Mosquitoes trapped in the Sepulveda Dam Recreation Area in late August were infected with the St. Louis encephalitis virus, the first appearance in the San Fernando Valley this year of the sometimes deadly organism, health officials say.
The Los Angeles County Department of Heath Services on Wednesday urged residents to eliminate mosquito breeding areas--any stagnant, unchlorinated water that has collected in containers ranging from tin cans and old tires to rain gutters and decorative ponds.
The virus is carried by birds and transferred by mosquitoes, which draw blood from the birds and can later bite humans.
A sample of 26 mosquitoes collected Aug. 31 in the recreational area, just northwest of the intersection of the San Diego and Ventura freeways, was examined by the state Viral and Rickettsial Disease Laboratory in Berkeley, said Frank Hall, director of vector control programs for the county health department.
The test results showed last week that the mosquitoes were carrying the virus, he said, the third consecutive year that it has appeared among mosquitoes in the Valley. The virus was detected earlier this summer in blood samples taken from pigeons in Norwalk and the La Brea Tar Pits area.
The disease results in inflammation of the brain and nervous system, causing high fever, headache and weakness. Usually the symptoms are mild. "Treatment is fairly simple," Hall said, but the disease is sometimes mistaken for influenza, and improper medication is given.
If untreated in severe cases, which are most likely in children or the elderly, the disease can cause delirium, seizures, paralysis, coma and death.
The virus was so rare in Los Angeles County until 1984 that health officials believe the handful of cases reported before then were infections acquired elsewhere, said Jim Thomas, an epidemiologist for the county health department.
But, in that year, there was an outbreak of 26 cases in Southern California. Sixteen of the cases were in Los Angeles County and half of those were in Reseda, Encino and Van Nuys. A 62-year-old Long Beach woman died.
Three more cases were reported in 1985.
In 1986, there were three more--a 66-year-old Norwalk man, a 59-year-old Covina man and a 37-year-old El Monte man, according to health department records.
Comes With Seasonal Change
All of the 1986 victims survived, but two of them suffered prolonged weakness and one suffers "residual tremors," Thomas said.
The disease usually appears in late summer or fall. Human cases usually do not appear until two to four weeks after discovery of the virus in mosquitoes, health officials have said.
The disease is more common in the southeastern United States and some parts of Latin America. Usually, about 30 to 40 cases a year are reported in the United States, Thomas said, with occasional outbreaks of up to 300. But, in 1975, there was a sudden jump to 1,815.
In California, the disease is most often found in heavily irrigated agricultural areas, such as the Imperial and Central valleys, where pools of water breed mosquitoes. Health authorities believe the disease is carried from those regions into Los Angeles by migrating birds.
The mosquitoes are crucial carriers in the spread of the disease among humans, who cannot get it directly from birds or from other human beings.
The disease acquired its name because it was first recognized and described by researchers in St. Louis in 1932, Thomas said.