PREVENTING DISEASE ENTRY

Specific measures should be taken to prevent pathogen entry from other farms and nonpig vectors. In addition, contact between the established herd and new additions to the breeding herd must be regulated to protect both.

Location of the herd is a major determinant of the ability to deny pathogen entry.⁴ Airborne spread is a major route of pathogen movement⁵ (Table 110-1). Spread of pseudorabies virus has been documented in consistent directions along prevailing seasonal winds.⁶ Porcine reproductive and respiratory syndrome virus (PRRSV) also may be spread by aerosols, although the data are inconclusive. Ideally, a facility should be located as far as possible from other facilities or farms, although in pig-dense areas the choices may be limited.

Table 110-1 Airborne Pathogen Spread

Nonpig vectors such as rodents, birds, insects, pet or stray dogs and cats, and humans can mechanically introduce pathogens from contact with pigs and their wastes on other farms. Constructing a barrier perimeter fence 12 to 15 m from buildings to restrict animals, vehicles, and people to one portal of entry is now standard for new construction in the swine industry.⁷ Prominent signs warning visitors to stay out without prior approval and to check in before making deliveries will reinforce the importance of biosecurity to herd employees and visitors.

Establishing an apron of gravel or limestone 0.6 to 0.9 m wide and 15.2 cm deep and eliminating weeds around hog buildings, along with strategic placement of traps and baits, help to rodent-proof a building.⁸ “Hardening” a building by tight construction and closing potential openings help to control rodents, flies, and birds.

Although pigs remain the primary source of introduction of new diseases, humans can be a significant mode of pathogen entry. Establishing shower-in–shower-out entry, with protective clothing provided by the farm, and disinfectant foot baths will control most of the potential for pathogen introduction by humans. Because of concerns that the human body may temporarily harbor certain swine pathogens that may not be eliminated by baths or disinfection, some farms have required that visitors must not have been on another hog farm at any time during the preceding 72 hours. Although such steps may decrease potential for disease entry, information regarding delay periods that are adequate or even reasonable is scarce. In addition, this requirement can significantly restrict the activities of veterinary consultants.⁹

Swine transport vehicles should be sanitized between loads. Movement of hogs should be one-way, with no reentry after the vehicle has passed the “point of no return.” Disease outbreaks have been traced back to hogs that went onto a truck, only to scurry back into the finishing floor.

Incoming breeding stock are potential sources of disease entry. Although breeding stock sources recognize that a “clean” reputation is vital to continued success, a wide spectrum of health programs exist on those farms. Minimum-disease status, sometimes referred to as high health, implies a low level of pathogens without specifying those not present. Some breeding stock companies utilize these terms for the herds in their distribution pyramid.⁵ Herds with specific pathogen–free designation are certified free of certain enumerated pathogens by virtue of cesarean derivation and immediate isolation of pigs. Those pigs are used to populate primary specific pathogen–free herds or to farrow pigs naturally, which are then used to populate secondary specific pathogen–free herds. Breeding stock from minimum-disease herds, some now preferentially located in areas not traditionally swine-dense, may be more at risk from diseases present in the herd of entry than the recipient herd is from the new animals. Quarantine and isolation of these pigs are essential, in conjunction with planned exposure to the organisms resident on the farm. Examples of recommended introduction protocols from two seedstock companies are shown in Table 110-2. In general, a 60-day quarantine period is recommended to reduce the possibility of PRRSV transmission. After 14 to 21 days, animals are serologically tested for any diseases of regulatory concern. Appropriate vaccines and exposure protocols are applied following the retesting procedure.

Table 110-2 Breeding Stock Company Protocols for Introducing Animals

Artificial insemination (AI) and embryo transfer technology may offer opportunities for reducing the need to introduce new animals into a breeding herd. At present, AI is a more practical technology than embryo transfer for routine use. In most situations, use of AI can improve health status.

Buying and introducing fewer boars because AI is being used constitute one reason AI can decrease disease risk. A 600-sow farm with 30 boars probably will buy 10 new boars per year. A 600-sow farm using AI and maintaining 6 boars is likely to purchase only 2 new boars per year. Fewer introductions of new animals reduce the risk of introducing disease with the new animals.

An off-site boar stud facility may offer an opportunity for improved biosecurity in some systems of production. An off-site facility requires at least the same level of biosecurity as that practiced in the breeding herd. To gain the full benefit of improved biosecurity with a boar stud, separation between collection technicians and processing technicians should be maintained in the AI laboratory. A pass-through window or a processing area separated from the collection area is needed. Collection technicians should never enter the laboratory area.

Buying semen rather than boars may further reduce the risk of introducing disease; however, the source herd should be checked in the same way as that used for screening live animals for purchase. Several species of bacteria are commonly found in boar semen. Many of them are commensal inhabitants of the preputial area and are not specific pathogens for the reproductive system but nevertheless may disrupt individual pregnancies. A major problem with many of the contaminating bacteria is that they reduce the shelf-life of semen. Some bacteria may have a spermicidal effect as well. Because the temperature at which swine semen is stored (15–20° C) does not inhibit bacterial growth, antibiotics are routinely added to semen extenders.

Many viruses have been shown to be present in boar semen. Only three, African swine fever virus, porcine parvovirus, and PRRSV, have been proved to be transmitted to sows through semen. The same precautions must be taken with the source of semen as are taken with live animals, to minimize the risk of bringing these viruses into the herd with semen. Because viruses and their by-products usually are not spermicidal, and antiviral agents are much more expensive than antibiotics, antiviral agents are not commonly added to semen extenders.

In addition to the potential for reducing the risk of disease from incoming animals, AI can reduce the spread of disease organisms within a breeding herd. For example, on a 600-sow farm, 30 boars are more likely to transmit disease organisms within a breeding herd than are 6 boars that never actually come into physical contact with the sows. At least some anecdotal evidence suggests that correct use of AI can limit vaginal discharge problems within a sow herd.