Summary

Many of the same biosecurity principles used to minimize introduction of new pathogens to a herd can be used to minimize the spread of pathogens within a herd. Producers and veterinarians are encouraged to take a scientific and herd-specific approach to biosecurity programme development. Risk assessment is recommended so that protocols can be based on actual risk instead of fear. Then, biosecurity procedures can be strategically implemented, based on effectiveness, compliance and economic benefit. One should expect variability in the effectiveness of procedures due to differences in herds, personnel and facilities. Once implemented, protocols should be regularly monitored for compliance and efficacy. Practices should be modified as scientific advances are made in the field and as new pathogens emerge. Practices that are not effective or not cost-effective should be eliminated.

Introduction

The first article in this series, "Biosecurity: Stopping the bugs from getting in" concentrated on means of preventing outside pathogens from infecting a herd. This article will focus on procedures to minimize pathogen spread among groups of pigs on a given farm site. Many of the same biosecurity principles discussed in the first article will hold true. Biosecurity procedures should be strategically implemented based on risk assessment of individual herds and farm sites. Farm personnel should participate in the development and implementation of procedures to improve buy-in and compliance. One should expect variability in the effectiveness of procedures due to differences in livestock, personnel and premises. Once implemented, protocols should be regularly monitored for compliance and efficacy. Practices should be modified as scientific advances are made in the field. Practices will need to be modified as the enzootic pathogens of the herd change. Finally, practices that are not effective or not cost-effective should be eliminated. This article will follow the same format as the first article.

Taking Inventory

The pathogens that are enzootic to the herd have already been identified. The next step is to identify the pathogens of interest not wished to spread amongst the herd. Then, inventory which pigs are colonized by these pathogens of interests, as well as which pigs are clinically affected by the pathogens of interest.

Identify the source of infection within the herd

Identify the source(s) of infection, within the herd, for each enzootic pathogen of interest. Identification of the source of the pathogens allows one to take steps to either eliminate the source of infection from the herd, or to place barriers between the source of infection and susceptible pigs. Potential internal sources of infection include: other swine in the herd (direct or indirect transmission), contaminated environment (unclean facilities or equipment), pests and pets (rodents, dogs and cats), aerosols, and personnel (zoonotic disease).

Setting Goals

As always, goals must be set before biosecurity interventions can be implemented.

Is the goal to prevent clinical manifestation of a disease in the herd even though the population of pigs is colonized by a given pathogen?

In this case, biosecurity interventions would be used to reduce the level of organisms within the herd to less than the infectious dose. For example, Mycoplasma hyopneumoniae is ubiquitous in most herds. Mycoplasma hyopneumoniae can be isolated from lungs of nursery pigs; yet mycoplasmal pneumonia, with exceptions of course, is generally most clinically relevant in growing/finishing pigs. Colonized pigs are likely the greatest source of infection for susceptible pigs within the herd. Young pigs become colonized with Mycoplasma hyopneumoniae via sow-to-pig transmission. Lateral transmission of Mycoplasma hyopneumoniae occurs among growing pigs. Thus, interventions to prevent the spread of Mycoplasma hyopneumoniae within a herd can focus on minimizing sow-to-pig transmission (early weaning), preventing/minimizing pig-to-pig transmission (all-in/all-out production) and enhancing the immunity of the herd (vaccination). Implementing these procedures is not likely to eradicate Mycoplasma hyopneumoniae from the herd, but could greatly reduce or eliminate clinical manifestations of mycoplasmal pneumonia.

Is the goal to develop a naive subpopulation of swine within the herd by preventing transmission of a given pathogen to that subpopulation?

In this case, biosecurity interventions would be used to eliminate the organism from the subpopulation. For example, many strategies (partial depopulation, vaccination, etc.) have been implemented (some reportedly successful) to produce progeny that are free of porcine reproductive and respiratory syndrome virus from sows colonized by the virus.

Caution must be taken when developing a subpopulation within a herd that has never been exposed to certain pathogens. Disease expression can be more severe in this group if subsequent accidental exposure to the pathogen occurs due to lack of acquired immunity. Vaccination of the naïve group can be used to enhance immunity in some cases. Regardless, strict biosecurity to prevent exposure of the naïve group to outside pathogens is recommended.

Is the goal to contain an outbreak of disease to a given location or subpopulation within the herd?

