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Biosecurity and Minimal Disease Herds
SWINE REPRODUCTION
0749-0720/92 $0.00
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BIOSECURITY AND MINIMAL DISEASE HERDS Camille Moore, DVM
The minimal disease concept is not a new one. This concept has been used in Europe since the beginning of the 1960s. 1 The goal of this approach is to produce healthier pigs in a manner to reduce the cost of production and upgrade the quality of the product sold to the consumer. In an attempt to give a definition of this type of production, many terms have been used. Before going further, we will try to clarify some of the terminology. The following definitions have been used: 9 Axenic: Germ-free pigs. They should be totally free of infection with microorganisms. This definition is theoretical, and this type of animal almost does not exist. Gnotobiotic pigs: Specially reared pigs whose micro flora and microfauna are completely known. This is usually for pigs who are born germ-free and then get contaminated with some diseases. Specific pathogen-free (SPF): The first generation of pigs produced after they have been derived by cesarean-section. Usually, we know that such pigs are completely free of some pathogens, and the list of the pathogens that they are free from is supplied. Minimal disease: A widely used definition for pigs that are free of some pathogens. This definition has been used a lot by breeding companies. However, it is not specific to any diseases, and it should not be used as a description for a specific herd health status. High health status: Used as a synonym of minimal disease herd. Refers to herds recognized for the absence of the major infectious diseases. However, a list of the diseases that are not present in a "high health" herd is needed, and this list may vary from veterinarian to veterinarian, from state to state, or from country to country. Accordingly, this definition is very unclear and unspecific and should not be used as a description for a specific herd health status. Medicated early weaning (MEW): The MEW technique is a way to produce
From St-Cesaire, Quebec, Canada
VETERINARY CLINICS OF NORTH AMERICA: FOOD ANIMAL PRACTICE VOLUME 8 • NUMBER 3 • NOVEMBER 1992
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pigs free of some specific diseases. Though not officially recognized as SPF, the pigs are by definition "specific pathogen free." SPF pigs are not absolutely free of all diseases. Some very specific diseases are targeted. The primary SPF herd would be a herd that has been populated only with animals derived through cesarean-section. A SPF secondary herd would be a herd that has been populated from a primary SPF herd. Usually, the aim of producing SPF piglets is to eliminate diseases that have a major impact on the profitability of the herd or to eliminate diseases that are required to be controlled by state or national laws. Up to now, it has been possible to produce pigs that are free of the following diseases:8,12 Foot and mouth disease, African swine fever, swine fever, Aujeszky's disease, transmissible gastroenteritis virus, swine pox, epidemic diarrhea, enzootic pneumonia, Actinobacillus pleuropneumoniae, Glassers disease, Streptococcus suis type I and II, atrophic rhinitis, Brucella suis, Salmonella cholerae suis, swine dysentery, sarcoptic mange, lice, and tuberculosis. Some other diseases mayor may not be present after these procedures. They are porcine intestinal adenomatosis (PIA) complex, Parvovirus, internal parasites, epidemic diarrhea, Clostridium perfringens type C, exudative epidermitis, hemagglutinating encephalomyelitis virus, and swine influenza. Up to now, it has been almost impossible to eliminate the following diseases: congenital tremor, cystitis-nephritis complex, E. coli enteritis, Klebsiella infection, and erysipelas. Before beginning a minimal disease approach, a few questions need to be asked. In a situation of depopulation and repopulation, we need to understand why it is dirty now, where the clean pigs will come from, if the producer will be able to keep the new herd clean, and if the location is suitable for the production of minimal disease pigs. Production of minimal disease animals requires a lot of discipline, and often the owner is the worst offender for all of the new rules. It should be kept in mind that the bigger the herd, the higher the risk of contamination. High health animals will never be a substitute for good management and good care of the animals. The use of such an approach requires a lot of dedication by the owner, a good record keeping system, good discipline, a complete disease control program, and a detailed production and management procedure. 1 CONSIDERATIONS BEFORE DEVELOPING A BIOSECURITY PLAN
To keep the good health status of the herd, it is necessary to develop a detailed biosecurity plan. However, before looking at all of the items of such a plan, it is necessary to know about diseases. Disease Transmission
In an attempt to keep the herd free from specific diseases, it is essential to know how these diseases are transmitted. It does not make any sense to try to eradicate a specific disease if that disease may be brought in the herd very easily after the clean-up procedure. The pig itself is the main source of disease organisms. Some organisms can stay alive for a very long period of time in a herd. Ascaris and coccidia can probably remain in an emptied herd for at least 5 years. 12 Therefore, it is almost impossible to produce pigs free of ascaris or coccidia after a depopulation/repopulation scenario. This is also probably true for bacteria that produce sporulated organisms, such as clostridium. 12
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Some diseases are airborne from herd to herd. Variation does exist in the potential for airborne transmission. Humid and cold weather are more favorable to disease transmission than dry and hot days. 3 The following diseases can apparently be transmitted by air: 1,8 Foot-and-mouth virus can travel for more than 25 miles. Enzootic pneumonia can be transferred up to 2 miles. Transmissible gastroenteritis (TGE), atrophic rhinitis, and pleuropneumonia can probably be transferred 0.5 mile. Swine infertility and respiratory syndrome (SIRS; mystery swine disease) can probably be transferred up to 2 miles. Influenza virus can be transferred up to 3 to 4 miles. Aujeszky virus can probably travel for 25 miles in the air. Other diseases, such as swine dysentery, are not likely to be transferred by air alone. However, such bacteria can easily be carried from herd to herd by house flies. Flies can travel up to 2 miles. Besides swine dysentery, they can also carry S. suis and TGE virus. 8 Much work has been done to understand the transmission of Mycoplasma hyopneumoniae. Usually, this bacteria does not survive for more than 48 hours outside the host. Survival time is longer during rainy weather, and the organism can be transferred in moist air. The rate of contamination is greater in very pig-dense areas. Many pigs are subclinical carriers for M. hyopneumoniae, and it can be easily transferred by pigs. 3 It is always better if depopulation/repopulation occurs during summer. Organism survival time is usually shorter during warm periods. The herd should be entirely empty of pigs for a period of at least 6 weeks. Good cleaning and disinfection should occur during this period. Rodent extermination should be done. Herd Location
