Measuring Time-to-Virus-Negative-Piglets Interval in PRRSv Positive Sow Herds

The point at which a PRRSV-positive breeding herd begins to consistently produce entire groups of PRRSV-negative weaned pigs is important. First, PRRSV-negative weaned pigs can begin to co-mingle. Second, PRRSV-negative weaned pigs can sig­nal when it is appropriate to reopen a closed breeding herd.
calendar icon 1 July 2016
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The objectives of this project were to assess a diagnostic monitoring protocol to improve the herd level PRRSV detection sensitivity and to get more information for action plan­ning in the breeding herd. A dual-subpopulation approach can broaden the scope of animals sampled when a positive herd is being monitored for time-to-virus-nega­tive-piglets interval; as well as, support better decision making for PRRSV-positive breeding herds.

How is it done?

In April 2008, a PRRSV elimination project was initi­ated on two newly infected and previously naïve sow farms within the same production system. Two subpopulations were monitored: 1) pre-colos­tral stillborn piglets that represent a period of potential virus circulation in the third trimester of gestation, and 2) piglets just prior to wean­ing that represent a period of potential virus circulation in late gestation and post-farrowing.

Subpopulation 1 monitoring: 10 stillborn piglets from each farm were sampled monthly: submitted to a diagnostic laboratory for both PRRSV ELISA and PCR testing.
Acceptable sample = wet umbilical cord + uninflated or partially inflated lungs + no milk in stomach

Subpopulation 2 monitoring: 60 blood samples from piglets prior to weaning taken monthly from each farm, the samples were submitted for PRRSV PCR testing.

What are the results?

Samples from stillborn piglets were collected 123, 162 and 201 days after the initial positive sow farm identi­fication. Positive results were detected in sows by PCR and ELISA. These stillborn piglet sample collections also correspond to days 118, 157 and 196 following the first whole-herd vaccination.

PRRSV viremia of in-utero infected stillborn piglets was detected in 6 of 40 (15%) serum samples. PRRSV viral RNA was detected in 6 of 40 (15%) skin swab samples.

Stillborn fetuses which were PCR-posi­tive in serum and/or on skin did not show evidence of sero-conversion to a degree detectable by ELISA.

Results of the se­rum and skin ELISA testing were 0% (0/40) positive, and no differences in S/P ratio between piglets that tested PCR positive and those that tested PCR negative.

What implications does this paper have?

It is interesting that swab samples collected from the surface of the skin in the axillary spaces of stillborn piglets tested PCR-positive. PRRSV most likely came from the amniotic fluid and not from the farrowing stall since farms are operated all in/all out, thoroughly washed with hot water, disinfected with a PRRSV-virucidal product and dried with a commercial drying agent.

The stillborn (pre-colostral) piglet sampling/testing procedure could provide producers another tool to complement PCR-testing of soon-to-wean piglets and provide greater confidence in detect­ing evidence for PRRSV circulation in the breeding herd.

In anticipation that PRRSV-negative soon-to-wean pigs would lag PRRSV-negative stillborn pigs by approxi­mately one lactation cycle, the interpretation of results and execution of subsequent actions using a combina­tion protocol that includes both monitoring of stillborn (pre-colostral) piglets and soon-to-wean piglets could be described.

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