West Nile virus

West Nile virus
Name	West Nile virus
Caption	Culex pipiens, a primary vector
Field	Infectious disease
Symptoms	Often asymptomatic; fever, headache, meningitis, encephalitis
Complications	Neurological damage, death
Onset	2–14 days post-exposure
Duration	Weeks to months for severe cases
Causes	Infection via mosquito bite
Risks	Outdoor activity, older age, immunocompromised
Diagnosis	Blood test, cerebrospinal fluid analysis
Prevention	Mosquito control, repellents
Treatment	Supportive care
Medication	None specific
Prognosis	Generally good; poor for neuroinvasive disease
Frequency	Seasonal outbreaks in Africa, Europe, Asia, North America
Deaths	~7% of neuroinvasive cases

West Nile virus. It is a single-stranded RNA virus belonging to the Flaviviridae family and the Flavivirus genus. The virus is maintained in a cycle primarily between mosquitoes and birds, but can infect humans and other mammals, often causing no symptoms. In a minority of cases, it can lead to severe neurological diseases such as encephalitis or meningitis.

Virology

The virus is an enveloped virus with an icosahedral capsid, approximately 50 nm in diameter. Its genome consists of a single strand of positive-sense RNA that encodes three structural proteins and seven non-structural proteins. It is closely related to other medically significant flaviviruses, including Japanese encephalitis virus, Saint Louis encephalitis virus, and Zika virus. The virus demonstrates significant genetic diversity, with two main lineages; lineage 1 is distributed globally and includes the strain introduced into North America, while lineage 2 is historically associated with sub-Saharan Africa but has caused outbreaks in Europe. Replication occurs in the cytoplasm of host cells.

Transmission and vectors

Primary transmission occurs through the bite of infected mosquitoes, primarily of the Culex genus, such as Culex pipiens and Culex quinquefasciatus. The virus is maintained in an enzootic cycle between these mosquitoes and reservoir bird hosts, including corvids and house sparrows. Humans, horses, and other mammals are considered dead-end hosts. Rare non-vector routes of transmission include blood transfusion, organ transplantation, transplacental infection, and possibly through breast milk. The Centers for Disease Control and Prevention monitors these routes. Outbreaks often correlate with mosquito abundance and specific climatic conditions.

Signs and symptoms

Approximately 80% of human infections are asymptomatic. About 20% develop West Nile fever, a febrile illness with symptoms such as fever, headache, myalgia, fatigue, and occasionally a maculopapular rash or lymphadenopathy. Less than 1% of infected individuals develop severe neuroinvasive disease, which may present as encephalitis, meningitis, or acute flaccid paralysis resembling poliomyelitis. Severe symptoms can include high fever, neck stiffness, disorientation, tremor, seizures, coma, and permanent neurological damage. Risk factors for severe disease include advanced age, immunocompromised status, and conditions like hypertension and diabetes.

Diagnosis and treatment

Diagnosis is typically confirmed by detecting IgM antibodies in serum or cerebrospinal fluid via ELISA; confirmation may involve plaque reduction neutralization test at the Centers for Disease Control and Prevention. Reverse transcription polymerase chain reaction can detect viral RNA, especially in immunocompromised patients. Magnetic resonance imaging may show enhancements in the basal ganglia or thalamus in severe cases. There is no specific antiviral drug; management involves supportive care, which may include hospitalization, intravenous fluids, analgesics, and antipyretics. For neuroinvasive disease, intensive care with mechanical ventilation may be required.

Prevention and control

Primary prevention focuses on personal protection against mosquito bites using repellents containing DEET, wearing protective clothing, and avoiding peak biting times. Community-level strategies involve integrated mosquito control: larvicide application to breeding sites like storm drains, and adulticide spraying. Surveillance programs, such as those coordinated by the World Health Organization, monitor dead birds, mosquito pools, and veterinary cases in horses. There are licensed vaccines for equines, but no human vaccine is commercially available, though several candidates are in clinical trials. Public health messaging is critical during outbreak seasons.

Epidemiology

The virus was first isolated in 1937 from a febrile woman in the West Nile subregion of Uganda. It has since been identified across Africa, Europe, Asia, and Australia. A significant turning point was its introduction into New York City in 1999, leading to rapid spread across the continental United States and into Canada and Latin America. Major outbreaks have occurred in Israel, Greece, Romania, and Russia. In the United States, states like Texas, California, and Colorado often report high case numbers. Incidence is seasonal, peaking in late summer and early fall, influenced by temperature, rainfall, and bird migration patterns.

Category:Flaviviruses Category:Arthropod-borne diseases Category:Notifiable diseases