In this case, biosecurity interventions would be used on a short-term basis to isolate the infected group of animals until they could be removed from the herd. Removal of the pathogen from the herd could occur either by euthanasia of infected pigs or by treatment of the infected population and subsequent marketing of recovered swine. An example would be an outbreak of Actinobacillus pleuropneumoniae in a group of finishing pigs in one room of a building. The room could be treated as an isolation unit with a designated caretaker(s). The caretaker would don room-designated clothing and boots at entry. Pigs in the room would be treated with an antibiotic and provided with supportive care. The pigs would ideally be cared for at the end of the day, after which the caretaker would shower. Recovered pigs would be marketed and the room thoroughly cleaned and disinfected before entry of the next group of pigs.

Is the goal to prevent the environment (facilities) from being a source of pathogens for groups of pigs housed in the same contaminated facilities over time?

In this case, biosecurity interventions would focus on sanitation of facilities and equipment. For example, coccidiosis can become a persistent problem in farrowing houses due to survival of oocysts in the environment. Strict sanitation targeting the oocysts is necessary to break the cycle of infection.

Assessing risks of transmission within a farm

Four basic steps must be taken for a pig to become infected with a pathogen of on-farm origin:

Risk assessment occurs by considering the probability that a live animal source (infected pig, person, dog) would be shedding the pathogen or that a non-animal source (feed, water trough, straw) would be contaminated with the pathogen. The duration of shedding by an infected animal and the length of survival of a pathogen on a contaminated fomite should also be considered.

Risk assessment occurs by establishing the mode(s) of transmission of the pathogens of interest. Biological transmission can occur directly when live, infected animals have direct contact with susceptible pigs or, indirectly, when infected animals shed the pathogen to the environment and susceptible pigs contact the contaminated environment. Mechanical transmission occurs when the pathogen is tracked into the area or spread via common equipment or facilities. Consider the probability of actual occurrence of transmission of the pathogen from the source to the pig.

Risk assessment in this area is difficult because of the many variables that impact calculation of an infectious dose. Pathogens vary in pathogenicity, infectiousness, contagiousness and viability, inside and outside of the host. Other factors such as the frequency of host exposure to the pathogen can impact the risk of infection (Thrusfield, 1995). Consider if and for how long an infected animal will shed an infectious dose of the pathogen. Also consider whether or not a non-animal source would be contaminated with an infectious dose of a given pathogen for the duration necessary to infect an animal that had contact with that source.

Risk assessment in this area is also difficult because the ability of a pathogen to infect a pig depends on the characteristics of the pig. Host susceptibility varies with age, immuno-competency, vaccination status, genetic pre-disposition, concurrent illnesses, stress, environment, management and nutrition (Thrusfield, 1995).

In summary, knowledge of the target pathogen and host will enable educated decisions regarding when and where biosecurity interventions should be placed to enable the greatest probability of success in preventing transmission of pathogens within the farm.

Determining the extent of the biosecurity programme to be implemented

Target pathogens have been identified. Intervention points have also been identified. Next, economic consideration should be given to the cost of treating diseases versus the cost of implementing each biosecurity procedure. Consider how personnel might best spend their time in achieving the goals of the farm. Then, determine the extent of the biosecurity programme, keeping in mind the level of risk that one is prepared to accept.

Identification of risk abatement procedures to minimize within-farm spread of pathogens

Biosecurity procedures include:

Transmission by direct contact between infected and susceptible pigs

Pigs can become infected through direct contact with their dam or other pigs. Usually, older pigs are colonised by more organisms so these older pigs are the source of infection for younger pigs.

