Secondary infections are common due to immunosuppressive effects of the virus. These can cause a wide range of presenting clinical signs. The disease appeared first in 1987 and has been recorded in the USA, Holland, France, Canada, Germany, UK, Japan, Korea and Austria. Cause The aetiological agent is the Arterivirus of the family Togaviridae. It is quite fragile and is rapidly inactivated in the environment.
Host species The only host species identified has been the domestic pig.
The disease spreads not only by direct contact but by aerosol spread. There has been transmission by wind over a distance of a few kilometres. Spread by infected semen is believed to have occurred. It is likely that infected pigs remain infectious for long periods, up to 8 weeks at least, after the end of clinical signs. Herds may be infected but show no clinical signs.