The 5 Mosquito-Borne Diseases Most Common in the US
A mosquito bite is usually nothing more than an itchy nuisance.
Occasionally, though, that bite carries something more serious. Here are the five mosquito-borne diseases most commonly reported across the United States.
1. West Nile Virus
West Nile virus is, by a wide margin, the most frequently reported mosquito-borne disease in the continental United States.
According to the CDC, the virus typically spreads when a mosquito bites an infected bird and then bites a person.
Most people infected with West Nile virus never develop symptoms at all. The CDC estimates that reported cases represent only 1-2% of actual infections, since the vast majority go unnoticed.
For the smaller share of people who do develop symptoms, the illness can range from mild fever and headache to a more severe neuroinvasive form affecting the brain or spinal cord.
Federal surveillance data from a recent reporting year found that 69% of confirmed West Nile cases were classified as neuroinvasive, and that 95% of those neuroinvasive patients required hospitalization, with roughly 11% of that group dying from the illness.
Arizona has recorded unusually high case numbers in certain years, a pattern researchers have linked to factors including population growth, new housing development in mosquito habitat, and weather conditions that favor mosquito breeding.
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2. Eastern Equine Encephalitis (EEE)
Eastern equine encephalitis is far rarer than West Nile virus, but it's considerably more dangerous when it does occur.
The CDC and Texas Department of State Health Services both note that EEE carries an estimated mortality rate of roughly 33% among people who develop severe symptoms, making it one of the deadliest mosquito-borne illnesses circulating in the country.
Survivors who develop severe disease often experience significant, lasting brain damage, even when the infection itself doesn't prove fatal.
EEE remains rare in absolute terms — federal surveillance data typically counts only a handful of confirmed cases nationally each year — but its severity means public health officials treat even isolated cases as a serious concern, particularly in the northeastern and Gulf Coast states where the virus is most often detected.
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3. La Crosse Encephalitis
La Crosse encephalitis is a less commonly discussed but still notable mosquito-borne illness native to the United States.
The disease disproportionately affects children, who tend to develop more severe neurological symptoms than adults exposed to the same virus.
Symptoms typically begin with fever, headache, nausea, and fatigue, and can progress in more serious cases to seizures, confusion, or other signs of central nervous system involvement.
The National Park Service notes that, like most viral mosquito-borne diseases in the US, there is no specific antiviral treatment for La Crosse encephalitis — medical care focuses on supportive treatment of symptoms rather than directly targeting the virus itself.
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4. Dengue Fever
Dengue occupies a slightly different category than the other diseases on this list, since most US cases are travel-associated rather than acquired domestically.
Texas DSHS explains that most American dengue cases occur in travelers and immigrants returning from tropical or subtropical regions where the virus circulates more widely, including parts of the Caribbean, Mexico, Central America, and South America.
That said, local transmission within the continental United States has occurred during specific outbreak periods, particularly in southern states where the Aedes mosquito species responsible for spreading dengue is established.
There are four distinct dengue virus types, and a notable feature of the disease is that a second infection with a different dengue virus type can produce considerably more severe illness than the first infection did — including dengue hemorrhagic fever or dengue shock syndrome in serious cases.
Symptoms typically begin four to ten days after a bite from an infected mosquito, and the same Aedes mosquitoes responsible for dengue can also transmit Zika and chikungunya viruses.
5. Chikungunya
Chikungunya rounds out the most commonly reported mosquito-borne illnesses circulating in or affecting travelers to the United States.
The virus is primarily spread through mosquito bites, though it can occasionally spread through blood exposure or, rarely, from a pregnant mother to her baby.
Symptoms commonly include fever alongside joint pain that can be severe enough to limit movement, a distinguishing feature compared to several of the other illnesses on this list.
Like dengue, most US chikungunya cases are associated with travel to regions where the virus circulates more actively, rather than mosquito bites acquired domestically.
How These Diseases Compare in Practice
Taken together, these five illnesses illustrate a consistent pattern: the most common mosquito-borne disease in the US (West Nile) isn't the deadliest one (EEE), and the diseases most associated with severe outcomes (EEE, severe dengue) are considerably rarer than the ones responsible for the bulk of annual case counts.
Globally, mosquito-borne illness remains a far larger problem than these US-specific numbers suggest — international health agencies estimate mosquitoes contribute to between 725,000 and 1 million deaths worldwide each year, the overwhelming majority from malaria, which is no longer locally transmitted within the United States but still affects more than 2,000 returning travelers diagnosed domestically each year.
For most domestic mosquito-borne diseases, no specific antiviral treatment exists, which makes prevention — reducing standing water around homes, using EPA-registered repellents, and wearing long sleeves during peak mosquito activity hours — the most consistently effective tool available to reduce individual risk.
