There are three very important mosquito-borne diseases that occur in Florida: Eastern Equine Encephalitis, St. Louis Encephalitis, and West Nile Fever/Encephalitis. All of these diseases are caused by viruses that are transmitted by the bite of an infected mosquito. Adult female mosquitoes feed on blood for the nutrients it provides for developing the eggs that they will deposit. When mosquitoes feed on blood, they salivate prior to and during feeding. If the mosquito feeds on a bird that is infected with one of the viruses, there is a chance that she will pick up the virus from the bird’s blood. After a couple of weeks, if the virus has survived in the mosquito and increased to high numbers, she is able to infect a new blood host, such as another bird, a human, or a horse. The virus is released through the saliva when she is feeding. If the virus infects a human or horse, for example, there is a possibility that the new host will develop symptoms of encephalitis and become ill; more often, however, these hosts are only slightly ill or experience no symptoms at all and develop antibodies to the virus. Some birds can harbor the viruses with no ill effects. However, West Nile virus has been fatal to many raptors and corvids (blue jays, crows, hawks).
West Nile Virus
West Nile virus is carried by mosquitoes and if transmitted to humans, it can cause severe encephalitis. It is closely related to St. Louis Encephalitis (SLE) virus, which is sometimes a problem in Florida. West Nile virus was first isolated in 1937 from a woman in the West Nile province of Uganda in Central Africa. Epidemics of West Nile have occurred in Israel, France, South Africa, and Romania. West Nile virus was first documented in the United States in New York City (NYC) during an epidemic in August 1999.
St. Louis Encephalitis
St. Louis encephalitis (SLE) virus is a flavivirus that is transmitted to humans and other vertebrates primarily by mosquitoes of the genusCulex. Infection with SLE results in unapparent infection in a variety of birds and mammals with a resultant period of viremia that lasts a matter of days. Humans represent an incidental, dead-end host. The clinical spectrum of human SLE infection includes unapparent infection, mild illness (febrile with headache), aseptic meningitis, and encephalitis which can progress to coma and death. Unapparent infection is most common in the young, whereas encephalitis, especially that progressing to coma and death, is more common in the elderly.
Eastern Equine Encephalitis
Eastern Equine Encephalitis (EEE) is a virus disease transmitted to horses and humans by mosquitoes. Birds are the source of infection for mosquitoes. The virus is found along the east coast from New England to Florida, the Gulf Coast, and some Midwestern areas. The principal vector in avian populations is the mosquito Culiseta melanura. This mosquito does not feed on humans or horses, but in rare cases the virus can escape from its marsh habitat in other mosquitoes that feed on both birds and mammals (including horses and humans) and then transmit the virus to mammals, including people. Horses and humans are dead-end hosts, meaning that they do not develop enough virus in their blood to transmit the virus (therefore sick horses or humans cannot transmit the disease to mosquitoes, only birds can).
Highlands J Virus
Highlands J virus (HJ) is a mosquito-transmitted alphavirus that is similar to Eastern Equine Encephalitis virus (EEE) in its natural cycle; it is transmitted from Culiseta melanura mosquitoes to songbirds in freshwater swamps. It has a low pathogenicity in mammals and is rarely seen in humans or horses. There have been outbreaks reported in penned birds but the symptoms are mild compared to EEE.