelaude et al., 2014; Van Meirhaeghe et al., 2018).

Direct contact between animals is considered to be the main transmission route of infectious agents. Therefore, more attention will be paid to biosecurity measures which avoid direct contact between animals, than to measures taken to prevent indirect transmission through, for example, work material or persons (Amass, 2003a; Pritchard et al., 2005; Amass and Baysinger, 2006).

An additional but still important factor is the frequency at which a pathogen can infect an animal population along a specific pathway. A less crucial transmission route may become very important the moment it creates an entrance for pathogens at the farm multiple times (Fèvre et al., 2006; Laanen et al., 2013).

The combined risk (chance of transmission x frequency) can be calculated using the following formula:

P = 1-(1-p)ⁿ

with p = the risk of disease transmission per event, n = the number of events and P = the combined risk (probability).

Example

If you assume that a certain transmission route has only one chance at 1000 (= 0.1%) to effectively transmit the disease and if you also know that this route occurs 50 times a year (e.g. weekly activity), the chance of disease transmission at the end of the year will be

1 - [(1 - 0.001)⁵⁰] = 4.88% (Dewulf, 2017).

On the other hand, the risk of disease transmission through a specific transmission route, which has a chance of 1 to 50 (= 2%) to transmit the disease, but which occurs only twice a year will be 1 - [(1 - 0.02)²] = 3.96%.

(Laanen et al., 2010)

Therefore, you should be well aware of the fact that any transmission pathway, also the less important ones, may pose a risk to the introduction and spread of infectious disease. In this way, vigilance on all biosecurity levels will always be very important for the farm.

The figure below illustrates the relative importance of the different pathways of pathogen transmission between farms (Laanen et al., 2010).

External biosecurity

Purchase of one-day-chicks 

Disease transmission from animal to animal

Direct contact between infected and susceptible animals is the most effective way to transmit a pathogen (Martin et al., 1987; Amass and Baysinger, 2006; Lister, 2008). Infected animals spread many pathogens through all sorts of excretions and secretions, including saliva or manure. When susceptible animals come into contact with this infected animals or with any of these excretions / secretions, pathogens can easily be transferred from one animal to another. In this way, a disease can spread rapidly through a particular poultry house (Carey et al., 2005; Graham et al., 2008; Lister, 2008; Dewulf, 2014). For example, the amount of Campylobacter spp. will expand very quickly within a poultry population through manure contamination of the drinking lines and coprophagy (eating faeces) (Graham et al., 2008; Lister, 2008). A pathogen can also be transferred through the eggs. This transmission route includes both the vertical transmission from hen to egg and the transmission via contaminated eggshells in the hatchery (Carey et al., 2005; Lister, 2008).

Limit the frequency of introduction

Both the frequency of introducing new animals and the number of animals acquired, will influence the risk of disease introduction to the farm, as pathogen transmission occurs very effectively via direct contact between animals. The risk of transmission will increase with the number of animals being introduced to the poultry farm (Fèvre et al., 2006; Laanen et al., 2013).

Transport and disease transmission

Livestock lorries drive from farm to farm and increase the risk of disease spreading (Anonymous, 2010; Gelaude et al., 2014). Several epidemiological studies have shown that transport can play a major role in introducing certain infectious agents to a farm (Rajkowski et al., 1998; Fritzemeier et al., 2000; Hege et al., 2002). Poultry can only be transported in vehicles that are thoroughly cleaned and disinfected. So all dead animals, all contaminated litter and all manure must be removed from the transport vehicle before transporting new live animals (Carey, 2005; Pritchard et al., 2005; Anonymous, 2010; Gelaude et al., 2014; Dewulf, 2014). It is also recommended to clean and disinfect the tires of the transport vehicle before entering the farm (Carey, 2005; Lister, 2008; Sims, 2008; Dorea et al., 2010).

A crucial point in the spread of infectious diseases through poultry transport, is the cleaning and disinfection between the different transports of poultry coming from other farms, and after the unloading of the animals at the slaughterhouse. If this is not performed thoroughly, it poses a high risk of disease transmission, since different poultry populations are indirectly in contact with each other (Rajkowski et al., 1998; Amass, 2005b; Dewulf, 2014; Gelaude et al., 2014). A higher biosecurity status on a poultry farm can be guaranteed if transport vehicles do not visit more than one farm a day and if these vehicles are only used for the same purpose (Van Steenwinkel et al., 2011).

Limit the number of source herds

It is of high importance to limit the number of source herds as much as possible to avoid the introduction of certain infectious diseases (Nespeca et al., 1997; Dewulf, 2014; Gelaude et al., 2014). By populating all the poultry houses of the farm with animals from the same source herd and derived from the same mother animals, a homogeneous poultry population can be created (Gelaude et al., 2014). Several studies have shown that introducing animals from different source herds increases the risk of disease introduction into the poultry farm (Hege et al., 2002; Lo Fo Wong, 2004).

Source herds with a high health status

New animals should always come from a farm with an equal or higher sanitary status (Pritchard et al., 2005; Kirwan, 2008; Lister, 2008; Dewulf, 2014). As each poultry farm uses its own management and thus focuses on different biosecurity measures, this sanitary status can vary considerably from one farm to another (Sims, 2008). One-day-old chicks can both be infected with pathogens during their stay in the hatchery as well as by vertical transmission through their development in the egg (from hen to chick) (Carey, 2005; Lister, 2008). Consequently, a higher or equal sanitary status in the source herd or in the hatchery is very important to prevent disease transmission within a broiler population (Sims, 2008).

Purchase of laying hens

Limit the number of source herds

It is of high importance to limit the number of source herds as much as possible to avoid the introduction of certain infectious diseases (Nespeca et al., 1997; Dewulf, 2014; Gelaude et al., 2014). By populating all the poultry houses of the farm with animals from the same source herd and derived from the same mother animals, a homogeneous poultry population can be created (Gelaude et al., 2014). Several studies have shown that introducing animals from different source herds increases the risk of disease introduction into the poultry farm (Hege et al., 2002; Lo Fo Wong, 2004). When layer pullets have been reared on separate premises and were exposed to infection, they can become carriers of diseases not existing on the layer farm. This way, new pathogens can be introduced to the production site.

Transport and disease transmission

Livestock lorries drive from farm to farm and increase the risk of disease spreading (Anonymous, 2010; Gelaude et al., 2014). Several epidemiological studies have shown that transportation can play a major role in introducing certain infectious agents to a farm (Rajkowski et al., 1998; Fritzemeier et al., 2000; Hege et al., 2002). Poultry can only be transported in vehicles that are thoroughly cleaned and disinfected. So all dead animals, all contaminated litter and all manure must be removed from the transport vehicle before transporting new live animals (Carey, 2005; Pritchard et al., 2005; Anonymous, 2010; Gelaude et al., 2014; Dewulf, 2014). It is also recommended to clean and disinfect the tires of the transport vehicle before entering the farm (Carey, 2005; Lister, 2008; Sims, 2008; Dorea et al., 2010).

A crucial point in the spread of infectious diseases through poultry transport, is the cleaning and disinfection between the different transports of poultry coming from other farms, and after the unloading of the animals at the slaughterhouse. If this is not performed thoroughly, it poses a high risk of disease transmission, since different poultry populations are indirectly in contact with each other (Rajkowski et al., 1998; Amass, 2005b; Dewulf, 2014; Gelaude et al., 2014). A higher biosecurity status on a poultry farm can be guaranteed if transport vehicles do not visit more than one farm a day and if these vehicles are only used for the same purpose (Van Steenwinkel et al., 2011).

