Porcine reproductive and respiratory syndrome is a haunting disease. Herds with pockets of active PRRS infection and inadequate breeding stock acclimatization programs may experience mini-outbreaks. These usually surface as clusters of sows aborting litters within a few days of each other, followed by months without noticeable problems.

The theory is that subpopulations of susceptible animals that come in contact with the virus are causing these mini-outbreaks, says Kelly Lager, veterinarian with USDA’s swine virology research unit in Ames, Iowa.

She notes that animals may fall into this subpopulation if:


  • They have not been infected with the on-farm virus.
  • They’ve been infected but their immunity has waned.
  • They were never vaccinated against PRRS.


“PRRS is a strange disease,” says Jim Collins, veterinary pathologist at the University of Minnesota. “It sometimes doesn’t spread as fast as we once thought, even if infected animals are next to uninfected ones. Animals can turn positive in later production stages like the finisher. And we still don’t understand the immunity mechanism.”