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St. Louis Encephalitis Virus

St. Louis Encephalitis Virus (SLEV) is transmitted to people by an infected mosquito and can develop in to a disease that attacks the brain tissue. The virus was first identified from victims of a 1933 epidemic in St. Louis, Missouri. The disease is commonly called sleeping sickness because in severe cases the patient suffers prolonged drowsiness.


SLEV is commonly found in Coachella Valley mosquitoes annually.

In nature, the SLEV is maintained in a primary enzootic transmission cycle, animal to animal, involving wild birds and mosquitoes. It is not known how the virus reappears each year. It has been speculated that migrant birds reintroduce the virus each spring from endemic tropical regions or that a local winter reservoir may exist in other vertebrates and arthropods.

The cycle begins when an uninfected female mosquito bites an infected bird during the period in which the bird has a viremia (virus in the blood). The virus replicates in the mosquito for 5 to 8 days. After this incubation period, the mosquito is infective for life and can transmit the virus to another bird, human, domestic or wild animal. Humans can be severely affected by the virus, but they are dead end hosts because not enough virus develops in their blood to infect other mosquitoes or people.

Symptoms of SLEV

SLEV infections can be unnoticeable, acute, or fatal. The majority of SLEV cases are mild or sub-clinical; a small number of people develop acute symptoms or die from the disease. The severity of the disease appears to be age dependent. The fatality rate for individuals under 40 is 1 to 5 %, while it ranges between 15 to 23 % for the elderly (60 and over).

Symptoms of infection appear 7 to 21 days after a bite from an infected mosquito. SLEV has three separate sets of symptoms: (1) feverish headache, (2) aseptic meningitis, and (3) encephalomyelitis. All SLEV infections are followed by immunity. Sequelae (permanent abnormalities) can follow an SLEV attack. Sequelae symptoms range from recurring headaches, dizziness, memory impairment, tremors, sensory motor disturbances, insomnia to speech disturbances.

Cases in California, as a whole, are most likely to occur during May through November. Approximately 80% of the cases occur during August and September. In Southern California, specifically, cases can occur year-round, but they are more likely to occur during the warmer months.

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