What you need to know about mosquito-borne diseases

Updated on: June 5, 2026

Quick overview

• West Nile virus is the most common mosquito-borne disease in the U.S.; most people never show symptoms
• Zika virus poses the greatest risk to pregnant women, due to links with birth defects
• There were over 4,000 locally acquired dengue fever cases in 2025, the second-highest figure since 2014
• Eastern equine encephalitis is rare but serious, with a mortality rate of around 30%
• La Crosse virus mainly affects children, though fewer than 1% of all infections are fatal

Mosquito-borne diseases are on the rise. As temperatures climb and mosquitoes spread into new territories, the diseases caused by mosquitoes are becoming harder to ignore. Not every mosquito carries a disease, but knowing the risks is your first line of defense. Mosquito control services from Western Exterminator can help protect you.

Jump to section:

• Zika virus
• West Nile virus
• Canine heartworm
• Malaria
• Dengue fever
• Eastern equine encephalitis
• Western equine encephalitis
• La Crosse virus
• St. Louis encephalitis
• How to reduce the chances of mosquito bites
• How we can help fight mosquitoes

How do mosquitoes transmit diseases?

Mosquitoes are one of the most effective disease vectors on the planet. When a female mosquito bites, she draws blood and injects saliva that can carry viruses or parasites picked up from a previous host. Once in your bloodstream, those pathogens can cause a range of mosquito-borne diseases, with symptoms ranging from mild flu-like symptoms to serious neurological illness.

Different mosquito species carry different diseases. Aedes aegypti, for example, is the primary carrier of Zika and dengue, while Culex species are more commonly linked to West Nile virus.

What diseases are carried by mosquitoes?

The diseases carried by mosquitoes range widely in severity, and the U.S. is home to several of them. Some, like West Nile virus, are relatively common and well-known. Others, like Eastern equine encephalitis, are rare but far more dangerous. Some of the diseases spread by mosquitoes, including malaria and dengue, are more typically associated with international travel, though domestic cases do occur.

Below, we break down the different types of mosquitoes and the diseases they spread, so you know what to watch for and when to be concerned.

Zika Virus

Zika was first discovered in African monkeys in the 1940s and later made its way to the U.S. through travel. It’s found in tropical and subtropical regions where mosquitoes feed all day and night. The virus is transmitted through the bite of an infected mosquito, most commonly from Aedes aegypti mosquitoes. Symptoms include fever, rash, and body aches, but most people who suffer from Zika don’t even realize they have it. This is alarming because the virus can also be transmitted through sex and blood transfusions. Medical professionals have not released a cure or a vaccine as of yet. 

Anyone can contract the Zika virus from Aedes mosquitoes, but it is particularly dangerous for pregnant women as they can pass it to their unborn children.

West Nile virus

Most people have already heard of West Nile virus (WNV), but did you know that the majority of humans who contract it are asymptomatic? Those who do develop West Nile fever have flu-like symptoms and weakness that can linger for weeks. Less than 1% of these patients develop invasive neurological trauma, such as meningitis, encephalitis, and paralysis, almost identical to polio, all of which can be fatal.¹

Healthy people who contract the disease usually recover in two weeks. Those with a compromised immune system—including elderly people and organ transplant patients—are at a much higher risk and can take much longer to recover, so it’s important to take all precautions when outdoors.

West Nile virus is transmitted to humans after a mosquito, acting as a vector, feeds on an infected bird. Over 300 species of birds are known carriers of the virus, most of which don’t appear sick. While birds aren’t known to directly spread the disease to humans, it’s important to avoid handling sick or dead birds.

Canine heartworm

Mosquitoes feed on warm-blooded animals, which in turn regulates their body temperature and aids egg production. Unfortunately, this puts our furry, four-legged friends at risk. 

Canine heartworm disease is a serious condition that can cause permanent damage to the heart, lungs, and arteries. Symptoms of canine heartworm don’t usually appear until the infection has spread beyond the heart. You may notice fatigue, lack of interest in food and exercise, weight loss, cough, and overall irritability. 

There is a cure for canine heartworm disease, but it’s expensive and complicated, so prevention is key. If your canine companion has not been treated for heartworms, schedule an appointment with your local veterinarian.

Malaria

Malaria is a parasite transmitted to humans through infected mosquitoes. When an adult mosquito pierces the skin with its specialized mouthpart, it can transmit this parasite directly into the bloodstream. Once inside the host, the malaria parasites invade the liver, where they multiply until maturity. They then move on to the red blood cells, where they battle with other cells and release daughter parasites. This is when victims begin to display the classic signs of malaria: flu-like symptoms, profuse sweating, organ failure, convulsions, and blood and neurological abnormalities. 

