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Diseases Mosquitos Transmit and Symptoms to Look For

Common Diseases That Mosquitoes Transmit

Magnified picture of mosquito

Magnified picture of mosquito

A 2018 statement from the CDC notes that cases of mosquitoes spreading disease have been rising steadily since 2004. In fact, researchers discovered two new germs transmitted by mosquito bites. Larger mosquito populations and habitat ranges contribute to these numbers, though widespread travel and commerce also play a part in spreading disease.

Worldwide, the significance and frequency of mosquito-borne disease transmission varies greatly. For example, in more developed countries, mosquito-borne diseases do occur, but the incidence of these diseases is much less than locations where these diseases are endemic (likely to be common and widespread).

Some mosquito-borne diseases that are prevalent in North America include:

Zika Virus

Zika is a virus that spreads to humans through Aedes mosquito bites. Its most common symptoms are:

  • Fever

  • Rash

  • Joint pain

  • Conjunctivitis

In most cases, symptoms associated with Zika are mild and cases requiring hospitalization are uncommon.

Severe Cases

While most cases are relatively mild, health officials also are examining possible links between Zika and long-term health concerns. After the first confirmed Zika virus case in Brazil, reports surfaced of Guillain-Barre syndrome, a rare immune system disorder, and birth defects such as microcephaly in newborn babies. According to the Pan American Health Organization (PAHO), evidence supports a link between the Zika virus and microcephaly.

Source: CDC -

For more information, visit our Zika Virus page.

Chikungunya Virus (CV)

Emerging Threat

Public health officials classify CV as an emerging infectious disease. CV is endemic in countries within Africa, Asia, Europe and around the Indian and Pacific oceans. Since 2006, the United States has averaged 28 imported cases of CV per year, all of these cases involving travelers returning from countries where the virus is common. More recently, as of late August 2014, 696 cases of CV associated with travelers were reported. Now only Montana, Wyoming, Utah, North Dakota and Iowa have not reported at least one case of CV.

However, in mid-July of 2014, the first locally acquired case of CV was reported in Florida. The incident is important because local transmission means that mosquitoes in the area have been infected with the virus and are spreading it to people who live locally.

Mosquito Species That Carry Chikungunya Virus

In the United States, CV is usually transmitted by Aedes aegypti and Aedes albopictus, mosquitoes that bite mostly during the daytime. The two species are common to the Southeastern U.S. and select parts of the Southwest. Aedes albopictus can also be found in northern parts of the country, including the lower-Midwest and the Mid-Atlantic. Within these areas, there are plenty of local mosquito populations that are firmly established.


People infected with CV typically develop:

  • Fever

  • Joint pain

  • Muscle aches

  • Headaches

  • Rash

Rarely Fatal

Infection with CV is rarely fatal when treated properly, but the joint pain is often severe and debilitating. There is no vaccine to protect against infection and no specific medications are used, except those to treat against the symptoms of CV. This virus is not spread from person to person. A serious concern is that a traveler to endemic areas could possibly be infected with CV and the dengue virus at the same time, since both are carried and transmitted by the same types of mosquitoes.

West Nile Virus (WNV)

West Nile virus (WNV) is another mosquito-transmitted disease and was first detected in the United States in New York in 1999. Since then, the West Nile Virus has reached the 48 contiguous states, as well as Canada and Mexico. Over 10,000 human cases of the virus were reported in 2003, 262 of which resulted in fatalities. There is no vaccine or specific treatment for WNV, but most people infected will show none or very mild symptoms. However, from 2009-2013, there was an average of nearly 1,200 cases of serious, neurological disease caused by this virus.

Which Mosquitoes Carry West Nile Virus?

While WNV has been found in about 75 mosquito species across the United States in the last 12 years, the mosquito species most commonly identified as transmitting the disease are Culex pipiens in the northern states, Culex tarsalis in the western states and Culex quinquefasciatus in the southern states. This virus is not transmitted from person to person, but it does infect birds and horses. The most likely non-mosquito contact that may threaten people is handling either live or dead birds that were infected with WNV. Always avoid direct contact with dead birds or any other dead animal by using protective gloves.

For more information, visit our West Nile Virus page.


Millions of Cases Each Year

Approximately 40-percent of the earth’s population lives in areas where it is possible to acquire dengue. According to the World Health Organization, 50 to 100 million cases of dengue occur each year, and the disease remains endemic in about 100 countries throughout the Americas, Asia, the Pacific, Africa, and the Caribbean. There are four viral types of dengue, but the most severe form of this disease is called Dengue Hemorrhagic Fever (DHF). DHF may result in death, especially among children, if not quickly and properly treated.

Mosquitoes & Dengue

Dengue is a viral infection transmitted by the bite of infected Aedes aegypti and Aedes albopictus mosquitoes and is a major cause of disease associated with infected travelers returning from endemic parts of the world. Outbreaks of dengue also have occurred in Florida, Hawaii, and along the Texas-Mexico border. The disease is important in both urban and suburban areas where people create good mosquito development sites by allowing containers to accumulate water and create ideal mosquito development sites.

For more information, visit our Dengue page.


A Dangerous Disease

Epidemiologists estimate that more than ½ million people die each year from malaria. Malaria occurs mostly in poor tropical and subtropical areas of the world and most of those who die are young children. In sub-Saharan Africa, malaria is commonly fatal to young children and pregnant women. Children usually have not developed the necessary immunities to fight the disease, and pregnant women often experience weakened immune systems along with decreased health in general.

