West Nile Fever

West Nile fever is an infection caused by the West Nile virus (WNV), typically spread by mosquitoes. This disease can range from asymptomatic to severe, involving the central nervous system.

Signs and Symptoms

Approximately 80% of those infected with WNV show no symptoms. About 20% develop symptoms ranging from mild to severe, appearing 3 to 14 days post-bite. Mild symptoms include fever, headache, chills, excessive sweating, weakness, fatigue, swollen lymph nodes, and flu-like symptoms.

Gastrointestinal symptoms like nausea, vomiting, loss of appetite, and diarrhoea may occur. Fewer than one-third of patients develop a rash. Severe cases, less than 1%, involve the central nervous system, leading to meningitis, encephalitis, or meningoencephalitis, with symptoms such as muscle weakness, stiff neck, confusion, seizures, and paralysis.

Medical conditions like cancer, diabetes, and age over 60 years increase the risk for severe symptoms.

Cause

Virology

WNV is part of the Japanese encephalitis antigenic serocomplex of viruses. The virus is a positive-sense, single-stranded RNA virus encoding seven nonstructural proteins and three structural proteins. It replicates rapidly and spreads more easily to birds at higher temperatures.

Transmission

WNV primarily spreads through mosquitoes, especially those feeding on infected birds. Rarely, it can be transmitted through blood transfusions, organ transplants, and from mother to baby during pregnancy, delivery, or breastfeeding. Vertical transmission is also observed, where the virus is passed from the female mosquito to her offspring.

Diagnosis

Diagnosis typically involves serologic testing of blood serum or cerebrospinal fluid (CSF) for virus-specific IgM and neutralising antibodies. Initial screening can use the ELISA technique. In cases of suspected WNV, specimens should be collected during both acute and convalescent phases of illness. Definitive diagnosis is based on detecting virus-specific IgM. In fatal cases, nucleic acid amplification, histopathology with immunohistochemistry, and virus culture of autopsy tissues are used.

Prevention

Preventing WNV involves reducing mosquito populations and avoiding bites. General measures include using insect repellent, wearing protective clothing, and using mosquito nets. Mosquito repellents such as DEET and picaridin are effective, and permethrin-treated clothing offers additional protection. Staying indoors during peak mosquito activity times, typically dawn and dusk, and ensuring living areas are well-screened or air-conditioned are also recommended.

Monitoring and Control

WNV can be monitored through trapping mosquitoes and testing blood samples from birds and animals. Control measures include eliminating mosquito breeding sites and applying larvicides and pesticides. However, environmentalists warn against pesticide spraying due to potential health risks and limited effectiveness.

Treatment

No specific treatment for WNV exists. Most cases recover without treatment, though over-the-counter pain relievers can help with mild symptoms. Severe cases may require hospitalisation, intravenous fluids, pain medication, respiratory support, and prevention of secondary infections.

Prognosis

While the overall prognosis is favourable, recovery can be prolonged, taking up to 60 to 90 days, with long-term fatigue and other symptoms. Neuroinvasive WNV infections can increase the risk for subsequent kidney disease. The general outlook is generally positive, but severe cases may lead to permanent brain damage or death.

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Self-assessment MCQs (single best answer)

What percentage of people infected with West Nile virus show no symptoms?

Which of the following is a common mild symptom of West Nile fever?

What is the primary mode of transmission for the West Nile virus?

Which diagnostic method is typically used for detecting West Nile virus?

What type of mosquito is most commonly associated with the spread of West Nile virus?

Which of the following is a severe symptom of West Nile fever involving the central nervous system?

What preventive measure can help reduce the risk of West Nile virus infection?

What is the primary vector for the West Nile virus?

Which of the following is NOT a common symptom of mild West Nile fever?

How long can recovery from West Nile fever take in severe cases?