Diseases carried by mosquitos have been in the news recently. Dr. Fauci had West Nile virus, which is carried by mosquitos. Some parks in Massachusetts have started an evening curfew because of cases of Eastern Equine Encephalitis, another mosquito borne disease. Mosquito borne diseases are rare in the US, but worldwide, mosquitos are the deadliest animal in the world. Worldwide, mosquito borne diseases kill 2.7 million people a year, 90% of which occur in Africa. As climate change causes increased global warming we are likely to see an increase in mosquito borne diseases in the US. In this post I will catalog all the mosquito borne diseases, which species of mosquitos carry them, and what areas have the most risk. Only a few species of mosquitos transmit diseases, but that information is primarily useful for epidemiologists and public health specialists. When a mosquito bites you, you are not going to know what species it is!
West Nile Virus
West Nile Virus is the most common disease transmitted by mosquitos, primarily by mosquitos of the culex species. Mosquitos get infected from birds and birds can get infected from infected mosquitos, so the disease passes back and forth between birds and mosquitos. People who get infected with West Nile cannot transmit it back to mosquitos that bite them.
Symptoms
Most people infected with West Nile virus do not have any symptoms but about 1 in 5 people who are infected develop a fever, headache, weakness, muscle pain, or joint pain, gastrointestinal symptoms and a transient rash. The illness usually lasts a few days, but can last for weeks. About 1 out of 150 infected people develop serious illness that invades the brain and spinal cord. This is called neuroinvasive illness. Sometimes this is fatal and if a person survives it they are likely to have permanent disability.
Species of mosquitos that carry it
Culex especially culex tarsalis
Treatment
There is no treatment or vaccine, so avoiding mosquito bites is the only way to keep from getting it when it has been identified in your area. I will write about the ways to avoid mosquito bites near the end of this post.
Who is at risk?
Cases are primarily in the summer and fall. Cases of West Nile disease have been reported across the continental United States. The CDC keeps track of human cases and cases identified in dead birds. West Nile virus occurs in epidemics in some years with few cases in others. So far in 2024 in the US there have been 289 cases of people with non-neuroinvasive West Nile illness and 189 cases of the serious neuroinvasive disease. Since 80% of infected people have no symptoms, that means the number of people with non-neuroinvasive infection so far this year is likely 5 x 289 = 1445. That is still a tiny percentage of the entire US population. Here is a link to the CDC web page showing 2024 West Nile cases by state and by county: West Nile Current Year Data (20240. If you live in an area where West Nile virus has has been identified in birds or humans, then you are at some increased risk.
Eastern Equine Encephalitis
The virus is maintained in fresh water hardwood swamps by birds and a particular species of mosquito called Culiseta melanura. This mosquito almost exclusively bites birds, so is not a significant risk to humans. The problem comes when infected birds are bitten by other species of mosquitos that do bite humans. Those species can infect humans.
Symptoms
Fortunately this is a rare disease. Thirty per cent of people infected with this virus die. Those who survive often have serious neurological problems. Symptoms are fever, headache, vomiting, diarrhea, seizures, behavioral changes, and drowsiness.
Species that transmit the disease to humans
Aedes, Coquillettidia, and Culex
Treatment
There is no vaccine or treatment for Eastern Equine Encephalitis. Avoiding mosquito bites is the only prevention. More about how to avoid mosquito bites later in this post.
Who is at risk?
Most cases are in the Eastern US. There have been only 4 cases so far this year according to the CDC data, but there was a death from a case just in the last few days in New Hampshire. The states that have recorded cases so far this year are Vermont, Massachusetts, New Jersey, Wisconsin and now New Hampshire. People who live in the northeast may need to be especially careful this year.
Cache Valley Virus
This virus is named for the Cache Valley in Utah where it was first recorded. It is very rare. Less than 10 cases have ever been reported. It has been reported in Illinois, Michigan, Missouri, New York, North Carolina, and Wisconsin. It is a severe disease.
Symptoms
Symptoms include stiff neck, confusion, loss of coordination, difficulty speaking, or seizures.
Species that transmit the disease
The virus has been found in several species (Anopheles, Culiseta, Coquillettidia). The main species that transmits the disease is not known.
Treatment
There is no vaccine or treatment. Avoiding mosquito bites is the only prevention. More about how to avoid mosquito bites later in this post.
Who is at risk?
This disease is so rare that there is no particular region of the US that is at risk. Mosquitos carrying the Cache Valley virus have been found in many additional locations in North America and in parts of Central America.
Jamestown Canyon Virus
The virus is maintained by mosquitos biting infected animals, mostly deer. Mosquitos cannot transmit disease from biting infected humans, so humans are considered a “dead end” host.
Symptoms
This is another relatively rare disease, but more frequent than Cache Valley Virus. Most infected people do not have symptoms, but a few people get severe neuroinvasive disease. The CDC counts only the severe cases, so the number of cases substantially underestimate the number of people infected. There are about 23 severe cases reported a year, mostly in the spring through fall. Symptoms include fever, fatigue and headache. Some people have respiratory symptoms such as cough, sore throat or runny nose. Symptoms of severe disease can include stiff neck, confusion, loss of coordination, difficulty speaking, or seizures. Death is rare from this disease.
