In the largest field study of its kind, a team of University of Minnesota researchers evaluated the impact of immunization in herd closure strategies on eliminating porcine reproductive and respiratory syndrome (PRRS) virus from breeding herds.

The most common herd closure strategy has been whole herd exposure, also known as “load, close, homogenize (LCH).” This method involves loading the herd with extra replacement gilts, exposing all breeding females with PRRS and interrupting introduction of replacement gilts in the sow herd for a period of six to 10 months. PRRS virus gradually dies out such that when the closure period ends, the virus has hopefully been eliminated from the herd.

Two methods used for whole herd exposure — modified-live-virus (MLV) vaccine or live virus inoculation (LVI) — were evaluated for the effectiveness of LCH programs in eliminating PRRS virus from breeding herds.

LCH programs were analyzed for the time it takes to produce PRRS-negative piglets at weaning (TTNP) between treatment groups and for its impact on total production losses in terms of number of pigs weaned/week.

Sixty breeding herds were enrolled in the study, 40 in the LVI program and 20 in the MLV program. The median time to PRRS-negative status was about 210 days (range 90-300 days).

LVI herds reached TTNP sooner than MLV herds, but had higher production losses. LVI herds reached TTNP at about 26 weeks vs. about 33 weeks for MLV herds. Production losses of MLV farms were about 1,300 pigs/1,000 sows, less than that of LVI farms. MLV farms recovered production levels about eight weeks before LVI herds.

Herds with prior PRRS virus infection became negative sooner, recovered production faster and had fewer total losses.

Researchers highlight that farms with several polymerase chain reaction (PCR) negative tests for PRRS in a row and with stable production levels might still have virus circulating in the herd at low levels. This means that PRRS monitoring must be done repeatedly over time and biosecurity measures in the farrowing house must be strictly enforced until at least four consecutive negative monthly tests are obtained.

Researchers: Daniel Linhares, DVM; Montse Torremorell, DVM; and Bob Morrison, DVM; University of Minnesota. For more information, contact Morrison by phone (612) 625-9276 or e-mail