Once again, this mosquito-borne disease spreads only through blood and from mother to fetus.

Dengue fever

Dengue fever is spread from infected humans to mosquitoes, which then transmit the virus to healthy humans. The dengue virus is closely related to those that cause West Nile and yellow fever. Over 400 million people worldwide come down with dengue each year, but only a fraction of those cases come to the U.S. According to the CDC, there were 1,263 travel-related cases and 4,049 cases of locally-acquired dengue fever in 2025.

Symptoms of dengue infection include nausea, vomiting, rash, fever, and aches in the joints and eye sockets. In more severe cases, dengue can cause intense joint pain, earning it the nickname 'breakbone fever' in some regions. However, around 40-80% of infected people are asymptomatic.² Fortunately, this virus can’t be spread from one person to another.

Eastern equine encephalitis

Eastern equine encephalitis (EEE) is one of the rarest mosquito-borne diseases in the United States, but also one of the most serious. According to the CDC, an average of just 11 human cases are reported nationwide each year, primarily between May and October.

The virus circulates in a natural cycle between mosquitoes and wild birds, particularly in and around freshwater hardwood swamps along the Atlantic and Gulf coasts and the Great Lakes region. Because the primary mosquito carrier — Culiseta melanura — feeds mainly on birds, human infections are uncommon. People are considered "dead-end hosts," meaning EEE cannot spread from person to person.

Western equine encephalitis

Very closely related to EEE, western equine encephalitis (WEE) is also transmitted to mosquitoes through infected horses and birds in wetlands. This virus can also attack the body in two ways: neurologically or systemically. The mild form may present itself as the flu, but the neuroinvasive form can lead to confusion, seizures, coma, and death. Survivors are often left with brain damage like cognitive impairment and behavioral changes.

La Crosse virus

This virus is very similar to the rest in that symptoms usually appear flu-like, if they appear at all. The La Crosse virus does the most serious damage to children because their bodies are not able to fight it off. To add to the concern, there are no known treatments or cures for the disease. Fewer than 1% of all La Crosse infections are fatal, making it one of the less deadly mosquito-borne diseases, even if severe cases can be serious.³

St. Louis encephalitis

Infected birds are carriers of the St. Louis virus, which is then passed to humans through hungry mosquitoes. The viral form, if not asymptomatic, presents as fatigue, headache, nausea, vomiting, and diarrhea. Neuroinvasive encephalitis usually occurs in elderly people, and can result in permanent brain damage or death.

How to reduce the chances of mosquito bites

Mosquito-borne diseases are largely preventable. A few simple habits can go a long way toward protecting you and your family during mosquito season. Here's what you can do:

• Avoid peak hours. Mosquitoes are most active at dusk and dawn. Stay indoors or in a screened area during these times, though some species bite throughout the day, so there's no completely safe window in the warmer months.
• Cover up. Wear loose-fitting, long-sleeved clothing where possible. Clothes pre-treated with permethrin offer added protection.
• Use insect repellent. Apply an EPA-registered repellent containing DEET, picaridin, or oil of lemon eucalyptus to all exposed skin. Follow label instructions carefully, especially around children. As a bonus, repellents also help protect against ticks, fleas, and other biting insects.
• Eliminate standing water. Mosquitoes breed in still water, so empty or remove birdbaths, buckets, old tires, and unused pots regularly. This is one of the most effective ways to reduce mosquito populations around your home.
• Exercise indoors when possible. Mosquitoes are drawn to heat, sweat, and carbon dioxide, making joggers a prime target. If you're heading outside for sports or recreation, apply repellent first.
• Check your screens. Inspect window and door screens for damage and replace any that are torn. Keep doors closed to stop mosquitoes from getting inside.
• Protect your pets. Keep pets up to date on vaccines and heartworm prevention. Speak to your veterinarian before using any mosquito repellent products on animals.

How we can help fight mosquitoes

Controlling mosquitoes is not just a personal precaution; it’s a critical public health measure to prevent the spread of diseases. If you live in a mosquito-ridden territory or see a spike in mosquitoes around your home or commercial property, call the professionals at Western Exterminator for a free inspection at 888-444-6138. We provide both residential and commercial pest control services throughout the western United States.

1 Clinical Signs and Symptoms of West Nile Virus Disease, CDC

2 Dengue Fever, CDC

3 Khan UM, Gudlavalleti A. La Crosse Encephalitis. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-.