Why do summer PRRS breaks last longer than winter breaks?

Summer outbreaks of porcine
reproductive and respiratory syndrome (PRRS) take 15 weeks longer to stabilize
than PRRS outbreaks beginning in the winter, according to data from the
Morrison Swine Health Monitoring Project (MSHMP).

“We had
around 35 weeks for the time to stabilize [PRRS] with winter and autumn breaks,
and around 50 weeks for summer breaks,” Juan Sanhueza, DVM, PhD, post-doctoral associate,
University of Minnesota, told Pig Health
Today. 

“It’s a large difference.”

This trend primarily occurred
in the hog-dense areas of Minnesota and Iowa.

Change in weather?

The MSHMP data also holds a
possible answer to why summer PRRS breaks take longer to eliminate than winter
PRRS breaks.

“We’re seeing in the data that
summer breaks [at first] are achieving stability at a similar rate with winter
breaks in time,” Sanhueza said. But then progress toward stability stalls. This
time period coincides with the arrival of winter.

“We don’t know if it is the
temperature that drops, the ventilation that drops, or what factors are
involved,” he said. “But it seems that during winter, this PRRS virus is able
to linger in the sow farm until the next season — spring or summer — when the
farm can perhaps finally get rid of it.”

The MSHMP data also shows the type
of PRRS virus is associated with increased time to stability. Outbreaks caused
by PRRS virus 174 took longer to reach stability, Sanhueza reported.

Filtered farm connection?

The data on PRRS outbreaks
exposed other questions. For example, the data showed more summer outbreaks of
PRRS in areas with a higher density of filtered farms.

“Something may be happening
during the summer in filtered farms that makes them more prone to have a PRRS
outbreak,” Sanhueza said. “Could be they are replacing filters at that time or
they stop filtering in the summer.”

Another observation was the
high rate of outbreaks in some swine-dense areas but not all of the high-dense
areas. Something other than pig density is driving the higher rate of PRRS
outbeaks, he theorized.

The next step for Sanhueza is
identifying all the factors to help growers get rid of the PRRS virus in a
shorter time, especially for the summer outbreaks.