Preferred herd location is chosen in relation to the diseases that we would like the herd to remain free of and the distance that such diseases can be transmitted. Theoretically pig barns should be as far as possible from other pig barns. Location is the most important factor to avoid recontamination. 3 This is the dominant factor for a herd to remain free of enzootic pneumonia. The second most important factor is the size of the neighbor farm. The site should be chosen based on the presence of other pigs in the area, the type of diseases that are being avoided, the size of the nearest infected farm for that specific disease, the type of production (weaners production unit or finishing unit), the air temperature and humidity pattern for the area, and prevailing winds. 9 1£ the location is in an area where winds are very important, the farm should be located down a valley and at the opposite side. 1£ the farm is located in an area of very hot weather, the herd should be located on the top of a hill. Roads that are close to the farm are also factors. Some other pigs could be carried close to the farm and become a source of contamination. The farm should be located on a dead-end road and as far as possible from major highways. Table 1 summarizes some distances between pig farms with which no transmission of diseases occurred. 9 M. hyopneumoniae transmission has never been reported over a distance of 3.2 km by air. 3 According to the available information, the following recommendations can be made: 1£ the aim is to keep a herd free of TGE, actinobacillus pleuropneumonia (APP), atrophic rhinitis (AR), and S. suis, the herd should be located at least 0.5
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Table 1. DISTANCE BETWEEN FARMS AT WHICH NO TRANSMISSION OF DISEASE WAS OBSERVED Disease
Distance
Time Free of Transmission
Aujeszky's Transmissible gastroenteritis Enzootic pneumonia Atrophic rhinitis Actinobacillus pleuropneumoniae Streptococcus suis Swine dysentery Mange
500 m 400 m 150 m 300 m 500 m 300 m 300 m 100 m
4 months, infected herd slaughter after 4 months, disease die out 4 years 3 years 6 years 12 years 4 years 5 years
mile from any other pig unit. If the aim is to keep a herd free of enzootic pneumonia and Aujeszky's disease, the herd should be located at least 2 miles from any other pig unit. Even though these types of precautions are recommended, in Denmark, where the minimum distance between herds is only 350 feet, they have been quite successful in keeping herds free of many diseases. 13 Owing to the lack of documentation on the role of other animals in disease transmission, it is difficult to make specific recommendations concerning distances between other livestock or poultry operations. Assessing Health Status
Many methods can be used to assess health status of a herd. Even if not completely objective, the continuous herd follow-up with clinical examination and monitoring for any change in clinical symptoms is still the most effective way to assess health status. 1,8,9 Slaughter checks are a useful tool to assess health status. However, the sample sizes used must be big enough for significant results. Guidelines related to the size of the sample necessary according to the degree of confidence expected have been published. l l In interpreting slaughter check results, we need to keep in mind that many lesions heal, and what we see at slaughter might not reflect what happened earlier in the herd. Therefore, in top pyramid herds, slaughter checks should be performed at least every 6 weeks. Serology is another tool to assess health status. However, serology has limitations. It is not always easy to interpret the results of serologic tests. We need to know the type of test used, its limitations, and the specificity and the sensitivity of each test used when interpreting serology. Inconclusive results will always exist. Sample size is as important for serology testing as it is for slaughter checks. With the emergence of new techniques, serology will become more and more reliable in the near future. In many countries or states, we can rely on associations that are doing herd monitoring and certification to assess health status. The Pig Health Control Association in the United Kingdom is a good example. They maintain a list of herds that are free of enzootic pneumonia, swine dysentery, atrophic rhinitis, Aujeszky's, mange, and S. suis type 11. 10 Table 2 gives specific criteria for monitoring some specific diseases. 9 Careful examination of vaccination programs and drugs used in the herd should also be performed. Theoretically, no drugs should be used. Finally, the owner should assess health status. He or she should daily check
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Table 2. SPECIFIC CRITERIA FOR MONITORING SOME SPECIFIC DISEASES Criteria
Disease Enzootic pneumonia Aujeszky's Transmissible gastroenteritis Actinobacillus pleuropneumoniae
Streptococcal meningitis Atrophic rhinitis Swine dysentery Mange
Life history, regular lung examination (serology becomes more and more reliable) History, serology History, clinical sign, serology (pay attention to the porcine corona respiratory virus in interpretation) History, regular lung examination, serology (depending on the technique used) 5-year history, no Streptococcus suis type II isolated from meningitis pigs History, nasal section at slaughter check, and nasal swabbing for toxigenic Pasteurella multocida History, no medication used History, scrapping and no parasitic used
and report any changes in coughing, short breath, loss of appetite, diarrhea, itching, sneezing, and all other visible changes in the herd. 1 Source of Replacement Animals
It is not always easy to find a source of minimal disease animals. All of the animals should come from only one supplier. Animals used should be derived from a pyramidal system. 1 This means that the animals flow from the top (nucleus) downstream to multiplier and commercial production. The type of production desired determines where in the pyramid to be located. All animals should always come to your herd from the next highest level in the pyramid. Each herd manager needs to decide the diseases from which the herd is wished to be free and the type of genetics he or she is looking for. With that in mind, a supplier herd can be chosen. The decision should be made based on the following factors:
The herd should be under routine veterinary observation. The record keeping system should be examined. No drugs should be used in the herd. Vaccination programs should be well examined. The herd history should be studied. All laboratory findings in that herd should be checked. Postmortem results should be examined. Replacement stock should always be bought from a closed herd. DESIGN CONSIDERATIONS
Even with a good location and the good type of animals, some considerations should be taken as far as buildings are concerned to make sure that the health status will remain clean. To ensure a good barrier to disease spread, total confinement should always be used. Some questions arise concerning the use of naturally ventilated buildings or open front buildings. If such buildings are used, they should be organized in a manner as to be entirely bird- and rodent-proof.
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General Considerations
A standard layout organization for a high health pig bam is shown in Figure 1. Care should be taken to orient the building according to wind direction. Enough places should be planned for all of the production stages. Hospital pens should always be planned. The organization of the building should take into consideration the pig flow. It is desirable to reduce the amount of movement needed for the pigs and also allow manpower specialization. Usually, the following distribution is used: Gilt acclimation place, breeding area, dry sow area, farrowing room, and nursery. If a grow-finish operation is to be on the same site, it should be located in another building. In setting up a minimal disease unit, five types of activities need to be planned: the production site itself, the office, the feed delivery area, the animal loading area, and the manure-handling system. These five activities should be located at different places, and a clear demarcation needs to exist between each of the areas. AII-inlAll-out Management
The use of an all-injall-out production system should be promoted in any type of pig production unit. It is even more important in high health-status herds.
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1~ Figure 1. Standard high-health pi g barn layout. 1 = access road, 2 = locked main gate; 3 = perim eter road; 4 = perimeter fence; 5 = quarantine; 6 = manure storage; 7 = feed bin; 8 = incoming animals; 9 = gilt acclimation; 10 = breeding; 11 = dry sow; 12 = farrowing; 13 = nursery; 14 = grow-finish; 15 = animal expedition facility; 16 = postmortem; 17 = incineratory; 18 = human entrance; 19 = showers; 20 = office; 21 = canteen; 22 = room for incoming material.
