Health Officials Fear an Outbreak of Encephalitis : Disease: A big increase in mosquito population is predicted. The insect carries virus from infected birds to humans.

This year’s heavy rains will probably produce the biggest mosquito population in 20 years, and county health officials are bracing for a possible outbreak of mosquito-transmitted St. Louis encephalitis.

“It’s shaping up to be the worst (season) we’ve had in 20 years,” said Robert Saviskas, executive director of the Los Angeles County West Vector Control District.

Calls reporting mosquitoes to the district were higher in February and March than at any other time in the last two decades, Saviskas said. The district, covering an area extending from the Westside to Ventura County, is one of five that monitors and attempts to control mosquitoes and other pests throughout Los Angeles County.

“We do get very concerned,” said Michele Jay, a research scientist for the California Department of Health Services.

Mosquitoes, she said, carry the virus responsible for St. Louis encephalitis, a viral brain inflammation transmitted from infected birds to humans through the blood carried by the insects.

Areas hit by the Northridge earthquake are particularly vulnerable, because broken swimming pools and back-yard spas make natural breeding grounds for mosquitoes.

“We can’t predict whether or not the virus will be present this year in the mosquito population, but we do predict that with this kind of rain, the mosquito population will be very large,” Jay said. “All of the county health departments are gearing up.”

Jack Hazelrigg, district manager for the Greater Los Angeles County Vector Control District, which monitors insects in the San Fernando Valley, said his agency is “very concerned about virus transmission.”

“High rainfall creates a habitat ideal for the mosquito suspected for transmitting St. Louis encephalitis,” he said.

It’s important to remember that there is as yet no evidence of an encephalitis breakout, said Dr. Laurene Mascola, chief of the acute communicable disease control unit of Los Angeles County. Mascola said that last year only three cases of the disease were reported in the county, and they may not have been caused by mosquitoes.

St. Louis encephalitis, one of many forms of virally transmitted encephalitis, can cause fatigue, headaches and seizures. It is not treatable with antibiotics, and although most people who are infected with it recover quickly, some suffer permanent brain damage or even die, said Dr. Carol Peterson, an epidemiologist responsible for monitoring the disease for Los Angeles County.

“Most of the cases are very mild,” Peterson said, so mild that people may not ever know they have contracted the disease. “But occasionally this is not true. It can be severe, with long-term mental changes.”

The only treatment is what doctors call supportive care, Peterson said, which involves making sure the patient is able to breathe well and take plenty of fluids. Oxygen is sometimes administered.

Young children and old people are particularly susceptible, Peterson said, and should stay away from infested areas, particularly at twilight, when the mosquitoes are active.

It’s impossible to tell whether the mosquitoes in a particular area are the common and benign Southern house mosquitoes or the Western encephalitis mosquito, which carries the disease, Hazelrigg said.

But there are precautions that state and county health officials recommend. Mosquitoes breed in standing water--even in drops of water as small as those inside the petals of a flower. So homeowners should remove any buckets or old tires that fill with water, and drain any stagnant pools.

According to Jay of the state Health Services department, Southern California has averaged one to three confirmed cases of St. Louis encephalitis each year since 1990. In 1984, when Los Angeles County had its last big outbreak of the disease, 16 people were hospitalized. Half of them lived in the Valley, and probably were bitten by mosquitoes that infest the area around Sepulveda Basin.

However, according to Mascola, it’s very hard to track the real incidence of the disease, and it may be more prevalent than the numbers indicate.

Doctors, Mascola said, tend not to order the tests necessary to diagnose encephalitis, and tend not to report it.

For example, she said, the county in 1986 randomly tested the blood of 1,800 people who came to county health clinics for blood tests. It turned out that 1.7% carried antibodies to encephalitis.

She encouraged doctors to send samples of suspected blood to the county for free analysis.

The county tries to monitor the likelihood of an encephalitis outbreak by keeping flocks of chickens in areas where they can be bitten by mosquitoes. If the chickens turn up with the disease, officials know that the virus is becoming prevalent. Also, state health officials routinely test mosquitoes captured at the county level to see whether they carry the disease.

Times researcher Stephanie Stassel contributed to this story.

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Mosquito Invasion Heavy rains have resulted in what could be the worst year for mosquitoes in two decades. One culprit is the western encephalitis mosquito, which could spread encephalitis. *

Description: A quarter- inch long, Slender, with a long, needlelike “beak” and narrow wings. Males, which live from seven to 10 days, feed on plant juices and do not bite humans. Females, which live 30 days or more, obtain protein needed to lay their eggs from the blood of birds and mammals, thus transmitting encephalitis in the process. *

Breeding A female mosquito makes a high- pitched sound with its wings to attract a male. Mate only once in a lifetime. Prefer clear, sunlit water, to which to breed, such as abandoned swimming pools and roadside ditches. *

Life Cycle Between 100 and 300 eggs are laid on water or damp ground in an arrangement that looks like small raft. They hatch in two or three days in warm weather. Larvae feed on small aquatic plants and animals, then molt into active pupae. Adults emerge two to five days later at surface from floating pupae. *

Range Found mostly in Western states. Sources: Greater Los Angeles County Vector Control District; Audubon Society Pocket Guide; Familiar Insects and Spiders and World Book.