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Western Equine Encephalitis
Western Equine Encephalitis is a mosquito-borne virus in California that reaches detectable levels every few years. This disease is commonly called sleeping sickness. It can be a serious condition affecting the brain. It causes greatest concern when it begins to affect humans and horses. Data kept on reported cases show that between 1945 and 1987, California had 751 human cases. The number of cases can vary dramatically from year to year. All age groups are susceptible to Western Equine Encephalitis (WEE), but most cases occur among the elderly and infants.
In 1930 a major encephalomyelitis outbreak occurred in horses in the San Joaquin Valley. The virus was isolated from the brain of a horse that same year. As the virus was first found in horses, it was given the name Western Equine Encephalitis. By 1938, a vaccine had been developed to protect horses. In subsequent years, the disease has been detected intermittently throughout the state.
The Culex tarsalis mosquito is the primary vector of WEE. This species is widely distributed throughout California. Its typical breeding sites include wetlands, duck clubs, rice fields and some irrigated crops and pastureland. At times, the Ochlerotatus melanimon mosquito becomes involved in the transmission cycle. Culex tarsalis or “the Encephalitis Mosquito”, becomes infected while feeding on birds that harbor the virus. The mosquito can then transmit the virus to other animals. Occasionally, this transmission occurs in humans. An infected human does not increase the risk of others becoming infected. Human to human transmission does not occur. Although breast fed infants, unborn fetuses, and blood and organ recipients can become infected without being bitten by a mosquito.
Disease symptoms occur 7 to 21 days after being bitten by an infected mosquito. Many infections result in no symptoms or signs of disease. Some infections will cause flu-like symptoms normally lasting 5 to 10 days. These symptoms may include headache, fever, nausea, and vomiting. Occasionally, meningitis-like symptoms will occur. This is a serious condition and victims should be under a doctor's care. Signs of illness may include flu-like symptoms and a notably severe headache and stiff neck lasting 3 to 10 days. In about 1% of WEE infections encephalitis will develop. This condition will require hospitalization. Symptoms can include delirium, stupor, convulsions, coma and death. Encephalitis is an inflammation of the brain and lasts 14 to 21 days. Acute effects of encephalitis can cause mental retardation, motor/muscle impairment and 2.5% to 10% of people exhibiting severe symptoms will die.
Most WEE cases begin during the summer months of July and August. 80% of human infections have occurred during these 2 months. At present no vaccine is available to protect humans from WEE infection. Once infected, there is no specific medical treatment other than supportive care.
There are some ways to reduce your exposure to mosquitoes and their bites. Drain all stagnant water on your property. Make use of mosquitofish in suitable mosquito sources. Make sure window and door screens are tight fitting and free of tears and holes. Wear long pants, long sleeved shirts and socks with shoes when outdoors. Apply an insect repellant, following all label directions. Reduce or eliminate outdoor activities at dawn and dusk, as these are peak mosquito feeding hours.
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