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Measurement of time-to-virus-negative-piglets interval in PRRSv positive sow herds

The accuracy and security provided by the combination of samples of weaned pigs and piglets at birth, either stillborn or live piglets, help us make a decision that will be crucial in any eradication plan.

Friday 6 November 2015 (2 years 10 months 16 days ago)
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Paper

Evaluation of a stillborn (pre-colostral) piglet sampling and testing procedure for possible inclusion with a conventional soon-to-wean piglet monitoring procedure to measure time-to-virus-negative-piglets interval in PRRSv positive sow herds. B Carmichael, D Polson, M Torremorell, and Holtkamp. American Association Of Swine Veterinarians, 2009

 

Paper summary

What are they studying?

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 with other sources or groups of PRRSV-negative weaned pigs. Second, PRRSV-negative weaned pigs can sig­nal when it is appropriate to reopen a closed breeding herd.

The objectives of this project were to assess a diagnostic monitoring protocol to improve the herd level of 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, as representatives of a period of potential virus circulation in the third part of gestation, and 2) piglets just prior to wean­ing, as representatives of a period of potential virus circulation in late gestation and post-farrowing through weaning.

Subpopulation 1 monitoring: 10 stillborn piglets from each farm were sampled monthly (axillary swabs and blood): 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 swabs.

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

The stillborn (pre-colostral) piglet sampling/testing procedure could provide producers with 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.

Anticipating that PRRSV-negative soon-to-wean pigs would lag behind 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.

 

Enric MarcoThe view from the field by Enric Marco

The key strategy for finally producing PRRSv negative piglets, apart from disuse of doses from positive boars, is the suspension of replacements or closure of the farm. These are the measures that will stop the virus circulating among the breeding pigs. Knowing when the first negative piglets are produced is key to continue the implementation of the measures that will lead to the elimination of the virus. In those farms where nursery and breeder pigs are housed in the same location, the time when the first negative piglets are produced is the signal to start depopulation among nursery pigs, which will imply, after the depopulation, that the piglets remaining in the farm stay negative (assuming, of course, there are no finishing pigs on the farm). It will also be the signal to make the decision of reopening the farm to negative replacements without the risk of them becoming positive.

Bleeding of weaned pigs was the most frequently used methodology. The risk of relying only on this kind of sample is missing the presence of the virus when their numbers are very low. The accuracy and security provided by the combination of samples of weaned pigs and piglets at birth, either stillborn —as the paper suggests— or live piglets, help us make a decision that will be crucial in any eradication plan. This new technique collecting blood or skin (axilla) samples from stillbirths provides technicians with new resources to obtain the necessary number of samples every time. However, these samples are no substitute for weaner samples, as discussed in the article, since some negative newborn piglets might become contaminated in the farrowing room. Needless to say, when such controls are carried out having a single negative serology is not enough; we'll need to obtain several consecutive negative results in order to be sure the virus is no longer circulating in the farm.

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