Native mosquitos pose a greater threat than invasive species

Mosquito abatement districts warn Californians the mild winter and extended drought are prime conditions for West Nile virus outbreaks

TULARE COUNTY – An invasive mosquito has been eating up ankles and headlines in the last few years but California’s mosquito abatement districts are warning residents to protect themselves from a native species that is far more of a threat when it comes to public health. 

On April 14, the Mosquito and Vector Control Association of California released a statement warning Californians that warm spring temperatures are prompting an early mosquito season and that drought conditions may lead to increased activity of Culex mosquitoes, which are native to the state and a carrier for West Nile virus. Mild winters and drought have been associated with West Nile virus disease outbreaks, according to the U.S. Environmental Protection Agency

West Nile virus is the most prevalent and serious mosquito-borne disease in California. There is no human vaccine for West Nile virus, a disease which can cause debilitating cases of meningitis, encephalitis, and even death. In 2021, there were 127 human West Nile virus disease cases from 28 counties in California, including 12 human deaths. Since 2003, more than 7,000 human disease cases have been reported, including more than 300 deaths.

In addition, as California residents brace for a third year of drought, mosquito experts are concerned that the lack of water is also conducive to the spread of invasive Aedes mosquitoes, whose eggs are resistant to drying out and can survive for many months. These mosquitoes are known as ninja mosquitos because they make many small, quick sips of blood instead of prolonged drinks, are smaller and harder to see and prefer to feed on humans instead of birds and livestock. Aedes are particularly hard to control and pose a potential public health threat because they have the potential to spread serious diseases like Zika, dengue, chikungunya, and yellow fever to people, and even animal heartworm to pets.

There have been no known human infections of these diseases in California since Aedes showed up in 2013 but there have been over 4,100 cases of West Nile virus which resulted in 202 deaths in that same time period. In Tulare County, there have been 117 cases of humans being infected with West Nile and one related death since 2013.

“Aedes aegypti is certainly a future threat we are working to minimize but Culex is a threat right now,” said Mir Bear-Johnson, assistant manager for the Delta Mosquito and Vector Control District, which covers northwestern Tulare County. “Aedes is more annoying but Culex is more dangerous.”

West Nile is a type of encephalitis, an infection in the brain. Though most individuals experience minimal to no effects, mild symptoms can include fever, headache, body aches, skin rash, and swollen lymph nodes, while severe symptoms include disorientation, coma, tremors, convulsions, paralysis and even death. Culex mosquitoes carry two other kinds of brain infections. 

The St. Louis Encephalitis Virus (SLEV) is in the same virus family as West Nile Virus and most people infected will have few to no symptoms. The most common symptoms are mild, flu-like symptoms, including fever and headache, from five to 15 days after being infected. Severe cases can affect the central nervous system, resulting in meningitis and/or encephalitis, and can result in death or long-term disability. Western equine encephalitis doesn’t show up for 4 to 10 days after transmission and common symptoms include fever, headache, vomiting, diarrhea, and drowsiness. More severe cases include seizures, can lead to coma, and one-third of those infected die. 

To raise awareness and educate Californians about the public health threat mosquitoes pose to our communities, Mosquito Awareness Week is observed April 17-23.

“Mosquitoes are more than a nuisance – they are a real public health threat,” said Senator Bill Dodd, who authored the state resolution recognizing Mosquito Awareness Week. “I encourage all California residents to take precautions to protect themselves and their families from mosquito-transmitted diseases. Dumping and draining standing water around your home and yard are easy ways to eliminate mosquitoes in your community.”

Bear-Johnson said Culex mosquitoes are sandy in color, primarily bite birds and prefer foul water, such as irrigation ditches, sewage plants, diary lagoons and storm basins. Delta Mosquito uses Encephalitis Virus Surveillance (EVS) traps in rural areas like orchards, crop fields or along river banks. They are used to catch female mosquitoes that are looking for a host to bite. EVS traps mostly catch native mosquito species that prefer to feed on birds. Female mosquitoes capable of transmitting West Nile Virus, St. Louis Encephalitis Virus, or Western Equine Encephalitis Virus are then tested for those viruses when at least 10 are caught in a single trap. Birds are an important part of the transmission cycle for these viruses and may die when they are infected. Dead birds can be tested for these viruses when they have been dead for less than 48 hours and have not died from obvious trauma.

However, the District needs the help of residents to identify green swimming pools, neglected hot tubs, and other sources through the rest of the season. You can anonymously report green pools, hot tubs, and other potential sources online using the service request form or by calling 559-732-8606 during regular business hours. The District does not charge for services.

In the meantime, mosquito experts offer the following tips to protect yourself this summer:

  • Apply insect repellent containing an EPA-registered active ingredient, including DEET, picaridin, oil of lemon eucalyptus, or IR3535, to clothes and exposed skin according to label instructions. Repellents keep mosquitoes from biting. It is important to follow EPA and CDC guidelines for the safe use of repellents on children.
  • Dress in long sleeves and pants, especially if outside at dawn and dusk when mosquitoes that can spread West Nile virus are most active.
  • Install screens on windows and doors and keep them in good repair to keep mosquitoes out of your home.
  • Eliminate all sources of standing water on your property, including in flowerpots, old tires, buckets, pet dishes, and trash cans. Mosquitoes lay their eggs in very small amounts of standing water.
  • Repair leaking faucets and broken sprinklers that can contribute to standing water around your home.
  • Clean rain gutters clogged with leaves.
  • Report neglected swimming pools and day-biting mosquitoes to your local mosquito and vector control agency at or

To learn more, check out the videos on the MVCAC website ( and for additional information on mosquitoes and mosquito-borne diseases, please visit the California Department of Public Health Mosquitoes and Mosquito-Borne Diseases webpage at

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