Mosquito-borne viral encephalitis
Mosquito-borne viral encephalitis is an infection caused by viruses transmitted through mosquito bites, leading to inflammation of the brain. In the United States, the primary types include eastern equine encephalitis, western equine encephalitis, West Nile virus, St. Louis encephalitis, and LaCrosse virus. Outside the U.S., Japanese and Venezuelan equine encephalitis are more common. While many who contract these viruses remain asymptomatic or experience mild symptoms such as fever and headache, some may develop serious conditions like seizures, coma, or even death. Risk factors for severe outcomes include age (particularly those under 15 or over 50), outdoor exposure, and compromised immune systems.
Diagnosis typically involves a thorough medical history, physical examination, and various tests such as blood work and imaging studies. Currently, there is no specific antiviral treatment; care focuses on managing symptoms until the immune system can clear the infection. Prevention strategies include minimizing mosquito exposure through protective clothing, repellents, and environmental management to reduce breeding sites. Vaccination is available for Japanese encephalitis for individuals traveling to high-risk areas.
Mosquito-borne viral encephalitis
- ANATOMY OR SYSTEM AFFECTED: Brain, central nervous system
Definition
Mosquito-borne viral encephalitis is an infection transmitted by mosquitoes that can lead to encephalitis, or inflammation of the brain. In the United States, there are five main types of mosquito-borne viral encephalitis: eastern equine encephalitis, western equine, West Nile, St. Louis, and LaCrosse. Outside the United States, the most common types of mosquito-borne viral encephalitis are Japanese and Venezuelan equine. The Zika virus is an additional mosquito-borne virus that can cause encephalitis. Mosquito-borne viral encephalitis is a potentially serious condition that requires care from a doctor.
Causes
Mosquito-borne viral encephalitis is caused by a bite from a mosquito that carries the virus from animals to humans. When mosquitoes bite an infected bird, horse, or other animal, they can pass the infection to humans. It usually takes between four and fifteen days for a person to have any symptoms after he or she has been bitten by an infected mosquito. Rarely, the infection can be passed through organ transplants or blood transfusions.
Risk Factors
The factors that increase the chance of developing mosquito-borne viral encephalitis include living in an area where outbreaks of viral encephalitis have occurred, spending much time outdoors for work or play, being fifty years of age and older or less than fifteen years of age, having a weak immune system, and using immunosuppressant drugs.
Symptoms
Most people who become infected with the viruses that can cause encephalitis do not develop any symptoms, and the infection runs its course without being dangerous. Many others develop only mild symptoms, including mild fever, headache, nausea, body ache, and restlessness.
Some who become infected with one of these viruses actually develop encephalitis, which can cause death or brain damage. The more serious symptoms of encephalitis include seizures, high fever, coma, weight loss, weakness, changes in mental state, stiff neck, tremors, paralysis, vision loss, and numbness. These symptoms, however, should not lead one to assume they have mosquito-borne viral encephalitis. The symptoms may be caused by other, less serious health conditions.
Screening and Diagnosis
Doctors will ask the patient about symptoms and medical history and perform a physical exam. Questions concern travel to areas that have had mosquito-borne viral encephalitis outbreaks, recent mosquito bites, and exposure to dead animals. Tests may include a neurological exam (a series of tests to measure reflexes, memory, and other brain functions), blood tests to look for signs of infection in the blood; a spinal tap (removal of a small amount of cerebrospinal fluid to check for signs of infection), a magnetic resonance imaging (MRI) scan (a scan that uses radio waves and a powerful magnet to produce detailed computer images), a computed tomography (CT) scan (a detailed X-ray picture that identifies abnormalities of fine tissue structure), and an electroencephalogram (EEG), a test that records the brain’s activity by measuring electrical currents through the brain.
Treatment and Therapy
No drug exists to treat mosquito-borne viral encephalitis, so doctors usually prescribe supportive care, which means treating the symptoms while the immune system fights the disease. Supportive treatment options include intravenous fluids, a respirator to help with breathing, anticonvulsants to treat seizures, sedatives to treat restlessness, pain relievers to treat headaches and fevers, and corticosteroids (anti-inflammatory drugs) to reduce brain swelling.
Prevention and Outcomes
The best way to reduce the chance of getting mosquito-borne viral encephalitis is to avoid being bitten by mosquitoes. One should limit outside activities where mosquitoes are present; wear long sleeve shirts and long pants at dusk and dawn, when mosquitoes are most active; use bug repellent that contains N-Diethyl-meta-toluamide (DEET); and empty sources of standing water around the home, such as bird baths and gutters, where mosquitoes may breed.
Other prevention tips include avoiding handling dead birds or other animals that can carry the virus and getting vaccinated (for Japanese encephalitis) if planning a long visit (greater than one month) to areas in Asia where outbreaks have occurred.
Bibliography
Booss, John, Margaret Esiri, and Margaret M. Esin, eds. Viral Encephalitis in Humans. Washington, D.C.: ASM Press, 2003.
Goddard, Jerome. Physician’s Guide to Arthropods of Medical Importance. 4th ed. Boca Raton, Fla.: CRC Press, 2003.
Marquardt, William C., ed. Biology of Disease Vectors. 2d ed. New York: Academic Press/Elsevier, 2005.
Meningitis and Encephalitis Fact Sheet.” National Institute of Neurological Disorders and Stroke,catalog.ninds.nih.gov/sites/default/files/publications/meningitis-encephalitis.pdf. Accessed 4 Feb. 2025.
Peters, C. J. “Infections Caused by Arthropod- and Rodent-Borne Viruses.” In Harrison’s Principles of Internal Medicine, edited by Anthony Fauci et al. 17th ed. New York: McGraw-Hill, 2008.
Rosenbluth, Teddy. "The Mosquito-Borne Virus Raising Alarms in the Northeast." The New York Times, 28 Aug. 2024, www.nytimes.com/2024/08/28/health/eastern-equine-encephalitis-prevention.html. Accessed 4 Feb. 2025.
"Zika Virus." Johns Hopkins Medicine, www.hopkinsmedicine.org/health/conditions-and-diseases/zika. Accessed 4 Feb. 2025.