West Nile virus (WNV) is a mosquito-borne illness that can lead to severe neurological disease in a small percentage of infected individuals. While most people infected with WNV experience no symptoms or mild fever, the neuroinvasive form of the virus can cause severe neurological disease.
Understanding Neuroinvasive West Nile Virus
West Nile virus belongs to the Flaviviridae family, which includes those causing Zika and dengue fever. Most WNV infections are asymptomatic. About one in five infected people might develop West Nile fever, with symptoms such as headache, fever, body aches, and fatigue.
In less than 1% of cases, the virus enters the central nervous system, resulting in neuroinvasive West Nile virus disease. This occurs when the virus crosses the blood-brain barrier, a protective mechanism that normally prevents harmful substances from reaching the brain and spinal cord. Once inside the central nervous system, WNV can cause inflammation of the brain (encephalitis), the membranes surrounding the brain and spinal cord (meningitis), or the spinal cord itself (poliomyelitis).
Transmission and Risk Factors
West Nile virus is primarily transmitted to humans through the bite of an infected mosquito. Mosquitoes, particularly Culex species, become infected when they feed on birds that carry the virus. Infected mosquitoes then transmit the virus to humans and other animals. Birds serve as the main reservoir hosts for WNV, and their migration plays a role in the virus’s geographic spread.
While mosquito bites are the most common transmission route, rare instances of WNV spread include blood transfusions, organ transplantation, and from mother to child during pregnancy, delivery, or breastfeeding. Human-to-human transmission through casual contact does not occur. Individuals at higher risk of severe neuroinvasive disease include people over 60 years old, those with weakened immune systems, and individuals with medical conditions such as cancer, diabetes, hypertension, or kidney disease.
Recognizing Symptoms and Seeking Diagnosis
Symptoms of neuroinvasive West Nile virus are severe and typically appear between 3 and 14 days after a mosquito bite. These can include a high fever, a severe headache, and a stiff neck, indicating inflammation of the brain or spinal cord. Other neurological symptoms may involve disorientation, stupor, tremors, convulsions, and muscle weakness, sometimes leading to paralysis. In the most severe cases, individuals may experience coma.
Seek medical attention if these symptoms arise. Diagnosing neuroinvasive WNV involves evaluating symptoms and history of potential mosquito exposure. Healthcare providers may order laboratory tests on blood or cerebrospinal fluid (CSF) to detect WNV IgM antibodies, indicating a recent infection. A lumbar puncture, or spinal tap, collects CSF for analysis. Imaging techniques, such as magnetic resonance imaging (MRI) of the brain, assess for inflammation or other changes in the central nervous system.
Managing the Infection and Recovery
There is no specific antiviral treatment for West Nile virus infection. Management of neuroinvasive WNV focuses on supportive care to manage symptoms and complications. Supportive care includes pain management for severe headaches, intravenous fluids to prevent dehydration, and respiratory support if breathing becomes difficult. Preventing secondary infections, such as pneumonia or urinary tract infections, is also important. Physical therapy may be necessary for individuals who experience muscle weakness or other neurological deficits.
Recovery from neuroinvasive WNV is often prolonged, taking weeks or months. Some individuals experience long-lasting neurological complications such as memory loss, hearing loss, balance issues, or persistent muscle weakness. The severity and duration of these long-term effects vary, with some experiencing permanent changes.
Protecting Yourself and Your Community
Preventing West Nile virus infection involves avoiding mosquito bites and controlling mosquito populations. Using insect repellents containing DEET, picaridin, or oil of lemon eucalyptus on exposed skin and clothing provides protection. Wearing long-sleeved shirts and long pants, especially during peak mosquito activity hours such as dawn and dusk, minimizes skin exposure.
At a community level, reducing mosquito breeding sites is important. This includes regularly emptying and cleaning containers that collect standing water, such as birdbaths, flower pots, and clogged rain gutters. Ensuring that window and door screens are in good repair helps prevent mosquitoes from entering homes. These measures reduce the risk of WNV transmission and the incidence of neuroinvasive disease.