Her entire lifetime may span only a few weeks, but the female Culex pipiens mosquito makes the most of it. Feeding on the blood of humans and animals gives this common carrier of West Nile virus the protein she needs to produce several hundred eggs every few days--eggs that evolve into biting adult mosquitoes seeking blood meals of their own. And with each bite, an infected mosquito may transmit West Nile virus (WNV).
Most people who become infected with WNV have no symptoms, according to the Centers for Disease Control and Prevention (CDC). About 20 percent develop West Nile fever with its mild, flu-like symptoms: fever, headache, body aches, and sometimes a rash and swollen lymph glands. In a small number of people--about 1 in 150--the virus causes life-threatening inflammation of the brain (encephalitis) or inflammation of the membrane surrounding the brain and spinal cord (meningitis).
There is no evidence to suggest that WNV can be spread through casual contact such as touching or kissing a person with the virus or by handling an animal with the virus, says the CDC. But public health officials are concerned that WNV may spread from person to person by other means.
"Based on ongoing investigations of cases reported this past summer and fall, we believe that West Nile virus can be transmitted by both organ transplantation and blood transfusion," says Jesse Goodman, M.D., M.P.H., director of the Food and Drug Administration's Center for Biologics Evaluation and Research. It is not yet clear how commonly this occurs. In addition, there is limited evidence suggesting the virus may also be transmitted from mother to infant through breast milk.
No tests exist yet for the large-scale screening of donor blood for WNV, nor are there any FDA-approved drugs to treat the virus or vaccines to prevent it. But the health and scientific communities are actively pursuing these medical advances. So while mosquitoes are laying low through the winter in most of the country, federal agencies, public health departments, research facilities, and blood banks are gearing up for the renewal of mosquito season and WNV activity in the spring.
An Insect-Spread Virus
WNV is one of a group of disease-causing viruses called flaviviruses, which are spread by insects, usually mosquitoes. Other flaviviruses include yellow fever, dengue, and St. Louis encephalitis viruses.
WNV primarily circulates between infected birds and mosquitoes that bite them. Only female mosquitoes bite and feed on blood; males feed on nectar. The infected mosquitoes can transmit the virus when they bite other animals or people. (See "West Nile Virus Transmission Cycle," page 26.)
More than 130 species of birds have been reported to be infected with WNV, according to the CDC. The virus also can infect horses and some other animals. (See "Protecting Your Pet Against West Nile Virus," page 27.)
Even in areas where WNV is circulating, not all mosquitoes become infected with it, and human infection does not occur in all individuals exposed to mosquitoes. A study done in 1999 among residents at the height of WNV activity in New York City showed that only 2.6 percent had been infected, says the CDC. Studies elsewhere have shown a lower infection rate, although the rates for summer 2002, the largest outbreak to date in the United States, are currently being calculated.
People who do get West Nile fever typically develop symptoms within three to 14 days after being bitten by an infected mosquito. Symptoms in people whose disease is limited to West Nile fever often go away without treatment in three to six days and do not seem to cause any long-term health effects. The risk of getting the more severe West Nile encephalitis from an infected mosquito is higher for people 50 years of age and older. Many recent cases in transplant and transfusion recipients have been severe and, although unproven, it is likely that people with compromised immune systems are at increased risk for the more severe symptoms of WNV infection. …