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Germs that infect humans

West Nile fever

West Nile fever

© Illustration Bruno Laporte

Microorganism: West Nile virus, family: Flaviviridae.

Disease: West Nile fever

Occurrence of the disease

History: the West Nile fever virus was isolated in 1937 in a woman in the West Nile district of Uganda.

Some specialists believe the West Nile virus to have been brought to North America by migrating birds. However, because most birds do not cross the Atlantic Ocean, others believe the virus to have come to North America via imported birds or infected humans.

Current situation: the first cases of West Nile fever in the western hemisphere were reported in 1999 in the state of New York. The epidemic affected 61 people and resulted in seven deaths. Birds and horses were also affected.

Mechanism of action of the microorganism: the virus enters the body through a mosquito bite and infects a variety of tissues. In severe cases, it may attack nerve cells. No cases of West Nile fever have been reported in Canada.

Symptoms of the disease: fever, headache, sore throat, back pain, fatigue, nausea, weight loss, stomach pain, diarrhea and breathing problems. In half the cases, infected individuals also have small pink skin eruptions. The virus may also cause severe encephalitis (inflammation of the brain). The disease is most severe in older women and in immunodeficient individuals.

Incubation period: unknown. Five to 11 days for Japanese encephalitis, a disease similar to West Nile fever.

Contagious period: West Nile fever is usually not transmitted between humans.

Hosts: some birds and mosquitoes.

Transmission: the West Nile fever virus is transmitted from birds to humans by mosquitoes, primarily those of the genus Culex.

Geographical distribution of the microorganism: Africa, Europe, the Middle East, central and west Asia, Oceania, and more recently, North America.

Prevention: control of mosquitoes is the most effective means of preventing transmission of the disease.

Vaccine: none.

West Nile fever resembles other diseases such as Japanese encephalitis and St. Louis encephalitis, and all three diseases are caused by members of the Flaviviridae family. A Japanese encephalitis vaccine, consisting of inactivated viruses obtained from mouse brain, is available.