PRRS is caused by a virus which was first isolated and classified as an arterivirus as recently as 1991. The disease syndrome had been first recognised in the USA in the mid 1980's and was called "mystery swine disease". It has also been called blue ear disease. The name porcine arterivirus has been proposed recently.
The virus of PRRS has a particular affinity for the macrophages particularly those found in the lung. Macrophages are part of the body defences. Those present in the lung are called alveolar macrophages. They ingest and remove invading bacteria and viruses but not in the case of the PRRS virus. Instead, the virus multiplies inside them producing more virus and kills the macrophages. Once it has entered a herd it tends to remain present and active indefinitely.
Up to 40% of the macrophages are destroyed which removes a major part of the bodies defence mechanism and allows bacteria and other viruses to proliferate and do damage.
A common example of this is the noticeable increase in severity of enzootic pneumonia in grower/finisher units when they become infected with PRRS virus.
It may take up to a year for all breeding stock, particularly in large herds, to become infected for the first time and although the virus appears to spread rapidly in a herd it may be some 4 -5 months before at least 90% of the sows have become sero-positive. Some sows remain naive. Furthermore, it is not uncommon for sow herds 1-2 years after infection to contain less than 20% of serological positive animals. This does not however necessarily mean they are not still immune nor does it mean that they have stopped passing on immunity to their offspring. Adult animals shed virus for much shorter periods of time (14 days) compared to growing pigs which can excrete for 1-2 months.
The clinical picture can vary tremendously from one herd to another. As a guide, for every three herds that are exposed to PRRS for the first time one will show no recognisable disease, the second would show mild disease and the third moderate to severe disease. The reasons for this are not clearly understood. However the higher the health status of the herd, the less severe are the disease effects. It may be that the virus is mutating as it multiplies, throwing up some strains that are highly virulent and some that are not.
PRRS infects all types of herd including high or ordinary health status and both indoor and outdoor units, irrespective of size.
