Mosquito Diseases You Need to Know About
Many cause only mild symptoms, but some can lead to serious illness. And increasingly, they're circulating right here in the U.S.
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Few of us will forget the Zika virus epidemic of 2016, which resulted in thousands of infants being born with birth defects after their mothers were infected during pregnancy.
During that time, pregnant women were warned against traveling to areas that had active Zika outbreaks, both in the U.S. and abroad.
That risk has subsided in the U.S., but there are still tens of thousands of Zika infections a year, primarily in South America. And there are other serious diseases that can be transmitted by a mosquito’s bite, such as the rare but deadly Eastern equine encephalitis (EEE), which has been linked to two deaths in the U.S. this year (in New Hampshire and New York).
Health experts regularly monitor several mosquito-borne viruses here, though they make headline news only sometimes. “Cases of mosquito-borne disease are on the rise in the United States,” says Kate Fowlie, a spokesperson for the Centers for Disease Control and Prevention (CDC).
- Mosquito-Borne Diseases: Eastern Equine Encephalitis La Crosse Virus West Nile Virus Others
- How to Keep Mosquitoes Away
Experts say that climate plays a role. Mosquitoes are sensitive to temperature changes, and warmer weather tends to lead to more outbreaks of mosquito-borne illness. International travel also provides opportunities for new diseases to be introduced in the U.S.
Eastern Equine and St. Louis Encephalitis
Eastern equine encephalitis, a rare but potentially deadly mosquito-borne disease, has been in the news recently. There have been at least two deaths linked to the viral illness in the U.S. this year, one in New York and the other in New Hampshire.
There are typically only a handful of neuroinvasive cases each year. So far in 2024 there have been 13 reported cases in the Northeast and Wisconsin. There were seven reported cases in 2023 and just one in 2022. (During the past 20 years, 2019 had the most reported cases, with 38.) Most cases occur in states on the East Coast, Gulf Coast, and Great Lakes region.
Though some people have only flu-like symptoms, others develop full-blown encephalitis. About one-third of all people who develop eastern equine encephalitis die from it. Others are often left with disabilities, including intellectual impairment, paralysis, or seizure disorders.
Another uncommon but potentially severe form of encephalitis—St. Louis encephalitis—can cause manageable symptoms such as fever, headache, dizziness, and nausea. But about 90 percent of older adults who get it develop meningitis as a result, and 5 to 15 percent of those who do die. Twenty neuroinvasive cases were reported last year, 33 in 2022, 17 in 2021, and 16 in 2020, most of them in Arizona, Arkansas, California, Mississippi, and Texas.
La Crosse Virus
La Crosse, one of what’s known as the California serogroup viruses, doesn’t cause symptoms in most people but can cause fever and flu-like symptoms in some. Its severe form, which can cause seizures, encephalitis, coma, and paralysis, tends to strike children younger than 16. Less than 1 percent of cases are fatal.
About 30 to 90 neuroinvasive cases are reported in the U.S. each year, according to the CDC. So far in 2024, 22 cases have been reported. Last year there were 32 cases, 22 in 2022, 40 in 2021, and 88 in 2020. Due to asymptomatic and less severe cases, this virus is substantially underdiagnosed and underreported, the CDC says.
It is seen mostly in upper Midwest, mid-Atlantic, and southeastern states.
West Nile Virus
West Nile virus can cause a few days or weeks of fever and other flu-like symptoms, vomiting, diarrhea, and rash, as well as fatigue and weakness that can last for weeks or even months during recovery. According to the CDC, about 20 percent of people who are infected develop a fever and other symptoms, and about 1 in 150 people infected with the virus develop a severe form of illness that can cause encephalitis or meningitis. About 10 percent of those with the severe form of West Nile die.
West Nile is by far the most common mosquito-borne disease in the continental U.S. In 2023, there were about 2,566 cases. There were fewer in 2022, 1,132 cases, according to a study published in May in the CDC’s Morbidity and Mortality Weekly Report (MMWR). Of those cases, 827 were considered neuroinvasive, with 772 hospitalizations and 91 deaths. The most neuroinvasive cases in that year were in California, Colorado, and New York.
But these are only the cases we know about, because people may have West Nile without realizing it. With West Nile and several other mosquito-borne diseases in the U.S., most of the people infected have few or no symptoms, says Robert Smith, MD, director of the division of infectious diseases at Maine Medical Center and principal investigator at the Vector-borne Disease Lab at Maine Medical Center Research Institute.
So the true numbers may be much higher. The MMWR study points out that other research suggests that for every single neuroinvasive case, there may be 30 to 70 more people with mild West Nile.
Other Mosquito Diseases
If you’re traveling internationally, you can get the lowdown on mosquito-borne illnesses in your destination before leaving home. The CDC has information for travelers about Zika, as well as for chikungunya, dengue, and malaria.
None are currently considered significant concerns in the continental U.S.
About 2,000 cases of malaria are usually reported in the U.S. every year, but a vast majority of those cases are contracted internationally. In 2023 there were a handful of locally acquired malaria cases in Florida, Maryland, and Texas. Still, the CDC’s health alert about those cases emphasized that “the risk to the U.S. public for locally acquired mosquito-transmitted malaria remains very low.”
Dengue is a significant problem in U.S. territories including American Samoa, Puerto Rico, and the U.S. Virgin Islands. Puerto Rico has declared a public health emergency due to an ongoing dengue outbreak, with 3,443 cases so far this year. Though less common, there have been locally acquired dengue cases in the continental U.S. as well as in Florida, Texas, Hawaii, Arizona, and California.
Chikungunya has occasionally caused a few domestically acquired illnesses inside the 50 states since 2014, when it caused a major outbreak in Puerto Rico. But there have been no locally acquired cases reported in U.S. states or territories since 2019.
Keep Mosquitoes and Their Diseases at Bay
If you suspect you’ve gotten sick as a result of a mosquito bite, let your doctor know. To help prevent these illnesses in the first place, take these steps:
Use insect repellent. In our tests of insect repellents, we’ve found that products that contain 25 to 30 percent deet provide the most consistently high levels of protection against mosquitoes. Apply repellent whenever you might be exposed to mosquitoes. Here are a few of our top-rated products:
Block their access. Wear long sleeves and pants when you know you’ll be exposed to mosquitoes. And check that the screens in your windows and doors are intact to keep the pests out of your home.
Clear standing water. Mosquitoes need standing water to breed. Regularly eliminate or drain and clean sources of standing water in your yard, such as in birdbaths, planters, tires, buckets, and more. The CDC recommends doing this weekly. (You can also use a mosquito trap in your yard—just make sure it’s one that actually works.)
Pay attention to local outbreaks. Heed any warnings of heightened risk by local health officials. In the 2019 outbreak of eastern equine encephalitis, for example, finding the virus in chickens prompted a warning to Florida residents to step up their anti-mosquito protection regimen.
Take care while traveling. Before you travel internationally, check with your doctor about whether you should have any vaccines beforehand, such as the shot for mosquito-borne yellow fever. The CDC offers a tool that lists any vaccines you might need for traveling to certain countries. Once you arrive, use insect repellent if diseases such as dengue or malaria are possible. Opt for lodging where windows and doors are screened, or sleep under a mosquito bed net.
