About Biosecurity_Pig

What is biosecurity?

Biosecurity at a pig farm includes all measures taken to minimize the risk of introduction and spread of infectious agents and thus, includes all actions for keeping the pigs 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 pathogens from entering or leaving 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 main objective of biosecurity is to keep the infection pressure at the farm as low as possible. Due to the reduction of the overall infection pressure, the immune system of a certain animal will be less stressed, resulting in a lower risk of a disease outbreak and, consequently, 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 correlation with the production results (such as daily growth) and the profitability of the farm. Along with this, the use of antibiotics can be greatly reduced (Laanen et al., 2011; 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 antibiotic resistance which is profitable both for animal health and human health (Angulo, 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 terms of 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 animals (Dewulf and Van Immerseel, 2018).

Direct contact between animals is considered to be the main transmission pathway 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, working 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 pathogen 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%.

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, one 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 breeding pigs, piglets and semen

Disease transmission from animal to animal

Direct contact between infected and susceptible animals is the most effective way to transmit a pathogen (Pritchard et al., 2005; Amass et al., 2006). The introduction of non-proprietary pigs or new genetic material (e.g. sperm) will pose a considerable risk to the on-farm pig population that has not yet developed an immunity against a particular pathogen or infectious agent (Filippitzi et al., 2017). Infected pigs spread the pathogens through all sorts of excretions and secretions, including saliva, manure, urine, semen ... When a sensitive pig is exposed to these excretions/secretions or when there is direct nose-nose contact between two animals, the pathogens can easily be transferred from one animal to another (Dewulf et al., 2017).

Avoid the purchase of animals

Pathogen transmission occurs very effectively via direct contact between infected and susceptible animals. Consequently, it is strongly advised to introduce as few animals as possible (preferably no animals) to the farm (Amass et al., 2006; Dewulf, 2014; Filippitzi et al., 2017). A fully closed herd or production system has a substantially lower risk of disease introduction because no animals are added to the on-farm pig population. (Amass, 2005a). The frequent addition of “naïve” animals may also favor the continuous circulation of herd-specific pathogens (Dewulf, 2014).

Limit the frequency of introduction

Both the frequency of introducing new animals and the number of animals (or genetic material such as semen) acquired, will influence the risk of disease introduction, 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 pig farm (Fèvre et al., 2006; Laanen et al., 2013).

Limit the number of source herds

It is of high importance to limit the number of source herds of animals and of semen, as much as possible to avoid the introduction of certain infectious diseases (Dewulf, 2014). Several studies have shown that introducing animals from different source herds increases the risk of disease introduction into the pig farm (Hege et al., 2002; Lo Fo Wong, 2004).

Source herds with a high health status

New animals or genetic material, should always come from a farm with an equal or higher sanitary status than the own farm (Pritchard et al., 2005; Kirwan, 2008; Dewulf, 2014). Farms with a 'high health status' focus on producing ‘healthy pigs'. This status may include the certified absence of a number of infectious diseases (specific pathogen free status (SPF)) and avoid the unintended introduction of new pathogens in the recipient herd (Laanen et al., 2010; Filippitzi et al., 2017).

In Denmark, one is already very familiar with this SPF-concept. This system indicates that the health level of the farm is constantly monitored and that everyone can consult the data/results of this SPF-monitoring program. The SPF-farms in Denmark often implement much stricter biosecurity protocols in order to comply with this concept and to ensure that the farm is at a lower risk for disease introduction (Filippitzi et al., 2017).

Respect a good quarantine

Newly purchased animals should always be introduced first in a quarantine stable. During the quarantine period, animals should be clinically inspected to assure that no symptoms of any diseases are present or will emerge. In this way, it is possible to learn something more about the immunity status of the animals (Barceló and Marco, 1998; Corrégé, 2002; Pritchard et al., 2005; Calvar et al., 2012; Dewulf, 2014).

The pigs can be evaluated thoroughly before they are introduced into the existing population and the chance of disease transmission will decrease. Animals can be sampled during the quarantine period to detect any current infections, for example by serology or manure examination. This is important for recognising subclinical carriers and at the same time, the pigs can be tested to find out if they are protected against the infectious agents that circulate at the farm. Moreover, the quarantine period can also be used to vaccinate the newly introduced animals in order to assure a sufficient level of immunity when brought into contact with the resident animals (Pritchard et al., 2005; Dewulf, 2014).

Quarantine period

In order to establish the ideal duration of the quarantine period, the incubation time of a specific disease (time between infection and onset of clinical symptoms) and the maximum excretion period must be taken into account (Amass, 2005a). A quarantine period should generally last at least 4 weeks, however for some diseases longer periods are required (PRRSV and PCV2 – 6 to 8 weeks; M. hyopneumoniae – 8 to 10 weeks) (Eijck, 2003; Pritchard et al., 2005).

