Porcine reproductive and respiratory syndrome (PRRS) virus is difficult to rid from herds because infection elicits a weak immune response that is not fully protective.

This results in persistent infection in a subset of pigs, thus providing a continued source of virus circulation within the herd.

Substantial research efforts have not yielded the exact components of a protective anti-PRRS virus immune response, particularly as it relates to persistence.

These shortcomings led researchers at the U.S. Department of Agriculture Agricultural Research Service's (ARS) Beltsville Animal Research Center (BARC) and Kansas State University to use “Big Pig” samples to determine whether immune markers that control PRRS virus persistence can be identified.

The Big Pig project was a PRRS CAP1 (Coordinated Agricultural Project)-supported, multi-institutional (university and commercial), multi-disciplinary experiment initiated in 2005. It was designed to analyze pig responses to PRRS disease, virus replication and immunity in 109 pigs (and 56 control pigs), sampled for up to 203 days post-infection.

Blood samples collected throughout the Big Pig project were evaluated for blood protein levels that might be predictive for pigs that clear PRRS virus rather than pigs that remain persistently infected with PRRS.

The hypothesis was that protective serum cytokine levels 7-42 days post-infection would help predict which pigs were more likely to have persistent viral infection (147-203 days post-infection).

Samples from pigs that apparently cleared PRRS virus from serum and tissues by 28 days post-infection were labeled the Non-Persistent (NP) pigs. Persistent (P) pigs were those that showed evidence of long-term, persistent PRRS infection at 150 days post-infection.

Sera from P and NP pigs collected over the course of the PRRS infection were tested for serum cytokine levels (substances that are secreted by cells of the immune system) following PRRS infection.

NP pigs appeared to have faster and higher levels of serum cytokine Interleukin-8 and anti-viral Interferon-gamma than the pigs with persistent infections. This immune cytokine trend correlated with the clearance of virus from serum and tissues.

Researchers noted this effect might indicate that the NP pig's immune response was faster and more effective than that for pigs with persistent infections, and possibly enabled the NP pigs to prevent PRRS virus infections from becoming persistent.

This conclusion sets the stage for identifying prognostic indicators of persistent infection and for targeting these proteins for anti-PRRS virus biotherapeutics or vaccines.

At present, it is estimated that 60% of U.S. hog operations are infected with PRRS. The National Pork Board calculates PRRS is the most economically significant disease facing the swine industry, costing $560 million annually.

Researchers: Joan K. Lunney, USDA, ARS, BARC; Bob Rowland, Kansas State University. Contact Lunney by phone (301) 504-9368, fax (301) 504-5306 or e-mail Joan.Lunney@ars.usda.gov. Contact Rowland by phone at (785) 532-4631 or e-mail browland@vet.k-state.edu.