Completing the task of PRRS elimination sounds like a great New Year’s resolution. However, like most resolutions, that wish will not be fulfilled. But our objective should be to make progress toward improving control of this costly and menacing virus in finishing pigs.

The industry now has access to PRRS-negative germplasm. Breed-to-wean sow herds can be stabilized using health and management techniques to produce negative weaners. New research has led to filtration of incoming air to minimize the risk of virus introduction.

The next major hurdle is developing strategies to best control PRRS virus in nursery-finisher pig flows.

Case Study No. 1

A 5,500-head, PRRS-naïve nursery-finisher site experienced some coughing and thumping. The owner decided against sampling for swine influenza.

Three days later, death loss had increased dramatically, including some big finisher pigs. Pigs were off feed, very inactive and thumping. Sampling confirmed the presence of PRRS; no influenza or other significant agents were found.

Pigs were treated for fever, but signs intensified. Pigs were very slow to recover. No intervention helped; 728 pigs died and 200 were culled. This virus had a virus sequence of 1-1-2. There were no known, similar viruses in the area, and the herd was not in a pig-dense area. Normally, weaned pigs are added every nine weeks to the continuous-flow site. One group of pigs was skipped and all subsequent weaners had been vaccinated with PRRS modified-live vaccine on arrival. Nursery mortality has returned to 2-3%, 3-4% in the finisher, but mortality levels have not dropped to performance achieved before PRRS.

Case Study No. 2

A 4,900-head, continuous-flow nursery receives pigs from a PRRS-stable sow herd. Enzyme-linked immunosorbent assay (ELISA) testing of a few samples has been negative since December 2009. Limited testing in downstream finishers has also been PRRS ELISA negative.

Because of the high level of PRRS activity in the area last winter, it was decided to monitor the nursery at least every six weeks with oral fluids testing. The first oral fluids sample collected Jan. 3, 2011, was negative. Due to respiratory signs, another sample was collected Jan. 28 and was positive by polymerase chain reaction (PCR). Serum was collected and the virus sequenced was 1-4-4, with a 100% match to last winter’s severe reproductive virus.

Surprisingly, mortality in the nursery closeout groups has not exceeded 3%, and most were under 2%. The nursery is well-managed and most pigs are individually treated. These pigs flow to seven different finisher sites!

Last June, a similar 1-4-4 virus was identified in a sow unit two miles from one of these finishers. In August 2011, a virus that was 99.3% similar to the nursery virus was identified in a finisher near the infected sow herd.

Because the virus has continued to circulate in the nursery, and it sequences very closely to the original virus, plans are to consider vaccination of incoming weaners with PRRS modified-live vaccine to help improve performance and reduce treatments in the nursery.

Discussion

In the first case study, the decision for proactive vaccination of subsequent groups of pigs was fairly easy, as the profound death loss was certainly not sustainable. There is no way to know for certain if the intervention has been truly challenged. However, this intervention does provide assurances.

In a controlled study, air exhausting from the barn with pigs vaccinated in the presence of field challenge had fewer days of PRRS-positive air samples outside barns compared to exhausted air from barns of unvaccinated pigs. So we would also expect that vaccinating negative pigs on arrival would reduce the amount of field virus shed at exposure, also providing some assurance for impacting PRRS area spread.

In the second case study, the problems did not develop into an unmanageable situation; thus, there has been less economic incentive to intervene.

As producers begin to work together in PRRS control areas or share information in the absence of an organized control project, perhaps more “big picture” decisions can be made. These decisions will encompass pig transport, vaccination, placement and attempts to reduce viral loads exhausting from ventilation systems. A recent example of this type of thinking resulted in a producer vaccinating incoming pigs from a PRRS break solely in hopes of reducing the amount of viral shedding to nearby producers.

Controlling PRRS by working together will eventually reduce the $660 million estimated annual cost of this disease to the industry.