Disease transmission by genetic material

Artificial insemination in sows is a widespread technique that is used as a reproductive tool in many intensive pig farms. This technique offers a number of advantages: the implementation of new superior genetic material, a better disease control and it is economically more beneficial. However, these benefits can also be considered as a disadvantage at some point, because semen can be seen as a transmission pathway for certain infectious diseases or genetic defects (Pritchard et al., 2005; Maes et al., 2008; Althouse and Rossow, 2011). Many pathogens have already been isolated from boar semen and for a number of these, their transmission via semen has been clearly established:

Classical swine fever virus (de Smit et al., 1999; Pritchard et al., 2005; Maes et al., 2008; Althouse and Rossow, 2011; Dewulf et al., 2017), porcine parvovirus (Lucas et al., 1974; Maes et al., 2008; Althouse and Rossow, 2011), porcine reproductive and respiratory syndrome virus (Swenson et al., 1994; Christopher-Hennings et al., 1995; Prieto et al., 1997; Christopher-Hennings et al., 2001; Pritchard et al., 2005; Maes et al., 2008; Althouse and Rossow, 2011), Aujeszky disease virus (Maes et al., 2008; Althouse and Rossow, 2011), brucellosis, … (Maes et al., 2008; Althouse and Rossow, 2011).

Therefore, the company that delivers the semen should be free from these diseases. So, high sanitation requirements are demanded at the artificial insemination centers to avoid disease transmission via semen.

Transport of animals, removal of carcasses and manure

Pig transport and disease transmission

Livestock lorries drive from farm to farm and increase in this way the risk of disease spreading. Pigs should 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 (Pritchard et al., 2005; Dewulf, 2014).

A crucial point in the spread of infectious diseases through pig transport, is the cleaning and disinfection between the different transports of pigs coming from other farms and after the unloading of pigs at the slaughterhouse. If this is not performed thoroughly, it poses a high risk of disease transmission, since different pig populations have indirectly contact with each other (Rajkowski et al., 1998; Amass, 2005b; Dewulf, 2014).

Downtime for pig transport

It is sometimes stated that a lorry for transport of livestock should have been empty for at least a couple of hours or days before it can enter the farm (Amass et al., 2007). Due to the nature/characteristics of intensive pig production, transport vehicles often get very busy, which makes this measure not to be applied often. If a transport vehicle is cleaned and disinfected properly, this biosecurity measure is soon omitted. However, if one is not absolutely sure whether a good cleaning and disinfection is performed or if the weather conditions hamper a thorough cleaning and disinfection, a downtime of 24 hours may be appropriate. All this can provide an additional risk reduction, however it is clear that a thorough cleaning, disinfection and drying step are the principal measures that cannot be replaced by a certain “downtime”.

Separation between the clean and dirty area

The principle of the clean and dirty road on a pig farm means that there is a clear separation between the clean and the dirty (risky) sections of the premises (Hémonic et al., 2010; Anonymous, 2010; Neumann, 2012; Filippitzi et al., 2017). Pig transport vehicles are constantly in contact with other farms and slaughterhouses so, this creates a quite extensive risk for disease transmission (Amass, 2005b; Neumann, 2012). All inbound and outbound traffic that serves multiple companies (feed, liquid manure, external transportation of animals,…) are always led 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; Maes, 2016).

Only the 'dirty road' is relatively easily accessible to visitors, suppliers and consumers. The cadaver collecting process 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. Liquid manure is always conveyed via the dirty road. Furthermore, it is advised to use farm-specific discharge pipes in order to prevent that pipes of the manure removal company, which recently have been in contact with manure on other farms, are also used on your farm (Pritchard et al., 2005).

Loading bay

Pigs that have been in contact with the lorry during loading may not be moved back to the stable in order to minimize the chance of introducing pathogens through an insufficiently cleaned lorry. Departing animals should be moved to a designated and demarcated loading area (the loading bay), as far as possible from the stables. In this way, it can be better prevented that pigs return to the stables and the clean/dirty principle remains in place. The loading bay should be cleaned and disinfected after every load of animals (Pritchard et al., 2005; Backhans et al., 2015).

Cadaver and disease transmission

Cadavers are always a major source of infectious material. Animals often die due to an infection and will potentially spread a lot of infectious material even after their death. It is therefore strongly advised to remove cadavers from the stables as soon as possible and to store them in a well-insulated place (Meroz et al., 1995; Pritchard et al., 2005).

The cadaver storage 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. This to avoid disease introduction through these potentially risky transports (Evans and Sayer, 2000; McQuiston et al., 2005; Pritchard et al., 2005; Anonymous, 2010; Maes, 2016).

Make sure that no vermin can reach the stored cadavers 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 (Pritchard et al., 2015; Filippitzi et al., 2017).

Cooled cadaver storage

A cooled cadaver storage room has several advantages over a non-cooled one. First, a fully closed system prevents the spread of infectious agents and it is effective in avoiding contact with vermin. Moreover, these cooled systems reduce the rotting process, ensuring less odor development and lowering the frequency of visits of the rendering company (Vangroenweghe et al., 2009a).