• Maintain an off-site isolation facility. Do not place new stock directly in the herd.
• Age segregation can be used to minimize pig-to-pig transmission. Pigs, as close to the same age as possible, can be grouped together and moved as cohorts through the system. All-in/all-out production allows for cleaning and disinfecting in between groups of pigs so, if a disease does break, it can be contained and transmission to the next group of pigs prevented.
• Early weaning, age segregation, and strategic medication or vaccination as needed, can be used to minimize dam-to-pig transmission of certain pathogens (Clark et al, 1994). The theory behind early weaning is that the piglet consumes colostrum. Colostrum protects the piglet from some of the pathogens that the sow carries for a short amount of time. If the pig is moved to a clean pathogen-free environment while he still has colostral protection, transmission of some pathogens from sow to pig can be prevented or minimized. In some cases, for example with Streptococcus suis, early weaning does not prevent transmission because the pig is already infected at birth. In other cases, early weaning just lowers the dose of pathogens transferred to the pig so the pig becomes a carrier, but does not show signs of disease. If the herd already has high health status using conventional weaning ages, a switch to early weaning probably will not benefit such a herd, and can hurt the herd in terms of reproductive performance.
• Parity segregation uses the same theory as age segregation. Most information on parity segregation is anecdotal at this time even though some farms have been using the protocol for over 10 years. In parity segregation, gilts are separated from sows that are P2 and older. This provides time to better develop immunity in gilts before they become infected with pathogens shed by older sows. Also, the pigs from gilt litters are separated from pigs from litters of older sows. The immune system of the gilt might not be as protective as the immune system of the sow because the gilt has been exposed to fewer pathogens. Thus, gilts might be more likely to infect their pigs. Further, the pigs from gilt litters might be infecting the other pigs in the nursery. Again, the system is mostly based on anecdotal and individual farm data at this time.
• Minimizing cross-fostering and limiting cross-fostering to the first 24 hours of life. Cross-fostering after 24 hours of birth can place pigs at risk for contact with a nurse sow shedding pathogens to which the pig did not receive colostral immunity.
• Use of hospital pens can partially segregate sick pigs from the rest of the group. However, these pigs will still share the same airspace with healthy pigs and unless solid pen gates are used, these pigs will have direct contact with pigs in adjacent pens.
• Maintain pigs with their cohorts in age, including those in hospital pens. Some farm practices include delaying weaning of poor-doing piglets or placing poor-doing pigs in a room of younger pigs of the same size instead of maintaining these pigs within their age cohorts. These practices can put the healthy pigs, to which these poor-doing pigs are exposed, at risk. The poor-doing pigs are likely infected by pathogens and can act as 'Typhoid Marys' within the herd.
• Strategic vaccination and medication protocols can limit direct transmission of pathogens among pigs by both limiting the number of organisms shed by infected pigs and by improving the immunity of exposed pigs to prevent infection.

• Sanitation of equipment (snares, boards, carts, feeders, water troughs, etc.) used within the herd, sanitation of facilities in-between groups of pigs, and sanitation of vehicles used within the site sanitation includes thorough cleaning (removal of visible organic material such as manure, urine, straw, shavings, dust, feed, etc.) followed by disinfection using an appropriate and effective disinfectant according to label directions. Use of recycled water in the cleaning process could potentially contribute to the spread of pathogens within the herd. Thorough cleaning is important as some swine pathogens including Streptococcus suis, rotavirus, and salmonella can be isolated from the dust in buildings. The goals of the sanitation programme are to lower the level of pathogens below an infectious dose at the time of pig exposure and to prevent the build-up of organisms over time. In practical farm situations, achieving sterility of large items and facilities is likely to be impossible.
• Fomites, such as common equipment, can be a potential route of indirect transmission. Maintaining group-specific equipment, cleaning and disinfecting common use equipment periodically, or cleaning and disinfecting equipment between groups of pigs are possible biosecurity procedures.
• Used needles can potentially spread pathogens from pig to pig. For example, one can spread greasy pig disease by using the same needle used to treat an infected pig subsequently on an uninfected pig. Experimentally, porcine reproductive and respiratory syndrome virus has been transmitted by needles (Otake et al, 2002). Practically, one should inject healthy pigs first and sick pigs last when using the same needle. One should never inject a sick pig and then use the same needle to inject a healthy pig. Seemingly, the same needle can be used for groups of pigs housed together with direct contact without dramatically increasing the risk of disease transmission within the group.
• Aerosol transmission of pathogens can potentially occur over short distances within a farm site. Spacing of buildings and strategic location of air inlets can lessen the probability of such transmission.
• Control of non-porcine animals is important. Rodents, birds, insects, feral animals, dogs and cats can all potentially mechanically transmit pathogens. In some cases, these animals can act as biological vectors. Control can be achieved through professional extermination, baiting and general maintenance. Cleaning up spilled feed and garbage will make the farm less attractive to pests.