Species that transmit the disease
Aedes, Culex, Coquillettidia
Treatment
There is no vaccine or treatment. Avoiding mosquito bites is the only prevention.
Who is at risk
Jamestown virus is found through most of the US, but Minnesota and Wisconsin have reported more than half of the cases. Again, there is no vaccine or treatment. Prevention is again the best option.
LaCrosse Virus
La Crosse virus circulates in the environment between tree hole breeding mosquitoes and small mammals, such as chipmunks or squirrels.
Symptoms
Most infected people do not have symptoms, but the disease can be severe especially in children under 16 years of age. Initial symptoms can include fever (usually lasting 2-3 days), headache, nausea, vomiting, fatigue , and lethargy. Symptoms of severe disease include high fever, headache, neck stiffness, stupor, disorientation, coma, seizures, muscle weakness, vision loss, numbness, and paralysis. Most patients recover but death from LaCrosse virus happens rarely.
Species that transmit the disease
Eastern tree hole mosquito (Aedes triseriatus). The tree hole mosquito is found almost exclusively in wooded or shady areas, and usually does not fly more than 200 yards from the area where it developed.
Treatment
There is no vaccine or treatment Avoiding mosquito bites is the only prevention.
Who is at risk?
Most cases occur in the upper Midwestern, mid-Atlantic, and southeastern states. LaCrosse virus is a rare disease. So far in 2024 there have been 13 cases of LaCrosse disease reported from Tennessee, North Carolina and South Carolina.
St Louis Encephalitis
St Louis Encephalitis virus circulates in the environment between mosquitos and birds.
Symptoms
Most infected people do not have symptoms. Severe disease usually occurs in older or immunocompromised people. Symptoms are fever, headache, dizziness, nausea, and generalized weakness. People with severe disease can develop stiff neck, confusion, disorientation, dizziness, tremors, and unsteadiness. 5-20% of infected people die from this disease. The risk of dying increases with age.
Species that transmit the infection
Culex
Treatment
There is no vaccine or treatment. Avoiding mosquito bites is the only prevention.
Who is at risk
In recent years sporadic cases and outbreaks have occurred in the Southwest. This is a rare disease and there have been no cases reported in 2024. In some years there are outbreaks primarily in urban areas. Again there is no vaccine or treatment for St Louis Encephalitis.
Dengue
Unlike many of the diseases discussed previously, Dengue virus circulates between humans and mosquitos. Mosquitos who bite infected people then spread the virus by biting other people. People can get Dengue multiple times.
Symptoms
Most infected people have no symptoms but 1 in 4 have symptoms which include Fever and bone and muscle pain. The pain can be severe. Dengue is also known as “break bone fever.” About 1 in 10 people who get sick have severe Dengue. Severe Dengue can result in shock, internal bleeding, and death. People with Dengue who have any of the following symptoms should go immediately to a hospital emergency department: belly pain or tenderness; vomiting; bleeding from the nose or gums; vomiting blood or blood in the stool.
Species that transmit the infection
Aedes.aegypti and Aedes.albopictus
Treatment
There is a vaccine for Dengue but it is only recommended for children who have already had one episode of Dengue. It is not available in the US. The only treatment is hospitalization and supportive care for severe illness.
Who is at risk?
Most Dengue in the US is in travelers from endemic areas. Endemic areas include the Caribbean, Central America, South America, Southeast Asia and the Pacific Islands. Many of these are popular tourist destinations, so the biggest risk for US citizens is travel to one of these regions. There have been some local outbreaks of Dengue in the US in Florida, Hawaii, Texas, Arizona, and California.
Treatment
Once again, other than the vaccine for children who have already had Dengue, treatment is supportive hospital care for people with severe Dengue. Travelers to endemic Dengue areas should use mosquito bite preventive measures, discussed further later in this post.
Chikungunya
Chikungunya virus, like Dengue, circulates in mosquitos and humans. Mosquitos become infected by biting a person with chikungunya virus and then spread the virus by biting other humans.
Symptoms
The most common symptoms are fever and joint pain and can include headache, muscle pain, joint swelling, or rash. Most patients feel better within a week, but joint pain can be severe and disabling and might persist for months. Death from chikungunya virus is rare.
Species that transmit the disease
Aedes.Stegomyia, Aedes.aegypti and Aedes.albopictus
Treatment
There is a vaccine for chikungunya. Travelers traveling to endemic areas may want to consider vaccination. There is no specific treatment. Avoiding mosquito bites is the only prevention.
Who is at risk?
There have been no cases of chikungunya in the US and US territories since 2019. Travelers to endemic areas are at risk. Endemic areas include Africa, the Americas, Asia, Europe, and islands in the Indian and Pacific Oceans. Travelers should check with the CDC travel website to see if outbreaks are occurring at their destinations.
Zika Virus
Like Dengue and chikungunya, Zika circulates between mosquitos and humans. If a mosquito bites a person infected with Zika virus in the first week of infection, the mosquito carries Zika to the next person it bites. Zika can also be transmitted through sex and can be transmitted from a pregnant mother to her fetus. Zika can cause severe birth defects when a pregnant mother is infected.