Transport vehicle of the rendering company

The truck of the rendering company transports several contaminated carcasses from several pig farms a day. Thus, this vehicle represents a major source of contamination for a farm. The cooled cadaver storage area (at the dirty section of the pig farm), should be located at the public road in order to avoid that the vehicle of the rendering company must enter the farm area. These transport vehicles have to stay as far away as possible from the pig stables (at the clean area of the pig farm) (Evans and Sayer, 2000; McQuiston et al. 2005; Pritchard et al., 2005; Anonymous, 2010; Maes, 2016).

Feed, water and equipment supply

Feed and disease transmission

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

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 a pig farm (Pritchard et al., 2005; Backhans et al., 2015). 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 (Hémonic et al., 2010; Anonymous, 2010).

Feed itself should generally not pose a risk due to the strict hygienic conditions of its production, however swill feeding (banned for decades under EU law) is a practice which has previously been associated with large outbreaks of infectious diseases, such as classical swine fever and African swine fever (Horst et al., 1997; Fritzemeier et al., 2000; Filippitzi et al., 2017; Dewulf et al., 2017).

Water and disease transmission

Pathogens can easily spread through contaminated drinking water to the different animal groups at the farm. 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 (Dewulf, 2014). The water well, the storage tank and the pipelines must be completely and properly closed in order to ban dust, pests or wild birds. In this way, vermin and dust cannot be a source of contamination for the drinking water of the animals (Lister, 2008).

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 pig population (Hémonic et al., 2010; Gelaude et al., 2014). Regular (preferably twice a year) examination of the drinking water quality both at the entrance and at the nipples is therefore definitely advisable, also a systematic cleaning of the pipes is advised (Pritchard et al., 2005; Hémonic et al., 2010; Dewulf, 2014; Gelaude et Al., 2014; Backhans et al., 2015).

Material and disease transmission

Infectious agents can find an entrance to a farm through the supply of materials. This happens especially when the material was previously in contact with pigs or when it was manufactured or packaged at other pig farms (Pritchard et al., 2005; Filippitzi et al., 2017).

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

Separation between the clean and dirty area

The principle of the clean and dirty road on a pig farm means that there is a clear separation between the clean and the dirty (risky) sections of the premises (Hémonic et al., 2010; Anonymous, 2010; Neumann, 2012; Filippitzi et al., 2017). Pig transport vehicles are constantly in contact with other farms and slaughterhouses so, this creates a quite extensive risk for disease transmission (Amass, 2005b; Neumann, 2012). All inbound and outbound traffic that serves multiple companies (feed, liquid manure, external transportation of animals,…) are always led 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; Maes, 2016).

Only the 'dirty road' is relatively easily accessible to visitors, suppliers and consumers. The cadaver collecting process 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. Liquid manure is always conveyed via the dirty road. Furthermore, it is advised to use farm-specific discharge pipes in order to prevent that pipes of the manure removal company, which recently have been in contact with manure on other farms, are also used on your farm (Pritchard et al., 2005).

Visitors and farmworkers

Disease transmission between human and animal

It is widely known that infectious agents can be transferred from humans to animals and vice versa. 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 away as possible from the animals. A human can serve both as a mechanical and as a biological vector for the transmission of infectious diseases at the farm (Amass et al., 2003b; Amass, 2005b; Lister, 2008; Anonymous, 2010; Dewulf, 2014; Maes, 2016).

Humans can act as mechanical vectors if they have been in contact with infected animals and subsequently switch to susceptible animals without taking any precautious measures (Amass et al., 2003b; Pritchard et al., 2005; Dewulf, 2014). The transmission happens mainly through leftovers of excreta from infected animals on footwear and clothing. This type of transmission has been proven through experiments for several infectious agents, among which E. coli (Amass et al., 2013b) and classical swine fever virus (Ribbens et al., 2007).

The chance of biological transmission between persons and pigs exists for infectious agents that can infect humans as well as pigs, such as the H1N1 influenza virus (Wentworth et al., 1997) or methicillin-resistant Staphylococcus aureus (MRSA) ST398. Both human and animal can be subclinical carrier of this MRSA ST398. Mutual transfer between humans and pigs via direct contact is assumed (Huijsdens et al., 2006).

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 people enter a farm (Amass, 2005b; Lister, 2008; Anonymous, 2010; Dewulf, 2014; Maes, 2016).

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 (Pritchard et al., 2005; Hémonic et al., 2010; Dewulf, 2014; Maes, 2016).

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 transfer through persons (Vangroenweghe et al., 2009a).

The location of the hygiene lock within the farm is extremely important because the stables or compartments may only be accessible by going through this lock. So, the hygiene lock has only one entrance and one exit and the room can be divided into a clean and dirty part (Vangroenweghe et al., 2009a; Hémonic et al., 2010; Filippitzi et al., 2017). 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., 2009a; Hémonic et al., 2010; Backhans et al., 2010).