Source herds with a high health status

New animals should always come from a farm with an equal or higher sanitary status (Pritchard et al., 2005; Kirwan, 2008; Lister, 2008; Dewulf, 2014). As each poultry farm uses its own management and thus focuses on different biosecurity measures, this sanitary status can vary considerably from one farm to another (Sims, 2008).

Multi-age farms

In some farms, layers of different ages are kept on site. Single age and all-in/all-out should be the aim. Older birds can be asymptomatic carriers of pathogens such as Salmonella, Gumboro, Mycoplasma, Newcastle and Infectious Bronchitis virus which can infect younger birds that have not yet developed resistance to these pathogens (Nespeca et al., 1997; East et al., 2006; East, 2007; De Gussem et al., 2013). Bringing new birds from outside, for example in case of spiking (= introducing new males in a breeder flock), is connected with a high biosecurity risk (Jones, 2009; Woodger and Wirral, 2009; Ssematimba et al., 2013; De Gussem et al., 2013).

Older birds infected with for example Mycoplasma or Salmonella, can spread the pathogens to flocks that contain younger birds, so the pathogens persist on the farm. Usually farms rearing replacement pullets and production farms are not at the same location. One should take care not to introduce diseases to the production site. Before transfer, monitoring for Salmonella and Mycoplasma is a general rule, but also internal and external parasites should be looked for and if needed the pullets need to be treated before moving to the production farm. The crates and containers used for transfer, should be clean and disinfected. A sanitary period between flocks is strongly recommended for cleaning and disinfecting of the houses. This can only be done efficiently if all houses are empty at the same time.

Production periods

After a first production period, layers are preferably slaughtered instead of kept for a second round. Several studies have shown that stress induced by high stocking density, induced moulting, while heat can induce immunosuppression that eases spread and persistence of Salmonella within a flock (Holt and Porter, 1992; Holt, 1995; Holt et al., 1998; Garber et al., 2003; Van Hoorebeke et al., 2012, Sasaki et al., 2012).

Depopulation of the poultry houses

Transport and disease transmission

Livestock lorries drive from farm to farm and increase the risk of disease spreading (Anonymous, 2010; Gelaude et al., 2014). Several epidemiological studies have shown that transportation can play a major role in introducing certain infectious agents to a farm (Rajkowski et al., 1998; Fritzemeier et al., 2000; Hege et al., 2002). Poultry can only be transported in vehicles that are thoroughly cleaned and disinfected. So all dead animals, all contaminated litter and all manure must be removed from the transport vehicle before transporting new live animals (Carey, 2005; Pritchard et al., 2005; Anonymous, 2010; Gelaude et al., 2014; Dewulf, 2014). It is also recommended to clean and disinfect the tires of the transport vehicle before entering the farm (Carey, 2005; Lister, 2008; Sims, 2008; Dorea et al., 2010).

A crucial point in the spread of infectious diseases through poultry transport, is the cleaning and disinfection between the different transports of poultry coming from other farms, and after the unloading of the animals at the slaughterhouse. If this is not performed thoroughly, it poses a high risk of disease transmission, since different poultry populations are indirectly in contact with each other (Rajkowski et al., 1998; Amass, 2005b; Dewulf, 2014; Gelaude et al., 2014). A higher biosecurity status on a poultry farm can be guaranteed if transport vehicles do not visit more than one farm a day and if these vehicles are only used for the same purpose (Van Steenwinkel et al., 2011).

Disease transmission between humans and animals

It is widely known that infectious agents can be transferred from humans to animals and vice versa or that humans can transmit infectious pathogens from animal to animal. Precisely for this reason, the number of visitors on a farm or a stable must be limited and it should be an objective to keep people as far as possible from the animals (Carey, 2005; Lister, 2008). Every time a lorry driver or a person from the catching team enters the farm and the stables, this poses a real risk of introducing infectious agents (Berndtson et al., 1996; Hald et al., 2000; Anonymous, 2010; Gelaude et al., 2014). A human can serve both as a mechanical vector and as a biological vector for the transmission of infectious diseases at the farm (Amass, 2005b; Lister, 2008). Visitors have played an important role in the spread of Avian Influenza in 2003, among other things (Thomas et al., 2005; Vieira et al., 2009).

Humans can act as mechanical vectors (for example, by carrying faeces on boots) if they have been in contact with infected animals and subsequently switch to susceptible animals without taking any measures The transmission happens mainly through leftovers of excreta from infected animals on footwear and clothing (Lister, 2008). Biological transmission may exist in pathogens that can infect both human and poultry, such as the Avian Influenza virus and Salmonella spp. (Grunkemeyer, 2011).

It is very important that contact with persons who get in touch with non-proprietary poultry, is avoided as much as possible (Berndtson et al., 1996; Van Steenwinkel et al., 2011). A study has shown (Van Steenwinkel et al., 2011) that there is frequent contact between professional poultry farms and hobby poultry farms. Hobby poultry farmers repeatedly visit the professional farms to buy eggs or animals. This all leads to major risks of disease transmission as the biosecurity level of hobby poultry farms is often much lower than the biosecurity level of the professional farms.

Farm clothing

As humans can serve as a mechanical vector for the transmission of infectious diseases, it is recommended to take specific biosecurity measures at the moment humans enter a farm (Lister, 2008; Anonymous, 2010).

When visitors and personnel enter the stables, they should always wear clean, herd-specific clothes and footwear to avoid disease transmission through leftovers of excreta from other infected animals (Nespeca et al., 1997; Lister, 2008; McDowell et al., 2008; Sims, 2008; Dorea et al., 2010).

Depopulation of the animals

There is a significant risk of introducing infectious diseases to a poultry farm due to the entry of the catching team and their equipment (Berndtson et al., 1996; Hald et al., 2000; Slader et al., 2002; Lister, 2008; McDowell et al., 2008). The baskets in which the poultry are transported, have been associated several times with the transmission of pathogens at the farm (Slader et al., 2002; Lister, 2008; McDowell et al., 2008). For this reason, it is recommended to carry out the depopulation of the poultry houses in as few steps as possible (preferably a complete stable in one day) and to provide farm-specific clothing for the entire catching team (Berndtson et al., 1996; McDowell et al., 2008; Sims, 2008). Furthermore, it is also important that the transport baskets are thoroughly cleaned and disinfected before being introduced to the farm. In a study about the transmission of Salmonella and Campylobacter through transport, it was found that many transport baskets are regularly reused between different farms without a proper cleaning or disinfection step (Slader et al., 2002).

Feed and water

Separation between the clean and dirty area

The principle of the clean and dirty road on a poultry farm means that there is a clear separation between the clean and the dirty (risky) sections of the premises (Carey, 2005; Al-Saffar et al., 2006; Ssematimba et al., 2013). Poultry transport vehicles are constantly in contact with other farms and slaughterhouses. This creates a quite extensive risk for disease transmission (Amass, 2005b; Pritchard et al., 2005; Anonymous, 2010; Gelaude et al., 2014). All inbound and outbound traffic that serves multiple companies (feed, manure, external transportation of animals…) are always lead via the dirty road. The clean road is preserved for the possible supply of animals and harmless products (internal movements at the farm) and only in fully cleaned and disinfected lorries (Pritchard et al., 2005; Al-Saffar et al., 2006).

