West Nile

By Peter Galvin, MD

West Nile virus is an infection that is spread to humans by mosquitoes of the Culex species. The virus is spread after a mosquito feeds on an infected bird and then bites a human. The viral particle contains a strand of RNA and West Nile is related to other mosquito-borne viral diseases such as zika, dengue, and yellow fever. West Nile virus is not spread person to person, nor is it sexually transmitted, but it can spread from mother to fetus as well as by breastfeeding. It can be spread by blood transfusion and organ donation, but in the U.S., blood products and transplantable organs are screened for it, but donors with a recent West Nile diagnosis should not donate blood for 120 days.

West Nile is the most common disease transmitted by mosquitos in the continental U.S. Cases are seasonal, usually beginning in the summer and continuing through the fall. In 2024, 1,466 cases were reported in the U.S., however reported cases account for a small percentage of total infections. That is because only about one in five people with West Nile have symptoms, which typically begin two to 15 days after being bitten. Symptoms may include fever, headache, rash, muscle pain, pain behind the eyes, nausea, vomiting, and diarrhea. Symptoms may last a few days to a few weeks. About one in 150 people develop severe neuroinvasive disease, which involves inflammation of the brain (encephalitis) and/or of the tissues surrounding the brain and spinal cord (meningitis). Symptoms may include neck stiffness, disorientation, convulsions, muscle weakness, paralysis, and coma. People with a compromised immune system and those aged 50 years and older are at the highest risk of neuroinvasive West Nile, which can cause permanent memory loss, weakness, and difficulty walking. About one in 10 people die from neuroinvasive disease.

West Nile is not directly named for the Nile River, but rather for the West Nile district in Uganda. Humans and horses are “dead-end” hosts, meaning they do not develop high enough viral levels for another mosquito to get it from biting them. Mosquitos can only pick the virus up from infected birds, either dead or alive. Testing for West Nile is done in specialized laboratories. The virus can be detected in blood and cerebrospinal fluid. There is no specific treatment for West Nile. Antiviral medications are not effective. Those with severe infections should be hospitalized for close monitoring and supportive treatment.

There is currently no vaccine to prevent West Nile infection. To decrease the risk of acquiring it, those living in or travelling to areas known for mosquitoes (i.e., coastal and swampy areas) should protect themselves from their bites by wearing loose-fitting, long-sleeved shirts or pants treated with permethrin and by using recommended insect repellants (with DEET or other active ingredients registered with the EPA). When sleeping outdoors, mosquito nets should be used; indoors, window screens are recommended. Drain sources of standing water (flowerpots, saucers, unused pools, and birdbaths) that may serve as mosquito breeding areas. Do not touch or handle dead birds, and reporting dead birds to the state health department can help local surveillance programs that monitor West Nile activity.

For more information go to the website of the Centers for Disease Control and Prevention at www.cdc.gov

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