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 defenses. 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.
http://www.thepigsite.com/diseaseinfo/97/porcine-reproductive-respiratory-syndrome-prrs