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Building design should be planned so that small rooms will be used at each level of production. These rooms do not allow contact with other rooms. After the rooms have been completely emptied, they are washed and disinfected. The purpose is to reduce build-up of bacteria and diseases. This type of system is more expensive because space usage is not optimal. Recently, the concept of multiple-site production has been developed. This technique uses either two sites for production, (site one is gestation and farrowing, and site two is nursery/growing/finishing) or three sites, (site one is gestation/ farrowing, site two is nursery, and site three is growing/finishing). In the threesite production system, piglets are weaned at 8 to 24 days of age and moved to another location. Pigs remain in the nursery for 6 to 8 weeks and are then moved to a growing/finishing complex. 4- 14 The use of this approach ensures the adherence to the all-in/all-out concept. This is also an insurance policy in the face of disease outbreaks, as the cycle of such disease can be broken. Material and Equipment
All material and equipment used in these building should be easy to wash and long-lasting. It is always a hazard to bring a service person into the unit to perform repairs. For this reason, good-quality material should always be used. All material that is brought into the facility premise after the unit is in operation should be new. Waste Management
The reservoir for affluent from the herd should always be located outside the fenceline. The pipe used to carry the manure should be rodent-proof to avoid introduction of rodents into the building. Great care should be taken of the equipment and vehicles used to dispose of manure. Ideally, this material should belong to the farm. If not, it should be completely washed and disinfected before use. BIOSECURITY POLICIES Personnel
The downtime" that should be observed by people going into a minimal disease herd is always tough to assess. This question goes back to disease transmission principles. It has been found that foot-and-mouth disease virus can be carried in the nose of humans for 11 hours. This virus also can survive under fingernails for a few days. It has been found that humans can carry the foot-andmouth disease virus for 1 to 2 days.7,13 Some analogies probably can be made between the foot-and-mouth disease virus and the TGE virus or pseudorabies virus. Two nights of downtime seems to be the practical minimum. It has been shown that M. hyopneumoniae can survive for 24 to 48 hours on clothes and for as long as 30 hours on filter paperl under lab conditions. Possibly, the bacteria survive for a shorter period of time in the field. Godwin's studies3 have not been able to make any correlation between downtime and reinfection with enzootic pneumonia. Workers in a minimal disease herd should never be in contact with other pigs. They should not be allowed to live at another pig farm. Visitors to a minimal /I
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disease herd should be kept to a minimum. They should have been free of pigs for at least 2 nights. A log book of all humans going into the herd should be kept. All visitors should write down the number of hours since they have been with pigs and the last place where they have been in contact with pigs. They should sign their name. Sign and Fence
A sign should be put close to the access road. This sign should say that pigs raised are under a health-monitoring system, and no one is allowed to go beyond that point without permission. The farm perimeter should be well identified. The use of a fenceline is recommended, especially if other pigs are close by (1 mile). In countries where wild animals or rodents are present, these fences should be built in a manner to restrict the access of these animals to the building area. These fences also keep all vehicles and humans away from the production buildings. Usually, fences are built 40 to 50 feet from the production unit. In areas where other pigs are 3 to 4 miles away, a fenceline might not be necessary. In these situations, some other devices, such as deep ditches, may be sufficient to restrict vehicle access. Shower and Clothes
The office and main entrance to the building should be located close to the fence. The office should supply a canteen to allow the staff to eat on the farm premises. The office should be located so that it is possible to see all incoming people and vehicles and also to control the pig loading bay. Only one entrance should be used. Before going into the herd area, a shower should be mandatory. Even though it is not clear that a shower can eliminate microorganisms that may be carried by humans, this is a good way to ensure that clothes are completely removed and a special clothes supply is used in the herd. Shower installation should be designed so that people will go through the shower and will not come back in the entrance or contaminated area to dress themselves. The shower is also a good physical barrier and a discouraging procedure for visitors who do not have a compelling reason to enter the herd. In the Danish SPF system, showers are not used. They have developed an entry system that could be applied to commercial producers who are using minimal disease animals. This system is .shown in Figure 2.7 The outside door must be locked at all times. A bell should be installed, as should a system to unlock the door without going back through the shower. All clothes used on the farm premises should be supplied by the farm. At no time should materials or animals go through the staff entrance. Animal Transportation
Loading and unloading animals is probably one of the most common ways to bring disease into minimal disease herds. If the herd is not completely closed and is being used as a multiplier or a commercial production unit, two different locations should be planned for animal receiving and animal expedition. A specific building for loading purposes needs to be built. This building must
BIOSECURITY AND MINIMAL DISEASE HERDS
Protective clothing for unit staff and visiting veterinarians & agricultural consultants should be kept here.
Footwear must be disinfected before
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This must be QD.!y entrance to pighouse.
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removed in "unclean footwear should be area".
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Water-tap with hose
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Wooden-grate passage should be entered QD.!y in stockinged feet (never in boots).
Hands must be washed and disinfected.
Figure 2. S.P.F. Danish entry.
be separated from the production unit. It should be located on the fenceline for protection. Its affluent drainage system should not go through the main building. It should be located close to the office to facilitate communications between truck drivers and herd staff. Animals that need to be sent are placed in the loading building by farm staff. Then, the door is locked, and farm staff should not go back into the loading facility. The truck driver loads the pigs into the truck without assistance from farm staff. There should be small pens in the loading building to facilitate loading by the driver. The loading facility must be washed, disinfected, and fumigated after each use. This facility can be used only once a day. If no other manpower is available, the facility can be cleaned at the end of the day by the farm workers. People who do this job must never go back into the farm premises the same day. The vehicle used for animal transportation should always arrive at the farm location empty. Godwin3 reported 13 cases where herds broke with enzootic pneumonia because a hauler came close to the bam (25 to 75 feet) with animals in the truck. The truck used for these transportations needs to be washed, cleaned, and disinfected before arriving on the farm premises. Feed If the feed used for the farm is purchased, a system should be planned for its delivery. Feed bins are installed around the farm perimeter fence, and the vehicle that delivers the feed stays outside the farm perimeter. If the feed bins are close to the farm, a system should be planned to allow the feed to be blown from outside the perimeter. If the feed is manufactured on the farm, the same precautions should be used for ingredient delivery. Food wastage (kitchen or industrial) should never be used in minimal disease herds. The use of animal by-products in feed formulations for minimal disease
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herds is still debatable. The type of diseases that can be carried by animal by-products is unclear. Until the time that better information is available, animal by-products (meat meal, bone meal, and so forth) should not be used. Vehicles
No vehicles should be admitted inside the herd perimeter. Employees' cars should be left as far as possible from the herd. Any time that it is possible, feed trucks should come to the herd first thing in the morning. Monday morning is the best day for feed delivery. Feed truck drivers should never come in contact with the feed. If a problem does occur during feed delivery, the driver should go back to the feed mill with the feed left. In hot regions, the use of an area with water and disinfection for the truck to go through is encouraged. Material and Equipment
