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El Nino Rains Bring Mosquitoes

May 10, 1998

LOS ANGELES (AP) _ Beefed-up bug patrols are prowling California’s parks, backyards and wetlands to curb an El Nino-fed profusion of mosquitoes that could spread potentially fatal encephalitis this summer.

A winter of soaking rains and heavy mountain snows has created ideal breeding conditions for mosquitoes sure to menace hikers, gardeners and anyone else who spends time outdoors _ especially at dusk or dawn.

But of greater concern than itchy bites is the threat of disease _ notably the brain-swelling sickness encephalitis _ and, to a lesser degree, malaria, plague and hantavirus.

``Disease is always a potential. ... We’ve geared up substantially this year with nearly double staff in the field,″ said Jack Hazelrigg, district manager for the Greater Los Angeles County Vector Control District.

``With this much standing water, we anticipate a banner crop of mosquitoes,″ said Vicki Kramer, chief of the Vector-Borne Disease Section of the California Department of Health Services.

Bug hunters are seeking out the ponds, pools, treeholes and standing water that provide just the right egg-laying conditions for several mosquito varieties. Their mission: get them before they get us.

The variety known as Culex tarsalis, which began emerging from hibernation weeks ago, is California’s primary carrier of encephalitis, which actually begins as a bird disease.

Although there have been no cases in the last three years, that could change given this season’s excess El Nino moisture. In California, encephalitis is continually passed between blood-sucking mosquitoes and their snack of choice, birds. Thus, a burst in the mosquito population provides greater odds that people could be bitten by an infected mosquito.

Peak mosquito season falls in late spring and summer; encephalitis cases typically crop up in late summer.

Western equine encephalitis, which can produce anything from a mild flu-like illness to coma and death, particularly affects small children, causing nerve or brain damage. St. Louis encephalitis, which generally produces a milder flu-like illness, can be fatal in the elderly.

Anopheles hermsi mosquitoes can spread malaria, but they aren’t a major risk because they first must bite an infected person _ and California doesn’t have many residents with malaria.

The major urban mosquito problem comes from Culex quinquefasciatus, which despite its intimidating name and bite, doesn’t carry disease.

Once the bug hunters corner their quarry, they pull out the arsenal: bacterial spray to destroy young mosquitoes, hormones to trap the larvae in perpetual childhood so they can’t develop into biters, fish to gobble them up.

One recent morning on bug patrol, Los Angeles County Vector Control specialist Rande Gallant stopped at a vacant backyard in the eastern San Fernando Valley. He dumped out several buckets that were full of larvae-filled stagnant water, then sprayed a bug-infested rubber tire.

About 15 miles to the west in the abandoned Chatsworth Reservoir, Gallant dipped a tin cup into a murky pond formed by hillside runoff and checked for any sign of mosquito larvae, also called ``wigglers.″

``There are a few little guys,″ he said.

But when Gallant knocked around some grass and no mosquitoes flew out, he knew the hormone he had applied the previous week kept the wigglers from maturing.

``It seems like we are finding a good number of them earlier than in years past,″ Gallant said. ``We will treat any kind we find, but we do try to focus on the kind that carry diseases.″

Mosquitoes aren’t the only disease-causing pests increased by El Nino’s rains.

Throughout the Southwest, the lusher-than-usual vegetation is likely to provide extra food for the deer mice that carry deadly hantavirus, although cases might take another year to appear. The pneumonia-like disorder was identified in the Four Corners states in 1993 after an El Nino winter.

Similarly, El Nino’s gift of greenery might increase plague, which is transmitted by fleas that catch a ride with mice.

``It does look like there’s some effect of precipitation on the plague, but it’s not a clear-cut thing,″ said Dr. Kenneth Gage of the Centers for Disease Control and Prevention’s Division of Vector-Borne Infectious Disease in Fort Collins, Colo.

Unlike the summer mosquito infestations in the eastern half of the country, California’s mosquito threat is nearly year-round _ as is encephalitis.

The greatest risk in California lies in the Central Valley, subject to both the mosquitoes that breed in fertile, irrigated farmland and those that lie dormant in the adjacent Sierra Nevada until mountain snows melt and let loose a population eager to bite and breed.

California has a sophisticated two-track monitoring system: Traps capture mosquitoes so scientists can test directly for encephalitis. And flocks of ``sentinel chickens″ are tested every two weeks. If infected, the bug hunters mount an offensive. So far, no infections have been reported.

Until the chickens show signs, ``it’s premature to say what the disease risk is going to be,″ said Roger Nasci, a CDC research entomologist in Colorado.

Some other states pounded by El Nino rains also could have more mosquito-borne diseases than usual.

Because Florida and the Southeast had increased levels of Eastern equine and St. Louis encephalitis last year and additional rain this year, ``there is an elevated level of concern across the Gulf Coast states and Florida ... for this mosquito season,″ Nasci said.

Although mosquito-transmitted encephalitis cases average about 100 to 200 a year nationwide, given the right conditions, ``the potential is there for explosive outbreaks,″ like the 2,000 cases found in the Mississippi Valley in 1975, Nasci said.

Pets are at risk of suffering and spreading bugborne diseases, too, said Dick Davis, a California public health biologist in Ventura County.

Dogs can pick up Lyme disease from ticks and heartworm from mosquitoes, he said, and cats exposed to the plague can become violently ill and transmit the disease to their owners ``faster than you would like to think about.″

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