In addition, in a pig farm with a high biosecurity status, one has to shower before accessing the stables. This measure is interesting as a sanitary measure but will also encourage visitors to put on some proprietary clothing and footwear (Vangroenweghe et al., 2009a; Hémonic et al., 2010; Maes, 2016).

Layout of the hygiene lock

By organizing and using a hygiene lock efficiently, the transmission of infectious agents by the farmer, visitors or the animals themselves can be restricted in a proper way (Maes, 2016).

When entering the hygiene lock the following steps should be respected:

1. When entering the lock, take off your jacket and footwear
2. Wash your hands with disinfecting soap
3. Step over the bench and put on a clean overall and boots
4. Disinfect the boots with the boot brush before entering the stable
5. When returning to the hygiene lock, clean and disinfect the boots with the boot brush
6. Stay with your wet boots on the grid
7. Put the boots on the appropriate shelve
8. Take off the dirty overall and put it in the laundry basket
9. Step over the bench and wash your hands before you put on your own jacket and shoes again

Hand hygiene

When visitors and personnel enter the stables, they should at least wash their hands properly. This latter is a simple and very useful measure, which is often forgotten. The hands of animal caretakers are surely an efficient way to transfer infectious agents through direct contact with the animals. So, when changing clothes and footwear in the hygiene lock, it has to be a habit to wash the hands both on arrival and on departure (Amass et al., 2003b; Vangroenweghe et al., 2009a; Anonymous, 2010).

A study by Lo Fo Wong et al. (2004) has shown that the chance of testing positive for Salmonella is reduced when you wash your hands consistently before entering a section with pigs.

Pig downtime

Often a pig-free downtime of 24 or even 48 hours is required for visitors at farms with a high health status before they can have access to the farm (Amass, 2005a; Pritchard et al., 2005; Vangroenweghe et al., 2009a). This is based on the argumentation that infectious agents excreted by pigs can survive on human beings for a specific period. During this period, persons could passively excrete this infectious material and transfer them to susceptible animals (foot and mouth disease and M. pneumonia, respectively 11 to 30 hours) (Sellers et al., 1971; Moore, 1992).

However, it is questionable if this pig-free period actually has its benefits. Several studies have shown that setting up a downtime has no effect on the transmission of different infectious agents when other appropriate biosecurity measures were implemented at the farm (proprietary clothes, washing of the hands, showering if necessary). This could be demonstrated for E. coli, foot and mouth disease, TGEV, M. hyopneumoniae and PRSSV (Alvarez et al., 2001; Otake et al., 2002; Amass et al., 2003b; Batista et al., 2004). This measure is often regarded as a kind of insurance for the pig owner or it is seen as a way to keep unwanted visitors away. However, if a large number of farms actually require a pig-free period (at least 24 hours), this will become difficult for many professionals to work efficiently (veterinarians, advisors, etc.) (Pritchard et al., 2005; Vangroenweghe et al., 2009a).

Take a shower before entering the stables

In companies with high health standards, visitors and personnel are often obliged to shower before entering the farm. The main benefit of this obligation is the certainty that all possibly contaminated clothing will be exchanged for farm-specific clothing and that the hands are washed thoroughly (Vangroenweghe et al., 2009a; Hémonic et al., 2010; Maes, 2016). In addition, it discourages less urgent visits (Moore, 1992; Amass and Clark, 1999).

Vermin and bird control

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 pig farm but also for the introduction of pathogens from a neighboring farm (Amass and Baysinger, 2006; Vangroenweghe et al., 2009a). 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 is clearly demonstrated that rodents are remarkable for the transmission of microorganisms such as B. hyodysenteriae (pig dysentery), leptospirosis, PRRSV, Salmonella, E. coli and L. intracellularis (Davies and Wray, 1995; Pritchard et al., 2005; Davies And Cook, 2008; Vangroenweghe et al., 2009a; Anonymous, 2010).

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 it (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 (Amass, 2005a; Lister, 2008; Hémonic et al., 2010; Dewulf, 2014; Backhans et al., 2015; 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 for rodents or birds (Lister, 2008; Anonymous, 2010).

Birds and disease transmission

In the same way as rodents do, birds can transmit pathogens directly or indirectly to a pig population. In addition, they can damage the farm buildings or equipment (Amass and Baysinger, 2006; Filippitzi et al., 2017). Examples of infectious diseases in which birds play an important role are Bordetella spp., Avian tuberculosis, Salmonella spp. and Avian influenza (Pensaert et al., 1981; Vangroenweghe et al., 2009a; Anonymous, 2010). To keep birds (and also rodents) out of the stables, it is recommended to cover all air inlets with nets (Vangroenweghe et al., 2009a; Anonymous, 2010).

