Ilheus encephalitis is caused by the Ilheus virus (genus Flavivirus) that is transmitted by different mosquito species. This encephalitis is endemic to eastern Brazil and other parts of south and Central America.
The virus of Ilhéus encephalitis (ILHV) was isolated for the first time in 1944 by Laemmert and Hughes from a pool of mosquitoes of the genera Aedes and Psorophora captured in the vicinity of Ilhéus in the State of Baia, Brazil. Ilheus virus has also been isolated from mosquitoes of the genus Haemagogus - Hg. janthinomys and Hg. spegazzinii in Panama and from Hg.leucocelaenus in Brazil.
Ilheus encephalitis virus is transmitted in enzootic cycle between birds and mosquitoes. It is not usually associated with human disease - infections cannot be correctly identified without a virus isolate because of the similar clinical symptoms and cross-reactivity in serologic assays as other flaviviruses.
Infected patients most commonly exhibit a mild febrile illness accompanied by headache, myalgia, arthralgia, and photophobia, symptoms that may result in clinical diagnosis of dengue, Saint Louis encephalitis, yellow fever or influenza. Some human infections may have no symptoms at all.