Only the dirty road is relatively easy accessible to visitors, suppliers and consumers. The cadaver collecting is for obvious reasons part of the dirty section (cadaver storage box, loading point). Barrels and other tools used for this may only be returned to the clean section after they have been thoroughly cleaned and disinfected (Pritchard et al., 2005; Al-Saffar et al., 2006). The removal of manure is always conveyed via the dirty road. This prevents your own farm from being contaminated trough material that has recently been in contact with manure from other farms (Pritchard et al., 2005).

If it is not possible to remove the manure along the dirty road, it is surely necessary to clean and disinfect all parts of the farm premises that have become dirty during the unloading of the stables. This can be done together with the cleaning and disinfection of the poultry stables.

Disease transmission between humans and animals

It is widely known that infectious agents can be transferred from humans to animals and vice versa or that humans can transmit infectious pathogens from animal to animal. Precisely for this reason, the number of visitors on a farm or a stable must be limited and it should be an objective to keep people as far as possible from the animals (Carey, 2005; Lister, 2008). Every time a lorry driver or a person from the catching team enters the farm and the stables, this poses a real risk of introducing infectious agents (Berndtson et al., 1996; Hald et al., 2000; Anonymous, 2010; Gelaude et al., 2014). A human can serve both as a mechanical vector and as a biological vector for the transmission of infectious diseases at the farm (Amass, 2005b; Lister, 2008). Visitors have played an important role in the spread of Avian Influenza in 2003, among other things (Thomas et al., 2005; Vieira et al., 2009).

Humans can act as mechanical vectors (for example, by carrying faeces on boots) if they have been in contact with infected animals and subsequently switch to susceptible animals without taking any measures The transmission happens mainly through leftovers of excreta from infected animals on footwear and clothing (Lister, 2008). Biological transmission may exist in pathogens that can infect both human and poultry, such as the Avian Influenza virus and Salmonella spp. (Grunkemeyer, 2011).

It is very important that contact with persons who get in touch with non-proprietary poultry, is avoided as much as possible (Berndtson et al., 1996; Van Steenwinkel et al., 2011). A study has shown (Van Steenwinkel et al., 2011) that there is frequent contact between professional poultry farms and hobby poultry farms. Hobby poultry farmers repeatedly visit the professional farms to buy eggs or animals. This all leads to major risks of disease transmission as the biosecurity level of hobby poultry farms is often much lower than the biosecurity level of the professional farms.

Feed and disease transmission

The feed can be a potential source of contamination in a poultry farm. Microorganisms such as Salmonella spp., Aspergillus spp. or E. coli can pollute the feed and can be a real danger to the present poultry population (Lister, 2008). Feed contamination can occur during production, transportation or storage (Lister, 2008; Anonymous, 2010).

In order to avoid spread of pathogens through feed transport vehicles, it is recommended to deliver the feed through proprietary pipelines from the dirty part of the premises. This prevents that the feed truck and the associated foreign pathogens can enter the poultry farm (Pritchard et al., 2005; Sims, 2008). Furthermore, it is also important to ensure that rats or other wild animals do not have access to the feed silos to prevent contamination of the feed by vermin (Nespeca et al., 1997; Al-Saffar et al., 2006; Charisis, 2008; Van Steenwinkel et al., 2011).

Water and disease transmission

Pathogens can easily spread through contaminated drinking water to the poultry population at the farm (Lister, 2008; Anonymous, 2010). The water may originate from different sources (surface, wells…) after which it is stored most of the time in a tank (not always the case) and supplied to the animals (Sims, 2008; Dewulf, 2014). The water well, the storage tank and the pipelines must be completely and properly closed in order to ban dust, vermin or wild birds. In this way, vermin and dust cannot be a source of contamination for the drinking water of the animals (East, 2007; Lister, 2008). A study has shown that the type of drinking water system in a poultry stable has an effect on the colonization of certain bacteria. Therefore, drinking cup systems will be a larger risk compared to drinking nipple systems. However, when using a drinking cup system, one can hardly avoid the contamination of the drinking water as the water often remains in the cup (Sommer et al., 2016).

The drinking water quality is influenced by the presence or absence of biofilms in the water pipelines. Biofilms form a protected environment for bacteria. For this reason, bacteria can survive longer and pose a danger for the on-farm poultry population (Gelaude et al., 2014). Regular (preferably twice a year) examination of the drinking water quality, both at the entrance and at the nipples/cups is therefore definitely advisable, just as a systematic cleaning of the pipes (Jeffrey, 1997; Pritchard et al., 2005; Anonymous, 2010; Dewulf, 2014; Gelaude et al., 2014).

Surface water originating from the close environment of the farm should not be used as drinking water as there is a chance that this water is contaminated by wild birds (Kapperud et al., 1993; East, 2007; Lister, 2008; Sims, 2008; Van Steenwinkel et al., 2011).

Transport of eggs

Transport and disease transmission

Livestock lorries drive from farm to farm and increase the risk of disease spreading (Anonymous, 2010; Gelaude et al., 2014). Several epidemiological studies have shown that transportation can play a major role in introducing certain infectious agents to a farm (Rajkowski et al., 1998; Fritzemeier et al., 2000; Hege et al., 2002). Poultry can only be transported in vehicles that are thoroughly cleaned and disinfected. So all dead animals, all contaminated litter and all manure must be removed from the transport vehicle before transporting new live animals (Carey, 2005; Pritchard et al., 2005; Anonymous, 2010; Gelaude et al., 2014; Dewulf, 2014). It is also recommended to clean and disinfect the tires of the transport vehicle before entering the farm (Carey, 2005; Lister, 2008; Sims, 2008; Dorea et al., 2010).

A crucial point in the spread of infectious diseases through poultry transport, is the cleaning and disinfection between the different transports of poultry coming from other farms, and after the unloading of the animals at the slaughterhouse. If this is not performed thoroughly, it poses a high risk of disease transmission, since different poultry populations are indirectly in contact with each other (Rajkowski et al., 1998; Amass, 2005b; Dewulf, 2014; Gelaude et al., 2014). A higher biosecurity status on a poultry farm can be guaranteed if transport vehicles do not visit more than one farm a day and if these vehicles are only used for the same purpose (Van Steenwinkel et al., 2011).

Disease transmission between humans and animals

It is widely known that infectious agents can be transferred from humans to animals and vice versa or that humans can transmit infectious pathogens from animal to animal. Precisely for this reason, the number of visitors on a farm or a stable must be limited and it should be an objective to keep people as far as possible from the animals (Carey, 2005; Lister, 2008). Every time a lorry driver or a person from the catching team enters the farm and the stables, this poses a real risk of introducing infectious agents (Berndtson et al., 1996; Hald et al., 2000; Anonymous, 2010; Gelaude et al., 2014). A human can serve both as a mechanical vector and as a biological vector for the transmission of infectious diseases at the farm (Amass, 2005b; Lister, 2008). Visitors have played an important role in the spread of Avian Influenza in 2003, among other things (Thomas et al., 2005; Vieira et al., 2009).

Humans can act as mechanical vectors (for example, by carrying faeces on boots) if they have been in contact with infected animals and subsequently switch to susceptible animals without taking any measures The transmission happens mainly through leftovers of excreta from infected animals on footwear and clothing (Lister, 2008). Biological transmission may exist in pathogens that can infect both human and poultry, such as the Avian Influenza virus and Salmonella spp. (Grunkemeyer, 2011).

