West Nile virus
West Nile virus is a mosquito-borne flavivirus that usually causes no symptoms, but in rare cases can invade the nervous system and cause serious disease.
What West Nile virus is
West Nile virus is a flavivirus that spreads mainly through infected mosquitoes. It can infect birds, people, horses, and other animals. Most human infections are silent, but the virus matters because a small number become neuroinvasive, meaning the infection affects the brain, spinal cord, or surrounding membranes.
How the cycle works
Birds are the main amplifying hosts for West Nile virus. Mosquitoes become infected after feeding on infected birds, and later bites can pass the virus to people or animals. Humans and horses are usually considered dead-end hosts because they generally do not develop enough virus in the blood to keep the mosquito cycle going.
Symptoms
Symptoms usually begin a few days after a bite from an infected mosquito, though timing can vary. Mild illness can include fever, headache, body aches, joint pain, vomiting, diarrhea, or rash. Fatigue and weakness can last longer than the initial fever.
Neuroinvasive disease
Severe West Nile disease can appear as encephalitis, meningitis, or acute flaccid paralysis. Warning signs include high fever, severe headache, stiff neck, confusion, tremors, muscle weakness, vision loss, numbness, paralysis, or coma. Older adults and people with weakened immune systems have higher risk of serious outcomes.
Diagnosis
Clinicians consider West Nile virus when symptoms, season, mosquito exposure, and local surveillance fit the pattern. Laboratory testing can look for virus-specific antibodies in blood or cerebrospinal fluid, especially when nervous-system symptoms are present. Because symptoms overlap with other infections, diagnosis usually combines lab results with clinical context.
Treatment
There is no specific antiviral treatment for West Nile virus disease in people. Care is supportive and can include pain control, fluids, monitoring, respiratory support, and hospital care for severe neurologic illness. Recovery from neuroinvasive disease can be slow, and some people have lasting weakness or other neurologic effects.
Prevention
There is no licensed human vaccine for West Nile virus, so prevention focuses on avoiding mosquito bites and reducing mosquito habitat. Repellent, long sleeves, window screens, air conditioning, draining standing water, and community mosquito-control programs all reduce risk. Blood and organ donor screening also helps prevent rare non-mosquito transmission.
Why it matters
West Nile virus connects human health, wildlife, climate, urban ecology, and public-health surveillance. A season with many infected mosquitoes, susceptible birds, warm weather, and human exposure can produce serious cases even when most infections are never noticed.
What public health teams watch
Surveillance may include human cases, mosquito pools, dead birds, animal cases, weather, and local mosquito abundance. These signals guide alerts, spraying decisions, blood-safety steps, and public messaging. The work is seasonal in many places, but the exact timing can shift with rainfall, temperature, and mosquito habitat.