Pets and disease transmission

Pets (dogs and cats) can act as an indirect vector for infectious agents when they have access to the stables. In this way, they can carry infectious material to the sensitive pig population at the farm. So, the control of rats or mice by pets is not an ideal method and is therefore absolutely discouraged (Vangroenweghe et al., 2009a).

Insects and disease transmission

Insects can also play an essential role in the transmission of multiple pig-associated pathogens at the pig farm. Several studies have shown that microorganisms such as Salmonella spp., TGEV, S. suis, PRRSV and PCV2 are carried and transmitted by specific invertebrates (often flies from the Muscidae family) (Otake et al., 2003; Dee et al., 2004; Pritchard et al., 2005; Forster et al., 2007; Blunt et al., 2011).

To ban insects from the farm buildings (not only in the stables, but also in the hygiene lock and other annexes), it is important to use different types of insecticides and other measures such as providing meshes on all windows or air inlets (Vangroenweghe et al ., 2009a; Gelaude et al., 2014).

Location of the farm

Airborne disease transmission

As many important pathogens can be transmitted by air, the location is considered to be a critical point in the external biosecurity at the farm (Julio and Santiago, 2003; Hartung and Schulz, 2007; Vangroenweghe et al., 2009a). M. hyopneumonia, classical swine fever and PRRSV are examples of pathogens in pigs that can be spread through the air (Ribbens et al., 2004; Desrosiers, 2005; Dee et al., 2009).

The type of nearby pig farms, the number of pigs in these neighboring farmhouses and the density of the pigs in these companies can be considered as really crucial for the introduction of infectious agents at the own farm (Pritchard et al., 2005; Backhans et al., 2015).

The distance to these neighboring pig 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. A study found out that the number of farms within a two-kilometer radius has a significant influence on the frequency of respiratory diseases at the farm (Rose and Madec, 2002). It is further suggested that a minimum distance of 500 meters between two different pig farms may significantly reduce the risk of spread of infectious diseases (Pritchard et al., 2005; Vangroenweghe et al., 2009a).

Manure and disease transmission

Pathogens such as E. coli, PRRSV, Salmonella and many others, can survive in faeces for a long period and can be transmitted through pig manure. The risk of disease transmission will be higher when an animal is exposed to faeces of a non-proprietary pig population. Therefore, it is advised not to spread manure from other pig farms in a radius of +/- three kilometers around a specific pig farm (Amass, 2005b). Furthermore, it is also recommended to use farm-specific materials or equipment to perform these tasks (Moore, 1992; Pritchard et al., 2005; Kirwan, 2008).

Wild boars

Wild animals, including wild boars, may pose a serious threat to a pig population, as these animals are a reservoir for infectious swine diseases (Ruiz-Fons et al., 2007; Filippitzi et al., 2017). If pigs are kept outside, the risk of disease transmission through direct contact with these wild boars will increase, especially if there are many wild boars in the environment of the farm (Ribbens et al., 2008; Vangroenweghe et al., 2009a). This applies for example to African and classical swine fever (Fritzemeier et al., 2000) and to Aujesky's disease (Artois et al., 2002). To avoid wild boars and other wildlife at the farm premises, it is recommended to have a good fence around the pig farm (Pritchard et al., 2005; Vangroenweghe et al., 2009a).

Internal biosecurity

Disease management

Returning to a younger age group

Returning piglets to a younger group is very risky and actually inadmissible. There is a reason why animals do not grow and develop in a proper way (Vangroenweghe et al., 2009a; Dewulf, 2014; Filippitzi et al., 2017). Slower growing pigs have a greater chance of developing infections as their immune system often does not work optimal. When a slower growing pig goes back to a younger group, the animal can introduce several infectious agents to the younger, more susceptible population. In this way, a disease can easily spread into a group of immunologically naive pigs (Dewulf, 2014; Belluco et al., 2015).

Nursery piglets are a very vulnerable group because they often have lower immunity, fight and bite when they are brought together and may carry a substantial amount of pathogens with them (Cameron et al., 2012; Johnson et al., 2012). When it is expected that the piglet has a low probability of becoming a profitable fattening pig, euthanasia is a better choice than letting it run around among its litter as a permanent infection source (Vangroenweghe et al., 2009a; Dewulf, 2014).

Sick bay

Diseased animals should be isolated as quickly as possible from the stables to avoid direct contact (nose to nose) or indirect (via excretions or secretions) with the other pigs in the compartment. The advice is to segregate these animals in a fully closed sick bay, in a separate room or building (Hémonic et al., 2010; Dewulf, 2014). Moving sick pigs to an unoccupied compartment within the same room or to the corridor of the department has little meaning because in this way the diseased animals can still have a significant influence on the other animals in the compartment. Carcasses are also a source of infection and must be removed from the stables as soon as possible (Dewulf, 2014). In order to avoid the transmission of pathogens between the diseased animal and the susceptible population, it is advised to manipulate or treat the diseased pigs always after the healthy pigs (Vangroenweghe et al., 2009a; Backhans et al., 2015).

