Other Mosquito-Borne Diseases

Of all the arthropods capable of transmitting disease to man, the mosquito is the most important. This is due in part to its high numbers, the need to blood feed among many female mosquitoes in order to develop their eggs and the mosquito's close association with man.

Mosquito-borne diseases are diverse and have a global distribution. The greater burden of disease currently exists in developing countries but historically the United States has suffered from a great many mosquito-borne diseases. The United States is still at risk from current mosquito-borne pathogens circulating within the country and pathogens imported through infected individuals who have traveled, without precaution, in countries endemic for these diseases.

Some pathogens carried by mosquitoes are specific to certain groups of mosquitoes e.g. malaria to Anopheles mosquitoes whilst others, such as West Nile virus, can be transmitted by many different types of mosquito.

The following is a list and brief description of some of the mosquito-borne diseases that can be transmitted to man.

St. Louis Encephalitis

What is St. Louis encephalitis?
It is a major disease caused by a mosquito borne virus that may affect the central nervous system in humans. Infected birds provide the source of viral infection for blood feeding mosquitoes (usually Culex mosquitoes). Birds will often show no symptoms of infection and once recovered will be immune to the virus for the remainder of their lives. Occasionally infected mosquitoes transmit the virus to humans and animals.

Where is it found and how common is it in humans?
The virus was first isolated in 1933 during an epidemic in St. Louis, Missouri and has since been detected in epidemics occurring across the country. One of the last major outbreaks occurred in Florida and Texas in 1990.The disease is not common in Virginia.

What are the symptoms in humans?
Mild symptoms include headache and fever. More serious symptoms include additional symptoms such as stiff neck, disorientation, coma, paralysis and death.

Who is at greatest risk?
Deterioration of infrastructure in highly populated areas may lead to the increase in the optimal habitats for Culex mosquitoes. As a result, people living or working in such areas during an outbreak are at great risk of contracting the infection.

There is a high fatality rate amongst the elderly during St. Louis encephalitis outbreaks and so this group must take care to avoid infection (see Prevention)

What can I do to prevent it?
There is no vaccine available for the virus and so the best prevention is to avoid areas undergoing known outbreaks and to avoid the bite of the mosquito. See Prevention

Where can I find more information?
Centers for Disease Control and Prevention Answers Your Questions About St. Louis Encephalitis
-Walter Reed Biosystematics Unit .

Eastern Equine Encephalitis

Picture of a horse headWhat is Eastern Equine encephalitis?
Eastern Equine encephalitis (EEE) is a viral infection that naturally cycles between the bird and mosquito populations. Occasionally, an infected mosquito may transmit the virus to horses, some other domesticated animals or in rare cases, humans. The resulting disease affects the central nervous system and is associated with a high rate of mortality and mild to severe neurological problems left in those who survive.

Where is it found and how common is it in humans?
EEE is found throughout the Americas. In North America, it occurs in Canada and along the eastern seaboard and Gulf Coast of the United States.

The mosquitoes most often found carrying the virus in Virginia are the bird biting mosquitoes, Culiseta melanura and are associated with swamps. Typically, such areas have a limited human population thus transmission from infected mosquito to human seldom occurs. Figures for Virginia in 2004, record few cases of EEE among horses and none amongst humans.

What are the symptoms in humans?
Flu like symptoms may occur 4-10 days after a bite from an infected mosquito. Symptoms include fever, muscle ache, headache, vomiting, seizures, fatigue or coma.

Who is at greatest risk?
Those living or visiting areas in which the virus is common are at greatest risk at contracting EEE.
Individuals under the age of 15 and over the age of 50 appear to be at greatest risk of developing severe disease.

What can I do to prevent it?
There is no vaccine to protect humans from the disease and so the best precaution is to avoid being bitten by mosquitoes particularly in areas where the infection is common. See Preventing West Nile virus

Where can I find more information?
CDC - Eastern Equine Encephalitis Fact Sheet
Virginia Department of Health - Arboviral Infections

La Crosse Encephalitis

What is it?
La Crosse encephalitis is a rare viral disease of the central nervous system spread through the bite of an infected mosquito and is named after La Crosse, Wisconsin, where it was first identified in 1963. The natural cycle of the infection typically involves the movement of virus between the tree hole mosquito, Ochlerotatus triseriatus, and woodland vertebrates such as squirrels and chipmunks. Occasionally, the virus may be transmitted by the bite of an infected mosquito to humans.

Where is it found and how common is it in humans?
The virus has been found throughout the Midwestern and Mid-atlantic States. There was one human case of La Crosse encephalitis detected in Virginia in 2004.

Who is at risk?
Children under the age of 16 are at greatest risk of disease. In addition, people who reside or work in or near woodland environments where the virus is present are also at risk of contracting the disease.

What are the symptoms?
Most people infected with La Crosse encephalitis will show no symptoms of disease. Others may show symptoms including fever, headache, nausea and vomiting. In more severe cases, convulsions, tremors and coma may occur. La Crosse encephalitis is associated with a low mortality rate and neurological complications which often resolve after several years.

How do I prevent it?
The best prevention is to avoid the bite of mosquitoes and reduce the number of mosquito breeding sites in your environment. See Preventing West Nile virus.

Where can I find more information?
CDC - Fact Sheet: La Crosse Encephalitis

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