Diagnostics

Definitive diagnosis of (Porcine Reproductive Respiratory Syndrome) PRRS virus still depends upon isolation of the virus. The best tissue sample is serum since the virus is found in the blood for over 40 days after infection in young pigs and up to 20 days in sows. Tonsils and lungs are the other two tissues that virus is easily isolated from. Recent work at Iowa State University indicates that the virus may be in tonsils for over 150 days experimentally, it remains to be demonstrated under field conditions. Dr. Pat Halbur, also from ISU, as developed a special test for detecting PRRS viruses from formalinized or "fixed" lung tissue. However, isolation of virus is difficult after the initial outbreak (except in pigs from persistently infected nurseries), therefore virus isolation is not the diagnostic test of choice for determining whether PRRS is still actively spreading in the herd. Excellent field research by Dr. Scott Dee of Morris, Mn. has shown that PRRS IFA (immunofluorescent antibody) titers are very reliable for determining whether the virus is continuing to circulate in breeding stock. Infected pigs loose their antibody response (go IFA "seronegative") 3 to 6 months later if they are not reexposed to the virus. Therefore a seronegative gilt or replacement boar may have been previously infected (it is not known how much of a risk they are, or for how long these seronegative animals may carry virus). Dr. Dee has observed that herds in which PRRS infection continues to spread have over 50% of their sows with IFA antibody titers. Usually PRRS is also circulating and causing increased levels of disease in nursery pigs in these herds as well. Many of these nursery and finisher pigs will have high PRRS IFA titers. PRRS IFA titers are reported as 1:16 (low), 1:64, 1:256 (intermediate), and 1:1024 (high). Virus is actively circulating if most animals have intermediate or high titers while the herd may be just breaking OR finally controlling the circulation of the virus if only some animals are positive with intermediate to low titers. To determine if your herd or a breeding stock herd has actively circulating PRRS collect blood samples for the PRRS IFA from 10 - 15 8 and 16 week old pigs and 30 breeding animals across parities and breeding groups. Herds in which PRRS is not circulating will have seronegative nursery and finisher aged pigs and < 20% of the sows will have PRRS IFA titers. Purchased gilts from these herds are most likely to be virus free and not be a risk, even if the herd previously had a clinical PRRS outbreak. Probably only a very few breeding stock herds remain that have never been infected by PRRS at one time or another.

Purchase of Breeding Stock

Whether your herd is currently seronegative for PRRS, has never gotten PRRS, or is currently seropositive and circulating PRRS, you should try to purchase stock from noncirculating breeding stock herds if at all possible. It is unclear how much cross protection there is between different strains of PRRS at this time. There are reports of PRRS reoccurring in previously infected herds. It is unclear though whether this is a result of infection of younger recently introduced replacement gilts or a true "reinfection" of older animals. If your sole breeding stock supplier does have PRRS circulating in the nursery or finisher, then you should strongly consider purchasing gilts and boars at a younger age and putting them in isolation for a minimum of 60 days and possibly for 120 days to allow them to control the infection and stop shedding virus. This will make these animals much less of a risk to introduce into your herd since they probably will not be shedding virus once they are seronegative.

Management for Minimizing PRRS Losses

To minimize piglet death losses and disease during and after a PRRS outbreak the following management practices are recommended and have been very effective. [These are based upon the fact that PRRS alone does not kill gnotobiotic (disease free caesarian-obtained) piglets. Piglets become severely ill and die usually as a result of secondary bacterial infections that are made possible by the destruction of specific immune cells in the pig's lungs.]

STOP CROSSFOSTERING pigs greater than 24 hours of age.

NEVER feed carcasses from dead sick or aborted pigs back to sows or gilts, even during the outbreak.

MOVE PIGS STRICTLY ALL IN - ALL OUT BY ROOM! Do this in farrowing and nursery. Continuous flow management during an outbreak is almost a guarantee of chronic PRRS infection problems to follow.

HOLD NO PIGS BACK TO YOUNGER GROUPS trying to get them to a minimum weigh for the nursery or finisher.

DESTROY VERY POOR DOING PIGS AND PIGS TOO SMALL TO MOVE TO THE NURSERY OR FINISHER EARLY. These pigs are sources of huge amounts of virus and bacteria for other pigs in the farrowing house or nursery!