• Caretaker vigilance is one of the best biosecurity measures that can be implemented. Optimisation of husbandry, nutrition, and management can reduce susceptibility of pigs to infection. Regular monitoring by personnel for clinical signs of disease enables prompt containment of outbreaks. Provision of veterinary care and disease surveillance on a regular basis can also prevent large outbreaks and speed the containment process.
• People have the potential to transmit swine pathogens, both mechanically and biologically. Mechanical transmission occurs when a person contacts infected pigs or contaminated facilities, becomes contaminated with the pathogen, and then tracks the pathogen around the farm. Biological transmission can occur with pathogens that infect people and pigs. A person that is sick with the pathogen can potentially shed that pathogen to the pigs that they contact. Ideally, personnel should move through the farm from the healthy pigs to the sick pigs, and within these categories, from the young pigs to the old pigs. However, such control of people movement is not likely to be practical. At least, people who have been working around rooms of clinically ill pigs and people who have handled mortalities or transported pigs to the dead pile should wash visible organic material from exposed areas of their body and don clean outerwear and footwear before subsequent handling of non-infected groups of pigs. Wearing gloves can decrease the gross contamination of hands, but does not prevent the need for washing.
• As stated above, in cases of an outbreak localized to a room(s) on the farm, personnel can treat such rooms as isolation areas with designated caretaker(s). The caretaker would don room-designated clothing and boots at entry. Room designated supplies and equipment can be used. Disposable items should be used when possible. Pigs in the room can be treated, if possible, and provided with supportive care. These pigs would ideally be cared for at the end of the day, after which the caretaker would shower. Recovered pigs would be marketed and the room thoroughly cleaned and disinfected before entry of the next group of pigs. If treatment is not possible or regulations specify, prompt euthanasia should occur to minimize transmission to the rest of the herd.

Prioritisation of risk abatement procedures

Use the characteristics of target pathogens, potential sources, feasibility of successful intervention, economic considerations and personal risk acceptance levels to select and customise preventive procedures for each target pathogen under specific conditions of the herd.

Evaluating the effectiveness of procedures

Personnel, facilities, equipment, pigs and pathogens are constantly changing, so procedures that might have worked in the past may not always be effective. Thus, existing and newly implemented biosecurity procedures should be periodically evaluated for effectiveness. The innate effectiveness of the procedure, employee compliance and economics should be evaluated. The biosecurity programme will need to be adapted by eliminating, modifying, or adding biosecurity procedures, as critical areas of risk and targeted pathogens change over time.

Monitoring and herd testing

Regular monitoring of pigs for optimal management and signs of infection is essential to the biosecurity programme. The sooner the disease is diagnosed, the better chance there is of preventing spread of the disease to other pigs. Checking pigs twice a day for signs of sick animals, death loss, fever, discolouration, decreased feed consumption, lameness and blisters is recommended.

Herd health monitoring utilizing clinical signs, serology, post-mortem examinations and slaughter checks can be used to detect clinical and sub-clinical exposure to target pathogens.

Sanitation protocol monitoring

Sanitation protocols can be objectively monitored by identifying specific target pathogens on fomites, or by using aerobic bacterial counts as a marker for contamination. Sentinel animals can also be used to monitor for specific pathogens following depopulation of a facility. However, this procedure can be costly.

To evaluate aerobic bacterial counts or numbers of target pathogens, collect a statistically valid number of representative swab samples of flooring, equipment, walls, etc. Measure and record the area sampled so that it is possible later to determine colony forming units (cfu) of bacteria or number of virions per cm². Sterile Replicate Organism Detection and Counting plates, RODAC ^TM (BBL^TM Sterile Pack Media: D/E Neutralizing Agar, BD Diagnostic Systems, Sparks, MD 21152) are commercially available for determining aerobic bacterial counts during environmental sampling. Purchase RODAC^TM plates with D/E agar to neutralize any residual disinfectant activity following sampling. Published targets for disinfection vary from 1 cfu/cm² (Tamasi, 1995) to 10³ cfu/cm² (Böhm, 1998).

Other factors can impact the efficacy of a sanitation protocol, including compliance, water quality and disinfectant preparation. Water quality can be assessed using test kits or commercial water quality analysis services. Disinfectants should be prepared according to label directions and used promptly. Activity of disinfectants after mixing varies with disinfectant class. For example, bleach solutions should not be used after 24 hours of preparation, while some phenol mixtures can be stable for weeks or months if stored in a clean, airtight container. Check with the manufacturer of specific disinfectants for stability times after mixing. In the author’s opinion, insufficient cleaning is the primary reason for disinfectant failure. A general target for the number of bacteria present following cleaning of surfaces and prior to disinfection is 10⁶ cfu/cm² (Böhm, 1998).

Failure of disinfectant to work after trouble-shooting, suggests that another disinfectant should be selected for use on the production unit.

Data collection

Record-keeping will not prevent a disease outbreak. However, good records can help trace the source of the outbreak and decide which biosecurity procedures are needed to prevent future outbreaks.

Glossary

Biosecurity: "Security from transmission of infectious diseases, parasites, and pests" (W.B. Saunders, 1999).

References