Overcoming the complex immune response to PRRS
Although PRRS was originally called Mystery Disease in part out of frustration at researchers' inability to discover its cause, the name still applies 20 years later concerning how the virus frustrates the animal's immune system.
University of Minnesota veterinary molecular biologist Michael Murtaugh explains it clearly: Despite the damage PRRS is capable of causing, all indications are that when the virus is first breathed into the pig's lung, the pig's system remains essentially unaware that a serious infection has occurred. Unlike the immune mechanism other viruses — like flu — immediately rally into action, the inner surveillance mechanisms that tell the animal it's been infected simply aren't switched on by PRRS. That mysterious failure to elicit a strong first-line response leads to a slow response or delay in the body's anti-viral defenses. By the time it occurs, the virus has moved on, sequestering itself away in other areas of the body, where it can persist for effectively the lifetime of the market-age hog. That “persistent infection” then serves as a subpopulation of infection to keep the virus circulating in a herd.
This unique ability to hide from the immune system has frustrated on-farm attempts to control or eliminate PRRS and to control related damage. In an unstable herd, PRRS virus regularly cycles from pockets of infected sows to uninfected sows or gilts. Those breeding animals then infect litters, which spread the virus down-stream as well as recycling the virus back to the breeding herd. However, researchers are showing that new tools such as mass-vaccination, or whole-herd, strategies can help manage out the losses.
Biosecurity
Why biosecurity remains difficult
Infection and re-infection of PRRS-free farms is a common event. Known and suspected routes of viral spread include:
Infected pigs
Semen
Needles
Coveralls and boots
Flies
On the air — although for only about a yard. The role of aerosolization continues to be investigated; current research indicates it can ride on the air but only over short distances.
Trucks. University of Minnesota's Scott Dee just led a study that underscores the risk posed by one of the most common vectors. His study contaminated 1- to- 150-scale livestock trailers stocked at a density typical of normal trucks with PRRS virus via infected pigs. It then employed differing degrees of scraping, cleaning, disinfecting, drying, freezing and thawing, before attempting to infect naïve sentinels by exposing them to the trailer. Only the replicate that washed, disinfected and dried the trailer succeeded in halting the infection's spread. Anything short of that was capable of passing on the virus.
Dee S.; et. al. New information on regional transmission of porcine reproductive and respiratory syndrome virus. Proc. Leman Swine Conf. 2003:68-70.
Dee S.; et. al. Evaluation of transport vehicles as a source of PRRSV. Can. J. Vet. Res. 2004 68(2):128.
More info
Murtaugh M.P. PRRS immunology: what are we missing? Proc. AASV 2004:359-368.
Goldberg T.L.; et. al. Quasispecies variation of porcine reproductive and respiratory syndrome virus during natural infection. Virology 2003 317:197-207.
Rowland R.R.R.; et al. Lymphoid tissue tropism of porcine reproductive and respiratory syndrome virus replication during persistent infection of pigs originally exposed to virus in utero. Veterinary Microbiology 2003 96:219-235.
Dee S. The science of PRRS: What do we really know about its transmission, diagnosis and control? Proc. AASV 2004:353-358.