West Nile Virus

The West Nile virus. Source: Wikimedia Commons' public-domain image from NIH

West Nile virus (WNR) is a virus that can cause an infectious disease in some people. It is spread by infected mosquitoes. Most people who get West Nile virus have no symptoms at all, or they may have a flu-like illness. Rarely (less than 1% of the time), the virus enters the nervous system and causes a very serious illness. The elderly and those with weakened immune systems are at greatest risk of developing this form of the disease.

West Nile virus first showed up in the United States in 1999, but it has long been native to Africa, West Asia, and the Middle East. It is now firmly established in the Western Hemisphere, including the United States.

Symptoms

The symptoms of WNV vary from person to person. Approximately 80 percent of people (about 4 out of 5) who are infected with WNV will not show any symptoms. In this case, the virus does no harm.

Up to 20% of infections cause mild or moderate symptoms that are similar to the flu. These symptoms are sometimes called West Nile Fever. The symptoms include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms may last just a few days, though some healthy people become sick for several weeks. This illness can be impossible to distinguish from other viral illnesses such as Dengue fever without additional testing.

About one in 150 infections cause a condition called West Nile encephalitis (also known as West Nile meningitis and West Nile poliomyelitis). This is the most serious form of the disease. Generally, this form affects people 50 years of age or older. At this point the infection is very serious because the virus has entered the brain. Symptoms of West Nile encephalitis include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, seizures(fits), muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks or a lifetime.

Cause

West Nile virus is a virus native to Africa, West Asia, and the Middle East. It is a flavivirus, which is a family of viruses that contain ribonucleic acid (RNA) instead of DNA as genetic material. Other flaviviruses include St. Louis encephalitis virus, Murray Valley encephalitis virus, Japanese encephalitis virus, dengue virus and yellow fever. West Nile virus infects humans, birds, mosquitoes, horses and some other mammals.

Transmission

The West Nile virus is transmitted (spread to people) in several ways.

Infected mosquitoes

Culex pipiens, a common mosquito responsible for transmitting WNV. Source: Wikimedia Commons, GNU Free Documentation License

Most human infections are caused by mosquitoes. Mosquitoes acquire the virus after biting a bird. The virus circulates in the mosquito for a few days before being deposited in the salivary glands. When the mosquito later bites a human or another animal, it injects saliva and the virus. Symptoms of infection appear after about 2 to 15 days.

Of the 200 species of mosquitoes that live in the United States, 36 have tested positive for WNV. The most common carrier is the Northern house mosquito, Culex pipiens.^[1]

Other means of transmission

Some extremely rare methods of transmission have been reported. In some cases, the WNV was transmitted during blood transfusions or organ transplants. One case the virus was transmitted from mother to child in pregnancy.^[2] A case has also been reported of an infant becoming infected after breast-feeding.^[3] The infant in this case did not get sick. Further investigation of several other similar cases concluded that this means of passing on the disease is probably extremely rare.^[4] Since 2003, all donated blood has been checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants, particularly in people with healthy immune systems, is now small.

How WNV is not spread

The virus is not spread through casual contact such as touching or kissing a person with the virus.

Diagnosis

To be diagnosed, WNV first has to be suspected. Since its symptoms can be vague and similar to many other diseases like the flu, WNV is not always considered. Certain risk factors, such as having lived or traveled in an area with WNV, make diagnosis easier. Diagnosis is aided further when severe symptoms appear, such as high fever, headache, neck stiffness, stupor disorientation, or seizures.

An effort to test for WNV is generally made in people who are older or have weakened immune systems, and who have moderate to severe symptoms that may be West Nile encephalitis. Most people with mild symptoms of possible West Nile fever do not need this test, because even if they do turn out to have WNV, they are likely to recover completely.

Tests

A test can be done to check for WNV in moderately to severely ill patients. Blood or cerebrospinal fluid (from a spinal tap) is taken, and the lab checks the sample for antibodies. These antibodies are produced by white blood cells in order to rid the body of the virus. They appear approximately eight days after infection.

Treatment

West Nile virus is treated with supportive measures that support body function as the virus is attacked by the immune system. As yet, no treatment is available that kills the virus. Treatments often include intravenous (IV) fluids, a breathing tube, and prevention of other infections (e.g., urinary tract infections or pneumonia).

Prevention

The best way to prevent spread of WNV is to protect against mosquito bites. Removing standing water from buckets, birdbaths, pet dishes, pots, tires, wading pools, and other reservoirs of water around the home help eliminate mosquito breeding areas. Mosquitoes can be repelled from skin using insect repellant that contains DEET or picaridin. According to the American Academy of Pediatrics, DEET is safe and effective in young children when used according to directions and when certain precautions are taken. ^[5] Pets should not be treated with DEET because they may ingest it after licking their fur. Mosquito nets are also effective, as is taking precautions during the early morning and evening when the mosquitoes are more likely to bite.

Risk

West Nile virus has received a lot of attention from the media since it was first detected in the United States. The media may have spread unjustified fear of the virus. The risk of developing serious complications from infection is extremely low, and many other diseases spread by mosquitoes are more harmful. The following should be kept in mind when assessing risk:

• By avoiding mosquito bites, most people will not become infected.
• Even in areas where cases have been reported, most mosquitoes are not infected.
• The majority of WNV cases do not produce symptoms. Only one in five infections result in sickness, and most symptoms are mild.

As a basis for comparison, 42,043 people died in car accidents in 2000,^[6] while West Nile virus infection killed 121 people in 2007.^[7] Malaria, another disease transmitted by mosquitoes, kills over 1,000,000 people each year around the world. Most of them are children.^[8]

References

1. ↑ National Wildlife Health Center Mosquitoes and West Nile Virus.Accessed May 7, 2008.
2. ↑ Centers for Disease Control and Prevention. Intrauterine West Nile Virus Infection --- New York, 2002. Accessed May 7, 2008.
3. ↑ Centers for Disease Control and Prevention. Possible West Nile Virus Transmission to an Infant Through Breast-Feeding--- Michigan, 2002. Accessed May 7, 2008.
4. ↑ Hinckley AF, O'Leary DR, Hayes EB. Transmission of West Nile virus through human breast milk seems to be rare. Abstract. Pediatrics. 2007 Mar;119(3):e666-71.
5. ↑ American Academy of Pediatrics. West Nile Virus Information. Accessed My 7, 2008
6. ↑ World Health Organization. Numbers and Rates of Registered Deaths. Accessed May 7, 2008
7. ↑ Centers for Disease Control and Prevention. West Nile Virus--Statistics, Surveillance, and Control. Accessed May 7, 2008.
8. ↑ Centers for Disease Control and Prevention. Malaria Facts. Accessed May 7, 2008.

External links

Food and Drug Administration: West Nile Virus press releases

CDC's West Nile Statistics

National Biological Information Infrastructure: Mosquito Control and West Nile Virus