Mosquito-Borne Dengue Fever Cases On The Rise: What To Know In CA

CALIFORNIA — More than 880 people nationwide, including 23 in California, have been sickened with mosquito-borne dengue fever this year, according to Centers for Disease Control and Prevention data.

The U.S. cases are among 3 million in the Americas. That's more than at any time in the Americas since 1980, according to a study in the journal Nature that said cases have climbed in the past few years as a result of hotter, more humid conditions that allow mosquitoes to thrive.

In a news briefing earlier this month, World Health Organization officials called dengue fever a “canary in the coal mine of the climate crisis.”

In California, that coal mine canary is the Aedes aegypti mosquito, better known as the ankle biter, whose population exploded in the aftermath of Tropical Storm Hilary.

RELATED: Get Ready For The Invasion Of The Ankle Biters

This year, 44 states and jurisdictions have reported cases of dengue fever. Florida has the most, with 226 cases. New York has the second-highest case rate, with 54. Notably, Texas has experienced cases of local transmission this year as did Maricopa County in Arizona last year. That means the illness spread locally when a mosquito bit an infected person, incubated the virus and bit someone else, infecting them.

The local transmission of dengue in dry climates is a major concern for health and vector control officials in California because it means the virus could spread here, too.

Relatively new to California, the Aedes' ability to spread diseases such as Dengue Fever and Zika locally remains unknown. So far, cases found in California have been linked to travelers abroad, said Los Angeles County's Kluh.

"We’ve mostly had imported Dengue cases reported by the health department. We have not observed any local transmission of any of theses diseases, and we are not sure why not," she said.

Kluh said vector control and health officials take these diseases very seriously with the invasive new mosquitoes.

"We are afraid of it," she said. "We definitely think it may be in our future because there is s0 much travel to outbreak areas."

In California, dengue cases from travel have been reported in Riverside, Los Angeles, Santa Barbara, Orange, Santa Clara, Alameda, Contra Costa, Marin, Sacramento, Sonoma, Fresno, Stanislaus, and San Joaquin counties this year.

RELATED: CA Endures Worst Mosquito Season In Years. When Will It End?

When an imported case is detected, vector control agencies around the state take aggressive measures to make sure mosquitoes don't bite the infected person and spread the disease to others, including spraying the sickened residents's yard and nearby areas. Travelers are urged to monitor themselves for signs of illness, wear mosquito-repellant and stay indoors while symptomatic.

Residents in most of California can call their local vector control agency to come to their yard if they suspect a mosquito infestation. The agencies can help identify the source, treat the problem or educate the homeowner about prevention methods.

Up to 400 million people worldwide are infected every year by a dengue virus, according to the CDC. About 100 million people get sick, and 40,000 die from severe dengue.

Mild symptoms of an infection can be easily confused with other illnesses, according to the CDC. The most common symptoms are a fever; nausea or vomiting; a rash; or aches and pains, including eye pain (typically behind the eyes), muscle, joint or bone pain.

Symptoms typically last from two to seven days, but illnesses can become severe within 24 to 48 hours after a fever has subsided, according to the CDC, which said “severe dengue is a medical emergency” that warrants a trip to a clinic or emergency room.

Symptoms of severe dengue include belly pain and tenderness; vomiting (at least three times in 24 hours), bleeding from the nose or gums; vomiting blood or blood in the stool; and feeling tired, restless or irritable.

People who have had dengue fever in the past are more likely to develop severe dengue. Infants and pregnant women are also at higher risk for developing severe dengue, according to the CDC.

There is no specific treatment for dengue. The best way to prevent dengue is to avoid mosquito bites, according to the CDC. Dengue vaccines are available and recommended for children ages 9-16 who have had a laboratory-confirmed case of dengue, if they live in areas where the virus is endemic. That includes several U.S. territories and freely associated states, but not the United States itself.

U.S. dengue cases have increased from 814 in 2021 to 2,261 in 2022, more than 50 percent of which were acquired in the United States, according to historical data from the CDC. Experts expect the rise to continue with urbanization and climate change.

Another study published in Nature called dengue fever “the greatest human disease burden of any arbovirus.” About half the world’s population lives in an area that is environmentally suitable for dengue transmission.

“In combination with these global trends, rising temperatures attributed to climate change have increased concerns that dengue will intensify in already endemic areas through faster viral amplification, increased vector survival, reproduction and biting rate, ultimately leading to longer transmission seasons and a greater number of human infections, more of which are expected to be severe,” the authors wrote. “Increasing temperatures may further exacerbate this situation by enabling greater spread and transmission in low-risk or currently dengue-free parts of Asia, Europe, North America and Australia.”

Some counties in Florida and Texas are experimenting with genetically modified mosquitoes, a strategy the CDC said has been successfully used in parts of Brazil, the Cayman Islands, Panama and India to control Aedes aegypti mosquitoes, which also spread Zika and chikungunya viruses.

Strategies in other regions with high dengue case rates include eliminating habitat where mosquitoes can lay their eggs, killing the eggs with larvicides and killing adult mosquitoes with insecticides.

Concerned about what the future holds, California officials are studying new approaches to control the spread of mosquito-borne diseases, said Kluh, of the Greater Los Angeles County Vector Control District.

One approach would be to release sterilized males much like a similar, successful approach to managing the fruit fly population. Male mosquitoes don’t bite. It’s the females that bite to provide blood meals for their eggs.

“In Los Angeles and Orange counties, we have partnered up to assess whether we can irradiate male mosquitoes and release those to mate with wild females, resulting in sterile eggs,” she said.

That and similar approaches for reducing the mosquito population are a ways off from becoming a reality, said Kluh. However, Kluh said researchers are optimistic about the approach if it can be scaled to have widespread impact.

For now, the best way for people to save themselves from pesky mosquito bites is to deprive them of their breeding grounds: stagnant water.

The Aedes don’t tend to travel far, so if people keep their yards clear of stagnant water and help their neighbors to do so, they can pretty much rid themselves of the ankle biters, according to the experts. Residents can also call their county vector control to come out and identify and treat the source for free.