Infection: West Nile Virus Infection
Infection: West Nile Virus Infection
West Nile virus (WNV) infection is a disease spread by mosquitoes. Severe forms of the infection may be called West Nile encephalitis (if brain tissue is inflamed) or West Nile meningitis (if the membranes covering the brain become inflamed).
West Nile virus infection is caused by a virus related to those that cause yellow fever and dengue. It can be classified as a zoonosis because it is a disease that can be spread to humans from animals. It is also called an emerging infectious disease (EID) because it has become more widespread in the last 20 years and is likely to become more common in the future. Most people who become infected with the virus have no noticeable symptoms; about 20 percent develop a mild flu-like illness that lasts about a week; a few people develop a serious illness as a result of WNV causing inflammation of the central nervous system (the brain and spinal cord).
WNV is increasingly widespread in both tropical and temperate regions of the globe. In North America, epidemics of WNV are most likely to occur in August and September, when mosquitoes are most active. The disease is not a serious infection in 80 percent of people exposed to it; out of 151 people who become infected with the virus, 110 will not notice any symptoms at all; forty will be sick for about a week; and one person will become severely ill. The risk of severe illness in North America is greatest among people over fifty, pregnant women, people who have recently had cancer chemotherapy, people with diabetes, and people with weakened immune systems. As far as is known, WNV affects all races and both sexes equally.
The West Nile virus is thought to have developed in Uganda about a thousand years ago. It was first identified as a distinct cause of disease in 1937 in a feverish woman in central Africa by researchers who were studying yellow fever. WNV was not described as a cause of severe disease in older adults until an outbreak took place in Israel in 1957. The virus reached North America some time in the 1990s; the first outbreak that attracted public attention occurred in New York, New Jersey, and Connecticut in 1999. The disease has since been reported across the United States. According to the Centers for Disease Control and Prevention (CDC), the total number of deaths in the United States from WNV between 1999 and 2007 is 1,060; most have been in elderly people.
West Nile virus infection is caused by a virus transmitted from infected birds to humans through mosquitoes that first bite the birds and then bite people. Crows, ravens, robins, and blue jays are the birds most likely to carry the virus; the CDC reported in 2007 that at least 138 different species of birds in North America have been found to carry WNV, and forty-three different species of mosquitoes. The first major outbreak of West Nile infection in the United States was preceded by the sudden death of large numbers of birds in the New York City area.
The Mysterious Death of Alexander the Great
One medical puzzle of ancient history is the death of Alexander the Great (356–323 BCE) at the young age of thirty-two. Alexander returned to Babylon in the spring of 323 BCE after his conquest of India. He fell sick with a fever and died after a two-week illness, which has been variously attributed to typhoid, influenza, liver failure caused by heavy drinking, and poisoning by enemies in his court.
In 2003, however, two researchers in Virginia and Colorado suggested that Alexander might have died from West Nile encephalitis. They noticed an interesting detail recorded by the Greek historian Plutarch: “When [Alexander] arrived before the walls of [Babylon], he saw a large number of ravens flying about and pecking one another, and some of them fell dead in front of him.” Epidemics of WNV are often preceded by the death of infected birds, so the American researchers thought that Alexander's symptoms might have been related.
Others disagree, saying that the time course of West Nile encephalitis does not fit what is known of Alexander's last days. The mystery is not likely to yield a final solution, however, as doctors in the ancient world did not use the same terms that are used today to describe symptoms of illness, and translators of ancient Greek documents do not always agree about the meaning of specific words.
The virus can be transmitted by mosquitoes to bats, horses, dogs, cats, squirrels, skunks, and rabbits as well as to people. The mosquitoes that carry the virus are called vectors, and the infected animals that harbor the virus are called hosts. The virus can also be transmitted by blood transfusions, organ transplants, and from a nursing mother to her baby through breast milk. People cannot get WNV, however, by touching or kissing someone with the virus; there are also no reported cases of people getting West Nile infections from household pets.
When a mosquito carrying WNV bites a human, the virus is transmitted directly into the bloodstream. Once in the bloodstream, WNV multiplies for an incubation period of three to fourteen days. The 20 percent of people who feel symptoms of WNV infection typically have chills, a low-grade fever, headache, sore throat, and nausea and vomiting. About half these patients will also develop a mild rash on the upper chest. In most cases the illness lasts about a week, although the person may feel tired for several weeks afterward.