All equipment and material brought into the herd should be cleaned and disinfected. To ensure compliance, a proper location to do so should be planned. This room should be located on the fence perimeter. A door gives access to the outside. New material or equipment is delivered in that room. If possible, a fogging system should be used to do the disinfection. If this is not possible, all of the new material and equipment should be put in a solution with disinfectant. The solution must be changed every day. Ideally, all material brought into the herd should be brand new. However, this is not always possible, especially with the service person's equipment. In this situation, equipment needs to be completely clean before it is brought into the disinfection room. Attention should be paid to material that could have been on another pig farm. Ideally, these types of material should not be allowed in. It is a good idea to have in the farm some of the general tools for service people. Dead Animal Disposal
Dead animal disposal is another risk for disease entry. In many cases, it is desirable to have a postmortem performed on dead animals. If laboratories do not exist close by and postmortem examinations need to be performed on the farm, it is a good idea to plan an area where they can be performed by the veterinarian. 12 This place should be planned outside of the farm perimeter to allow a veterinarian who may not have been pig-free to perform the postmortem examinations. The best way to dispose of dead animals is the use of an incinerator. However, in many areas these devices are not allowed. Also, these devices usually are expensive to use. If the only way to dispose of dead carcasses is through a rendering company, the vehicle owned by those companies should not be allowed to come closer than 1 mile to the herd. To ensure that, a transportation mode should be planned for the dead carcasses from the production unit to a place where they can be picked up by the rendering company. Pet Animals and Rodent Control
No cats, dogs, or other pet animals should be allowed on the farm premises at any time. Rats and mice have been caught and the diseases they can carry
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studied. 6 Bordetella bronchiseptica, Pasteurella sp, E. coli, Campylobacter jejuni, Treponema sp, Rotavirus, and TGE titers were found. The same workers have been able to infect rats with Aujeszky's virus and classical swine fever virus. 6 These findings prove the importance of having an adequate rodent control program. The buildings need to be rodent-proof. A program for monitoring rodent presence should be in place. As soon as the presence of rodents is noticed close to the barn, some action should be taken to destroy them. Quarantine
Producers who are associated with genetics companies and many commercial producers find it impossible or impractical to keep a closed herd. Unfortunately, these people have to continuously bring new animals into the unit. Even if one is always dealing with the same supplier and even if the sources are well monitored, a quarantine facility and program should be put in place. It should be kept in mind that for many diseases, the incubation period is long and animals could be brought in before any sign is seen or diagnosis is made in the source herd. Also, remember that contamination could have occurred during transportation. Ideally, quarantine facilities should be located on another farm. Practically, this is almost impossible. Therefore, quarantine facilities should be located as far as possible from the main building. No direct communication should be used between the quarantine facility and the main building. Quarantine buildings should have their own manure-handling facilities. 1 Feeding and examining the animals without having to come into direct contact with them should be possible. It is recommended to have a filter on the air outlet of the quarantine. Farm staff should go in the quarantine area only at the end of the work day. Foot baths should be installed, and all clothes should be changed before going into the quarantine unit. Direct contact with animals should be kept to a minimum. Farm staff should never come back to the farm the same day that they have been at the quarantine unit. Animals should remain in quarantine for 6 weeks. Serology should be done for the diseases that are suspected. The blood testing should not be done before 21 days after introduction. When possible, acclimatization programs for new incoming animals should start in the quarantine facilities. To do so, manure and afterbirth from the farm is carried into the quarantine facilities. However, this should not occur during the first 3 weeks of quarantine. Quarantine should always be used on an all-inJall-out basis. Quarantine facilities need to be emptied, cleaned, washed, and disinfected between each group. GENETIC IMPROVEMENT
Even for herds that are kept closed, genetic improvement is needed. Some safe techniques exist to bring new genes into a herd. Artificial Insemination
Artificial insemination (AI) is the easiest and most available method to bring new genes into a herd. However, AI brings only half of the genetic components, and if the staff is not well-trained, it may yield lower reproductive performance. 1 As far as disease transmission is concerned, AI should never be done by an
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outside person (inseminator). The AI pack should be sealed and brought directly from the AI station. AI should never come from a nonaccredited AI center and should never be purchased directly from another herd. The AI pack should be well-disinfected before it is brought into the herd. To avoid effect on semen quality, disinfection should be performed at a temperature of 18° to 20°C. Theoretically, semen can carry many microorganisms. Semen can carry Parvovirus, hog cholera virus, influenza virus, Mycoplasma sp, and Lepta Sp.5,12 In the face of an acute disease of the donor animal, other microorganisms could probably be transferred. Many bacteria are in the normal flora of the prepuce. 5 For all of these reasons, a diluent with a broad spectrum antibiotic should always be used. Semen should always come from an accredited AI center, and the health policy and follow-up of that AI center should be checked before the purchase of semen. Embryo Transfer
Embryo transfer is another technique that can be used for the introduction of new genes. This technique calls for a high level of technical expertise and demands a lot of synchronization. For these reasons, this technique is not widely used. Embryo transfer has the advantage that genes can be transferred from country to country. Embryo transfer also allows the transfer of all of the genetic component. As far as disease transmission is concerned, the diseases that can be transferred by AI can also be transferred by embryo transfer .12 When frozen embryos become available, this method will become easier to use and much safer as far as disease transmission is concerned. Medicated Early Weaning
The MEW technique was developed as an alternative method to hysterectomy and fostering. 2 Even though more convenient, this method is less safe than hysterectomy and fostering, embryo transfer, or AI to bring new genes into an existing herd. This methodology is based on the fact that piglets (with a few exceptions) are microbiologically sterile before birth, and it takes a few weeks for the microflora to be developed. Also, during their first week of life, pigs are well protected by colostrum against many diseases. Sows, usually second litter and older, that are going to be used for MEW are moved to another location at 100 days of gestation. Sows are medicated 5 days prior to farrowing to 5 days after. Piglets are medicated from birth to 20 days of age. At 4 to 5 days postfarrowing, the biggest piglets are weaned and moved to a third location. Piglets are raised in that third isolated location until they reach market weight.2,4,14 Since the primary work of Alexander in 1980, another technique called modified MEW has been developed and used. 4,14 Usually, this technique is less sure and is oriented towards the elimination of one specific agent. Hysterectomy and Fostering
Hysterectomy and fostering is probably the safest method to introduce new genes into a herd. With this method, a cesarian section is performed on the donor sow. These cesarian sections usually occur under an isolator. Piglets derived from cesarian sections are kept in an isolator and moved to the target herd. At that time, they are fostered to another sow to be raised. 12
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If the donor sows come from a standard herd, the sows should be moved to another location 8 weeks prior to farrowing. At that location, they should be blood-tested to determine those diseases to which they has been exposed. Some diseases, such as Leptospira sp, Parvovirus, and inclusion body rhinitis virus, have been proven to cross the placenta. Others, such as hog cholera, Brucellosis, tuberculosis, Aujeszky's, and pig pox could potentially cross the placental barrier.12 For these reasons, the donor sow should be moved to another location and blood-tested. The recipient sow should be moved into quarantine. Fostered piglets are put on this sow, and the sow and the new litter should stay in quarantine for 3 to 4 weeks. Ideally, some sentinal pigs should be brought into the quarantine and mixed with these pigs prior to their introduction into the herd.
SUMMARY
The minimal disease concept is a way of raising pigs so that some specific diseases are absent. Many bacteria and viruses can be transferred by pigs, air, or mechanical contact. To avoid contamination, the herd location should take into consideration disease transmission possibilities. Herd health status and source herd health status should be continuously monitored. To maintain herd health status, specific rules need to be followed for herd construction and establishment, compound perimeter, people movement, down time, animal transportation, feed use and delivery, vehicle movement, material, dead animal disposition, and rodent control. All new incoming animals should go through quarantine, and in some herds, safer methods such as AI, embryo transfer, MEW, or hysterectomy and fostering need to be used.
