Health & Medicine

As the weather heats up, it’s time to fend off mosquitoes carrying West Nile virus

Significant rainfall last winter ended drought conditions in all of California for the first time since 2011, but mosquito experts fear those same downpours have left a breeding ground for the blood-sucking flies that spread West Nile virus.

Public health officials in California reported 11 deaths from the disease and 243 infections in 2018. Yolo County had the third-highest rate of infection last year at 4.97 cases per 100,000 residents, coming in behind Glenn County at 6.95 per 100,000 and Butte County at 5.27 per 100,000.

The West Nile virus causes brain inflammation, said Dr. Stu Cohen, chief of infectious diseases at Sacramento’s UC Davis Medical Center, and that inflammation damages the brain and central nervous system. Despite a number of attempts to find a cure, he said, none has been found.

Some people may be infected with West Nile and never show symptoms, said Dr. Arthur Jey, an emergency medicine doctor at Sutter Medical Center, Sacramento, but for those who do, medical providers focus on helping people survive them. Those most vulnerable to the virus tend to be the very old or the very young, he said.

To avoid getting the disease, local residents should look for stagnant water where mosquitoes can breed, said Luz Robles, the public information officer at Sacramento-Yolo Mosquito & Vector Control District.

“It’s imperative people do their part to make sure they’re not breeding mosquitoes on their property,” Robles said. “Inspect ... for any stagnant water such as flower pots, wheelbarrows, tires, dog dishes, bird baths, anything that can hold even a small amount of stagnant water can and will become a breeding source.”

If residents cannot remove the water themselves, they can call the vector control to schedule a free service call. An inspector usually can be dispatched within a day or two, Robles said.

The district offers a number of free services for property owners to control mosquito growth: Workers will identify potential areas where mosquitoes can breed. They can put mosquito fish into rice fields, horse troughs, neglected swimming pools, ponds or other places of standing water. They can also spray bushes or other areas known to be mosquito breeding grounds.

If a property has an outdoor water leak, many residents may qualify for a free house call and repairs through Sacramento’s Department of Utilities. To check whether a property is within the area being served, call 916-808-5605, email or visit

A harbinger of West Nile virus are birds that have fallen ill or died from the disease, Robles said, so Sacramento-area residents should call 877-968-2473 if they spot one. They can also file a report online at or

“Let’s say somebody from Natomas or the Arden area calls in with a dead bird; we would pick up that bird,” Robles said. “If that bird tests positive for West Nile virus, then what we do is we send out traps that collect mosquitoes around a 1-mile radius of where that bird was found. Once that trap is full, we bring back those mosquitoes to the lab and test those as well. If they test positive for the virus, we know there is obviously something happening within that area.”

The vector control inspectors will continue to check that area to find the source of the problem. Typically, most mosquito infestations can be treated by removing water, using ground sprays or adding mosquito fish, Robles said, but on rare occasions, aerial spraying may be necessary in urban areas.

“Usually, aerial spraying starts with localized efforts, in parks, greenbelts, open areas where mosquito activity is detected,” Robles said. “However, once there’s a widespread area such as what happened last year where we were getting a lot of West Nile virus activity in the Pocket as well as Elk Grove, then sometimes we have to make the decision to interrupt the West Nile virus activity cycle and do large-scale aerial spraying over urban areas.”

That’s usually conducted at dusk when mosquitoes are most active. People may not be seeing many mosquitoes now, even if they have standing water in their yard, Robles said, but the summer heat will change that.

“In warm temperatures, mosquitoes complete their life cycle faster,” she said, “so for instance, in the peak of summer, when mosquitoes lay an egg, it can go from an egg to an adult mosquito in as little as four to seven days because heat accelerates a mosquito’s life cycle.”

Abatement efforts are critical to keeping mosquitoes in check, Jey and Cohen said, but there are other preventive measures that Californians can take: Wear clothing that covers legs and arms, especially at dusk or when near stagnant water. Fix screens on windows or doors to keep pests out of the home. Use a natural repellant such as oil of lemon eucalyptus or a product approved by the U.S. Environmental Protection Agency.

The Sacramento-Yolo vector control team also distributes free repellant samples to community-based organizations serving the homeless, Boy Scouts, Girl Scouts and other groups that cannot escape mosquito exposure.

At its worst, West Nile virus can cause overwhelming fatigue, terrible muscle aches and headaches, Jey said, and if that happens, seek medical help. To diagnose whether a person is infected, physicians will do a lumbar puncture and take spinal fluid for analysis, he said.

People who have recovered from a severe West Nile infection have reported ongoing problems with fatigue and neurological damage, Cohen said, and as with stroke, it’s best to start rehabilitation as early as possible. Cohen said that he has treated 10 to 15 people with West Nile virus and he doesn’t think people are exposed to more virulent strains. Rather, he said, it’s about the medical condition of the person who’s infected.

“The people who end up having more neurologic disease are older, they’re immune-compromised, they’re diabetics,” he said. “It’s not so much that the virus is different. I think it’s that the person who gets infected can’t really respond as well. Look at who ends up dying from West Nile infections. They tend to be people at least 65 or older. Most are in their 70s or 80s. I’m not sure that it’s a more virulent form of the virus. I think it’s that the mix of host and pathogen is a bad match.”

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