Evaluate routine use of antibiotics for controlling secondary bacterial infections. In some herds treatments actually increased preweaning mortality rates!

Return to normal management practices 2 months after clinical signs of PRRS have ended, but continue strict all in - all out pig movement procedures. There is nothing that can be done to minimize reproductive losses (abortions, stillbirths, and mummies) once the outbreak has begun. Fetuses are infected shortly after the sow becomes infected.

The virus may continue to circulate months after the original outbreak if large numbers of gilts are introduced into the herd. The breeding herd shows abortions, decreased farrowing rates, weak born pigs with increased preweaning mortality from gilt litters, and continual disease problems in nursery-aged pigs are frequently seen. In these kinds of herds management must be changed so virus stops circulating in the sow herd, as determined by a PRRS IFA seroprevalence of < 10%. Bringing in enough gilts for 3 to 4 months and closing the herd to new introductions will help control sow virus circulation. We have also determined that feedback of sick or weak-born pigs after they die to sows as a "autogenous vaccine" also continues the circulation of PRRS in the herd. Finally, we have found that the severity and duration of PRRS associated losses in suckling pigs (and therefore reexposure of sows to PRRS) is directly affected by farrowing house management practices.

Once there are no clinical signs of reproductive PRRS in the herd, and < 20% of the sows are seropositive for PRRS antibody, then PRRS can be eliminated from the endemically infected chronically diseased nursery. Dr. Scott Dee from the University of Minnesota has demonstrated that PRRS circulation can be stopped in the nursery by depopulating the nursery for 2 weeks with complete clean-up. Afterwards, he reports exceptional pig performance since PRRS and the associated secondary bacterial diseases have been eliminated.

Vaccination

I have purposely left discussion of vaccination until last, because the only widely available PRRS vaccine is only approved for pigs up to 18 weeks of age. We have learned much about control of PRRS and OTHER swine diseases using strategic AND PROFITABLE management changes. Therefore, while vaccination does have a role, and may possibly be very effective against PRRS, it is economically short sighted to solely rely on vaccine to cure our PRRS-initiated disease ills.

The currently available non-autogenous vaccine is a modified-LIVE virus vaccine. Field experiences are still being determined as of this writing, but according to the manufacturer vaccinated animals do not become viremic, show few if any clinical signs after challenge with wild-type virus, and shed less virus after challenge. They indicate that pigs can be vaccinated at weaning to control nursery and finishing related PRRS problems. The vaccine should be effective in herds which PRRS is circulating in the nursery AFTER the original outbreak. During the original outbreak piglets are becoming infected in the uterus or in the farrowing house, and vaccination of these pigs or their weaning-group-mates will not be very beneficial. All in-all out pig movement through farrowing and nursery stages during the outbreak will be much more effective in preventing the establishment of endemic PRRS in the nursery phase (see PRRS MANAGEMENT above) later. The manufacturer also reports poor response in herds DURING PRRS OUTBREAKS. Again, the vaccine should be a valuable tool for controlling circulating PRRS problems in the nursery, after the reproductive disease outbreak has ended. The vaccine appears experimentally to be very good at stimulating a protective immunity against virus challenge. While it is not approved yet for use in PREGNANT animals or any pigs > 18 weeks of age, I would recommend to start vaccinating replacement gilts and boars at 18 weeks of age, at least 60 days before breeding. These animals should maintain at least some protection against PRRS infection for a year. By that time the vaccine hopefully will be cleared for use in breeding swine. It is obvious that once vaccines are approved, that breeding stock should be vaccinated against PRRS to prevent devastating reproductive disease losses. Other modified-live and killed PRRS vaccines are being developed but it is uncertain when they will be available.

Points to Remember

Know the PRRS virus circulation status of your herd.

Know the PRRS virus circulation status of potential breeding stock suppliers.

PRRS is not easily transmitted by any other means other than direct pig to pig contact.

Do not tolerate continued PRRS associated disease losses in the nursery. Eliminate the circulation of PRRS in the nursery by following a nursery depopulation plan coupled with ALL IN ALL OUT management.

Begin vaccinating replacement breeding stock at 18 weeks of age, and do not breed them within 60 days of vaccination.