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West Nile Virus returns to the state, county
The recent heavy rainfall has created ideal mosquito breeding conditions. - photo by Contributed

The threat of mosquito-borne illnesses like West Nile Virus and St. Louis encephalitis has the Turlock Mosquito Abatement District engaging in an aggressive strategy to rid the region of as many of the virulent pests as possible.

The effort has become more critical as yellow fever mosquitoes were found in Merced County last season and the radius in which they were found is growing. This mosquito, Aedes aegypti, is capable of transmitting several human diseases, including dengue, chikungunya, yellow fever and Zika. Since 2013, Aedes aegypti has been detected in 13 other California counties, primarily south of Merced.

“That’s got our red flag up,” said TMAD General Manager David Heft.

Moreover, the California Department of Public Health announced four people in Southern California have been confirmed as having West Nile Virus.

“West Nile virus activity in the state is increasing, so I urge Californians to take every possible precaution to protect against mosquito bites,” said CDPH Director and State Public Health Officer Dr. Karen Smith.

Mosquitoes become infected with West Nile Virus when they feed on infected birds. Infected mosquitoes can then spread West Nile Virus to humans and other animals when they bite, according to the Centers for Disease Control.

Approximately one in five people who are infected with West Nile virus will develop symptoms such as fever, headache, body aches, joint pains, vomiting, diarrhea, or rash. Less than one percent will develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues). About 10 percent of people who develop neurologic infection due to West Nile virus will die, according to the CDC.  People over 50 years of age and those with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and organ transplants, are at greater risk for serious illness. 

There are no medications to treat or vaccines to prevent West Nile Virus infection. People with milder illnesses typically recover on their own, although symptoms may last for several weeks or months. In the neuroinvasive forms, patients can suffer severe and sometimes long-term symptoms.

As of June 15, there are 12 counties in California that have confirmed the presence of West Nile Virus, with the four illnesses occurring in Los Angeles, Kern and Riverside counties. A total of 21 dead birds have tested positive for the virus in California, including one in Stanislaus County.

By this time last year West Nile Virus had been detected in 14 counties and there were two human cases and 25 dead birds, according to the CDPH.

“Last year we had an early season, but this year we are seeing a more normal trajectory and are really just getting into the season now,” Heft said.

TMAD recently completed their survey of neglected swimming pools in the area, which are havens for breeding Culex mosquitoes that carry West Nile Virus and St. Louis Encephalitis. 

The Culex mosquitoes like to breed in standing water, particularly in reedy areas, like irrigated pastures.

“As the summer gets hotter the stages of breeding get quicker, which means we have a shorter window to catch them before they become an issue,” Heft said.

The Culex mosquitoes are usually out at dawn and dusk, but if people are being bitten during the afternoon hours, TMAD still wants to know about it.

Aedes aegypti is a small black and white day-biting mosquito that prefers to feed on humans. It lives in urban habitats and lays its eggs just above the water surface in small containers such as flower pots, plant saucers, pet bowls, and bird baths. 

CDPH recommends that individuals protect against mosquito bites and WNV by practicing the “Three Ds”:

1. DEET – Apply insect repellent containing DEET, picaradin, oil of lemon eucalyptus or IR3535 according to label instructions. Repellents keep the mosquitoes from biting you. Insect repellents should not be used on children under two months of age.

2. DAWN AND DUSK – Mosquitoes that transmit WNV usually bite in the early morning and evening so it is important to wear proper clothing and repellent if outside during these times. Make sure that your doors and windows have tight-fitting screens to keep out mosquitoes. Repair or replace screens that have tears or holes.

3. DRAIN – Mosquitoes lay their eggs on standing water. Eliminate all sources of standing water on your property by emptying flower pots, old car tires, buckets, and other containers. If you know of a swimming pool that is not being properly maintained, please contact your local mosquito and vector control agency.

Reporting and testing of dead birds is an important step in preventing West Nile Virus. A confirmed case of the virus in dead birds or mosquito samples helps to identify areas that need treatment to reduce mosquito activity. To report a dead bird, call the California State hotline at 1-877-WNV-BIRD or report it online at Birds of particular interest are crows, ravens, magpies, jays and raptors (hawk or eagle).

The Turlock Mosquito Abatement District is available to help with neglected pools in the prevention of mosquito development. To request District service, call 634- or visit the District website at