Vaccination and health status

Having a stable farm immunity prevents the continuous circulation of infectious agents. In a farm where many foreign animals are introduced to the population, it will be much more difficult to maintain a stable immunity among the pigs compared to a closed herd that never imports animals (Amass and Baysinger, 2006).

By vaccinating a pig population while following a well-founded protocol, a more balanced and better farm immunity can be created (Filippitzi et al., 2017). The vaccination of animals will provide protection against the development of clinical symptoms, but will also suppress the transmission of pathogens in the population (Dewulf, 2014). Therefore, vaccination can reduce the risk of disease or mortality within the pig population but it will also benefit the general pig welfare (Morton, 2007). Most of the time, it is the farm veterinarian who will set up a suitable vaccination protocol, taking into account, among other things, the present diseases at the farm and the cost-effectiveness of the various vaccines (Backhans et al., 2015; Filippitzi et al., 2017).

In addition to vaccination, it is also valuable to know the disease status of the pig farm. In this way, a good population health can be ensured and it gives the opportunity to intervene in time where it is needed (possibly by (re)introducing a particular vaccination protocol).

Farrowing and suckling period

Washing of the sows

Infectious agents can be transmitted horizontally from sow to piglet via the skin and teats (in addition to vertical transmission via the placenta or milk) (Amass et al., 1996; Filippitzi et al., 2017). Before the sows are placed in the farrowing pen, they should be dewormed, treated against scabies and washed in order to prevent the transmission of pathogens from the sow barn to the farrowing pen. When washing the sows it is important that this happens before they enter the farrowing pen to avoid the contamination of these pens though the process of washing (Vangroenweghe et al., 2009a).

Transferring piglets in the farrowing unit

Mixing litters in the farrowing house is really dangerous as infectious agents can be transferred from an infected sow or carrier to a susceptible piglet, which does not have the right maternal antibodies (Zimmerman et al., 2012). Secondly, it can be said that the number of "contact piglets" of one infected piglet increases significantly when different litters are mixed (Vangroenweghe et al., 2009a). A study, performed on several Dutch farms, showed that moving more than 5% of the piglets in the farrowing house and mixing litters later than 48 hours after throwing, enlarged the probability of problems with PRRS at the farm (Duinhof et al., 2006).

Equipment to treat the piglets

In the farrowing house, a number of actions are performed with the suckling piglets for which specific material is required (castration, ear tags, injection of iron,...). This material can form a transmission pathway for infectious agents if they are not thoroughly cleaned and disinfected after use (Alvarez et al., 2002; Vangroenweghe et al., 2009a; Filippitzi et al., 2017). For example, when castrating piglets, it is necessary to disinfect the castration blade before it is reused for a next piglet. In order to speed up this work, two blades can be used alternately (at the same time, the blade that is not being used can be immersed in a small box containing disinfectant) (Hémonic et al., 2010).

Nursery unit

All-in/all-out

The all-in/all-out principle is probably one of the most important biosecurity measures to break the infectious cycle of pathogens from one production round to another (Clark et al., 1991). This measure implies that pigs are held together in different groups according to their age and their weight. This different groups cannot be mixed during their stay at the pig farm and when a group of pigs is moved to a next production stage (e.g. from farrowing house to the nursery unit), the whole stable will be completely emptied (Maes et al., 2008; Anonymous, 2010; Backhans et al., 2015). This principle creates the opportunity to clean and disinfect each compartment thoroughly in order to avoid cross-contamination between consecutive production rounds (Dewulf, 2014). To be able to fully comply with this system, a farm should have a sickbay or a separated stable to isolate animals. In the end, the group size has to be adjusted to each compartment of the farm (Vangroenweghe et al., 2009a).

Keeping groups together

An all-in/all-out system (in which a group of pigs of the same compartment is moved together through the different production stages) is surely preferred over mixing different groups during the transition to the next production stage (Maes et al., 2008; Hémonic et al., 2010). It is advisable to keep the groups together as much as possible rather than sorting all animals in terms of their size. The latter will result in a lot of mixing which substantially increases the likelihood of spreading infections (Dewulf, 2014).

Extra hygiene lock for the nursery unit

It may be appropriate for the most receptive age groups (such as the nursery unit) or risk groups (such as the quarantine department) to implement an additional hygiene lock at the entrance of the unit. Nursery piglets are very susceptible to infections and therefore it is really important to protect these animals against the excessive infection pressure from other compartments of the farm by a separated hygiene lock. On the other hand, problems in the nursery unit can also be very risky for the other animals of the pig farm that you need to take additional measures (such as a hygiene lock) to reduce disease transmission.

Returning to a younger age group

Returning piglets to a younger group is very risky and actually inadmissible. There is a reason why animals do not grow and develop in a proper way (Vangroenweghe et al., 2009a; Dewulf, 2014; Filippitzi et al., 2017). Slower growing pigs have a greater chance of developing infections as their immune system often does not work optimal. When a slower growing pig goes back to a younger group, the animal can introduce several infectious agents to the younger, more susceptible population. In this way, a disease can easily spread into a group of immunologically naive pigs (Dewulf, 2014; Belluco et al., 2015).

