Answered by: Enric Mateu Published on: May 27, 2016
According to the usual nomenclature1 farm is classified as stable if there are evidences of at least “a 90-day period of sustained lack of viremia in weaning-age pigs and no clinical signs of PRRS in the breeding herd“. In practical terms, this means that no viral circulation occurs in maternities and if there is viral circulation this is confined to other sections of the farm. A farm fulfilling these requirements is not free of the virus yet.
Vaccination against PRRSV is intended to reduce the clinical and economic impact of the infection and to reduce as well the transmission of the virus as much as possible. Bearing this in mind, to have a sows’ stock as homogeneous as possible in terms of the immunity against the virus is a desirable goal. In principle, blanket vaccination is more likely to produce that homogeneity than post-farrowing vaccination.
Certainly, if sows are vaccinated very close to the farrowing date with a modified live vaccine, the virus may eventually cross the placenta and produce the birth of viremic piglets. In principle, the practical impact of this with genotype 1 (European-type) vaccines, seem to be less important than with type 2 vaccines (North American type) although precise comparative data are lacking.
The so called 6-60 protocols were designed with two objectives: to provide (or sustain) immunity from the very start of gestation and to reinforce it before the critical phase for PRRSV transplacental infection avoiding vaccination of late gestation sows. This protocol of vaccination however does not produce better homogeneity than blanket vaccination and is being increasingly abandoned.
1. Holtkamp DJ, Polson DD, Torremorell M, et al. 2011. Terminology for classifying swine herds by porcine reproductive and respiratory syndrome virus status. J Swine Health Prod, 19:44:56