References 1. Alexander TJL: Methods of disease control. In Leman AD, Straw BE, Glock RD, et al: Diseases of Swine, ed 6. Ames, Iowa State University Press, 1986, pp 778-802 2. Alexander TJL, Boon G, Lysons RJ, et al: Medicated early weaning to obtain pigs free from pathogens endemic in the herd of origin. Vet Rec 106:114-119, 1980 3. Goodwin RFW: Apparent reinfection of enzootic-pneumonia-free pig herds: Search for possible causes. Vet Rec 116:690-694, 1985 4. Harris DL: Alternative approaches to eliminating endemic diseases and improving performances of pigs. Vet Rec 123:422-423, 1988 5. Larsen RE, Hurtgen JP, Hilley HD, et al: Diseases transmissible with artificial insemination. In Proceedings of 19th Annual George A. Young Conference, Lincoln, NE, 1978, pp 59-69 6. Le Moine V, Vannier P, Jestin A: Microbiological studies of wild rodents in farms as carriers of pig infectious agents. Prev Vet Med 4:399-408, 1987 7. Madsen P: Denmark's success with SPF. Pig International Jan:12-14, 1988 8. Muirhead MR: Herd biosecurity. In International Pig Letter Symposium, Quebec, Canada, 1991, p 41-44 9. Muirhead MR: The high health status herd. Pig Vet J 22:38-50, 1988 10. Pig Health Control Association: Annual report, abstracts of publications and herd lists 1986 11. Pointon AM, Morrison RB, Hill G, et al: Monitoring pathology in slaughtered stock: guidelines for selecting sample size and interpreting results. National Animal Health Monitoring System, Washington, DC United States Department of Agriculture, 1990 12. Taylor DJ: Disease in high health pig herds. In Proceedings of the Annual Meeting Corporation professionnelle des medecins veterinaires du Quebec, Montreal, 1990
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13. The National Conupittee for Pig Breeding and Production [English translation of -Svineavl og-produktion ]. Copenhagen, Denmark, 1987 14. Wiseman B, Morrison R: Establishing disease-free herds. Am Assoc Swine Pract Newsletter 2-4:1-8, 1990
Address reprint requests to Camille Moore, DVM P.O. Box 1313 St-Cesaire Quebec, JOL 1TO Canada
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BIOSECURITY AND MINIMAL DISEASE HERDS Camille Moore, DVM
The minimal disease concept is not a new one. This concept has been used in Europe since the beginning of the 1960s. 1 The goal of this approach is to produce healthier pigs in a manner to reduce the cost of production and upgrade the quality of the product sold to the consumer. In an attempt to give a definition of this type of production, many terms have been used. Before going further, we will try to clarify some of the terminology. The following definitions have been used: 9 Axenic: Germ-free pigs. They should be totally free of infection with microorganisms. This definition is theoretical, and this type of animal almost does not exist. Gnotobiotic pigs: Specially reared pigs whose micro flora and microfauna are completely known. This is usually for pigs who are born germ-free and then get contaminated with some diseases. Specific pathogen-free (SPF): The first generation of pigs produced after they have been derived by cesarean-section. Usually, we know that such pigs are completely free of some pathogens, and the list of the pathogens that they are free from is supplied. Minimal disease: A widely used definition for pigs that are free of some pathogens. This definition has been used a lot by breeding companies. However, it is not specific to any diseases, and it should not be used as a description for a specific herd health status. High health status: Used as a synonym of minimal disease herd. Refers to herds recognized for the absence of the major infectious diseases. However, a list of the diseases that are not present in a "high health" herd is needed, and this list may vary from veterinarian to veterinarian, from state to state, or from country to country. Accordingly, this definition is very unclear and unspecific and should not be used as a description for a specific herd health status. Medicated early weaning (MEW): The MEW technique is a way to produce
From St-Cesaire, Quebec, Canada
VETERINARY CLINICS OF NORTH AMERICA: FOOD ANIMAL PRACTICE VOLUME 8 • NUMBER 3 • NOVEMBER 1992
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pigs free of some specific diseases. Though not officially recognized as SPF, the pigs are by definition "specific pathogen free." SPF pigs are not absolutely free of all diseases. Some very specific diseases are targeted. The primary SPF herd would be a herd that has been populated only with animals derived through cesarean-section. A SPF secondary herd would be a herd that has been populated from a primary SPF herd. Usually, the aim of producing SPF piglets is to eliminate diseases that have a major impact on the profitability of the herd or to eliminate diseases that are required to be controlled by state or national laws. Up to now, it has been possible to produce pigs that are free of the following diseases:8,12 Foot and mouth disease, African swine fever, swine fever, Aujeszky's disease, transmissible gastroenteritis virus, swine pox, epidemic diarrhea, enzootic pneumonia, Actinobacillus pleuropneumoniae, Glassers disease, Streptococcus suis type I and II, atrophic rhinitis, Brucella suis, Salmonella cholerae suis, swine dysentery, sarcoptic mange, lice, and tuberculosis. Some other diseases mayor may not be present after these procedures. They are porcine intestinal adenomatosis (PIA) complex, Parvovirus, internal parasites, epidemic diarrhea, Clostridium perfringens type C, exudative epidermitis, hemagglutinating encephalomyelitis virus, and swine influenza. Up to now, it has been almost impossible to eliminate the following diseases: congenital tremor, cystitis-nephritis complex, E. coli enteritis, Klebsiella infection, and erysipelas. Before beginning a minimal disease approach, a few questions need to be asked. In a situation of depopulation and repopulation, we need to understand why it is dirty now, where the clean pigs will come from, if the producer will be able to keep the new herd clean, and if the location is suitable for the production of minimal disease pigs. Production of minimal disease animals requires a lot of discipline, and often the owner is the worst offender for all of the new rules. It should be kept in mind that the bigger the herd, the higher the risk of contamination. High health animals will never be a substitute for good management and good care of the animals. The use of such an approach requires a lot of dedication by the owner, a good record keeping system, good discipline, a complete disease control program, and a detailed production and management procedure. 1 CONSIDERATIONS BEFORE DEVELOPING A BIOSECURITY PLAN
To keep the good health status of the herd, it is necessary to develop a detailed biosecurity plan. However, before looking at all of the items of such a plan, it is necessary to know about diseases. Disease Transmission
In an attempt to keep the herd free from specific diseases, it is essential to know how these diseases are transmitted. It does not make any sense to try to eradicate a specific disease if that disease may be brought in the herd very easily after the clean-up procedure. The pig itself is the main source of disease organisms. Some organisms can stay alive for a very long period of time in a herd. Ascaris and coccidia can probably remain in an emptied herd for at least 5 years. 12 Therefore, it is almost impossible to produce pigs free of ascaris or coccidia after a depopulation/repopulation scenario. This is also probably true for bacteria that produce sporulated organisms, such as clostridium. 12
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Some diseases are airborne from herd to herd. Variation does exist in the potential for airborne transmission. Humid and cold weather are more favorable to disease transmission than dry and hot days. 3 The following diseases can apparently be transmitted by air: 1,8 Foot-and-mouth virus can travel for more than 25 miles. Enzootic pneumonia can be transferred up to 2 miles. Transmissible gastroenteritis (TGE), atrophic rhinitis, and pleuropneumonia can probably be transferred 0.5 mile. Swine infertility and respiratory syndrome (SIRS; mystery swine disease) can probably be transferred up to 2 miles. Influenza virus can be transferred up to 3 to 4 miles. Aujeszky virus can probably travel for 25 miles in the air. Other diseases, such as swine dysentery, are not likely to be transferred by air alone. However, such bacteria can easily be carried from herd to herd by house flies. Flies can travel up to 2 miles. Besides swine dysentery, they can also carry S. suis and TGE virus. 8 Much work has been done to understand the transmission of Mycoplasma hyopneumoniae. Usually, this bacteria does not survive for more than 48 hours outside the host. Survival time is longer during rainy weather, and the organism can be transferred in moist air. The rate of contamination is greater in very pig-dense areas. Many pigs are subclinical carriers for M. hyopneumoniae, and it can be easily transferred by pigs. 3 It is always better if depopulation/repopulation occurs during summer. Organism survival time is usually shorter during warm periods. The herd should be entirely empty of pigs for a period of at least 6 weeks. Good cleaning and disinfection should occur during this period. Rodent extermination should be done. Herd Location
