St. Louis encephalitis (SLE), West Nile (WN), eastern equine encephalitis (EEE) and Highlands J (HJV) are mosquito-transmitted viruses that involve birds in the transmission cycle.  Humans are not a part of their natural life cycle and human infections are simply accidental “collateral damage.” As a group these are considered to be “encephalitis viruses” since encephalitis is the common severe manifestation of infection with any of these viruses.

SLE and WN are of particular concern for residents of Indian River County because, unlike Zika, dengue and chikungunya viruses, they are considered to be endemic to the county (that is, present all of the time). These viruses are intimately associated with the natural wetlands and agricultural environment of the region. The kinds of mosquito that transmit these viruses bite at night, between dusk and dawn.



cnigF1 FMELIn the latter half of the 1900s, St. Louis encephalitis (SLE) became the predominant mosquito-borne disease of man in Florida and was responsible for recurring epidemics in the south and central portions of the state. Major epidemics occurred in 1959, 1961, 1962, 1977, and 1990 and 1997. The 1959-1962 outbreaks in the Tampa Bay area involved 55 fatalities amongst 315 cases. In contrast the most recent epidemic (1997) yielded 9 cases throughout the state, with only one fatality. The 1990 epidemic was largest (223 documented cases) and most widespread (cases in 28 counties), with 11 fatalities. The 1990 epidemic was first recognized in Indian River County, which also experienced the highest impact of any affected county (19 confirmed cases and one fatality, despite a relatively low population).

Most people infected with SLEV experience mild symptoms, if any. A smaller group may experience more severe symptoms (including fever) and require medical assistance. The most affected people display neuroinvasive symptoms such as encephalitis or paralysis and are at risk of death. SLEV is normally associated with wild birds and several species of mosquito, most notably Culex nigripalpus in south and central Florida. 



Like St. Louis encephalitis virus, most people infected by WNV experience mild or unrecognized symptoms while a minority experience illness with fever. More rarely victims develop encephalitis, paralysis and may die. The greatest number of human cases of West Nile disease seen in Florida was during 2003, a total of 94 cases. Despite having been first detected in Indian River sentinel chickens in 2001, and many times in the following years, there has not yet been a human case of disease caused by West Nile virus in this county.



As its name suggests, eastern equine encephalitis (EEE) is a disease affecting horses as well as humans. EEEV is infrequently detected in Indian River County, only one human EEE case having been recorded. EEEV activity is much more common in northern and central Florida. The life cycle of this virus is poorly understood in Florida and probably involves different mosquito species than in more northern states. The virus is associated with birds and mosquitoes frequenting freshwater swamps.



Highlands J virus (HJV) is most notable for the need to distinguish this virus from its close relative, eastern equine encephalitis virus, when testing sentinel chicken blood samples for evidence of recent infection. While HJV is not known to cause disease in man, a single horse fatality was documented. Like EEEV, HJV is associated with freshwater swamps.