West Nile Virus

West Nile virus is an RNA virus and a member of the Flaviviridae family:

• It is transmitted by culicine mosquitoes, which generally prefer to bite birds but will sometimes bite and infect horses or humans¹.
• It is a neuropathogen of humans, horses, birds (wild birds appear to be optimal hosts), dogs, and other mammals. It was first identified in Uganda in 1937.
• Animal to human transmission is not known. Human to human transmission is rare, but there have been cases of infection occurring transplacentally, via breast feeding, from blood transfusions (testing has been introduced in the UK for those visiting USA/Canada, and replaces the previous restriction on donation for 28 days), and following organ transplantation.

• West Nile virus has been found in Africa, Europe, the Middle East, west and central Asia, and more recently, North America.
• The peak transmission season is during the summer months.
• There have been increasing frequency of outbreaks in humans and horses, an apparent increase in severe human disease, and high bird death rates accompanying human outbreaks.
• Sporadic cases and outbreaks of disease in humans and horses have occurred in Europe since the 1960s but the risk of contracting West Nile virus throughout Europe is considered to be very low.
• There has been no active disease in birds and no reported human cases to date in the UK. The risk of West Nile virus causing human cases in the UK is considered to be very low.

• The incubation time is usually between 3 and 15 days.
• 80% of those infected have no symptoms and 20% have just a mild influenza-like illness (fever, headache, generalised muscle pains).
• Less than 1% of cases develop more severe disease. Severe infection may cause encephalitis², meningitis or a polio-like infection with flaccid paralysis. Patients may suffer headaches, fever, stiff neck, sore eyes, disorientation, muscle weakness, convulsions and coma.

• Should be considered as an unlikely but possible cause of encephalitis and viral meningitis in the summer months in the UK.
• The diagnosis is more likely if the person has very recently returned from travel to a warmer climate.

• IgM antibodies (enzyme-linked immunosorbent assay - ELISA) measured in blood or CSF and the test is positive in most infected people within eight days of the onset of symptoms.
• Because of serological cross-reactivity with the viruses mentioned in the introduction, both acute and convalescent phase serum is often required, or use of plaque reduction neutralisation test (PRNT).
• Unlike herpes simplex virus encephalitis, CT and MRI do not show specific localisation.

• There is no specific antiviral treatment for West Nile virus infection.
• In more severe cases, high dependency care, including ventilation, is required.
• Trials with ribavirin had no effect on mortality, and results of interferon alpha-2b in USA are not yet known.
• Rehabilitation of survivors with neurological sequelae is required.

• Most deaths have been reported in those over 50 years old, who generally suffer more severe disease than younger patients.
• Overall fatality rates vary from 4-14%, rising to 48% of encephalitis patients in an outbreak in Russia.

• Reduce mosquito bites: Limit outdoor exposure during peak times of mosquito feeding (usually from dusk to dawn), wear loose fitting, light-weight clothing that covers up skin as much as possible, use an effective insect repellent (those containing DEET are considered the most effective), air conditioning, insect-proof screens on windows and doors, bed nets and spraying the room with insecticide.
• No vaccine is available. Low incidence would not make one cost-effective.
• Surveillance programmes for mosquito, bird, and horse infection, control of larval breeding sites, and targeted spraying of mosquitoes if disease activity is detected.