Mortalities are sporadic and the disease tends to occur in localised areas where suitable vectors are common. The disease occurs in many parts of the world, particularly the Americas but also South Africa and in parts of Asia, Europe and Australia. Cause The aetiological agent is a Lentivirus belonging to the family Retroviridae. Host species The disease affects only equidae. Transmission Infected horses are persistent carriers of the virus.
Natural spread is mechanical, mainly by large biting insects.
Mosquitoes can also transmit the infection. Iatrogenic transmission occurs by use of infected blood products or inadequately sterilised surgical instruments, hypodermic needles and the like. Maternal transmission also occurs. Essentially, though, it is a chronic infection. Risk of introduction Currently live horses can be imported from Australia, UK, USA, Ireland and Canada.