Access control

In order to maintain a high level of biosecurity at a poultry farm, it is important to limit the access to the stables as much as possible (Carey, 2005; Sims, 2008; Van Steenwinkel et al., 2011). Visitors can carry pathogens into the farm and within the farm between houses on footwear, clothes and hands. People can use and move equipment contaminated with dust, faeces or bird feathers (Lister, 2008; De Gussem et al., 2013). Some pathogens as Mycoplasma can survive for a few days on hair and clothes and can be transmitted mechanically (Christensen et al., 1994). Therefore, it is recommended to prevent all unauthorized persons from going into the animal houses and to ensure that external staff spends as little time as possible in and around the farm premises (Carey, 2005; Al-Saffar, 2006; Charisis, 2008).

It is strongly advisable to provide a proper access control in order to have a sufficient supervision on the persons who want to enter the poultry houses. Each visitor should sign up before entering the farm premises and the animal houses (Carey, 2005; Anonymous, 2010). By establishing a fence around the farm, you can create an effective entry barrier (Nespeca et al., 1997; Sims, 2008; Van Steenwinkel et al., 2011).

Removal of manure and carcasses

Separation between the clean and dirty area

The principle of the clean and dirty road on a poultry farm means that there is a clear separation between the clean and the dirty (risky) sections of the premises (Carey, 2005; Al-Saffar et al., 2006; Ssematimba et al., 2013). Poultry transport vehicles are constantly in contact with other farms and slaughterhouses. This creates a quite extensive risk for disease transmission (Amass, 2005b; Pritchard et al., 2005; Anonymous, 2010; Gelaude et al., 2014). All inbound and outbound traffic that serves multiple companies (feed, manure, external transportation of animals…) are always lead via the dirty road. The clean road is preserved for the possible supply of animals and harmless products (internal movements at the farm) and only in fully cleaned and disinfected lorries (Pritchard et al., 2005; Al-Saffar et al., 2006).

Only the dirty road is relatively easy accessible to visitors, suppliers and consumers. The cadaver collecting is for obvious reasons part of the dirty section (cadaver storage box, loading point). Barrels and other tools used for this may only be returned to the clean section after they have been thoroughly cleaned and disinfected (Pritchard et al., 2005; Al-Saffar et al., 2006). The removal of manure is always conveyed via the dirty road.

Cadaver and disease transmission

Cadavers or carcasses are always a major source of infectious material. Animals often die due to an infection and will potentially spread a lot of infectious material. It is therefore strongly advised to remove cadavers from the animal houses as soon as possible and to store them in a well-insulated place (Meroz et al., 1995; Nespeca et al., 1997; Evans and Sayer, 2000; Pritchard et al., 2005; Anonymous, 2010). Dead animals should be removed from the stable at least twice a day, and if there are many cases of acute mortality, this frequency should be increased.

The cadaver storage should be located as far away as possible from the stables, in a place where the rendering company can collect the cadavers without entering the farm to avoid disease introduction through these potentially risky transports (Evans and Sayer, 2000; Gibbens et al., 2001; Anonymous, 2010).

Make sure that no vermin can reach the cadavers (by a well-closed, cooled storage), because they could spread the infectious material (Evans and Sayer, 2000). After the collection of the cadavers, it is advisable to thoroughly clean and disinfect the cadaver storage. The person manipulating the cadavers should always wear disposable gloves for their own safety as well as to avoid further spread of pathogens (Anonymous, 2010; Gelaude et al., 2014; Pritchard et al., 2015).

Cooled cadaver storage

A cooled cadaver storage area has several advantages over a non-cooled one. Firstly, a fully closed system prevents the spread of pathogens and it is effective in avoiding contact with vermin. Moreover, these cooled systems reduce the rotting process, ensuring less odour development and a lower frequency of visits of the rendering company (Vangroenweghe et al., 2009; Anonymous, 2010).

Transport vehicle of the rendering company

The truck of the rendering company transports contaminated carcasses from several poultry farms a day. Thus, this vehicle represents a major source of contamination for a farm. The cooled cadaver storage area (at the dirt section of the poultry farm), should be located at the public road in order to avoid that the vehicle of the rendering company must enter the farm area. This transport vehicle has to stay as far away as possible from the animal houses (at the clean area of the poultry farm) (Evans and Sayer, 2000; McQuiston et al., 2005; Pritchard et al., 2005; Anonymous, 2010).

Visitors and farmworkers

Disease transmission between humans and animals

It is widely known that infectious agents can be transferred from humans to animals and vice versa or that humans can transmit infectious pathogens from animal to animal. Precisely for this reason, the number of visitors on a farm or a stable must be limited and it should be an objective to keep people as far as possible from the animals (Carey, 2005; Lister, 2008). Every time a lorry driver or a person from the catching team enters the farm and the stables, this poses a real risk of introducing infectious agents (Berndtson et al., 1996; Hald et al., 2000; Anonymous, 2010; Gelaude et al., 2014). A human can serve both as a mechanical vector and as a biological vector for the transmission of infectious diseases at the farm (Amass, 2005b; Lister, 2008). Visitors have played an important role in the spread of Avian Influenza in 2003, among other things (Thomas et al., 2005; Vieira et al., 2009).

Humans can act as mechanical vectors (for example, by carrying faeces on boots) if they have been in contact with infected animals and subsequently switch to susceptible animals without taking any measures The transmission happens mainly through leftovers of excreta from infected animals on footwear and clothing (Lister, 2008). Biological transmission may exist in pathogens that can infect both human and poultry, such as the Avian Influenza virus and Salmonella spp.(Grunkemeyer, 2011).

It is very important that contact with persons who get in touch with non-proprietary poultry, is avoided as much as possible (Berndtson et al., 1996; Van Steenwinkel et al., 2011). A study has shown (Van Steenwinkel et al., 2011) that there is frequent contact between professional poultry farms and hobby poultry farms. Hobby poultry farmers repeatedly visit the professional farms to buy eggs or animals. This all leads to major risks of disease transmission as the biosecurity level of hobby poultry farms is often much lower than the biosecurity level of the professional farms.

Access control

In order to maintain a high level of biosecurity at a poultry farm, it is important to provide access to the stables to as few people as possible (Carey, 2005; Sims, 2008; Van Steenwinkel et al., 2011). Visitors can carry many pathogens with them and these pathogens will contaminate the present poultry population. Therefore, it is recommended to prevent all unauthorized persons from going into the animal houses and to ensure that external staff spends as little time as possible in and around the farm premises (Carey, 2005; Al-Saffar, 2006; Charisis, 2008).

It is strongly advisable to provide a proper access control in order to have a sufficient supervision on the persons who want to enter the poultry houses. Each visitor should sign up before entering the farm premises and the animal houses (Carey, 2005; Anonymous, 2010). By establishing a fence around the farm, you can create an effective entry barrier (Nespeca et al., 1997; Sims, 2008; Van Steenwinkel et al., 2011).

Poultry 'downtime'

Often a poultry-free downtime of 24 or even 72 hours is required for visitors and personnel before they can have access to the poultry farm (Charisis, 2008; Lister, 2008).

This proposition is based on the fact that pathogens, which are excreted by poultry, could survive in humans for a particular period of time. During this period, humans could also excrete these infectious agents passively and they could transmit the pathogens to the susceptible animals through direct contact. However, when all other preventive biosecurity measures (such as the use of farm-specific clothing and footwear, hand hygiene or even taking a shower before entering the stables) are properly applied, the "animal-free" period has only a limited additional protective value.

Farm clothing

As humans can serve as a mechanical vector for the transmission of infectious diseases, it is recommended to take specific biosecurity measures the moment humans enter a farm (Lister, 2008; Anonymous, 2010).

When visitors and personnel enter the stables, they should always wear clean, herd-specific (farm-specific) clothes and footwear to avoid disease transmission through leftovers of excreta from other infected animals (Nespeca et al., 1997; Lister, 2008; McDowell et al., 2008; Sims, 2008; Dorea et al., 2010).