Preferred herd location is chosen in relation to the diseases that we would like the herd to remain free of and the distance that such diseases can be transmitted. Theoretically pig barns should be as far as possible from other pig barns. Location is the most important factor to avoid recontamination. 3 This is the dominant factor for a herd to remain free of enzootic pneumonia. The second most important factor is the size of the neighbor farm. The site should be chosen based on the presence of other pigs in the area, the type of diseases that are being avoided, the size of the nearest infected farm for that specific disease, the type of production (weaners production unit or finishing unit), the air temperature and humidity pattern for the area, and prevailing winds. 9 1£ the location is in an area where winds are very important, the farm should be located down a valley and at the opposite side. 1£ the farm is located in an area of very hot weather, the herd should be located on the top of a hill. Roads that are close to the farm are also factors. Some other pigs could be carried close to the farm and become a source of contamination. The farm should be located on a dead-end road and as far as possible from major highways. Table 1 summarizes some distances between pig farms with which no transmission of diseases occurred. 9 M. hyopneumoniae transmission has never been reported over a distance of 3.2 km by air. 3 According to the available information, the following recommendations can be made: 1£ the aim is to keep a herd free of TGE, actinobacillus pleuropneumonia (APP), atrophic rhinitis (AR), and S. suis, the herd should be located at least 0.5
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Table 1. DISTANCE BETWEEN FARMS AT WHICH NO TRANSMISSION OF DISEASE WAS OBSERVED Disease
Distance
Time Free of Transmission
Aujeszky's Transmissible gastroenteritis Enzootic pneumonia Atrophic rhinitis Actinobacillus pleuropneumoniae Streptococcus suis Swine dysentery Mange
500 m 400 m 150 m 300 m 500 m 300 m 300 m 100 m
4 months, infected herd slaughter after 4 months, disease die out 4 years 3 years 6 years 12 years 4 years 5 years
mile from any other pig unit. If the aim is to keep a herd free of enzootic pneumonia and Aujeszky's disease, the herd should be located at least 2 miles from any other pig unit. Even though these types of precautions are recommended, in Denmark, where the minimum distance between herds is only 350 feet, they have been quite successful in keeping herds free of many diseases. 13 Owing to the lack of documentation on the role of other animals in disease transmission, it is difficult to make specific recommendations concerning distances between other livestock or poultry operations. Assessing Health Status
Many methods can be used to assess health status of a herd. Even if not completely objective, the continuous herd follow-up with clinical examination and monitoring for any change in clinical symptoms is still the most effective way to assess health status. 1,8,9 Slaughter checks are a useful tool to assess health status. However, the sample sizes used must be big enough for significant results. Guidelines related to the size of the sample necessary according to the degree of confidence expected have been published. l l In interpreting slaughter check results, we need to keep in mind that many lesions heal, and what we see at slaughter might not reflect what happened earlier in the herd. Therefore, in top pyramid herds, slaughter checks should be performed at least every 6 weeks. Serology is another tool to assess health status. However, serology has limitations. It is not always easy to interpret the results of serologic tests. We need to know the type of test used, its limitations, and the specificity and the sensitivity of each test used when interpreting serology. Inconclusive results will always exist. Sample size is as important for serology testing as it is for slaughter checks. With the emergence of new techniques, serology will become more and more reliable in the near future. In many countries or states, we can rely on associations that are doing herd monitoring and certification to assess health status. The Pig Health Control Association in the United Kingdom is a good example. They maintain a list of herds that are free of enzootic pneumonia, swine dysentery, atrophic rhinitis, Aujeszky's, mange, and S. suis type 11. 10 Table 2 gives specific criteria for monitoring some specific diseases. 9 Careful examination of vaccination programs and drugs used in the herd should also be performed. Theoretically, no drugs should be used. Finally, the owner should assess health status. He or she should daily check
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Table 2. SPECIFIC CRITERIA FOR MONITORING SOME SPECIFIC DISEASES Criteria
Disease Enzootic pneumonia Aujeszky's Transmissible gastroenteritis Actinobacillus pleuropneumoniae
Streptococcal meningitis Atrophic rhinitis Swine dysentery Mange
Life history, regular lung examination (serology becomes more and more reliable) History, serology History, clinical sign, serology (pay attention to the porcine corona respiratory virus in interpretation) History, regular lung examination, serology (depending on the technique used) 5-year history, no Streptococcus suis type II isolated from meningitis pigs History, nasal section at slaughter check, and nasal swabbing for toxigenic Pasteurella multocida History, no medication used History, scrapping and no parasitic used
and report any changes in coughing, short breath, loss of appetite, diarrhea, itching, sneezing, and all other visible changes in the herd. 1 Source of Replacement Animals
It is not always easy to find a source of minimal disease animals. All of the animals should come from only one supplier. Animals used should be derived from a pyramidal system. 1 This means that the animals flow from the top (nucleus) downstream to multiplier and commercial production. The type of production desired determines where in the pyramid to be located. All animals should always come to your herd from the next highest level in the pyramid. Each herd manager needs to decide the diseases from which the herd is wished to be free and the type of genetics he or she is looking for. With that in mind, a supplier herd can be chosen. The decision should be made based on the following factors:
The herd should be under routine veterinary observation. The record keeping system should be examined. No drugs should be used in the herd. Vaccination programs should be well examined. The herd history should be studied. All laboratory findings in that herd should be checked. Postmortem results should be examined. Replacement stock should always be bought from a closed herd. DESIGN CONSIDERATIONS
Even with a good location and the good type of animals, some considerations should be taken as far as buildings are concerned to make sure that the health status will remain clean. To ensure a good barrier to disease spread, total confinement should always be used. Some questions arise concerning the use of naturally ventilated buildings or open front buildings. If such buildings are used, they should be organized in a manner as to be entirely bird- and rodent-proof.
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General Considerations
A standard layout organization for a high health pig bam is shown in Figure 1. Care should be taken to orient the building according to wind direction. Enough places should be planned for all of the production stages. Hospital pens should always be planned. The organization of the building should take into consideration the pig flow. It is desirable to reduce the amount of movement needed for the pigs and also allow manpower specialization. Usually, the following distribution is used: Gilt acclimation place, breeding area, dry sow area, farrowing room, and nursery. If a grow-finish operation is to be on the same site, it should be located in another building. In setting up a minimal disease unit, five types of activities need to be planned: the production site itself, the office, the feed delivery area, the animal loading area, and the manure-handling system. These five activities should be located at different places, and a clear demarcation needs to exist between each of the areas. AII-inlAll-out Management
The use of an all-injall-out production system should be promoted in any type of pig production unit. It is even more important in high health-status herds.
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1~ Figure 1. Standard high-health pi g barn layout. 1 = access road, 2 = locked main gate; 3 = perim eter road; 4 = perimeter fence; 5 = quarantine; 6 = manure storage; 7 = feed bin; 8 = incoming animals; 9 = gilt acclimation; 10 = breeding; 11 = dry sow; 12 = farrowing; 13 = nursery; 14 = grow-finish; 15 = animal expedition facility; 16 = postmortem; 17 = incineratory; 18 = human entrance; 19 = showers; 20 = office; 21 = canteen; 22 = room for incoming material.