Nursery piglets are a very vulnerable group because they often have lower immunity, fight and bite when they are brought together and may carry a substantial amount of pathogens with them (Cameron et al., 2012; Johnson et al., 2012). When it is expected that the piglet has a low probability of becoming a profitable fattening pig, euthanasia is a better choice than letting it run around among its litter as a permanent infection source (Vangroenweghe et al., 2009a; Dewulf, 2014).

Stocking density

The stocking density in a pig stable includes the available amount of space that a pig requires to satisfy all basic needs (feed, drink, move, rest, etc.). This parameter has a significant impact on the health status of the animals as it is closely related with the stable climate and the prevailing infection pressure within a farm (Dewulf et al., 2007). Therefore, a higher stocking density will considerably increase the rate of digestive disorders and/or respiratory problems in the stables (Pointon et al., 1985; Maes et al., 2000a; Maes et al., 2000b; Stärk, 2000; Laanen, 2011). If pigs are housed very close together, the individual animal will also experience more stress, which makes the animal more susceptible to infections and it will secrete more infectious germs. Moreover, with a higher stocking density there are more animals, which are exposed to one infected pig. Because of this, many infected pigs on a small surface area will cause an increase in overall infection pressure (Dewulf et al., 2007; Laanen, 2011).

The guidelines for the stocking density in pig stables are determined in the EU legislation. However, these norms are based on outdated research as well as insights and have not evolved with the recent evolutions in the industry. Therefore, the values are not in line with the optimal guidelines and should be considered as absolute minimum requirements rather than the ideal values (Dewulf et al., 2007). Studies have shown that the optimal values for stocking density are on average 24% above the legal requirements (Laanen et al., 2011).

Finishing unit

All-in/all-out

The all-in/all-out principle is probably one of the most important biosecurity measures to break the infectious cycle of pathogens from one production round to another (Clark et al., 1991). This measure implies that pigs are held together in different groups according to their age and their weight. This different groups cannot be mixed during their stay at the pig farm and when a group of pigs is moved to a next production stage (e.g. from farrowing house to the nursery unit), the whole stable will be completely emptied (Maes et al., 2008; Anonymous, 2010; Backhans et al., 2015). This principle creates the opportunity to clean and disinfect each compartment thoroughly in order to avoid cross-contamination between consecutive production rounds (Dewulf, 2014). To be able to fully comply with this system, a farm should have a sickbay or a separated stable to isolate animals. In the end, the group size has to be adjusted to each compartment of the farm (Vangroenweghe et al., 2009a).

Keeping groups together

An all-in/all-out system (in which a group of pigs of the same compartment is moved together through the different production stages) is surely preferred over mixing different groups during the transition to the next production stage (Maes et al., 2008; Hémonic et al., 2010). It is advisable to keep the groups together as much as possible rather than sorting all animals in terms of their size. The latter will result in a lot of mixing which substantially increases the likelihood of spreading infections (Dewulf, 2014).

Returning to a younger age group

Returning piglets to a younger group is very risky and actually inadmissible. There is a reason why animals do not grow and develop in a proper way (Vangroenweghe et al., 2009a; Dewulf, 2014; Filippitzi et al., 2017). Slower growing pigs have a greater chance of developing infections as their immune system often does not work optimal. When a slower growing pig goes back to a younger group, the animal can introduce several infectious agents to the younger, more susceptible population. In this way, a disease can easily spread into a group of immunologically naive pigs (Dewulf, 2014; Belluco et al., 2015).

Stocking density

The stocking density in a pig stable includes the available amount of space that a pig requires to satisfy all basic needs (feed, drink, move, rest, etc.). This parameter has a significant impact on the health status of the animals as it closely correlates with the stable climate and the prevailing infection pressure within a farm (Dewulf et al., 2007). Therefore, a higher stocking density will considerably increase the rate of digestive disorders and/or respiratory problems in the stables (Pointon et al., 1985; Maes et al., 2000a; Maes et al., 2000b; Stärk, 2000; Laanen, 2011). If pigs are housed very close together, the individual animal will also experience more stress, which makes the animal more susceptible to infections and it will secrete more infectious germs. Moreover, with a higher stocking density there are more animals, which are exposed to one infected pig. Because of this, many infected pigs on a small surface area will cause an increase in overall infection pressure (Dewulf et al., 2007; Laanen, 2011).

The guidelines for the stocking density in pig stables are determined in the EU legislation. However, these norms are based on outdated research as well as insights and have not evolved with the recent evolutions in the industry. Therefore, the values are not in line with the optimal guidelines and should be considered as absolute minimum requirements rather than the ideal values (Dewulf et al., 2007). Studies have shown that the optimal values for stocking density are on average 24% above the legal requirements (Laanen et al., 2011).

