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Mosquito-Borne Illnesses

Mosquito-Borne Diseases

The temperature is rising, the outdoors are all the more enticing, and mosquitoes have awoken from their cold-weather slumber. Whether you’re deep in the forest on a camping trip, working in the yard, or spending a day the beach, mosquito bites are the undesired aside that comes with enjoying a natural environment. Even less desirable is the scare of West Nile virus, and the threat of other diseases carried by these blood-sucking critters.

In 2012, the Centers for Disease Control and Prevention cited more outbreaks of West Nile than ever, with 5,387 cases illness in 48 states and 243 fatalities. Fifty-one percent of cases broke the blood-brain barrier, resulting in neuroinvasive diseases such as meningitis and encephalitis. With no vaccine or treatment for humans, the irritating itch of mosquito bites has grown into a widespread concern that may be life-threatening, or at least sickening enough to ruin a vacation.

Mosquitos have been known to carry a variety of other diseases in addition to West Nile, including other encephalitides, malaria, Rift Valley Fever, yellow fever, and dengue. Although many of these illnesses are of greater concern when traveling abroad, like West Nile virus, no one can predict when another deadly disease will be imported.

Diseases Carried By Mosquitoes


Eastern Equine Encephalitis

A rare illness given to humans via mosquito bite, eastern equine encephalitis is typically seen in only a few people per year in the United States. The majority of cases occur in the Atlantic and Gulf Coast states, though it is found throughout the Americas and the Caribbean. Many infected humans survive without major symptoms. In certain instances of severity, flu-like symptoms of headache, sore throat, chills, high fever, nausea and vomiting may occur without warning. The disease has to potential to worsen dramatically, resulting in seizures, coma, and/or disorientation. As such, it is one of the most serious diseases carried by mosquitoes, and may be fatal.

About half of those infected have died from EEE. Some survivors of serious symptoms may suffer long-term brain impairment and require assisted living. No treatment has been identified for EEE in humans, though a vaccine exists for horses.


Japanese Encephalitis

The main cause of vaccine-preventable encephalitis in Asian and western Pacific countries, the risk of acquiring Japanese encephalitis (JE) is usually low for travelers, though it can vary based on season, location, length of stay, and what you do. JE virus usually results in mild symptoms or none at all for humans, though a slim percentage  of cases grow into encephalitis, an inflammation of the brain. When this type of infection is present symptoms will rapidly fluctuate into high fever, headache, disorientation, tremors, convulsions, and coma. Approximately 1 in 4 cases can cause death, and there is no specific treatment, though vaccines are available and doctors can help you manage your symptoms.


La Crosse Encephalitis

Even less frequently occurring than the previous two, La Crosse encephalitis (LAC) is still prevalent in certain areas of the U.S., especially the 13 states east of the Mississippi River, mainly the Appalachian region. Linked to the tree-hole mosquito, LAC is usually found in children and adolescents under age 16 and can be asymptomatic. In some cases, symptoms similar to others listed above of nausea, headache, fever, and others may occur. As with other diseases in this family, when the virus enters the cerebrospinal fluid from the bloodstream, it may result in encephalitis and life-threatening symptoms can occur.

No vaccines exist nor specific treatments, but medical care is always worth pursuit to control symptoms and avoid permanent damage, long-term disability, or death. It’s daunting to think that an insect bite could lead to such consequences, but diseases prevail and impact lives quite seriously.


St. Louis Encephalitis

Most typically found along the Gulf of Mexico, notably in Florida, St. Louis encephalitis (SLE) has resulted in several epidemics, most recently in 1990. Though most people who acquire the illness have no evident symptoms, individuals who get sick often experience fatigue, nausea, vomiting, headache, and fever. As you may have expected, SLE can also develop into encephalitis if the virus enters the fluid inside the spinal cord and surrounding the brain. In these rare instances, debilitating effects or death can occur, as your central nervous system is gravely impacted.

No vaccines or treatments are available, but you can prevent mosquito bites and seek medical attention if you experience symptoms.


