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Arboviral or Mosquito-borne Encephalitis

Division of Epidemiology
275 East Main Street
Frankfort, KY  40621
502-564-3418 or 3261


What is mosquito-borne encephalitis?

Mosquito-borne (also called arboviral) encephalitis is a viral infection that is transmitted to people by the bite of an infected mosquito. In the United States the following viruses have been identified: St. Louis encephalitis (SLE), eastern equine encephalitis (EEE), western equine encephalitis (WEE), Venezuelan equine encephalitis (VEE), LaCrosse (LAC) and other California serogroup viruses, and most recently, West Nile virus (WNV) in New York.

How is mosquito-borne encephalitis spread?

These viruses have complex life cycles that usually involve a nonhuman primary vertebrate host and a primary arthropod vector. Wild birds are the hosts for most of these viruses, with a variety of species of mosquitoes being the vectors. Chipmunks and tree squirrels are the reservoir hosts for LaCrosse virus, with the treehole mosquito being the vector. Most cases of arboviral encephalitis occur from June through September, when mosquitoes are most active. Humans and domestic animals, like horses can develop illness but are considered "dead-end" hosts, meaning they cannot spread the virus to other people or animals.

Who is at risk for infection?

Persons involved in outdoor work and recreational activities.
SLE and WNV occur more frequently in older persons
LaCrosse is usually found in children less than 16 years of age that live in woodland areas.
EEE and WEE occur sporadically in children and the elderly.
What are the symptoms of mosquito-borne encephalitis?

The majority of cases are asymptomatic or may result in a nonspecific flu-like syndrome. The incubation periods vary slightly from three to fifteen days depending on the specific virus. Onset may be insidious or sudden with fever, headache, muscle aches, tiredness and possibly prostration. Some cases can progress to encephalitis with focal paralysis, seizures, and coma with a fatal outcome or permanent neurologic sequelae. Only a small proportion of infected persons progress to the encephalitis stage.

How is mosquito-borne or arboviral encephalitis diagnosed?

Diagnosis is confirmed by laboratory tests on blood, serum, or spinal fluid. Some of the tests require two blood samples taken about two weeks apart.

What is the treatment for encephalitis caused by the arboviruses?

Treatment is supportive, dealing with problems caused by the virus in the brain. Antibiotics are not effective because the infectious agent is a virus.

How can mosquito-borne or arboviral encephalitis be prevented?

There are no vaccines approved for humans in the United States.
There are vaccines available for use in horses.
Avoid Exposure to Mosquito-borne Diseases by Avoiding Mosquitoes!
Mosquitoes breed in standing water. To eliminate breeding sites:
Remove water-holding containers (tires, buckets, etc.) from your property.
Change the water in birdbaths at least once a week.
Clean and chlorinate swimming pools or drain and cover if not in use.
Unclog gutters and downspouts.
Make sure that doors and windows have tight-fitting screens.
If outside when mosquitoes are most likely to bite (in the evening, at night, or early in the morning), wear protective clothing such as long pants, long-sleeved shirts, and socks. Consider the use of an insect repellant containing 10% or less DEET (N, N-diethyl-methyl-meta-toluamide) for children and no more than 30% for adults.
Do not use DEET on infants. Follow manufacturer’s directions carefully.




Last Updated 1/8/2008