West Nile virus: epidemiology, transmission, and clinical manifestations

West Nile Virus is form of encephalitis only found in the United States within the last three years. It is spread by mosquitoes. The mosquito bites an infected animal, often a bird, and then bites a human being, infecting that person with the virus. The first case of West Nile Virus was reported in New York City in 1999, but by the end of last summer had reached virtually all but a few western states (Gorman, 2002).

West Nile Virus has infected far more people than has been reported in the media because most people have either very mild symptoms or none at all. About 20% have headache and fever without any further severe illness or complications (CDC, 2002). Only one in 150 people who contract the disease become sick enough to have the blood tests necessary for accurate diagnosis (Mechcatie, 2001). Most people who become seriously ill with West Nile Virus are older. They first had a fever with some gastro-intestinal discomfort. They may have swollen glands and a rash on their bodies. For some reason, the disease then makes them sicker including neurological symptoms, including muscle weakness or paralysis. These patients will also have a high fever, stiff neck, mental confusion and even convulsions. Some have had to be put on ventilators because they could no longer breathe for themselves (Mechcatie, 2001)

For the great majority of people, symptoms will only last a few days, but those who develop the severe form of the disease may be sick for a month or more, and they may have some permanent neurological damage (CDC, 2002).

Incubation, or the period between when the person is bitten and when first symptoms appear, is usually between 3 and 14 days (CDC, 2002). Because of the disease's spread and the seriousness of it in its more severe form, physicians have been warned to be alert for it in their patients, especially in those states where the virus is spreading. That makes a diagnostic problem for doctors because West Nile Virus can look like any number of other diseases. The CDC warns doctors, "No single sign "will let the physician know the patient has West Nile as opposed to an enteroviral or other cause of encephalitis."(Mechcatie, 2001) The symptoms are vague, physicians cannot tell from a physical examination alone whether the person has the disease or not. In addition, it's possible to contract other forms of encephalitis as well.

However, the Center for Disease Control notes one symptom all people should be aware of. If they experience unexplained lethargy, they should check with their doctor immediately, as it could be a sign of the more serious form of the disease. While all deaths have occurred in older people or people with impaired immune system, early treatment is important for all people who have developed the neurological form of West Nile Virus.

The CDC has provided funds to all states with West Nile Virus to do the necessary tests for confirmation of a diagnosis. Those tests include blood tests and an examination of cerebrospinal fluid. Both tests are necessary, because one test will respond positively for dengue fever, yellow fever and other kinds of encephalitis as well as West Nile. The CDC recommends follow-up tests after several weeks.

Humans are not the only animals affected by West Nile Virus. Birds, especially American crows, are highly susceptible to the virus and are believed to be the reason for the rapid spread of the disease across the country, because crows have the ability to fly significant distances. Out of over 4,000 birds infected with West Nile Virus in 2000, almost 90% were crows (Mechcatie, 2001).

Since as yet there is no vaccine against the disease for human, the only way to prevent the disease is to prevent mosquito bites. The CDC has a number of suggestions for people to minimize their risk for acquiring West Nile Virus.

When people are outdoors, they should use insect repellent with DEET (N, N-diethyl-meta-toluamide). This chemical has been proven in research to be the most effective mosquito repellant. In addition, people are advised to cover as much skin as possible, using full-length pants and long-sleeved shirts. The clothes should be sprayed with insect repellant also, or mosquitoes may be able to bit them through their clothing. If the clothes aren't sprayed, then the skin under the clothes should be (CDC, 2002). They also recommend staying indoors when mosquitoes are most likely to be out: dawn, and dusk through early evening.