Hand hygiene

When visitors and personnel enter the poultry houses, they should at least wash their hands properly. This latter is a simple but very useful measure, which is often forgotten. The hands of animal caretakers are surely an efficient way to transfer pathogens through direct contact with the animals. So, when changing clothes and footwear in the hygiene lock, it has to be a habit as well to wash the hands both on arrival and on departure (Lister, 2008; Vangroenweghe et al., 2009; Anonymous, 2010).

Contact with non-proprietary poultry

It is very important that there is as little contact as possible between the on-farm poultry population and persons who have close connections with non-proprietary poultry (most of the time hobby poultry). These non-proprietary animals can serve as a vector for the transmission of multiple infectious poultry diseases (Van Steenwinkel et al., 2011; Ssematimba et al., 2012). A study has shown that frequent contact is made between industrial poultry farm and hobby poultry farmers. Hobby poultry farmers repeatedly come to industrial farms to buy eggs or animals. This all entails a major risk, as the biosecurity level of hobby poultry holders is often a lot lower than the biosecurity level at industrial farms (Van Steenwinkel et al., 2011).

Furthermore, it is strongly discouraged for professional poultry farmers or personnel of the poultry farm to keep hobby poultry at home (Nespeca et al., 1997; Al-Saffar et al., 2006; Charisis, 2008).

The number of animal caretakers for each poultry house should be limited. Especially, when an animal care taker is responsible for several poultry houses at the same time. In this way, pathogens can be exchanged very easily between the different poultry populations within a farm or between the poultry populations of several farms (Kapperud et al., 1993; Refrégier-Petton et al., 2001).

The hygiene lock

The hygiene lock, where visitors should put on company clothes and shoes, is especially intended to decrease the risk of mechanical transfer of diseases through persons (Evans and Sayer, 2000; Vangroenweghe et al., 2009).

The location of the hygiene lock within the farm is extremely important because the stables can only be accessible after going through this lock. So, the hygiene lock has simply one entrance and one exit and the room can be divided into a clean and dirty part. Furthermore, attention should be paid to the presence of a sink, as hand hygiene (cleaning and disinfecting of your hands) is really essential for the on-farm biosecurity (Vangroenweghe et al., 2009).

Material supply

Material and disease transmission

Pathogens can find an entrance to a farm via the supply of materials. This happens especially when the material was previously in contact with poultry or when it was manufactured or packaged at other poultry farms (Pritchard et al., 2005).

To prevent pathogens from transferring from one company to another, it is advised to use proprietary, farm-specific material. It is also recommended to provide this material to anyone who needs it at the farm (Lister, 2008; Gelaude et al., 2014; Sommer et al., 2016). However, if non-proprietary material has to be introduced to the farm or to certain poultry houses, this can be done via specific hatches with disinfectant UV-radiation (Filippitzi et al., 2017).

Infrastructure and biological vectors

Outdoor access

By giving a poultry population outdoor access, the risk of direct or indirect (via faeces) contact with wild living birds or other animals increases, together with the risk of disease transmission (Al-Saffar et al., 2006; Anonymous, 2010). Therefore, it is very important that poultry farms with outdoor access have an excellent fence. In this way, wild animals cannot come in contact with the on-farm poultry population (Anonymous, 2010; Van Steenwinkel et al., 2011).

Litter and disease transmission

Litter from a poultry house can be highly contaminated at the end of a production round with all kinds of infectious agents such as Avian Influenza, E. coli, Infectious Bronchitis virus and many others (Swayne and Suarez, 2000; Alexander, 2007; Lister, 2008). In many places, contaminated litter is spread on the surrounding agricultural fields. This will significantly increase the risk of disease transmission for the farms in the vicinity of those fields (Swayne and Suarez, 2000; Alexander, 2007; Charisis, 2008; Lister, 2008; Ssematimba et Al., 2012). In addition, contaminated litter is better not stored on the poultry farm itself (Charisis, 2008; Lister, 2008; Anonymous, 2010).

The risk of infection by spreading litter on the surrounding fields will be further affected by the wind direction, the presence of vermin or wild birds and the spreading through personnel or equipment (Vieira et al., 2009).

Rodents and disease transmission

Rodents play a significant role in both the mechanical and biological transmission of certain infectious agents. These species will be important for the spread of certain pathogens within a poultry farm but also for the introduction of pathogens from a neighboring farm (Amass and Baysinger, 2006; Meerberg and Kijlstra, 2007; Lister, 2008). Vermin, like rats, will often serve as a reservoir for farm-specific pathogens, and they will spread these pathogens in the environment (Andres and Davies, 2015). It was already clearly demonstrated that rodents are remarkable for the transmission of microorganisms such as Salmonella spp. and Campylobacter spp. (Liljebjelke et al., 2005; Meerberg and Kijlstra, 2007; Lister, 2008; Backhans and Fellstrom, 2012).

In the end, rodents can also cause damage to the equipment (electricity, isolation, structure,…) and the farm buildings, or they can be a source of feed waste when they have access to the feed (Carey, 2005; Meerberg and Kijlstra, 2007; Backhans and Fellstrom, 2012).

Measures for rodent control

To control vermin, an efficient control program is required. This is often developed in collaboration with specialized companies (Nespeca et al., 1997; Amass, 2005a; Carey, 2005; Lister, 2008; Sims, 2008; Van Steenwinkel et al., 2011; Dewulf, 2014; Filippitzi et al., 2017).

It is important to prevent that vermin can house in the neighborhood of the stables. This can be achieved by avoiding the presence of all types of hiding places near to the stables (e.g. plants, piles of dirt…). In addition, the feed should be stored in closed reservoirs with no access to rodents or birds (Nespeca et al., 1997; Charisis, 2008; Lister, 2008; Anonymous, 2010; Madec et al., 2010).

In order to minimize the contact with rodents and other wild animals, a proper fence or strong closure can be placed around the poultry houses or around the farm (Nespeca et al., 1997).

Birds and disease transmission

Wild, free-living birds can transmit pathogens directly or indirectly to the on-farm poultry population. Moreover, these birds can also damage the farm premises or equipment (Amass and Baysinger, 2006; Filippitzi et al., 2017). Examples of infectious diseases in which wild birds can have an important role are Avian influenza, Newcastle Disease, Mycoplasma spp., Campylobacter spp., and Salmonella spp. (Al-Saffar et al., 2006; Alexander, 2007; Charisis, 2008; De Jong et al ., 2009; Gelaude et al., 2014).

To keep birds (and rodents) out of the animal houses and to avoid direct or indirect contact with the poultry population, it is advisable to cover all air inlets with nets (Carey, 2005; Lister, 2008; Sims, 2008; Vangroenweghe et al., 2009; Madec et al., 2010). Ponds or other sources of stagnant water should also be covered with nets to keep migrant birds away, and to prevent them from using this water as a resting place (Carey, 2005; Lister, 2008). In addition, surface water from the environment should not be used as drinking water as it is likely contaminated by wild birds (Kapperud et al., 1993; East, 2007; Charisis, 2008; Lister, 2008; Sims, 2008; Van Steenwinkel et al., 2011). The presence of trees or other bushes in the vicinity of poultry houses should be avoided as they can provide protection to wild birds from the sun or rain (Anonymous, 2010).

