On a farrow to nursery farm with 200 sows, 20 blood samples were taken from sows, gilts, and piglets. All samples were negative for PRRS by ELISA except one, which was from a 5th parity sow. No pathology associated with PRRS was observed, and no vaccination against this disease was done. The farm has good performance, 90% fertility and 14 piglets weaned per sow. How should we interpret these analytical results?
No ELISA test is 100% specific. Specificity is the test’s probability of correctly detecting negative animals.
When referring specifically to the PRRS virus, the best ELISA tests on the market are capable of correctly detecting 94-99% of negative animals, but there is always a small percentage that can give a positive ELISA reaction without actually having been infected by the virus: these are known as false positives.
The causes can be multiple and in many cases, it is related to the presence of proteins in the blood of the animal detected positive that can be identified as antibodies against PRRS.
If the farm has not had clinical signs suggestive of the disease, the positive is most likely a “false positive“.
The way to confirm this would be to retest the sample using another type of antibody test, either another type of ELISA or another blood antibody test such as an IPMA.
A second method would be to do a second sampling and perform the same type of analysis, if the first positive sample was real it would indicate that the virus has reached the farm and we should expect a spread of the disease and therefore an increase of animals with positive results to the ELISA test; if that were not so it would be confirmed that the first result has been a “false positive”.
If you want to know more about diagnostic and monitoring read out chapter:
References:
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1. Tyler JF, Cullor JS. Titers, tests and truisms: rational interpretation of diagnostic serologic testing. JAVMA Vol. 194 Nº 11 June 1, 1989. Pp. 1550-1557.
2. Kinga Biernacka K, et al. Comparison of six commercial ELISAs for the detection of antibodies against porcine reproductive and respiratory syndrome virus (PRRSV) in field serum samples. Research in Veterinary Science 121 (2018) 40–45.
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