PRRS: Is elimination attainable?

It is the most expensive disease of modern swine production. A full-blown outbreak of porcine reproductive and respiratory syndrome can devastate a herd, and pinpointing where the virus originated can be a frustrating proposition. The virus can reappear on a farm that went to great lengths to eliminate it.

Each year, PRRS costs the U.S. swine industry an estimated $600 million. Much has been learned about the virus since it emerged in the mid-1980s, but there are also major gaps in knowledge, such as the potential for aerosol transmission between farms.

Abortions, pigs born weak and ill, and secondary bacterial infections characterize the syndrome. Older pigs develop pulmonary signs and secondary infections. Dr. Steve Henry, a practitioner in Abilene, Kan., said the disease has had a more profound psychologic effect on him than any other he has encountered.

"The drama is in the mortality of a full-blown outbreak in a naïve herd," Dr. Henry said, "with 50 to 75 percent of piglets dying in their first few days of life, sows dying due to toxicosis from the dead fetuses in their uterus, and pigs crippled and necessarily euthanatized because of secondary bacterial polyarthritis."

Swine veterinarians are cooperating with industry and academia on PRRS research projects. Two national efforts have been launched to coordinate financial and human resources for targeted work. Dr. Eric Neumann of the National Pork Board described them during a half-day general session at the AASV meeting that featured some top PRRS experts.

The most sweeping initiative is a $4.4 million Competitive Research Grant just announced in April by the Department of Agriculture's Cooperative State Research, Education, and Extension Service. In 2003, CSREES issued a broad call for research proposals on PRRS, avian influenza, and Johne's disease. Its intent was to select a single proposal relating to one of those diseases and fund it at $4 million over four years. The agency instead funded two proposals at that level—one on PRRS and one on Johne's.

The PRRS project was developed by the North Central Multi-State Research Committee 229, which fosters interaction between PRRS researchers at a dozen land-grant universities. With the grant, it will create a work plan to help the industry pursue creative solutions to PRRS.

The potential exists for funding beyond the $4.4 million. PRRS is seen as a problem needing funding of as much as $15 million over time, with producers and large pork production systems possibly contributing.

The other key national initiative is the National Pork Board's National PRRS Initiative (www.porkboard.org/prrs), which got under way in spring 2003. A $2 million allocation from the Pork Checkoff will fund creation of tools and strategies for successful management of PRRS on farms. Grants will be awarded, most of them on a competitive basis.

An industry-funded program that began in 2003 is the Advancement in PRRS Research Award. Boehringer Ingelheim is awarding $75,000 in grants annually to help find new ways to control, manage, and eradicate the PRRS virus. The grants are given for research that will be completed within a year and produce results for use in the field quickly.

Boehringer Ingelheim announced the 2004 recipients at the AASV meeting. Sharing the funding were Dr. Cameron Schmitt, a practitioner in Pipestone, Minn., for "Production of Neutralizing Antibody Against PRRSV and Effect on Rate and Duration of Piglet Infection"; Dr. Scott Dee, University of Minnesota, for "An Evaluation of Protocols To Sanitize PRRSV-contaminated Commercial Swine Transport Vehicles in the Absence of Drying"; and Dr. Kyoung-Jin Yoon, Iowa State University, for "Immunological Significance of PRRSV Genetic Variation."

Besides Dr. Neumann, five others made presentations at the PRRS portion of the AASV general session.

Dr. Hank Harris gave "a speculative philosophic perspective" in which he said that the swine industry has made tremendous advances in PRRS control in a short time. He noted that RNA viruses are difficult to control, and vaccines for PRRSV are marginally effective or must be modified continually. He believes that, as with other swine diseases, producers may need to become the driving force behind PRRSV eradication, with veterinarians taking the lead in reducing/eliminating it from farms.

Dr. Scott Dee, University of Minnesota, said the essential component to controlling PRRS is producing pigs free of virus at weaning. The technology needed to eliminate the virus from farms is well-documented, he said, but frequently, farms with unrelated strains become reinfected. Dr. Dee said that the use of PRRSV-positive serum to inoculate naive gilts and pregnant sows in increasing, but it appears to promote third-trimester abortion, elevated preweaning mortality, and mortality in lactating sows.

In his overview on immunology, Michael P. Murtaugh, PhD, University of Minnesota, said that the difficulties in achieving consistent control and prevention of PRRS with live, attenuated vaccines underscore the incomplete understanding of the virus's immunology. He said it is important to apply the quantitative measures we have of viral infection to reservoirs besides blood. He believes that more specific and quantitative assessments will lead to progress.

Dr. Howard T. Hill, Iowa Select Farms, described 10 PRRS control strategies the industry uses, saying there's a place for each one, depending on the client's needs. Because none of the strategies has shown great success, swine practitioners sometimes resort to whole-herd exposure by injecting herd-specific, live virulent PRRS virus. This hastens the time a sow farm produces viremic pigs. Dr. Hill said that when doing this, it is important to develop a plan to ensure that the right viral strain is being injected.

Dr. Henry said some producers and veterinarians have advanced beyond managing PRRS infections to eliminating the virus from herds. His practice works with 32 herds that have been affected by the virus; 27 were managed to remove infective virus and, over time, became seronegative. The other five were infected old herds that were replaced with seronegative, new animals. The simple steps he outlined are not exposing pigs to PRRS vaccines, entering replacement breeding animals into the herd at a young age, operating nursery through finishing on sites removed from the breeding herds, and minimizing unnecessary procedures that might transmit infectious bodily fluids among pigs.

"Ultimately, Dr. Henry concluded, "PRRS must be eradicated, if herds are to be successful economically."