It is strongly discouraged to keep hobby poultry on the premises of a commercial poultry farm, as these birds can be a major source of infection for the present commercial poultry (East, 2007; Van Steenwinkel et al., 2011). Furthermore, it is also important that the personnel of the farm does not keep hobby poultry at home. By coming in contact with their poultry at home, they can very easily introduce infectious agents to the farm (Charisis, 2008; Van Steenwinkel et al., 2011).

Pets and disease transmission

Pets (dogs and cats) can act as an indirect vector for infectious agents when they have the possibility to get into the poultry houses. In this way, they can carry infectious material to the sensitive poultry population at the farm (Vangroenweghe et al., 2009; Van Steenwinkel et al., 2011). So, the control of rats or mice by pets is not an ideal method and is therefore absolutely discouraged (Vangroenweghe et al., 2009). In addition, dogs and cats can also be a carrier of certain infectious diseases, such as Avian Influenza. These pathogens can be transferred to the poultry population through direct contact between the animals or indirectly through the feed (if pets have access to the feed storage places) (Röhm et al., 1995; Belser et al., 2008).

Other farm animals and disease transmission

Various infectious agents can be transferred between different animal species. For example, the presence of other farm animals on a poultry farm (pigs, cattle, etc.) may constitute a potential source of infection for the existing poultry population (Nespeca et al., 1997; Van Steenwinkel et al., 2011; Gelaude et al., 2014).

It has already been shown that Campylobacter jejuni can be transmitted between poultry and pigs or between poultry and cattle (Giessen et al., 1996, 1998; Boes et al., 2005). Avian Influenza has also been associated with keeping pigs in the vicinity of poultry houses (Peiris et al., 2001; Ninomiya et al., 2002; Charisis, 2008).

Furthermore, it is strongly discouraged to keep different types of poultry on the same farm premise because some pathogens are more or less pathogenic to a particular poultry species (Tablante, 2008). For example, a specific infectious agent may be less pathogenic to a chicken while it is highly pathogenic and dangerous to a turkey. This is the case with Histomonas meleagridis, also called blackhead disease (Mc Dougald, 2005). Another example is the different sensitivity to avian influenza. Chickens or turkeys are much more sensitive to this virus compared to ducks or pigeons (Van Steenwinkel et al., 2011).

Location of the farm

Birds and disease transmission

Wild, free-living birds can transmit pathogens directly or indirectly to the on-farm poultry population. Moreover, these birds can also damage the farm premises or equipment (Amass and Baysinger, 2006; Filippitzi et al., 2017). Examples of infectious diseases in which wild birds can have an important role are Avian influenza, Newcastle Disease, Mycoplasma spp., Campylobacter spp., and Salmonella spp. (Al-Saffar et al., 2006; Alexander, 2007; Charisis, 2008; De Jong et al ., 2009; Gelaude et al., 2014).

To keep birds (and rodents) out of the animal houses and to avoid direct or indirect contact with the poultry population, it is advisable to cover all air inlets with nets (Carey, 2005; Lister, 2008; Sims, 2008; Vangroenweghe et al., 2009; Madec et al., 2010). Ponds or other sources of stagnant water should also be covered with nets to keep migrant birds away, and to prevent them from using this water as a resting place (Carey, 2005; Lister, 2008). In addition, surface water from the environment should not be used as drinking water as it is likely contaminated by wild birds (Kapperud et al., 1993; East, 2007; Charisis, 2008; Lister, 2008; Sims, 2008; Van Steenwinkel et al., 2011). The presence of trees or other bushes in the vicinity of poultry houses should be avoided as they can provide protection to wild birds from the sun or rain (Anonymous, 2010).

It is strongly discouraged to keep hobby poultry on the premises of a commercial poultry farm, as these birds can be a major source of infection for the present commercial poultry (East, 2007; Van Steenwinkel et al., 2011). Furthermore, it is also important that the personnel of the farm does not keep hobby poultry at home. By coming in contact with their poultry at home, they can very easily introduce infectious agents to the farm (Charisis, 2008; Van Steenwinkel et al., 2011).

Airborne disease transmission

Since many important pathogens can be transmitted by airborne transmission, the location is considered to be a critical point in the external biosecurity at the farm (Hartung and Schulz, 2007; Vangroenweghe et al., 2009). Mycoplasma spp., Newcastle disease virus and Avian influenza virus are examples of pathogens in poultry that can be spread through the air (East et al., 2006; Tellier, 2006; Bradburry and Morrow, 2008; Sims, 2008).

The distance to neighboring poultry farms, the presence of animal transport along the public road in the environment of the farm and the dominant wind direction at the farm will further determine the probability of airborne disease transmission (Nespeca et al., 1997; Graham et al., 2008; Vieira et al., 2009; Lister, 2008; Van Steenwinkel et al., 2011). It is suggested that a minimum distance of 500 meters between two different poultry farms (preferably more than 1 km) may significantly reduce the risk of spread of infectious diseases. This distance also applies to the location of a farm with respect to hobby poultry farms (Lister, 2008; Van Steenwinkel et al., 2011).

Litter and disease transmission

Litter from a poultry house can be highly contaminated at the end of a production round with all kinds of infectious agents such as Avian Influenza, E. coli, Infectious Bronchitis Virus and many others (Swayne and Suarez, 2000; Alexander, 2007; Lister, 2008). In many places, contaminated litter is spread on the surrounding agricultural fields. This will significantly increase the risk of disease transmission for the farms in the vicinity of those fields (Swayne and Suarez, 2000; Alexander, 2007; Charisis, 2008; Lister, 2008; Ssematimba et Al., 2012). In addition, contaminated litter is better not stored on the poultry farm itself (Charisis, 2008; Lister, 2008; Anonymous, 2010).

The risk of infection by spreading litter on the surrounding fields, will be further affected by the wind direction, the presence of vermin or wild birds and the spreading through personnel or equipment (Vieira et al., 2009).

Internal biosecurity

Disease management

Vaccination and health status

Vaccinating a susceptible poultry population is an important part of good disease management, together with a proper euthanasia policy, removing the cadavers from the animal houses and controlling the stocking density (Carey, 2005; Gelaude et al., 2014). When poultry is vaccinated in a correct manner, there will be less losses due to illness or mortality (Morton, 2007; Cserep, 2008). In addition, vaccination has a positive effect on animal welfare and can lead in combination with other measures to the eradication of a particular infectious disease (Capua and Maranoon, 2006; Morton, 2007; Cserep, 2008).

The risk of a disease outbreak is usually smaller in areas where the poultry population is vaccinated against a particular infectious disease (for example Avian influenza) compared to areas or farms where no vaccination is performed. This can be explained by the fact that correct vaccination decreases the number of animals, which secrete the virus, and reduces the overall infection pressure for this disease in the vaccinated area (Van der Goot et al., 2005; Ellis et al., 2006).

It is crucial that the poultry owner has some notion of the maternal immune level of the animals when the poultry population is vaccinated (Al-Saffar et al., 2006; Butcher and Miles, 2012). In the case of strong maternal immunity, the action of the vaccine may be buffered by the present maternal antibodies (causing a great reduction of the function of the vaccine). In the case of weak maternal immunity, a severe reaction can be triggered by applying the vaccine. Protective antibody titers are generally formed only 12 days after vaccination. Therefore, animals with a weak maternal immunity can be susceptible to certain pathogens for up to 12 days after vaccination (Butcher and Miles, 2012).

In addition to vaccination, it is also valuable to know the disease status of the poultry farm. In this way, a good population health can be ensured and it gives the opportunity to intervene on time where it is needed (possibly by (re)introducing a particular vaccination protocol) (Carey, 2005; Al-Saffar et al., 2006).