Malaria in the United States

Each year in the United States, there are reports of about 1,500 cases of malaria. In nearly every reported case, the disease was brought in by travelers returning from tropical and subtropical visits. However, infrequent outbreaks of malaria transmission by local mosquito populations do occur. Local transmission occurs when people become infected while in an endemic area of the world and then transmit malaria to local residents through the bite of local mosquitoes when they get back home.

Malaria is transmitted by various species of Anopheles mosquitoes. Patients who have malaria are usually very sick with high fever and very serious flu-like illness. Treatment consists of taking anti-malarial drugs and receiving supportive care.

For more information, visit our Malaria page.

Yellow Fever

A Tropical Virus

While not endemic within North America, yellow fever cases are occasionally reported. The cases are usually the result of infected travelers returning home from travels to endemic regions of the world. Yellow fever is a viral infection that is transmitted by the bite of an infected yellow fever mosquito, Aedes aegypti. It occurs in the tropical areas of Africa and South America and historically is viewed as the most lethal of all mosquito-borne diseases, as well as a disease that has adversely affected the people who live in its endemic areas. A vaccine is currently an important component in yellow fever prevention programs.

For more information, visit our Yellow Fever page.

Eastern Equine Encephalitis Virus (EEEV)

Rarely Seen in the U.S.

EEEV is a rare illness in humans, and only about six cases are reported in the United States each year. Over the past 45 years, the states with the most reported cases have been Florida, Georgia, Massachusetts, New Jersey, Louisiana, South Carolina, North Carolina and Michigan. Cases of EEEV most commonly occur around freshwater hardwood swamps near the Atlantic and Gulf Coast and throughout the Great Lakes region. Individuals that contract the disease show no signs of illness.

A Deadly Disease With Dangerous Symptoms

However, cases involving encephalitis can result in serious brain damage or fatality. While the number of cases reported each year is small, about 1/3 of those who are infected by EEEV die. Aedes, Coquillettidia and Culex mosquitoes commonly transmit the disease. EEEV also affects horses, and the infection can prove fatal. Currently, there is no vaccine for EEEV, and care is centered around treatment of symptoms. Other mosquito borne diseases in the same virus family as EEEV and found in the Americas include Western equine encephalitis virus and Venezuelan equine encephalitis virus.

For more information, visit our EEEV page.

Japanese Encephalitis Virus (JEV)

Common in Asia

While this disease rarely is reported in North America, JEV is an important disease in Asia and the western Pacific. For most travelers to Asia, the risk for JEV is very low, but varies based on destination, duration of travel, season and activities. Infected Culex mosquitoes transmit JEV to humans living in areas where the disease is endemic. Cases often arise from rural agricultural areas associated with rice cultivation and crop irrigation. The virus usually peaks between summer and fall in temperate regions of Asia. In subtropical and tropical regions, transmission occurs year round, with the majority of cases occurring during rainy seasons.


Initial symptoms of JEV infections include:

  • Fever

  • Headache

  • Vomiting

Most cases will not require any treatment. As with most other encephalitis viruses, serious central nervous system disorders may occur. In JEV patients who develop encephalitis, 20 – 30 percent may die.

A vaccine is available in the U.S. While the number of cases reported in the U.S. is extremely low, the seriousness of developing encephalitis makes JEV an important mosquito borne disease.

La Crosse Encephalitis Virus (LACV)

Spread by Aggressive Mosquitoes

In the United States, between 80 and 100 cases of LACV arise each year. LACV is transmitted to humans by the bite of an infected Aedes triseriatus, commonly called the eastern treehole mosquito. This species of mosquito is an aggressive, daytime-biter especially around property that is located in or near forests. The mosquito’s common name is very descriptive since Aedes triseriatus female mosquitoes normally lay their eggs in pools of water accumulated in tree holes.

What Areas Are At Risk?

Man-made objects that retain water, like discarded tires, also serve as ideal breeding grounds for the mosquito. Most cases of LACV disease occur in the upper mid-western, mid-Atlantic and southeastern states primarily from late spring through early fall. However, in the southern portions of the Gulf States, cases may occur in winter.


The majority of people infected with LACV show no noticeable symptoms. However, some individuals may become seriously ill and develop severe encephalitis. There is no specific treatment for LACV infection; rather care is based on treating the symptoms. Less than 1 percent of LACV cases result in death.

St. Louis Encephalitis Virus (SLEV)

What Areas of the U.S. Are At Risk?

SLE is primarily occurs in the eastern and central states where urban-located outbreaks have recurred since the 1930s. The largest epidemic of serious SLEV-caused neuro-invasive disease ever reported occurred in the United States in 1975, with nearly 2,000 cases located in the central states in the Ohio-Mississippi River Basin. SLEV cases are also reported along the Gulf of Mexico coast.


In the United States, fewer than 1 percent of SLEV infections show apparent symptoms, and most infections remain undiagnosed. However, SLEV should never be taken lightly because the disease is very serious among the elderly with complications affecting the central nervous system (encephalitis). Generally, SLEV symptoms are milder in children than older adults. In children and young adults, about 40-percent of those infected with SLEV develop fever and headache. In elderly adults, about 90-percent of infections develop into encephalitis.


Overall death from SLEV cases is 5-15 percent, with the risk of fatalities increasing with age. There is no vaccine or specific treatment for SLEV infection; care is based on treating the symptoms.

What Types of Mosquitoes Carry SLEV?

The majority of the United States experiences SLEV transmission from late summer to early fall. However, the Southern U.S. reports cases of SLEV year round. (SLEV) is transmitted to humans by the bite of its principal vectors, Culex pipiens and Culex quinquefasciatus in the eastern states, Culex nigripalpus in Florida, and Culex tarsalis and Culex pipiens species complex in western states.


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