Mosquito sitting on someone's arm

West Nile Fever

What is West Nile fever?

West Nile fever is an infection contracted from the Culex virus, which belongs to the Japanese encephalitis antigenic complex of the family Flaviviridae.

How is West Nile fever spread?

West Nile fever is most commonly spread by mosquitoes, which feed on infected animals (primarily birds) and then pass the virus on to humans and other animals through their bite.

A small proportion of human infections have also been caused by organ transplant, blood transfusions and breastfeeding and in one instance by transplacental (mother-to-child) transmission.

West Nile fever is most commonly contracted during warm weather when mosquitoes are most active, especially in Africa between June and September and the incubation period ranges from two to 14 days.

Who is at risk of contracting West Nile fever?

First identified in the West Nile district of Uganda in 1937, West Nile fever has now been reported in more than 50 countries around the world including Africa, parts of Europe, the Middle East and West Asia. The largest outbreaks have occurred on major bird migratory routes in Greece, Israel, Romania, Russia and the USA.

While fewer than 1% of people infected in these parts of the world develop symptoms, age is a significant risk factor and the older you are (particularly over 60), the more likely you are to develop symptoms. Certain medical conditions also increase your risk of developing severe symptoms including people with kidney conditions, diabetes, hypertension, cancer and impaired immune systems.

What are the symptoms of West Nile fever?

While 8 out of 10 people infected with West Nile fever do not develop any symptoms at all, 1 in 5 develop a fever with symptoms such as headaches, body aches, joint pains, vomiting, diarrhoea or rashes.

Roughly 1 in 150 people develop more severe illnesses affecting the central nervous system such as encephalitis and meningitis, displaying serious symptoms such as high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis.

How is West Nile fever diagnosed?

West Nile fever is diagnosed by a simple blood test, which detects genetic material or antibodies in the blood associated with the virus. If the patient’s symptoms are brain-related, a doctor may also order a lumbar puncture, MRI and other scans as necessary.

Are there any treatment options for West Nile fever?

Currently there is no vaccine for West Nile fever and treatments are only to alleviate symptoms including over-the-counter medicines to relieve headache and fever such as aspirin and paracetamol.

In severe cases, patients may need to be hospitalised and receive supportive treatment such as intravenous fluids, pain medication, respiratory support and medications such as antibiotics to prevent secondary infections.

Can West Nile fever be prevented?

Without a vaccine for West Nile fever, the best means of protection from the virus is to reduce the likelihood of outbreaks. This can be achieved by:

  • Animal health surveillance to detect new cases in birds and horses and provide early outbreak warnings.
  • Public health educational messages focusing on reducing the risk of mosquito transmission through the use of mosquito nets and insect repellent, wearing long-sleeved clothing and avoiding outdoor activity at peak biting times (dawn and dusk).
  • Community programmes encouraging at-risk communities to destroy mosquito breeding sites in residential areas.
  • Wearing gloves and other protective clothing while handling sick animals or their tissues, particularly during slaughtering and culling procedures.
  • Reducing the risk of transmission through blood transfusion and organ transplant.
  • Conducting mosquito surveillance and control programmes in areas where the virus occurs, including source reduction, water management and chemical and biological control methods.
  • Preventing infection in healthcare settings by ensuring healthcare workers caring for patients with West Nile fever or handling specimens adopt standard infection control precautions.

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