The discovery of mosquitoes and birds carrying a virus known as St. Louis encephalitis has prompted county health officials to intensify efforts to eradicate mosquito populations along the San Gabriel River in four Southeast area cities.
Residents living near the river in Downey, Norwalk, Santa Fe Springs and Pico Rivera also have been advised to curtail outdoor activities at night or take precautions like wearing long-sleeve clothing and using bug sprays to discourage mosquitoes.
In addition, residents have been asked to eliminate mosquito breeding sources such as standing water in planter boxes, old tires, cans and buckets.
St. Louis encephalitis is transmitted to humans through the bite of a mosquito which becomes infected after biting birds carrying the virus. The virus cannot be transmitted directly from person to person or from birds to humans, said Art Tilzer, an official with the county Department of Health Services.
Generally, those infected with St. Louis encephalitis experience mild symptoms--headaches, low-grade fever and nausea, Tilzer said. But in a small number of cases, the virus can cause confusion, paralysis and even coma, he said. Older people are more likely to experience symptoms and severe illness from the virus.
Health officials targeted the southeast corner of the county after a 66-year-old Norwalk man was admitted to a local hospital July 13 with a serious case of virus. The Norwalk man recovered, but his illness prompted officials to begin catching and testing mosquitoes for the virus on a regular basis. Birds, the other primary carrier, were also tested.
As recently as last week, both mosquitoes and birds tested positive for the virus in several areas near the San Gabriel River, said Frank W. Pelsue, general manager of the Southeast Mosquito Abatement District.
The area of concern, he said, stretches from Firestone Boulevard in Downey to Whittier Narrows, north of Pico Rivera.
The San Gabriel River is considered an ideal habitat for mosquitoes. To breed in large numbers, mosquitoes need lots of water, food and warm weather. Because of above average rainfall in the region in recent winters, there has been plenty of water in the river, even during the summer months. Nutrients that mosquitoes feed on also have been plentiful. And with daytime summer temperatures near the river reaching the upper 80s and low 90s, mosquitoes are thriving, Pelsue said.
Birds, including a large number of migratory species, also use the river, particularly the Whittier Narrows Wildlife Sanctuary.
Tilzer and others believe birds first introduced St. Louis encephalitis to Los Angeles County four years ago. Before then, he said, there were no known cases of the virus here. But since 1983, there have been 21 cases, including two this year--the Norwalk man and a 59-year-old Covina man, who also was hospitalized and later released. No deaths have been attributed to the disease so far, Tilzer said.
Common in South, East
The virus is common throughout much of the south and eastern United States as well as in numerous Latin American countries, Pelsue said. In this state, the virus has been found in heavily irrigated agricultural areas such as the Imperial Valley and the Central Valley. Therefore, it is possible that birds migrating north and south may have carried it into Los Angeles County over a period of time, he said.
"Since the virus only remains active in a bird for a few days, there was probably a hopscotch effect," Pelsue said. "One bird probably brought it part way and then it was transmitted to another."
To contain the virus, health officials have stepped up mosquito spraying efforts. Along the river where there is standing water and in ponds and near culverts, crews have been spraying an oily substance known as Golden Bear 11-11. The substance, which is not toxic to humans, smothers mosquito larvae, making it difficult for them to breathe, Pelsue said.