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West Nile Virus

Overview

West Nile Virus is a viral infection which is transmitted by Culex mosquitos and certain birds. The infection is related to Japanese encephalitis but produces febrile illness and arthropathy. It is also a member of the flavivirus genus and belongs to the Japanese encephalitis antigenic complex of the family Flaviviridae. The majority of individuals infected remain asymptomatic whilst 20-30% present with febrile signs and symptoms. Severe illness can occur in fewer than 1% of infected individuals who develop symptoms of neurological illness.

 

West Nile Virus is found in Africa, Europe, the Middle East, North America and West Asia. In 1999 West Nile Fever was imported in New York which produced a large scale and dramatic outbreak. This outbreak spread throughout the United States of America in following years, highlighting importation and establishment of vector-borne pathogens outside endemic areas represent a serious danger. The largest outbreaks occurred in Greece, Israel, Romania, Russia and USA where there are major bird migratory routes. In addition to humans, the virus can cause severe death and disease in horses.

 

Signs and symptoms

The incubation period usually lasts around 3-14 days after which individuals may present with:

  • Fever

  • Headache

  • Fatigue

  • Myalgia

  • Nausea

  • Vomiting

  • Skin rash (usually on trunk)

  • Swollen lymph glands

 

Severe neuro-invasive disease can develop in <1% of cases. Signs and symptoms include:

  • Headache

  • High fever

  • Stiff neck

  • Stupor

  • Disorientation

  • Tremors

  • Convulsions

  • Muscle weakness

  • Paralysis

  • Coma

Severe disease can occur in any individual but there is increased risk in individuals aged over 50 and those who are immunocompromised.

 

Causes

West Nile virus is maintained in a mosquito-bird-mosquito transmission cycle. The principle vectors of West Nile virus are mosquitoes of the Culex genus, especially those of the Pipiens species. The transmission cycle is maintained throughout mosquito populations through vertical transmission from adults to eggs.

Birds are the reservoir hosts and infection with West Nile virus in Europe,

Africa, the Middle East and Asia is rare. Alternatively, the virus is

highly pathogenic for birds located in areas of America.

Different bird species have varied potential for maintaining the

transmission cycle. Horses are dead end hosts similarly to

humans. Although they can become infected, they cannot transmit

the infection further as they do not develop high levels of the virus in

the bloodstream.

 

Humans are infected with the virus through the bite of an infected mosquito.

Additional transmission routes have been reported but account for very few cases.

These include via blood transfusions, organ transplants, exposure to the virus in a laboratory setting and from mother to baby via pregnancy, delivery or breastfeeding.

 

Risk factors / at risk groups

  • Travel to endemic area

  • Elderly

  • Comorbidities of cancer, diabetes, hypertension, kidney disease

  • Receiving an organ transplant in an endemic country

 

Diagnosis / microbiology testing

  • IgG antibody sero-conversion (or significant increase in antibody titres) by                                                                       enzyme-linked immunosorbent assay (ELISA)

  • IgM antibody capture enzyme-linked immunosorbent assay (ELISA)

  • Neutralisation assays

  • Viral detection by reverse transcription polymerase chain reaction (RT-PCR) assay

  • Virus isolation by cell culture.

 

Treatment

The majority of infected individuals recover from West Nile virus without treatment. Severe cases may require hospitalisation with intravenous fluids and analgesics. There is no current antiviral recommended for West Nile virus. Antivirals would be provided in the cases of encephalitis and meningitis complications.

 

Vaccine / preventative measures

There is currently no vaccine available for West Nile virus so prevention depends on community-based vector control and personal measures.

 

Preventative measures include:

  • Applying insecticides to mosquito breeding sites

  • Avoid outdoor activities at times mosquitos are most active

  • Wear long-sleeved tops and long trousers where possible

  • Stay in air-conditioned or screened accommodation

  • Sleep under a mosquito net if outdoors

  • Apply insect repellent (DEET) to exposed areas of skin

  • Apply permethrin-containing repellent to clothing and bed netting

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