27 July 2004: West Nile Virus Risk

West Nile virus (WNV) infection has been confirmed in two Irish travelers who recently visited the Algarve region of Portugal. In collaboration with the Irish National Disease Surveillance Centre, the cases were confirmed at the HPA specialist laboratory at Porton Down. One of the travellers has completely recovered and the other is improving; neither required hospitalisation.

WNV is a mosquito-transmitted flavivirus; other flaviviral infections include yellow fever, dengue and Japanese encephalitis. Birds are the natural hosts of WNV, and the disease is transmitted to humans by several species of mosquitoes that mainly feed between dusk and dawn. There is no person to person transmission.

The majority of WNV infections (around 80%) are asymptomatic. Less than 20% will experience a mild, self-limited flu-like illness with fever, headache, myalgias and rash. Less than 1% of those infected will develop a more severe neurological syndrome of meningitis and/or encephalitis. Patients with neurologic disease may have headache, neck stiffness, disorientation, muscles weakness, seizures, paralysis or coma. WNV infection is fatal in less than 1 out of 1,000 infections.

The overall risk of WNV in Southern Europe is extremely low. These two cases are believed to be the first human cases of WNV from Portugal, although evidence of infection in humans living in southern Portugal was found through serological surveys in the early 1970s 1. WNV has been reported over the years from many areas of the world including Africa, the Middle East, west and central Asia, and in a number of European countries. A small outbreak of human WNV was reported last summer in Southern France 2. However, the highest recorded activity for WNV currently is in North America where nearly 10,000 cases were reported in the U.S. in 2003 and about 1,300 cases in Canada. As of 20 July 2004 all of the 182 cases of WNV infection in North America this year have been in the US.

Health care professionals should be aware of the signs and symptoms of West Nile Virus and be sure to include a travel history when interviewing patients. Specialist advice should be sought when persons suspected of having WNV infection are evaluated, and specific testing can be performed (for testing, see the link below for the HPA).

Travellers are again reminded to practice insect bite avoidance measures, in particular:

• Limit outdoor exposure during peak times of mosquito feeding - usually the hours from dusk to dawn
• Wear loose-fitting, long-sleeved clothing and long trousers to prevent mosquito bites
• Apply DEET-based insect repellents according to manufacturers' instructions to exposed skin, taking care to avoid eyes and mucous membranes
• Clothing can be treated with a suitable insecticide solution
• Indoor accommodation should be screened or air-conditioned
• Mosquito coils or vapour release devices can be used in enclosed areas

Links to further information

• NaTHNaC: Awareness of insect bite avoidance measures
• HPA guidance on West Nile virus.
• European Commission information on West Nile virus
• United States Centers for Disease Control guidance on West Nile virus.
• Health Canada guidance on West Nile virus.

References

1. Filipe A R, Serological survey for antibodies to arboviruses in the human population of Portugal. Trans Roy Soc Trop Med Hyg. 1974; 68:311-314.
2. Mailles A, Dellamonica P, Zeller H, et al. Human and equine West Nile virus infections in France, August - September 2003. Eurosurveillance Weekly 2003;7.