PRRS is caused by a virus which was first isolated and classified as an arterivirus as recently as 1991. 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. Some sows remain naive.