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Mosquito activity on the rise this summer
Drought could have impact on West Nile transmission
The Turlock and East Side mosquito abatement districts reported six mosquito samples have tested positive for West Nile Virus in Stanislaus County. - photo by Photo Contributed

As the temperature climbs and mosquito activity increases, local health departments are reminding residents to take precautions against catching West Nile virus, as the California Department of Public Health reported the first death from West Nile Virus for the season.

The death was reported in San Luis Obispo County.

“West Nile virus activity in the state is increasing, so I urge Californians to take every possible precaution to protect against mosquito bites,” said Dr. Tomás J. Aragón, Director of the California Department of Public Health and State Public Health Officer.

As of July 9, WNV has been detected in 45 dead birds from 6 counties and 177 mosquito samples from 13 counties. Hot temperatures this month are contributing to increasing numbers of mosquitoes and the increased risk of virus transmission to humans. So far this season, activity is within expected levels. The risk of disease due to WNV usually increases at this time of year and is highest throughout the summer and early fall.

In 2020, mosquito samples with West Nile virus were found in May, which was the earliest the mosquito abatement districts had logged virus activity. This year has been the opposite, with virus activity just now showing up in mosquito collections in Stanislaus County and not at all in Merced County.

“We know that WNV transmission will occur each year, we just don’t have a great model as to why last year we had high activity starting in May and this year didn’t see anything until mid-July,” said Turlock Mosquito Abatement District General Manager David Heft.

“We are seeing the activity later than normal, but just because we haven’t detected it yet, doesn’t mean that it is not here,” said Rhiannon Jones, the general manager of the Merced County Mosquito Abatement District.

The drought conditions and restrictions on irrigation has had an impact on mosquito populations, but it could actually lead to a higher rate of infected mosquitoes.

“We’ve experienced drought conditions such as these before and strangely, although we see a reduction in mosquito numbers, we can see an increase in WNV transmission,” Heft said. “We believe the lack of water may actually bring the mosquitoes and birds closer together and amplify the transmission of the virus. So, with this hot weather, we are tentatively expecting a higher percentage of mosquitoes carrying the virus this summer. Although the numbers may be down, the mosquitoes that are present may have a higher likelihood of carrying the virus. This makes it challenging from a control aspect as the overall population is already low, making it so important to wear repellent if outdoors during dusk/dawn.”

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 1 in 5 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 1 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.

People 50 years of age and older have a higher chance of getting sick and are more likely to develop serious illness when infected with WNV. Studies also indicate that those with diabetes and/or hypertension are at greatest risk for serious illness.

It’s not just the transmission of West Nile Virus that has the area’s mosquito districts concerned. The mosquito breed responsible for transmitting the Zika virus has been detected in Stanislaus and Merced counties. Aedes aegypti mosquitoes, which are invasive for the region, are capable of transmitting viruses such as chikungunya, dengue, yellow fever, and Zika. While the Aedes aegypti mosquito has the potential to transmit deadly viruses, none of these viruses are currently known to be transmitted locally in California.

In contrast to the native amber-colored Culex mosquitoes, whose peak biting times are dawn and dusk, Aedes aegypti mosquitoes are black and white, bite aggressively during the day, and feed almost exclusively on humans. Additionally, the larvae of Aedes aegypti mosquitoes require much less water. Females lay their eggs just above the water line in small containers and vessels that hold water, such as dishes, potted plants, bird baths, ornamental fountains, tin cans, or discarded tires. The eggs can survive for up to eight months after the water dries out.

“It’s an invasive species that has made it’s way to the area,” Jones said. “It’s a species that doesn’t take much water to complete its life cycle. And they have a preference for feeding off people.”

Jones said the diseases these mosquitoes are known to transmit are endemic in other countries and the concern is that an individual could go to one of theses places, get infected, come back here and then be bitten by a mosquito.

“Then we would have local transmission and that is what we want to avoid,” Jones said.

This variety of mosquito was found in Turlock recently, with more than 300 found in one Turlock backyard. The primary source was a Turlock Irrigation District utility vault that contained standing water from law sprinklers. Several treatments were done in the neighborhood and educational material was provided to the residents, Heft said.  

“These mosquitoes require a much different approach to controlling due to their biology and the fact that they live in such close association with people,” Heft said. “They can live entirely indoors. These mosquitoes require a much greater investment from the public in controlling and making sure that no standing water, no matter how small, exists on their property.”

The districts anticipate more WNV and mosquitoes in the coming months and would like to remind residents they can help by taking the following precautions:

— Dump or drain standing water. These are places mosquitoes like to lay their eggs.

 — Defend yourself against mosquitoes by using repellants containing DEET, picaridin or oil

of lemon eucalyptus.

— Avoid being outdoors at dusk and dawn. These are the times when WNV carrying

mosquitoes are generally most active.

— Report neglected swimming pools to your local mosquito abatement district.

— Use tight fitting door and window screens to keep mosquitoes from entering your


Contact your veterinarian for information on vaccinating equine against WNV.

The districts will continue with their surveillance programs identifying mosquito breeding sources and mosquito borne disease activity. They will treat according to their surveillance results.

Reporting and testing of dead birds are important steps 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).

Contact the Turlock Mosquito Abatement District at (209) 634‐1234 ( or the Merced County Mosquito Abatement District at (209) 722-1527