Cadaver and disease transmission

Cadavers or carcasses are always a major source of infectious material. Animals often die due to an infection and potentially spread thereby a lot of infectious material. It is therefore strongly advised to remove cadavers from the animal houses as soon as possible and to store them in a well-insulated place (Meroz et al., 1995; Nespeca et al., 1997; Evans and Sayer, 2000; Pritchard et al., 2005; Anonymous, 2010). Dead animals should be removed from the stable at least twice a day, and if there are many cases of acute mortality, this frequency should be increased.

The cadaver storage room should be located as far as possible from the stables in a place where the rendering company can collect the cadavers without entering the farm to avoid disease introduction through these potentially risky transports (Evans and Sayer, 2000; Gibbens et al., 2001; Anonymous, 2010).

Make sure that no vermin can reach the cadavers (by a well-closed, cooled storage) because it could spread the infectious material (Evans and Sayer, 2000). After the collection of the cadavers, it is advisable to thoroughly clean and disinfect the cadaver storage room. The person manipulating the cadavers should always wear disposable gloves for their own safety as well as to avoid further spread of pathogens (Anonymous, 2010; Gelaude et al., 2014; Pritchard et al., 2015).

Stocking density

The stocking density of a poultry population will notably affect the size of a disease outbreak (Sims, 2008; Van Steenwinkel et al., 2011). When poultry is housed close together, stress will be induced in these animals. As a result, their susceptibility to various infectious diseases increases and the poultry population will secrete more pathogens. For this reason, the overall on-farm infection pressure will increase dramatically at long last (Gelaude et al., 2014). It has been shown already in an old study that reducing the stocking density of a poultry population is a great measure for controlling infectious diseases (Kouwenhoven et al., 1978).

When the on-farm stocking density is too high, it will have an effect on the transmission of certain infectious diseases, but it will also affect the production results of a poultry farm. It has been shown that there are more problems with bone and muscle production in a poultry population with a high stocking density, which results in more fractures and cases of bowed legs. There will be also more problems with footpad dermatitis and the daily weight gain of the animals will be less good compared to a poultry stable with a lower stocking density (Van Poucke et al., 2010).

Legal maximum standards for the stocking density in broilers (Van Gansbeke and Van den Bogaert, 2001):

Age and sensitivity

Animals of different ages may have different levels of sensitivity to certain pathogens (Dewulf, 2014; Filippitzi et al., 2017). On the one hand, this has to do with the fact that certain receptors are only present at specific times in the life of a chicken. On the other hand, the maternal immunity (the protection of the chicks by maternal antibodies in the egg) will disappear over time (Dewulf, 2014). Therefore, it is crucial to keep different age groups separate in order to avoid the transmission of pathogens to each other (Dewulf, 2014; Filippitzi et al., 2017). In addition, the work on the farm should be performed from the youngest poultry population to the oldest poultry population (Carey, 2005). The most ideal situation for a poultry farm is the presence of only one age at the same time in the same animal house or even better, one age throughout the whole farm (Nespeca et al., 1997; East et al., 2006; East, 2007).

Cleaning and disinfection

Transport and disease transmission

Livestock lorries drive from farm to farm and increase in this way the risk of disease spreading (Anonymous, 2010; Gelaude et al., 2014). Several epidemiological studies have shown that transport can play a major role in the introducing of certain infectious germs to a farm (Rajkowski et al., 1998; Fritzemeier et al., 2000; Hege et al., 2002). Poultry may only be transported with vehicles that are thoroughly cleaned and disinfected. In addition, all dead animals, all contaminated litter and all manure must be removed from the transport vehicle (Carey, 2005; Pritchard et al., 2005; Anonymous, 2010; Gelaude et al., 2014; Dewulf, 2014).

It is also recommended to clean and disinfect the tires of the transport vehicle before entering the farm (Carey, 2005; Lister, 2008; Sims, 2008; Dorea et al., 2010).

Cleaning and disinfection of the animal houses

In order to control infectious diseases at a poultry farm and break through the infection cycle of a pathogen, these separate steps in the cleaning of the animal houses are very important: thoroughly cleaning (dry and wet), thoroughly disinfecting and allowing adequate time for the stables to dry out (Meroz et al., 1995; Luyckx et al., 2015b).

Having a vacancy period between two production rounds is a good measure to reduce the number of present pathogens in the animals houses. However, the efficiency of a vacancy period depends on the type of pathogen. For pathogens like Coryza and Mycoplasma spp., this vacancy period can be very important because these pathogens do not survive for a long time in the environment. For the infectious germs which can remain for a long period in the environment, the cleaning and disinfection is really crucial to fight against those germs (e.g. gumboro, coccidiose) (Butcher and Miles, 2012). It is recommended to maintain a vacancy period for at least one week after cleaning and disinfection (Meroz et al., 1995).

However, in a European study on risk factors for Campylobacter infestation in poultry houses, it has been shown that a vacancy period of more than 10 days has a negative effect on the colonization of this infectious germ. This can be explained, for example, by the fact that companies with a longer vacancy period (> 10 days) often maintain a less thoroughly cleaning and disinfection of their stables (Luyckx, 2016; Sommer et al., 2016).

If the cleaning and disinfection has not been carried out thoroughly, infectious germs can persist in the poultry houses. This can cause many problems with the poultry population in the next production round (by contact with residual manure, dust or feathers) (Hoff and Akin, 1986; Meroz et al. ., 1995; Carey, 2005; Tablante, 2008). However, some infectious germs can survive for a long time in the environment without the presence of animals (Jeffrey, 1997; Butcher and Miles, 2012). Drinking water nipples, drainage holes and cracks in the floor of the stables are the most critical locations where bacteria can be found after cleaning and disinfection. As follows, special attention should be paid to the cleaning and disinfecting at these places (Mueller-Doblies et al., 2010; Luyck et al., 2015b).

An optimal cleaning and disinfection protocol consists of seven steps:

1. dry cleaning to remove all organic material,

2. soaking of all surfaces preferably with detergent,

3. high pressure cleaning with water to remove all dirt. This step will go much easier, faster and effective if a good soaking step is performed before,

4. drying of the stable to avoid dilution of the disinfectant applied in the next step,

5. disinfection of the stable to achieve a further reduction of the concentration of the germs,

6. drying of the stable to assure that animals afterwards cannot come into contact with pools of remaining disinfectant,

7. testing of the efficiency of the procedure through sampling of the surface (hygienogram). If all previous steps are performed correctly, it is not necessary to provide an additional vacancy period (Luyckx, 2016).

Cleaning and disinfecting should be considered as two separate steps. Each step needs a specific chemical product, which cannot be in contact with each other (Anonymous, 2002). Considerable attention should be paid to removing all the organic material during the cleaning process as organic material can deactivate the used detergent. In addition, the presence of fat will provide a good protection for bacteria and other microorganisms (Böhm, 1998; Anonymous, 2002).

Not only the inside of the poultry houses (including the drinking water pipelines, feed silos and the present feed system) should be thoroughly cleaned and disinfected, also the surroundings around the stables should be take care of in a proper way (paved parts around the stable, loading places, etc.) (Studer et al., 1999).

Hygienogram

A good cleaning and disinfection is not always easy at a poultry farm. To evaluate the efficiency of the cleaning protocol, a hygienogram can be used. To make a hygienogram, samples are taken with the aid of RODAC pressure plates from all possible areas within the farm. These plates measure and quantify the presence of bacterial contamination (total aerobic flora) present after the cleaning and disinfection of the farm buildings (Vangroenweghe et al., 2009; Luyckx et al., 2015a). The results are expressed in colony-forming units (CFU) per plate.

