Mosquito Diseases You Need to Know About

Many bring only mild symptoms, but some can cause serious illness

When you shop through retailer links on our site, we may earn affiliate commissions. 100% of the fees we collect are used to support our nonprofit mission. Learn more.

mosquitoes GettyImages-161144917

Few of us will forget the Zika virus epidemic of 2016, which resulted in hundreds 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 since subsided in the U.S., but it remains a concern in some other areas, along with several other diseases that can be transmitted by a mosquito's bite.

And health experts are paying attention to several other mosquito-borne viruses here. “Cases of mosquito-borne disease are on the rise in the United States,” notes Kate Fowlie, a spokesperson for the Centers for Disease Control and Prevention (CDC).

A 2018 CDC study noted that new diseases transmitted by both ticks and mosquitoes are a growing public health threat in the U.S.

Experts say climate issues play 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.

more on mosquitoes and ticks

Mosquito-borne illness often causes only mild symptoms, such as fever, headache, body aches, joint pain, rash, vomiting, and nausea—all of which can be look like symptoms of other ailments.

But occasionally, they can progress to more serious illnesses, causing severe symptoms such as paralysis, inflammation of the brain (encephalitis), inflammation of the membranes that surround the brain and spinal cord (meningitis), and other neurological symptoms.

Scientists refer to cases of mosquito-borne disease with these types of symptoms as “neuroinvasive.” They can be fatal.

And when a mosquito-borne disease does become severe, there’s typically no effective treatment apart from supportive healthcare, such as medication for pain. (Malaria, for which several treatments are available, is a key exception.) That makes prevention critical.

Still, the risk of catching a mosquito-borne illness in the U.S. is low. For many diseases, only a few cases are reported across the entire country each year. And several smart steps can help you prevent mosquito bites in the first place.

Here, we’ll explain what you need to know about mosquito diseases in the U.S. and internationally—plus how to keep yourself from contracting one.

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 1 in 150 people infected with the virus develop encephalitis or meningitis. And about 10 percent of those people die.

West Nile is by far the most common mosquito-borne disease in the continental U.S., accounting for some 2,647 illnesses last year, according to a study published this month in the CDC’s Morbidity and Mortality Weekly Report (MMWR). And 1,658 of them were considered neuroinvasive. California, Illinois, Nebraska, Texas, and Pennsylvania had the highest numbers of cases of West Nile during 2018.

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 those people infected have little or no symptoms, says Robert Smith, M.D., 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 anywhere from 30 to 70 more people with mild West Nile.

La Crosse Virus

La Crosse, a member of what’s known as the California serogroup viruses, can cause fever and flu-like symptoms. Its severe form, which can bring seizures, encephalitis, coma, and paralysis, tends to strike children younger than 16. Fewer than 1 percent of cases are fatal.

Although La Crosse was the second most common mosquito-borne disease reported in the U.S. in 2018, health officials received notice of just 86 cases. But 83 of those were classified as neuroinvasive, and the CDC says that less severe illness due to this virus is probably substantially underreported. La Crosse virus is seen mostly in the upper midwestern, mid-Atlantic, and southeastern states.

Eastern Equine and St. Louis Encephalitis

Eastern equine encephalitis, a rare but potentially deadly mosquito-borne disease, has been in the news recently: Florida health officials warned that the virus was discovered in chickens there in July, and Michigan’s Department of Health and Human Services found two cases in horses this month.

Only a handful of cases occur each year. Just six neuroinvasive cases were reported in 2018, and five in 2017, mostly in states on the East Coast, Gulf Coast, and Great Lakes.

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. Only five neuroinvasive cases were reported last year, six in 2017, and seven in 2016—most of them in California, Arkansas, and Texas.

Mosquito Diseases Outside the U.S.

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 U.S. Even Puerto Rico, which was hit hard by Zika in 2016, has reported only 23 cases transmitted by local mosquitoes so far in 2019 (compared with more than 35,000 cases in 2016).

More than 1,000 cases each of dengue and malaria are usually reported in the U.S. every year, but the vast majority are contracted internationally.

Chikungunya has occasionally caused a few domestically acquired illnesses inside the 50 states since 2014, when it caused a major outbreak in Puerto Rico. However, just eight locally acquired illnesses were reported in the territory in 2018, compared with 4,242 illnesses in 2014.

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, 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 in order 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.

Pay attention to local outbreaks. Heed any warnings of heightened risk by local health officials. In the recent case of eastern equine encephalitis, for example, finding the virus in chickens prompted the 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 the 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 widows and doors are screened, or sleep under a mosquito bed net.

Catherine Roberts

As a science journalist, my goal is to empower consumers to make informed decisions about health products, practices, and treatments. I aim to investigate what works, what doesn't, and what may be causing actual harm when it comes to people's health. As a civilian, my passions include science fiction, running, Queens, and my cat. Follow me on Twitter: @catharob