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Building design should be planned so that small rooms will be used at each level of production. These rooms do not allow contact with other rooms. After the rooms have been completely emptied, they are washed and disinfected. The purpose is to reduce build-up of bacteria and diseases. This type of system is more expensive because space usage is not optimal. Recently, the concept of multiple-site production has been developed. This technique uses either two sites for production, (site one is gestation and farrowing, and site two is nursery/growing/finishing) or three sites, (site one is gestation/ farrowing, site two is nursery, and site three is growing/finishing). In the threesite production system, piglets are weaned at 8 to 24 days of age and moved to another location. Pigs remain in the nursery for 6 to 8 weeks and are then moved to a growing/finishing complex. 4- 14 The use of this approach ensures the adherence to the all-in/all-out concept. This is also an insurance policy in the face of disease outbreaks, as the cycle of such disease can be broken. Material and Equipment
All material and equipment used in these building should be easy to wash and long-lasting. It is always a hazard to bring a service person into the unit to perform repairs. For this reason, good-quality material should always be used. All material that is brought into the facility premise after the unit is in operation should be new. Waste Management
The reservoir for affluent from the herd should always be located outside the fenceline. The pipe used to carry the manure should be rodent-proof to avoid introduction of rodents into the building. Great care should be taken of the equipment and vehicles used to dispose of manure. Ideally, this material should belong to the farm. If not, it should be completely washed and disinfected before use. BIOSECURITY POLICIES Personnel
The downtime" that should be observed by people going into a minimal disease herd is always tough to assess. This question goes back to disease transmission principles. It has been found that foot-and-mouth disease virus can be carried in the nose of humans for 11 hours. This virus also can survive under fingernails for a few days. It has been found that humans can carry the foot-andmouth disease virus for 1 to 2 days.7,13 Some analogies probably can be made between the foot-and-mouth disease virus and the TGE virus or pseudorabies virus. Two nights of downtime seems to be the practical minimum. It has been shown that M. hyopneumoniae can survive for 24 to 48 hours on clothes and for as long as 30 hours on filter paperl under lab conditions. Possibly, the bacteria survive for a shorter period of time in the field. Godwin's studies3 have not been able to make any correlation between downtime and reinfection with enzootic pneumonia. Workers in a minimal disease herd should never be in contact with other pigs. They should not be allowed to live at another pig farm. Visitors to a minimal /I
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disease herd should be kept to a minimum. They should have been free of pigs for at least 2 nights. A log book of all humans going into the herd should be kept. All visitors should write down the number of hours since they have been with pigs and the last place where they have been in contact with pigs. They should sign their name. Sign and Fence
A sign should be put close to the access road. This sign should say that pigs raised are under a health-monitoring system, and no one is allowed to go beyond that point without permission. The farm perimeter should be well identified. The use of a fenceline is recommended, especially if other pigs are close by (1 mile). In countries where wild animals or rodents are present, these fences should be built in a manner to restrict the access of these animals to the building area. These fences also keep all vehicles and humans away from the production buildings. Usually, fences are built 40 to 50 feet from the production unit. In areas where other pigs are 3 to 4 miles away, a fenceline might not be necessary. In these situations, some other devices, such as deep ditches, may be sufficient to restrict vehicle access. Shower and Clothes
The office and main entrance to the building should be located close to the fence. The office should supply a canteen to allow the staff to eat on the farm premises. The office should be located so that it is possible to see all incoming people and vehicles and also to control the pig loading bay. Only one entrance should be used. Before going into the herd area, a shower should be mandatory. Even though it is not clear that a shower can eliminate microorganisms that may be carried by humans, this is a good way to ensure that clothes are completely removed and a special clothes supply is used in the herd. Shower installation should be designed so that people will go through the shower and will not come back in the entrance or contaminated area to dress themselves. The shower is also a good physical barrier and a discouraging procedure for visitors who do not have a compelling reason to enter the herd. In the Danish SPF system, showers are not used. They have developed an entry system that could be applied to commercial producers who are using minimal disease animals. This system is .shown in Figure 2.7 The outside door must be locked at all times. A bell should be installed, as should a system to unlock the door without going back through the shower. All clothes used on the farm premises should be supplied by the farm. At no time should materials or animals go through the staff entrance. Animal Transportation
Loading and unloading animals is probably one of the most common ways to bring disease into minimal disease herds. If the herd is not completely closed and is being used as a multiplier or a commercial production unit, two different locations should be planned for animal receiving and animal expedition. A specific building for loading purposes needs to be built. This building must
BIOSECURITY AND MINIMAL DISEASE HERDS
Protective clothing for unit staff and visiting veterinarians & agricultural consultants should be kept here.
Footwear must be disinfected before
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This must be QD.!y entrance to pighouse.
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Wooden-grate passage should be entered QD.!y in stockinged feet (never in boots).
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Figure 2. S.P.F. Danish entry.
be separated from the production unit. It should be located on the fenceline for protection. Its affluent drainage system should not go through the main building. It should be located close to the office to facilitate communications between truck drivers and herd staff. Animals that need to be sent are placed in the loading building by farm staff. Then, the door is locked, and farm staff should not go back into the loading facility. The truck driver loads the pigs into the truck without assistance from farm staff. There should be small pens in the loading building to facilitate loading by the driver. The loading facility must be washed, disinfected, and fumigated after each use. This facility can be used only once a day. If no other manpower is available, the facility can be cleaned at the end of the day by the farm workers. People who do this job must never go back into the farm premises the same day. The vehicle used for animal transportation should always arrive at the farm location empty. Godwin3 reported 13 cases where herds broke with enzootic pneumonia because a hauler came close to the bam (25 to 75 feet) with animals in the truck. The truck used for these transportations needs to be washed, cleaned, and disinfected before arriving on the farm premises. Feed If the feed used for the farm is purchased, a system should be planned for its delivery. Feed bins are installed around the farm perimeter fence, and the vehicle that delivers the feed stays outside the farm perimeter. If the feed bins are close to the farm, a system should be planned to allow the feed to be blown from outside the perimeter. If the feed is manufactured on the farm, the same precautions should be used for ingredient delivery. Food wastage (kitchen or industrial) should never be used in minimal disease herds. The use of animal by-products in feed formulations for minimal disease
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herds is still debatable. The type of diseases that can be carried by animal by-products is unclear. Until the time that better information is available, animal by-products (meat meal, bone meal, and so forth) should not be used. Vehicles
No vehicles should be admitted inside the herd perimeter. Employees' cars should be left as far as possible from the herd. Any time that it is possible, feed trucks should come to the herd first thing in the morning. Monday morning is the best day for feed delivery. Feed truck drivers should never come in contact with the feed. If a problem does occur during feed delivery, the driver should go back to the feed mill with the feed left. In hot regions, the use of an area with water and disinfection for the truck to go through is encouraged. Material and Equipment
All equipment and material brought into the herd should be cleaned and disinfected. To ensure compliance, a proper location to do so should be planned. This room should be located on the fence perimeter. A door gives access to the outside. New material or equipment is delivered in that room. If possible, a fogging system should be used to do the disinfection. If this is not possible, all of the new material and equipment should be put in a solution with disinfectant. The solution must be changed every day. Ideally, all material brought into the herd should be brand new. However, this is not always possible, especially with the service person's equipment. In this situation, equipment needs to be completely clean before it is brought into the disinfection room. Attention should be paid to material that could have been on another pig farm. Ideally, these types of material should not be allowed in. It is a good idea to have in the farm some of the general tools for service people. Dead Animal Disposal
Dead animal disposal is another risk for disease entry. In many cases, it is desirable to have a postmortem performed on dead animals. If laboratories do not exist close by and postmortem examinations need to be performed on the farm, it is a good idea to plan an area where they can be performed by the veterinarian. 12 This place should be planned outside of the farm perimeter to allow a veterinarian who may not have been pig-free to perform the postmortem examinations. The best way to dispose of dead animals is the use of an incinerator. However, in many areas these devices are not allowed. Also, these devices usually are expensive to use. If the only way to dispose of dead carcasses is through a rendering company, the vehicle owned by those companies should not be allowed to come closer than 1 mile to the herd. To ensure that, a transportation mode should be planned for the dead carcasses from the production unit to a place where they can be picked up by the rendering company. Pet Animals and Rodent Control