Agar contact plates (such as RODAC plates) are often used as a standard evaluation for the cleaning and disinfection, but in fact, contamination levels can be better evolved by enumeration of bacteria via swabs (Luyckx et al., 2015a).

This is the hygienogram scoring system for a poultry farm (Anonymous, 2017):

The hygiene lock

The hygiene lock, where visitors should put on company clothes and shoes, is especially intended to decrease the risk of mechanical disease through persons (Hald et al., 2000; Evans and Sayer, 2000; Vangroenweghe et al., 2009).

The location of the hygiene lock within the farm is extremely important because the stables may only be accessible after going through this lock. So, the hygiene lock has simply one entrance and one exit and the room can be divided into a clean and dirty. Furthermore, attention should be paid to the presence of a sink, as hand hygiene (cleaning and disinfecting of your hands) is really essential for the on-farm biosecurity (Vangroenweghe et al., 2009).

Separation between the clean and dirty area

The principle of the clean and dirty road on a poultry farm means that there is a clear separation between the clean and the dirty (risky) sections of the premises (Carey, 2005; Al-Saffar et al., 2006; Ssematimba et al., 2013). Poultry transport vehicles are constantly in contact with other farms and slaughterhouses. This creates in that way a quite extensive risk for disease transmission (Amass, 2005b; Pritchard et al., 2005; Anonymous, 2010; Gelaude et al., 2014). All inbound and outbound traffic that serves multiple companies (feed, manure, external transportation of animals…) are always lead via the dirty road. The clean road is preserved for the possible supply of animals and harmless products (internal movements at the farm) and only in fully cleaned and disinfected lorries (Pritchard et al., 2005; Al-Saffar et al., 2006).

Only the 'dirty road' is relatively easily accessible to visitors, suppliers and consumers. The cadaver collecting is for obvious reasons part of the dirty section (cadaver storage box, loading point). Barrels and other tools used for this may only be returned to the clean section after they have been thoroughly cleaned and disinfected (Pritchard et al., 2005; Al-Saffar et al., 2006). The removal of manure is always conveyed via the dirty road (Pritchard et al., 2005).

Boot washers and disinfection baths

In order to prevent the spread of pathogens through footwear, boot washers and disinfecting baths can be placed at the entrance of each poultry house (Nespeca et al., 1997; Vangroenweghe et al., 2009). If disinfection baths are not properly used and maintained, it is really wasted money because these baths can even be a possible transmission pathway for infectious germs (Vangroenweghe et al., 2009).

An efficient disinfection can only be achieved if dirt and faeces are removed from the boots in advance. This can be done with a boot washer and water (preferably with adding a detergent) (Amass et al., 2000; Anonymous, 2010). Next, the boots have to be placed in a visually clean solution with an disinfectant. This protocol requires that the concentration of the disinfectant and the duration of the cleaning have to be followed as instructed by the manual of the disinfectant (Amass et al., 2000). The disinfectant in the disinfection baths should be refreshed at regular intervals, especially when the liquid has become visibly dirty, as direct contact with organic material will inactivate the disinfecting effect (Vangroenweghe et al., 2009; Anonymous, 2010).

Materials and measures between compartments

Material and disease transmission

Germs can find an entrance to a farm via the supply of materials. This happens especially when the material was previously in contact with poultry or when it was manufactured or packaged at other poultry farms (Pritchard et al., 2005).

To prevent that pathogens will be transferred from one company to another, it is advised to use proprietary, farm-specific material. It is also recommended to provide this material to anyone who needs it at the farm (Lister, 2008; Gelaude et al., 2014; Sommer et al., 2016). However, if non-proprietary material has to be introduced at the farm or to certain poultry houses, this can be done via specific hatches with disinfectant UV radiation (Filippitzi et al., 2017).

Equipment in the various animal houses

The transmission of pathogens can easily occur indirectly through all the material used in a poultry farm (Laanen, 2011; Gelaude et al., 2014; Filippitzi et al., 2017). A floating panel or shovel can be quickly contaminated with manure that contains all kinds of infectious germs. To avoid internal disease transmission, it is important to work with different materials in the different animal houses of the herd and to avoid using the same material in different age groups. Therefore, it is advisable to use equipment that is clearly recognizable (different colors) to prevent moving it from one section (stable or animal house) to another (Vangroenweghe et al., 2009; Laanen, 2011; Gelaude et al., 2014).

Farm clothing

As humans can serve as a mechanical vector for the transmission of infectious diseases, it is recommended to take specific biosecurity measures the moment they enter a farm (Lister, 2008; Anonymous, 2010).

When visitors and personnel enter the stables, they should always wear clean, herd specific (farm-specific) clothes and footwear to avoid disease transmission through leftovers of excreta from other infected animals (Nespeca et al., 1997; Lister, 2008; McDowell et al., 2008; Sims, 2008; Dorea et al., 2010).

Egg management

Eggs

Misshapen, dirty or cracked eggs are unsuitable for hatching and should be removed because they are a risk of contamination (Hafez, 2007; de Gussem et al., 2013; De Lange, 2015).

Disease transmission between human and animals

It is widely known that humans can serve as vector for infectious germs and as such can transfer pathogens from humans to animals or between animals. For this reason, the number of visitors on a farm or a stable must be limited. People should be kept away from the animals as much as possible and any unnecessary visit should be avoided (Carey, 2005; Lister, 2008). A human can serve both as a mechanical and as a biological vector for the transmission of infectious diseases at the farm (Amass, 2005b; Lister, 2008). Visitors have played an important role in, among other things, the spread of Avian Influenza in 2003 (Thomas et al., 2005; Vieira et al., 2009).

Humans can act as mechanical vectors (for example, by carrying faeces on boots) if they have been in contact with infected animals and subsequently switch to susceptible animals without taking any measures. The transmission happens mainly through leftovers of excreta from infected animals on footwear and clothing (Lister, 2008). Biological transmission of pathogens that can infect both human and poultry can occur, such as the Avian Influenza virus and Salmonella spp.(Grunkemeyer, 2011).

Material and disease transmission

Germs can find an entrance to a farm via the supply of materials. This happens especially when the material was previously in contact with poultry (Pritchard et al., 2005).

To prevent transfer of pathogens from one company to another, it is advised to use, farm-specific material. It is also recommended to provide this material to anyone who needs it at the farm (Lister, 2008; Gelaude et al., 2014; Sommer et al., 2016). If non-proprietary material has to be introduced at the farm or to certain poultry houses, this can be done through a specific sanitary lock with disinfectant UV radiation (Filippitzi et al., 2017).

Farm equipment

The transmission of pathogens can easily occur indirectly through all the material used in a poultry farm if it is not thoroughly cleaned and disinfected after use (Laanen, 2011; Gelaude et al., 2014; Filippitzi et al., 2017). Ideally, egg trays for transport are disposable and should not return to the egg room. Shared egg trays were identified as risk factors for the spread of Avian Influenza (Singh et al., 2018). If no disposable material is used, thorough cleaning and disinfection needs to be performed before used again.

Contact with poultry house

It is very important that there is as little contact as possible between the persons employed in the poultry houses and those working in the egg collecting room. Since external contamination of the eggs from the environment can occur and bacteria can migrate to the internal egg through the pores or cracks in the egg shell. Horizontal transmission can also occur through other vectors: dust, insects, rodents, trays, transport equipment. If the same persons are employed in the poultry house and the egg collection room, necessary hygiene measures should be taken before moving from one unit to the other.