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West Nile Virus

West Nile virus is a newly emergent virus of the family Flaviviridae, found in both tropical and temperate regions. It mainly infects birds, but is also the cause of a number of conditions in humans, horses, and some other mammals. It is transmitted by bites of infected mosquitoes.

Photographs from a high resolution electron microscope published in the journal Science reveal the virus as spherical with a slightly bumpy surface but no projecting protein arms. It is said to have a strong resemblance in appearance to dengue fever virus.

west nile virus symptoms

In most people (80%), infection causes no symptoms. In others, west nile symptoms can be seen as mild flu-like symptoms known as West Nile fever. The virus is able to pass the blood-brain barrier, and the most serious effects (in 0.7% of the infected) are encephalitis (inflammation of the brain) and meningitis (inflammation of the lining of the brain and spinal cord), both of which can be fatal. Persons over 50 years of age are at higher risk of developing severe disease, the symptoms of which include fever, nausea, stiff neck and changes in mental status. In rare cases (first reported October 2002), patients develop sudden-onset paralysis.

Symptoms of west nile develop 3-14 days after infection. No effective treatment is known.

The disease can be diagnosed by employing an ELISA test detecting IgM antibodies against the virus. However, several related viruses also cause encephalitis and result in similar antibodies.

The virus is mostly maintained in birds. Female mosquitos, mainly of the species Culex pipiens, Culex restuan and Culex quinquefasciatus, bite infected birds, carry the virus in their salivary glands, and infect other birds when they bite again. Culex pipiens is thought to be the main mosquito species transmit the virus from birds to mammals. In mammals the virus does not multiply as readily, and it is believed that mosquitos biting infected mammals do not further transmit the virus. A paper in the journal Science in 2004 found that Culex pipiens mosquitos existed in two population in Europe, one which bites birds and one which bites humans. In North America 40% of Culex pipiens were found to be hybrid of the two types which bites both birds and humans, providing a vector for West Nile virus. This is thought to provide an explanation of why the West Nile disease has spread more quickly in North America than Europe.

It was initially believed that direct human-to-human transmission was impossible, but in 2002 the Centers for Disease Control and Prevention (CDC) discovered the transmission of West Nile virus through blood transfusion and organ transplants as well as through breast milk and prenatal infection.

There is no vaccine for humans. A vaccine for horses based on killed virus exists; some zoos have given this vaccine to their birds, although its efficacy there is unknown.

Posted by Staff at May 18, 2005 1:07 AM

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