Officials Warn Mosquito-Borne Dengue Fever Cases on the Rise in California

FILE - In this Aug. 26, 2019 file photo, a biologist examines a mosquito in Salt Lake City. The Canadian Press/AP-Rick Bowmer

Close to 1,000 people nationwide, including 23 in California, have been infected with a mosquito-borne fever this year, according to the World Health Organization.

Since the start of the year there have been a “significant magnitude” of dengue fever outbreaks in the Americas, with cases reaching just over 3 million, compared to last year’s numbers of 2.8 million, according to the organization.

Dengue fever is a mosquito-borne illness that mostly occurs in tropical and subtropical areas of the world and causes a high fever and flu-like symptoms. Its severe form, also called dengue hemorrhagic fever, can cause serious bleeding, with a sudden drop in blood pressure and death.

The fever is spread from a breed of mosquito, aedes aegypti, known as “ankle biters.” These mosquitoes also spread zika, chikungunya, and other viruses.

An Aedes Aegypti mosquito is photographed on human skin in a lab of the International Training and Medical Research Training Center (CIDEIM) in Cali, Colombia, on Jan. 25, 2016. Luis Robayo/AFP/Getty Images

The organization also reported that, of the 3 million dengue cases in the Americas reported by July this year, 45 percent were laboratory confirmed, and less than one percent were classified as severe.

The report also states that the highest number of dengue cases to date this year are in Brazil, Peru, and Bolivia, with 1,302 deaths from dengue.

According to a July study in the journal Nature, current infections of dengue are higher than any time in the Americas since 1980. The study also stated that 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 last month, the World Health Organization also described the latest spike in cases as a “canary in a coal mine”—meaning an early warning—relative to climate change and rising temperatures. Experts additionally say the mosquito’s population is likely to grow further in California in the wake of Hurricane Hilary in late August.

The aedes’ ability to spread diseases such as dengue fever and zika locally in the Golden State remains unknown. So far, cases found in California have been linked to travelers from abroad, according to vector control officials.

Travelers are urged to monitor themselves for signs of illness, wear mosquito-repellant, and stay indoors while symptomatic.

A patient suffering from dengue fever sits under a mosquito net inside a dengue and malaria ward at the Sindh Government Services Hospital in Karachi, Pakistan, on Sept. 21, 2022. Akhtar Soomro/Reuters

Symptoms of Dengue Fever

According to the World Health Organization, most infected with dengue will have mild or no symptoms. If they do occur, they usually appear between four and 10 days after a mosquito bite and last from two to seven days. Those infected typically improve in one to two weeks.

Experts say symptoms include headache, high fever, muscle and joint pain, nausea, vomiting, swollen glands and rash and that severe cases are rare, which require medical intervention and can worsen rapidly.

Those previously infected are said to be more likely to develop a severe infection, according to health officials.

Those with severe dengue may experience severe abdominal pain, rapid breathing, increased thirst, blood in vomit or stool, pale and cold skin, persistent vomiting, blood in the gums or nose, and weakness.

There is no current dengue vaccine for adults. However, a pediatric dengue vaccine has been approved for children between the ages of 9 and 16 who have had a laboratory-confirmed previous dengue virus infection and are living in an area where it occurs frequently. Such areas, the CDC says, include the U.S. territories of American Samoa, Puerto Rico and the U.S. Virgin Islands.

A municipal council worker dispenses insecticide using a fogging machine during a dengue-prevention spraying in Ampang, in the suburbs of Kuala Lumpur on Feb. 11, 2014. Mohd Rasfan/AFP/Getty Images

How to Prevent Dengue Fever

Experts say the best way to prevent infection is to prevent mosquito bites. Recommended steps include wearing protective clothing, applying insect repellent with ingredients like DEET, Picaridin or IR3535, using mosquito nets, and using window screens.

Regions like California have also considered using genetically engineered mosquitoes to limit the spread of dengue in which mosquitos are bred to prevent females, who are the only mosquitos that bite, from maturing to adulthood. Such practices have been approved by the Environmental Protection Agency for use in counties in Texas and Florida.

Researchers say such mosquitos have a gene that prevents female mosquitoes from living to adulthood, so they can’t continue to reproduce—which in turn reduces their population. Officials from the Environmental Protection Agency claim such mosquitos pose “no risk” to people, animals, or the environment, according to an agency study.

Regions like Orange County, New York have taken steps such as spraying insecticides in residential areas to prevent the spread and using drones to spray insecticides over wetlands and other areas prone to larvae and adult breeding grounds.