The article says it was pretty active all over the Midwest this past winter, too.
February 16th, 2012
PRRS popping up in Ohio hog herds
By Matt Reese

The soggy winter is at least partially to blame for the Porcine Reproductive and Respiratory Syndrome (PRRS) virus problems that have been plaguing hog producers in eastern Indiana and western Ohio in recent months.

The cagy virus is tough to corner because it can change so quickly.

?PRRS is programmed to very rapidly mutate. That is one of the problems in trying to develop a good vaccine that can work against the virus,? said Dr. Bill Minton, of Minton Veterinary Service in Mercer County. ?
Along with being a problem itself, PRRS can also worsen symptoms of other pathogens.

?A lot depends on whatever secondary pathogens are also present. If you have PRRS, and if you also have the influenza or bacterial infections, these can all compounded by the PRRS virus,? Minton said. ?It attacks the immune system.?
?We know this virus can travel 5.5 miles in the right conditions, which are mild to moderate wind, overcast skies and high humidity,? he said. ?We have certainly had that in Ohio this winter. It has been a perfect storm.?...