No cats, dogs, or other pet animals should be allowed on the farm premises at any time. Rats and mice have been caught and the diseases they can carry
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studied. 6 Bordetella bronchiseptica, Pasteurella sp, E. coli, Campylobacter jejuni, Treponema sp, Rotavirus, and TGE titers were found. The same workers have been able to infect rats with Aujeszky's virus and classical swine fever virus. 6 These findings prove the importance of having an adequate rodent control program. The buildings need to be rodent-proof. A program for monitoring rodent presence should be in place. As soon as the presence of rodents is noticed close to the barn, some action should be taken to destroy them. Quarantine
Producers who are associated with genetics companies and many commercial producers find it impossible or impractical to keep a closed herd. Unfortunately, these people have to continuously bring new animals into the unit. Even if one is always dealing with the same supplier and even if the sources are well monitored, a quarantine facility and program should be put in place. It should be kept in mind that for many diseases, the incubation period is long and animals could be brought in before any sign is seen or diagnosis is made in the source herd. Also, remember that contamination could have occurred during transportation. Ideally, quarantine facilities should be located on another farm. Practically, this is almost impossible. Therefore, quarantine facilities should be located as far as possible from the main building. No direct communication should be used between the quarantine facility and the main building. Quarantine buildings should have their own manure-handling facilities. 1 Feeding and examining the animals without having to come into direct contact with them should be possible. It is recommended to have a filter on the air outlet of the quarantine. Farm staff should go in the quarantine area only at the end of the work day. Foot baths should be installed, and all clothes should be changed before going into the quarantine unit. Direct contact with animals should be kept to a minimum. Farm staff should never come back to the farm the same day that they have been at the quarantine unit. Animals should remain in quarantine for 6 weeks. Serology should be done for the diseases that are suspected. The blood testing should not be done before 21 days after introduction. When possible, acclimatization programs for new incoming animals should start in the quarantine facilities. To do so, manure and afterbirth from the farm is carried into the quarantine facilities. However, this should not occur during the first 3 weeks of quarantine. Quarantine should always be used on an all-inJall-out basis. Quarantine facilities need to be emptied, cleaned, washed, and disinfected between each group. GENETIC IMPROVEMENT
Even for herds that are kept closed, genetic improvement is needed. Some safe techniques exist to bring new genes into a herd. Artificial Insemination
Artificial insemination (AI) is the easiest and most available method to bring new genes into a herd. However, AI brings only half of the genetic components, and if the staff is not well-trained, it may yield lower reproductive performance. 1 As far as disease transmission is concerned, AI should never be done by an
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outside person (inseminator). The AI pack should be sealed and brought directly from the AI station. AI should never come from a nonaccredited AI center and should never be purchased directly from another herd. The AI pack should be well-disinfected before it is brought into the herd. To avoid effect on semen quality, disinfection should be performed at a temperature of 18° to 20°C. Theoretically, semen can carry many microorganisms. Semen can carry Parvovirus, hog cholera virus, influenza virus, Mycoplasma sp, and Lepta Sp.5,12 In the face of an acute disease of the donor animal, other microorganisms could probably be transferred. Many bacteria are in the normal flora of the prepuce. 5 For all of these reasons, a diluent with a broad spectrum antibiotic should always be used. Semen should always come from an accredited AI center, and the health policy and follow-up of that AI center should be checked before the purchase of semen. Embryo Transfer
Embryo transfer is another technique that can be used for the introduction of new genes. This technique calls for a high level of technical expertise and demands a lot of synchronization. For these reasons, this technique is not widely used. Embryo transfer has the advantage that genes can be transferred from country to country. Embryo transfer also allows the transfer of all of the genetic component. As far as disease transmission is concerned, the diseases that can be transferred by AI can also be transferred by embryo transfer .12 When frozen embryos become available, this method will become easier to use and much safer as far as disease transmission is concerned. Medicated Early Weaning
The MEW technique was developed as an alternative method to hysterectomy and fostering. 2 Even though more convenient, this method is less safe than hysterectomy and fostering, embryo transfer, or AI to bring new genes into an existing herd. This methodology is based on the fact that piglets (with a few exceptions) are microbiologically sterile before birth, and it takes a few weeks for the microflora to be developed. Also, during their first week of life, pigs are well protected by colostrum against many diseases. Sows, usually second litter and older, that are going to be used for MEW are moved to another location at 100 days of gestation. Sows are medicated 5 days prior to farrowing to 5 days after. Piglets are medicated from birth to 20 days of age. At 4 to 5 days postfarrowing, the biggest piglets are weaned and moved to a third location. Piglets are raised in that third isolated location until they reach market weight.2,4,14 Since the primary work of Alexander in 1980, another technique called modified MEW has been developed and used. 4,14 Usually, this technique is less sure and is oriented towards the elimination of one specific agent. Hysterectomy and Fostering
Hysterectomy and fostering is probably the safest method to introduce new genes into a herd. With this method, a cesarian section is performed on the donor sow. These cesarian sections usually occur under an isolator. Piglets derived from cesarian sections are kept in an isolator and moved to the target herd. At that time, they are fostered to another sow to be raised. 12
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If the donor sows come from a standard herd, the sows should be moved to another location 8 weeks prior to farrowing. At that location, they should be blood-tested to determine those diseases to which they has been exposed. Some diseases, such as Leptospira sp, Parvovirus, and inclusion body rhinitis virus, have been proven to cross the placenta. Others, such as hog cholera, Brucellosis, tuberculosis, Aujeszky's, and pig pox could potentially cross the placental barrier.12 For these reasons, the donor sow should be moved to another location and blood-tested. The recipient sow should be moved into quarantine. Fostered piglets are put on this sow, and the sow and the new litter should stay in quarantine for 3 to 4 weeks. Ideally, some sentinal pigs should be brought into the quarantine and mixed with these pigs prior to their introduction into the herd.
SUMMARY
The minimal disease concept is a way of raising pigs so that some specific diseases are absent. Many bacteria and viruses can be transferred by pigs, air, or mechanical contact. To avoid contamination, the herd location should take into consideration disease transmission possibilities. Herd health status and source herd health status should be continuously monitored. To maintain herd health status, specific rules need to be followed for herd construction and establishment, compound perimeter, people movement, down time, animal transportation, feed use and delivery, vehicle movement, material, dead animal disposition, and rodent control. All new incoming animals should go through quarantine, and in some herds, safer methods such as AI, embryo transfer, MEW, or hysterectomy and fostering need to be used.
References 1. Alexander TJL: Methods of disease control. In Leman AD, Straw BE, Glock RD, et al: Diseases of Swine, ed 6. Ames, Iowa State University Press, 1986, pp 778-802 2. Alexander TJL, Boon G, Lysons RJ, et al: Medicated early weaning to obtain pigs free from pathogens endemic in the herd of origin. Vet Rec 106:114-119, 1980 3. Goodwin RFW: Apparent reinfection of enzootic-pneumonia-free pig herds: Search for possible causes. Vet Rec 116:690-694, 1985 4. Harris DL: Alternative approaches to eliminating endemic diseases and improving performances of pigs. Vet Rec 123:422-423, 1988 5. Larsen RE, Hurtgen JP, Hilley HD, et al: Diseases transmissible with artificial insemination. In Proceedings of 19th Annual George A. Young Conference, Lincoln, NE, 1978, pp 59-69 6. Le Moine V, Vannier P, Jestin A: Microbiological studies of wild rodents in farms as carriers of pig infectious agents. Prev Vet Med 4:399-408, 1987 7. Madsen P: Denmark's success with SPF. Pig International Jan:12-14, 1988 8. Muirhead MR: Herd biosecurity. In International Pig Letter Symposium, Quebec, Canada, 1991, p 41-44 9. Muirhead MR: The high health status herd. Pig Vet J 22:38-50, 1988 10. Pig Health Control Association: Annual report, abstracts of publications and herd lists 1986 11. Pointon AM, Morrison RB, Hill G, et al: Monitoring pathology in slaughtered stock: guidelines for selecting sample size and interpreting results. National Animal Health Monitoring System, Washington, DC United States Department of Agriculture, 1990 12. Taylor DJ: Disease in high health pig herds. In Proceedings of the Annual Meeting Corporation professionnelle des medecins veterinaires du Quebec, Montreal, 1990
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13. The National Conupittee for Pig Breeding and Production [English translation of -Svineavl og-produktion ]. Copenhagen, Denmark, 1987 14. Wiseman B, Morrison R: Establishing disease-free herds. Am Assoc Swine Pract Newsletter 2-4:1-8, 1990
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