Effect of vaccination against PRRSV on virus shedding

The view from the field by Enric Marco

Remarkably, when talking about PRRS many of the conventions are no use to us. For example, when talking about biosecurity, measures applied may work better or worse depending on the wild-type strain circulating in the vicinity of the farm because, as we know, not all strains have the same ability to spread through the air. When talking about vaccines, they do not work as we expect them to do. While it is possible that they may reduce shedding to some extent or, in some cases, shorten the duration of outbreaks or even the severity of symptoms, they do not confer protection clearly reducing clinical signs in vaccinated animals as other vaccines do, such as the vaccine against the virus of Aujeszky's disease or circovirus (PCV2). These peculiarities prevent the unification of criteria on how we should proceed.

However, as we said in previous reviews, we are clear about which areas we need to act on in order to restore normality to the farm. Similarly, we know that, in high density areas, action plans must be joint if progress is to be made in reducing the risk of infection on the holding. The joint measures to be applied, including but not limited to, are: centralized information management (outbreak reporting, virus sequencing to track outbreaks); actions on animal movements (restrictions on the source of replacement gilts or fattening piglets); biosecurity improvement and monitoring; and the implementation of common prophylactic plans. The aim of implementing joint prophylactic plans in these areas is simply to try to reduce the risk of virus vehiculation. However, such measures are recommended based on prior experience fighting other infections, namely Aujeszky's disease. Animals vaccinated and infected with Aujeszky's disease have been experimentally proven to greatly reduce viral excretion; for some vaccines, this reduction can be by 3 to 6 log₁₀. In the specific case of PRRS vaccination, viraemia in infected animals has also been proven to experiment a reduction, albeit of a much smaller magnitude, not even reaching a 1 log₁₀ and affecting, in many cases, only to the duration of the viraemia. Therefore, estimating the real impact of this measure on a regional control plan is really difficult. Since vaccination of sows is widespread, the additional cost of this measure is minimal. However, the article mentions an interesting fact: Even though the phase of viraemia is not affected (at least in this work), the possibility of airborne transmission is indeed reduced in vaccinated populations, even after being infected, which can obviously be a great help in joint control plans. It provides, therefore, a different methodology to all the ones that have been applied in some of the areas where implementation of these regional plans has been initiated. As is apparent from the article, such actions could have a real impact as long as the entire population is vaccinated (i.e., finishing pigs and sows). Naturally, the cost of these actions is high (especially when double vaccination is performed on fattening pigs), so the availability of cost-benefit studies would facilitate their implementation.