Measures between compartments, working lines and use of equipment 

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 pig. On the other hand, the maternal immunity (the protection of the piglets by maternal antibodies in the milk) will disappear over time (Dewulf, 2014). Therefore, it is crucial to keep different age groups separate in order to avoid the transmission of pathogens among each other (Dewulf, 2014; Filippitzi et al., 2017). In addition, the work on the farm should be performed from young piglets to old pigs and from healthy animals to sick animals (first the sucking piglets, afterwards the pregnant sows, nursery pigs, fattening pigs, quarantine and eventually the sick animals). In this way, disease transmission can be prevented (Filippitzi et al. 2017).

Older pigs are often better protected against certain pathogens, but they can still be carriers of different pathogens. Young pigs have not yet developed immunity against these pathogens and they are therefore at risk of getting infected in case of contact with these older animals. Consequently, it is strongly discouraged to replace animals in a young pig population with older animals, as well as to house old and young pigs in the same area (Dewulf, 2004).

Working lines

An important basic rule to prevent the spread of diseases between different age groups, is determining and upholding working lines within the farm. A fixed course is created that is always used to visit and work in the stables. During the rounds to the stables, this established route should be strictly followed and all actions at the pig farm should be performed in this strict order. In addition, the material should be provided according to these working lines so there is no need to return to a previous compartment (Vangroenweghe et al., 2009a; Laanen, 2011). It is recommended to start with the youngest animals, then go on with the pregnant sows, the nursery unit and the fattening pigs. After this, you can enter the quarantine building and finally end up with the sick animals (Vangroenweghe et al., 2009a; Laanen, 2011; Filippitzi et al., 2017).

Equipment in the various compartments

The transmission of pathogens can easily occur indirectly through all the material used in a pig farm (Laanen, 2011; Gelaude et al., 2014; Filippitzi et al., 2017). A driving board or shovel can be quickly contaminated with manure that contains all kinds of infectious agents. To avoid disease transmission, it is important to work with different materials in different compartments 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 to another (Vangroenweghe et al., 2009a; Laanen, 2011; Gelaude Et al., 2014).

Farm-specific material

Pathogens can be easily introduced to your farm through all kinds of materials. This is especially the case when the equipment has recently been used for other pigs or has been manufactured and packaged at other pig farms (Pritchard et al., 2005; Filippitzi et al., 2017). To prevent transmission of infectious agents from one farm to another, it is advised to use farm-specific equipment and to make this material available for anyone who needs it at your farm (Lister, 2008; Gelaude et al., 2014). For example,each farm should have its own pig noose and it should not be exchanged with other pig farms. Internal biosecurity measures should also be taken to prevent disease transmission between the various susceptible age groups because such a tool can be quickly contaminated with mucus, saliva and other organic material (Vangroenweghe et al., 2009a).

Needles and syringes

The use of injection syringes in a pig population to administer for example medication or iron, can play a significant role in the disease transmission of many infectious diseases if these needles and syringes are not regularly replaced (Hémonic et al., 2010; Filippitzi et al. , 2017). Needles can be contaminated by pathogens on the skin of the pig or through infectious agents in the bloodstream of the animal. This way, these equipments can be a real risk for disease transmission between different pigs (Hémonic et al., 2010). Ideally, a new needle should be used for all new injections in a pig (disposable needles) but in most pig farms a needle is used until it is totally blunt. To reach a compromise, a needle can be changed for every new litter or for every 10 animals (sows) (Vangroenweghe et al., 2009a).

Cleaning and disinfection

Cleaning and disinfection of the stables

In order to control infectious diseases at a pig farm and break through the infection cycle of a pathogen, the following three separate steps in the cleaning of the farm buildings are very important: thoroughly cleaning, thoroughly disinfecting and allowing adequate time for the stables to dry out (Amass and Clark, 1999). In addition, it should be mentioned that this cleaning protocol could never succeed without first actively removing all the faeces from the stables. When infectious agents still remain in the building as a result of insufficient cleaning, this can lead to lots of problems in the pig population of the next production round (Dewulf, 2014). A particular study demonstrated that this transmission of pathogens could take place very easily and quickly. It has been shown that pigs get already infected after spending only two hours in a stable contaminated with Salmonella Typhimurium (Hurd et al., 2001).

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 pathogens,

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 disinfection 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).

Hygienogram

A good cleaning and disinfection is not always easy at a pig 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., 2009a; Vangroenweghe et al., 2009b; Luyckx et al., 2015). 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 evaluated by enumeration of bacteria via swabs (Luyckx et al., 2015).

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

Boot washers and disinfection baths

In order to prevent the spread of pathogens through footwear, boot washers and disinfecting baths can be placed between the different sections or compartments at a pig farm. 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 agents (Vangroenweghe et al., 2009a).

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). Next, the boots have to be placed in a visually clean solution with a 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., 2009a).