The viruses are restricted to the Americas, with cases reported from the USA, Canada, Venezuela, Brazil and Argentina. Outbreaks have occurred in Central America. In North America the disease tends to be seasonal due to the changes in mosquito and other insect populations. Spread of the disease between areas by migratory birds may be possible, and encephalitis has been detected in infected birds.
Wild birds act as a reservoir for the virus, and spread is by insects.
Mosquitoes appear the main vector, but mites, ticks and lice may also be implicated. The movement of infected mosquioes between distant areas by high winds has been recorded. Clinical signs with the different virus are indistinguishable. The incubation period may be 1-3 weeks after which there is a mild loss of appetite, low fever and depression. The animals may initially be hypersensitive to external stimuli, with periods of excitement and restlessness.