Equidae and man are end hosts; mosquitoes are vectors; reservoir hosts are birds, small rodents, or the leopard frog. 3 serotypes from Eastern USA, Western USA and Venezuela (termed EEE, WEE and VEE). They were part of the old arbovirus (Arthropod borne) family.
Pathogenesis:
- Mammalian infection is from an infected mosquito bite, virus being in the saliva. More rarely, aerosol infection occurs between mammals.
- Biphasic infection :1^st viremia and visceral replication occur 2-5 d.p.i. with pyrexia, depression and anorexia, and virus in aerosol and in urine. Recovery is possible at this stage.
- 2^nd CNS replication at 6-8 d.p.i with paralysis of the lips, drooping eyelids and incoordination. This is often fatal. Neuronophagia, perivascular cuffing and gliosis in the CNS.