VECTOR CONTROL PROGRAM

What is encephalitis?

Encephalitis means "an inflammation of the brain". It is the primary mosquito borne disease of man now known to occur in the United States. It is a viral disease that affects the central nervous system. There are various types of encephalitis that are normally transmitted from wild bird to wild bird and less commonly from wild bird to man or his domestic animals. There are other types of encephalitis not transmitted by mosquitoes.

Of the various types of encephalitis, the two most common in California are Western Equine Encephalitis (WEE) and St. Louis Encephalitis (SLE). Wee is generally not fatal to man but is extremely fatal to horses. SLE is the most imnportant mosquito borne disease remaining in the United States. A new Encephalitis, West Nile Virus, is expected to arrive in California in the Summer of 2003. For more information on West Nile Virus, click here.

Where does it come from?

Encephalitis is transmitted by mosquitoes, primarily by Culex tarsalis. The mosquito feeds on an infected wild bird and then transmits the virus to other wild birds and domestic fowl. The primary reservoirs (animals that harbor the virus) are wild migratory birds, black birds, sparrows and possible wild rodents.

How humans contact encephalitis

Humans, horses and other animals may also become infected from the bite of an infected Culex tarsalis mosquito. However, the virus is in very low concentration in the blood stream of these hosts and the mosquito cannot become infected from them. In other parts of the country other mosquito species may beocme involved in tranmission of the virus.

Symptoms in Humans

Fortunately not all persons bitten by infected mosquitoes become sick. Most symptoms of the disease are mild, generally with a few days of fever and severe headache. More severe symptoms include sudden malise, chills, severe headache, high temperature, nausea, convulsions and speech and visual difficulities. While most symptoms are mild with good recovery, these diseases are dangerous. Not everyone exhibits all of the symptoms; some exhibit only a few. Laboratory analysis of the blood is needed to determine the presence of the virus.

Prevention

Some effective preventive measures are:

1. Control of mosquito breeding sites.
2. Control of adult mosquitoes.
3. Complete screening of residences and or sleeping quarters.
4. Remaining inside during evening hours.
5. Using insect repellents.

For more information on mosquito control, click here.

There is an effective vaccine for the prevention of encephalitis in horses, but so far routine immunization of people has not been considered effective. There is no specific treatment available as yet. Most cases of encephalitis recover fully with bed rest and supportive therapy under the supervision of a physican.

For Further Information Call:

Ventura County
Environmental Health Division