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Just when many U.S. sow herds are reaching a state of PRRS stability, the virus is cropping up elsewhere along the production chain – in finishing pigs.

PRRS-positive finishers with active circulation are common, according to Reid Philips, DVM, technical manager for respiratory biologics at Boehringer Ingelheim Vetmedica, Inc. (BIVI).

Pigs coming from stable or negative sources are often exposed and infected with PRRS virus during the finishing phase of production, creating clinical disease, he says.

“These pigs are unprotected and moving into pig-dense or high-risk sites for PRRS,” notes Philips. “In many cases, producers have a window of opportunity to vaccinate these pigs prior to exposure to field virus. Vaccination must be timed four weeks ahead of exposure to field virus for optimum protection.”

Philips and his colleagues at BIVI conducted a large field study to evaluate how well vaccine controlled PRRS in grow-finish pigs.

The study involved 120,000 pigs; half received Ingelvac PRRS ATP intramuscularly at 7 weeks of age. Half served as unvaccinated controls.

A three-site production strategy was used in the large commercial system:

Site One – Sow herd: A PRRS positive and stable breeding herd. There is no evidence of circulating virus and weaned pigs were negative according to routine PCR sampling.

Site Two – Nurseries: PRRS inactive, or no evidence of seroconversion. Pigs were routinely bled at 10 weeks of age to assess and classify nursery status by PRRS ELISA.

Site Three – Finishers: PRRS active; serologic and clinical evidence of PRRS virus circulation due to horizontal transmission at the site, typically beginning four weeks post-entry.

Seven replicated groups of vaccinated pigs and non-vaccinated controls were placed side by side. The authors noted that the integrity of each treatment group was preserved when pigs were transferred to the finishing units so there was no commingling within facilities: pigs did share common airspace by site.

Periodic serologic monitoring confirmed ongoing endemic circulation of field virus at the finishing site.

Production performance data of vaccinates demonstrated an advantage over non-vaccinates. Production performance benefits from vaccination were an increase in ADG, and an improvement in feed conversion. This field study is evidence of cross protection from appropriately placed vaccine and corresponding biologic and economic benefit.

Assuming $45 per cwt. live hog market price and $120 per ton for finishing feed costs, the total benefit per pig vaccinated with Ingelvac PRRS ATP for the trial was $3.03 per pig, according to study authors (see economic benefit table on page 3).

They noted that a limitation to the study was the small number of replicates; however, once the data was analyzed, the system owners elected to implement the vaccination strategy.

This paper was presented during the 19th International Pig Veterinary Society Congress, 2006, in Copenhagen, Denmark.