West Nile encephalitis or meningitis develops when the virus reaches the tissues of the brain or the tissues that cover the brain and the spinal cord and multiplies there. People with this severe form of West Nile infection develop a stiff neck, severe headaches, and feel mentally confused. They have weak or partially paralyzed muscles and may lose consciousness.
The symptoms of WNV are similar to those of other viral infections. A definite diagnosis can be provided by testing blood or spinal fluid for antibodies against the virus. Since 2003, the Food and Drug Administration (FDA) has approved two rapid diagnostic tests for detecting the West Nile virus; one, developed in Australia, gives results in two hours. The other test was developed in Canada and was approved for use in the summer of 2007; it gives results in only fifteen minutes.
There is no specific treatment for West Nile virus. Most people who develop fever, nausea, or other symptoms recover without seeing a doctor because the symptoms are similar to those of other mild viral infections. The National Institute of Allergy and Infectious Diseases (NIAID) is currently testing a new drug treatment for WNV that was expected to complete its Phase I clinical trial by the end of 2008.
People who develop West Nile encephalitis or West Nile meningitis should be taken to the hospital for specialized treatment to help them breathe. They may also be given intravenous fluids to prevent dehydration and intensive nursing care.
Most people who have a mild form of West Nile virus infection recover in about a week with no long-term health problems. Those with severe infections, however, have a 10 percent risk of dying. Those who survive may suffer permanent brain damage or paralysis similar to that caused by polio.
There is no vaccine against WNV for humans that has completed clinical trials, although a vaccine for horses has been developed and approved for use. The most important precautions that humans can take are controlling mosquitoes and preventing themselves from being bitten by mosquitoes. The CDC recommends the following steps:
- Once a week, drain water from birdbaths, outdoor buckets, flower pots, swimming pool covers, and other water-filled containers where mosquitoes can lay their eggs and breed. Empty children's wading pools when they are not being used.
- Use insect repellent when outdoors; apply it to exposed skin and spray clothing with repellent.
- Wear long-sleeved shirts and long pants when the weather permits.
- Try to stay indoors in early morning and early evening, because mosquitoes are most active at those times of day.
- Keep window and door screens in good repair so that mosquitoes cannot get inside the house.
- Cover baby strollers or carriers with mosquito netting when taking the baby for a walk outdoors.
- Do not handle any dead birds found outside. Contact the local health department first and ask for instructions on reporting and disposing of them.
It is likely that a vaccine for humans and a specific treatment for West Nile virus will be approved by the FDA within the next few years. About
450 different chemicals that appear to be effective against viruses have been identified since 2003 as promising enough to justify further testing. There were also two potential vaccines against WNV undergoing clinical trials at the NIAID Vaccine Research Center in Maryland in 2008.
SEE ALSO Encephalitis; Meningitis
WORDS TO KNOW
Dengue: A tropical disease caused by a virus similar to the virus that causes West Nile infection. It is also spread by mosquitoes.
Emerging infectious disease (EID): A disease that has become more widespread around the world in the last twenty years and is expected to become more common in the future.
Encephalitis: Inflammation of the brain.
Host: An organism that is infected by a virus, bacterium, or parasite. Birds, humans, and other animals may be hosts of West Nile virus.
Meningitis: Inflammation of the membranes that cover the brain and line the brain and spinal cord.
Vector: An animal that carries a disease from one host to another.
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Centers for Disease Control and Prevention (CDC) BAM! (Body and Mind) website. The Buzz-z-z on West Nile Virus. http://www.bam.gov/sub_diseases/diseases_westnile.html (accessed April 10, 2008). BAM! Is the CDC's web-site for children in elementary school and junior high.
Centers for Disease Control and Prevention (CDC). West Nile Virus Home Page. http://www.cdc.gov/ncidod/dvbid/westnile/index.htm (accessed April 10, 2008).
United States Geological Survey (USGS). West Nile Virus Disease Map, Human. http://diseasemaps.usgs.gov/wnv_us_human.html (accessed April 9, 2008). Viewers can click on their state on the map of the United States to obtain updated information about reported cases of West Nile virus in their area. There is also a link to a disease surveillance map of Canada.