Would you recommend the vaccination of a PRRS seropositive herd, even there are no clear reproductive and respiratory signs, or would you recommend specific management measures?

Answered by: Tomasz Stadejek I Published on: May 5, 2016

Not always we can observe clear clinical symptoms of PRRS. More often, in endemically infected farms, the disease affects the production parameters and its diagnosis can be made based on detailed analysis of those. If precise figures are not known or the losses are not evident it is good to evaluate the virus circulation, either with serological methods (ELISA) or PCR.
If PRRSV starts to circulate pre-weaning or early post weaning it is indicative for unstable reproductive herd. It means that there is vertical transmission from sows to piglets. Even if the losses in the reproduction are minimal (only gilts maybe affected) the price is paid in piglets. Not necessarily directly due to PRRSV but more often due to multifactorial infections as PRRSV modulates immune system. It would be strongly recommended to vaccinate replacement gilts at quarantine, and the reproductive herd, in order to eliminate vertical transmission and increase maternal immunity in piglets. If the virus starts to circulate later in nursery, vaccination of piglets could be considered.
It has to be kept in mind that the immunity against PRRSV is slow to develop. Piglets should be vaccinated at least 4 weeks before the infection is expected to occur. The efficacy of modified live vaccine in piglets can be compromised by maternal antibodies. The virus contained in such vaccine must be able to effectively replicate in vivo. High levels of maternal antibodies can neutralize the vaccine virus before it is able to properly stimulate the immune system of a piglet.
There are several management practices that could be applied together with, or instead of vaccination program, to break the infectious chain in a herd.Which method is best justified in a given farm situation has to be assessed individually, considering all the possible constraints regarding the implementation of the management changes, and their cost. So, the seropositive status of a farm is too unspecific to be able to decide for, or against vaccination.
The virus reservoir has to be known based on detailed ELISA or PCR analysis and then that action against it could be planned. Whether or not PRRSV control or elimination in endemically infected farm will bring significant improvement in production parameters depend on many factors besides PRRSV.
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