There are several challenges that the porcine reproductive and respiratory syndrome virus throws at you along the way. But the key to the battle is persistence, says Jeff Zimmerman, veterinarian at Iowa State University.

Even though many of the disease's mechanisms are still a mystery, there are many clues to PRRS persistence in chronically infected carrier animals.

Boars are at greater risk than sows to become persistently infected with PRRS.
PRRS virus does not necessarily persist for the life of the pig. However, animals can remain positive for up to 210 days.

It's unknown how long after infection that a pig remains a carrier. Limited data suggests most hogs remain infected for at least 90 days after exposure to the virus.

The presence of subpopulations of persistently infected pigs within a herd can explain early weaning protocols' failure to eliminate PRRS. These pigs are difficult to identify and can trigger an outbreak when PRRS-naive pigs meet in nurseries, finishing or breeding barns.

With persistent infection in a herd, it is possible to have serologically PRRS-negative animals that come up positive with a polymerase chain reaction test. Those animals are seroconverting for PRRS.