Western Equine Encephalitis

Western equine encephalitis (WEE) is not commonly found in humans, though infection may result in flu-like symptoms as mentioned with other arboviral encephalitides. While not typically a concern for our species, if one of us does become infected, it can cause just as severe symptoms as other forms of this virus, including coma, encephalitis, and death. The CDC reports only 639 cases since 1964.


West Nile Virus

The largest mosquito-borne concern for humans, North Americans in particular as of late, is the West Nile virus (WNV), which has become a major epidemic causing thousands of serious infections and hundreds of deaths over the years. Beginning with a single mosquito bite (though that rarely happens), WNV is similar to SLE and JE, transmitted between birds and mosquitoes to humans, resulting in the characteristic flu-like symptoms. When WNV escalates, it can become  full-blown encephalitis (brain inflammation), meningitis (swelling of meninges), meningoencephalitis (swelling of both brain and meninges), West Nile poliomyelitis (swelling of the spinal cord), or acute flaccid paralysis (sudden weakness in limbs or breathing muscles).

Symptoms of WNV infection may include fever, headache, muscle aches, exhaustion, and back pain. If it develops into one of the aforementioned diseases, you may experience neurological symptoms such as coma, convulsions, tremors, confusion, headache, high fever, or partial paralysis. Though mild symptoms typically disappear on their own, as any virus, if your symptoms are that of a severe infection, contact a doctor immediately – you may need to be hospitalized.


Dengue Fever

An infectious disease found in the tropical regions of the Americas, Asia, and Africa, dengue fever is a virus carried by the Aedes mosquito with increasing risk in the United States. Those who live along the Texas-Mexico border are at a higher risk of the virus, and in 2009, an outbreak in Key West sickened a number of Floridians. Symptoms of the illness generally appear four to six days after infection and may last up to ten days. These include sudden, high fever, severe headaches, pain behind the eyes, intense joint and muscle ache, vomiting, skin rash (appears 3-4 days after acquiring fever), and mild bleeding (in the gums, nose, or easy bruising).

Infection with dengue fever can also develop into dengue shock syndrome (DSS) if dengue hemorrhagic fever is present, which includes liver enlargement and circulatory system shut down. This can lead to excessive bleeding, shock, and even death, especially in those with weakened immune systems. If you feel you may have dengue fever, take acetaminophen for pain, rest up, and stay hydrated. Consult your physician as soon as your are able to treat your symptoms and avoid complications.


Yellow Fever

An ancient illness, also native to tropical regions of Africa and the Americas, yellow fever is adamantly combated by travel laws, and thus a vaccine is given to anyone visiting certain regions. However, the disease can be acquired domestically. Each year, an estimated 200,000 cases are reported with about 30,000 deaths across and average of 33 countries. It can be acquired in both urban and jungle environments and can lead to a wide range of symptoms including fever, headache, chills, back pain, appetite loss, vomiting in mild, 3-4 day-long infections, to bleeding in the mouth, eyes, and gastrointestinal tract, the toxic phase which affects 15 percent of cases.



Though the invention of pesticides has cleared much of the risk of malaria, 300-500 million cases and one million deaths are reported annually throughout the world. Another disease of the tropics, mosquitoes carrying malaria place about 40 percent of the world’s population at risk. However, antimalarial medications have been developed and available for the past 50 years. Researchers in the United States and Britain have identified the malaria parasite genome, an advancement which may help reduce outbreaks with future studies.






1. Centers for Disease Control and Prevention. CDC West Nile Virus Homepage.

2. Mosquito.org. Mosquito-Borne Diseases.

3. Centers for Disease Control and Prevention. Eastern Equine Encephalitis.

4. Centers for Disease Control and Prevention. Japanese Encephalitis.

5. Centers for Disease Control and Prevention. La Crosse Encephalitis.

6. Centers for Disease Control and Prevention. St. Louis Encephalitis.

 7. Mayo Clinic. West Nile Virus.

8. WebMD. Dengue Fever: Symptoms, Causes, and Treatment.

9. Wikipedia. Yellow Fever.


Image by James Gathany, CDC via Wikimedia Commons.