- Mosquitoes can carry viruses or parasites that cause a number of diseases in people.
- When a mosquito carrying a virus or parasite bites a person, it can pass on the microbe.
- The most common mosquito-borne diseases in the U.S. are West Nile, malaria, and eastern equine encephalitis.
More than just a summertime annoyance, mosquitoes can also spread disease. When mosquitoes bite and drink blood from a person or animal, they can pick up a virus or parasite. If they bite another person or animal, they can pass on that microbe.
The recent rise in mosquito-borne illnesses across the U.S. has many people concerned about the potential risks to their health and how they can best protect themselves.
“Minimizing excessive mosquito exposure is probably the only thing people can do to protect themselves,” said Marc Siegel, MD, a professor of medicine in the Division of Infectious Diseases at the George Washington School of Medicine and Health Sciences.
“So avoid the times that mosquitoes tend to feed more, which is dusk to dawn,” he told Healthline.
If you do have to be outside, wear pants and a long-sleeved shirt or jacket, and wear mosquito repellent, he said—especially in wooded or swampy areas.
However, Siegel points out that not every mosquito that bites a person will be carrying a virus or parasite.
Over 200 types of mosquitoes live in the continental United States and U.S. territories, according to the CDC. Yet, only about 12 can spread a virus or parasite that makes people sick.
“Most mosquito bites are not going to result in West Nile virus because the vast majority of mosquitoes are not infected with West Nile virus,” he said.
Climate change, though, has led to the spread of mosquito-borne illnesses to new parts of the world, which will increase people’s exposure to these diseases.
Below are the four most common types of mosquito-borne illnesses, the risks they can pose, and how health experts recommend you can best protect yourself.
West Nile virus
Birds are the natural hosts of West Nile virus. Mosquitoes that become infected by biting and drawing blood from a bird can pass on the virus when they bite a person.
Only about one in five people infected with West Nile virus will have symptoms, which can include fever, headache, body aches, and vomiting. In addition, about one in 150 infected people will develop a serious, sometimes deadly, illness.
West Nile is the most common mosquito-borne disease in the continental United States, according to the CDC. There have been 289 West Nile cases reported to the CDC in the country so far this year. In 2023, there were over 2,566 cases in the country.
This year, cases have occurred in 33 states in the Northeast, South, Midwest, and Southwest.
“There are probably a higher number of total infections, many of which are not diagnosed,” said David Sullivan, MD, a professor in the Department of Molecular Microbiology and Immunology at the Johns Hopkins Bloomberg School of Public Health.
However, “this is still less than the chance of being hit by lightning,” he told Healthline.
There are no specific medications to treat West Nile virus disease or vaccines that can prevent it.
People of any age can develop severe illness if they are infected with West Nile virus. But those 60 years of age or older are at greater risk for more serious disease.
Dr. Anthony Fauci, once the top US infectious disease expert, recently became infected with West Nile virus. After being hospitalized for a week, Fauci, 83, is recovering at home, reports STAT.
Other people with a greater risk for severe illness include those with cancer, diabetes, high blood pressure, an immunocompromising condition, or other medical conditions.
In an effort to reduce the risk of West Nile virus to the public, the New York City Health Department has been spraying pesticides in parts of the city to control the mosquito population.
Eastern equine encephalitis
Eastern equine encephalitis, or EEE, is a rare but potentially deadly disease caused by a virus passed onto people through the bite of an infected mosquito. Most people infected with the virus don’t develop symptoms.
Those who do become infected may develop symptoms such as fever, chills, body aches, and joint pain. Some people may develop more serious inflammation of the membranes surrounding the brain and spinal cord. About one-third of people who develop severe eastern equine encephalitis die from their illness.
“In worse cases, the virus infects the tissue of the brain, and that’s when we call it encephalitis,” said Siegel. “That is more serious and is much more likely to lead to death or long-term neurological deficits in patients who survive.”
Sullivan said both very young children and older adults are at higher risk of severe illness from eastern equine encephalitis. In contrast, West Nile virus tends to affect older adults more than young children, he said.
Most cases of eastern equine encephalitis occur in eastern or Gulf Coast states. So far this year, four cases have been reported to the CDC, with 38 reported in 2019.
The virus passes between mosquitoes and birds found in freshwater hardwood swamps. Another mosquito species is needed to pass the virus from infected birds to people or horses.
There is no vaccine treatment for eastern equine encephalitis. People with severe symptoms will be given medications and other therapies to manage their symptoms.
In Oxford, Massachusetts, about 50 miles west of Boston, a man in his 80s was exposed to the virus, health officials announced on Aug. 16.—the first reported human case of eastern equine encephalitis this year in the state.
To minimize further risks to the public, Oxford health officials recommended an outdoor curfew from dusk to dawn, the peak biting time for many mosquitoes.
Similarly, Plymouth, about 40 miles south of Boston, said all public parks and fields will be closed from dusk to dawn after a recent eastern equine encephalitis infection diagnosed in a horse.
Malaria
Around 247 million malaria cases occurred worldwide in 2021, with around 619,000 malaria deaths. This disease is caused by a parasite that infects a certain kind of mosquito. Malaria cases mainly occur in tropical parts of the world, especially Africa.
Malaria was once common in parts of the United States but was eliminated from the country in the mid-1950s. However, around 2,000 malaria cases are imported into the United States each year by people traveling to other parts of the world.
In 2023, several locally acquired malaria cases occurred in Florida and Texas for the first time since 2003.
Symptoms of malaria, which can include flu-like illness and headache, range from mild illness to severe disease.
Prescription drugs are available to treat and cure malaria. A malaria vaccine is also available in parts of the world where malaria regularly occurs, such as in parts of Africa.
Dengue
Each year, up to 400 million people worldwide are infected with a dengue virus, according to the CDC. Not everyone gets symptoms, which can include aches, eye pain, muscle or joint pain, nausea, vomiting, and rash.
Dengue is most commonly found in tropical and subtropical areas of the world. So far this year, over 4,000 Dengue cases have been reported to the CDC, with cases occurring in most states.
However, most cases reported in the continental United States occurred in travelers infected in other areas. Limited local spread of dengue has been reported in Arizona, California, Florida, Hawaii, and Texas.
There is no specific treatment for dengue, but people may be given medication to relieve pain. In the United States, a dengue vaccine is available for use in children ages 9 through 16 with laboratory confirmation of a previous dengue infection who live in an area where dengue is common.
Takeaway
Over 200 types of mosquitoes live in the continental United States and US territories. Only about 12 can spread a virus or parasite that causes illness in people. When mosquitoes bite and draw blood from an animal or person, they may pick up a disease-causing microbe.
The most common mosquito-borne diseases in the United States are West Nile, malaria, dengue, and eastern equine encephalitis. Some cases occur in returning travelers who were infected in other parts of the world.
West Nile is the most common mosquito-borne disease in the United States, with thousands of reported cases each year. Around 80% of people infected with West Nile virus will have no symptoms. However, some people with symptoms may develop a serious, sometimes deadly, illness.