Symptoms
Most infected people have no symptoms, and when symptoms occur they tend to be mild. They include fever, rash, headache, joint pain, conjunctivitis (red eyes) and muscle pain. Symptoms are rarely severe enough to need hospitalization. The biggest problem with Zika is the birth defects caused by infection of pregnant women.
Species that transmit the disease
Aedes. aegypti and Aedes. albopictus
Treatment
There is no vaccine or treatment for Zika virus.
Who is at risk?
There have been no local cases of Zika in the US or US territories since 2019. People most at risk are travelers to countries that have outbreaks. Outbreaks occur in Mexico and South America and in certain countries in Africa and India. Other countries including the US have the Aedes species that can transmit Zika. Travelers to these areas should check with the CDC about current Zika outbreaks.
Yellow Fever
Yellow fever virus is maintained in forests between mosquitos and monkeys. People who work in forests where monkeys live can contract the virus this way. Mosquitos biting infected people can also spread the virus when mosquitos bite other people. Epidemics in urban areas occur this way.
Symptoms
Most people infected with yellow fever virus will either have no symptoms or mild symptoms and completely recover. Symptoms can include sudden onset of fever, chills, severe headache, back pain, general body aches, nausea, vomiting, fatigue, and weakness. Most people who develop symptoms improve within one week. A few people will develop a more severe form of the disease. Severe symptoms include high fever, yellow skin or eyes (jaundice), bleeding, shock, and organ failure. Among those who develop severe disease, 30-60% die
Species that transmit the disease
Primarily Aedes Aegypti
Treatment
There is an effective vaccine for prevention of yellow fever. Travelers going to an endemic yellow fever region should receive the vaccine before they travel. There is no treatment once someone has yellow fever.
Who is at risk?
There is no local transmission of yellow fever in the US. Yellow fever in travelers only rarely occurs. Yellow fever is endemic in Africa and South America.
Malaria
Malaria is caused by a parasite that resides in the liver and then infects red blood cells. It is maintained by circulation between mosquitos and humans. Mosquitos get infected from biting humans with malaria and then can transmit the parasite by biting other humans.
Symptoms
Malaria symptoms range from very mild illness to severe disease and even death. Early symptoms can include: Fever and flu-like illness, Chills, Headache, muscle aches, and tiredness. Nausea, vomiting, and diarrhea may also occur. If not treated quickly, the infection can become severe. Severe symptoms can include kidney failure, seizures, mental confusion, coma and death.
Species that transmit the disease
Anopheles
Treatment
Malaria can be treated successfully with several different drugs depending on the type and resistance. There are drugs that travelers can take if they are traveling to an area where malaria is present that will prevent infection. The newest development is a malaria vaccine that can be given to children in endemic malaria regions.
Who is at risk?
There is virtually no local transmission of malaria in the US in modern times. In a typical year, the U.S. reports about 2,000 cases of malaria, almost all of which are in travelers who have been to an endemic region. Malaria occurs only in tropical regions and below 6,500 feet above sea level. Most cases of malaria occur in sub-Saharan Africa, but it also occurs in parts of Oceania (such as Papua New Guinea) and in parts of Central and South America and Southeast Asia. Worldwide, malaria has caused 608,000 deaths in the past year. Malaria is one of the world’s most severe public health problems, with nearly half of the world’s population at risk for infection.
How to prevent mosquito bites.
Indoors
Make sure all windows have screens and any holes in the screens are patched. Use air conditioning if you have it during the spring and summer when mosquitos are most active.
Outdoors
Mosquitos lay eggs in standing water. Remove or empty any receptacles outside that allow water to pool such as saucers under flower pots. Tightly cover any water storage containers. If you have a birdbath, use a battery or solar powered agitator. Mosquitos only lay eggs in still water.
Residential outdoor misting systems that use permethrin provide good mosquito control and are safe for humans, birds and animals. They are somewhat expensive, however.
Use an EPA-registered insect repellant with one of the following ingredients:
- DEET (Has been shown to be non-toxic for humans and repels but does not kill insects)
- Picaridin (known as KBR 3023 and icaridin outside the United States)
- IR3535
- Oil of lemon eucalyptus (OLE)—A plant-derived ingredient (must be applied more frequently than DEET)
- Para-menthane-diol (PMD)
- 2-undecanone—A plant-derived ingredient
Wear loose long sleeved shirts and long pants when outside when mosquitos are active
Treat items such as boots, pants, socks, and tents with permethrin or buy permethrin-treated clothing and gear.
Bottom Line
Mosquitos transmit many diseases, but all of these are quite rare in the US. Global warming may well increase mosquito transmitted diseases in the US in the future. The vast majority of mosquito bites in the US are just a nuisance and do not transmit disease. In other parts of the world mosquito disease transmission is a major public health problem. Travelers to areas where mosquito borne diseases are prevalent should use mosquito bite prevention strategies, especially EPA-approved insect repellents. Travelers to areas endemic for yellow fever should be vaccinated prior to travel. Travelers to malaria prevalent regions should start preventive medicine 1 week before and continue 1 week after travel.
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