Clinical and productive effects of PRRS in pigs on a positive farm will basically depend on the immunization status of the herd and the virulence of the PRRS virus strain infecting the farm. The immunization status of the herd would depend on the previous PRRS virus circulation on the farm and the vaccination strategies applied.
Once the PRRS virus is introduced into a farm, the circulation of PRRS in pigs of the herd will depend on different factors (Figure 1) such as the production system (i.e: 1-site vs. 2-sites, AIAO vs. continuous flow…), internal biosecurity, replacement gilt management, vaccination and immunization strategies or the presence of subpopulations of negative sows. Generally, it will tend to circulate indefinitely if no control measures are applied and periodical clinical outbreaks can be expected. However, some rare cases of spontaneous elimination of the virus have been described, especially on very small farms:
Re-emergence or re-circulation of the same PRRS strain already present on the farm: In this situation, the presence of negative subpopulations can play a crucial role in the clinical presentation and virus circulation. Negative subpopulations can be susceptible to the infection with the PRRS virus present on the farm, experiencing clinical problems and shedding the virus. Since a positive herd population presents homologous immunity against the circulating PRRS, clinical presentations tend to be a temporary mild impairment of productive and reproductive parameters.
Lateral infection introducing a new PRRS virus strain into a positive farm: Depending on the virulence and the immunization level of the herd and the presence of negative subpopulations as well, clinical PRRS problems can range from no effect to a severe outbreak.
New introduction of the PRRS virus into a negative farm: PRRS infection introduced into a naïve farm will most probably cause serious clinical outbreaks, especially if the PRRS virus strain is highly virulent. Typical reproductive disturbances such as late term abortions and increased numbers of stillborn and weak-born piglets at farrowing. At the same time, increased preweaning mortality can? be observed. Moreover, after this reproductive clinical episode, the PRRS virus will be spread rapidly to weaners and growers.