If there is a PRRS rebrake in a positive farm, affected animals will always seroconvert? To what extent and for how long?

Answered by: Enric Mateu Published on: June 2, 2016
This is a very difficult question to answer without additional data.
In principle, the clinical re-emergence of the same PRRSV isolate in a farm is usually produced because there are subpopulations of pigs without adequate immunity (or even naïve pigs).
Accordingly, clinical signs are seen in animals without previous active immunity. In such a case, seroconversion occurs shortly (7-14 days) after the onset of the infection. A different matter is the introduction of a new PRRSV strain in a previously infected/vaccinated herd. When this occurs, the result of this new introduction can range from almost nothing to severe clinical disease, depending on the virulence of the new strain and the protection afforded by the pre-existing immunity.
Under such circumstances, namely the introduction of a new virulent isolate, animals do not have necessarily to show seroconversion in ELISA. As a matter of fact, when we performed experiments of heterologous challenge of pigs with pre-existing immunity, seroconversion was not always seen.
This seems to have an idiosyncratic component but also is related to the viral isolates involved. If you fear that a vaccinated/immune farm is suffering an outbreak, focus on diseased animals and newborns, use PCR for detection of the virus and sequence in order to compare with the isolate previously present in the farm.

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