Mosquito-borne virus has become widespread since 1999.
By Lisa Ellis InteliHealth Staff Writer
The name evokes images of African deserts and rain forests, but West Nile virus quickly has made itself at home across the United States.
The first U.S. cases appeared in New York in 1999. Since then, the virus has spread from coast to coast, with more than 30,000 reported cases. Public health experts agree that this mosquito-borne infection won't go away.
West Nile virus is responsible for a variable number of human deaths and illnesses each year. Most people who acquire West Nile virus, however, don't get sick. Most who do get sick don't become profoundly ill.
The CDC estimates that less than 1% of people infected with West Nile virus become severely ill. These cases can involve inflammation of the brain or the tissues that cover it (encephalitis or meningitis). Some of the most serious cases can also result in a type of muscle paralysis that resembles polio.
According to the CDC, about 20% of those infected will develop West Nile fever. This illness is similar to the flu. Symptoms can include fever, headache, fatigue, muscle pain, eye pain, rash, nausea and vomiting. West Nile fever can be debilitating. People who are affected may miss school or work.
Most Americans who are infected with West Nile virus do not have any symptoms. Many people probably will never know that they were infected. Even so, because of the small chance of serious illness, it is still important to take precautions by avoiding mosquito bites.
How West Nile spreads
West Nile is spread through the saliva of mosquitoes. In this country, the main culprits are Culex mosquitoes. Other kinds of mosquitoes also may be involved.
Culex mosquitoes favor foul, stagnant water in leaf-choked gutters, storm drains and disused pools. Other kinds of mosquitoes prefer the edges of shallow ponds, wetlands, flooded areas next to rivers, or even water-filled tree holes or tin cans.
Only the female mosquito feeds on blood. Culex mosquitoes tend to prefer bird blood, but they are not always so picky.
If given the choice, they'll probably go after a bird. Mostly, West Nile virus bounces back and forth between birds and Culex mosquitoes. But if they're hungry and you're in the way, some of these mosquitoes will also feed on people, dogs, raccoons, just about any vertebrate animal.
Once inside a suitable bird, the virus may multiply rapidly. Some birds die. Others tolerate it quite well and may then infect many more mosquitoes. It is likely that many tens of thousands of birds die each year in the United States because of this virus. Crows are particularly susceptible. This virus is said to have devastated crow populations in many areas.
In addition to mosquito transmission, in 2002 nearly two dozen people got sick from West Nile virus spread by blood transfusion. In 2003, blood banks began testing for the virus. This has reduced the risk of transfusion-related infections.
How did it get here?
The West Nile outbreak in the United States began in the late summer of 1999 in the New York City area.
Originally, CDC officials thought they were dealing with St. Louis encephalitis. This is another virus transmitted by mosquitoes. Then officials realized that birds also were dying. St. Louis virus normally doesn't kill birds. Tests revealed that the culprit was something unexpected West Nile virus.
This virus was well known in Africa, Europe, the Middle East and parts of Asia. How did it get to the United States?
Researchers don't really know. It may have been carried by a person who was not aware of being infected, a migrating bird blown off-course by a storm, or a bird (or other animal) smuggled into the country.
Future outbreaks in any particular area are likely to occur in cycles.
When mosquitoes infected with West Nile first arrive in a new region, the birds in that region have not been exposed to the virus. Lots of birds quickly get infected when the mosquitoes feed on the birds.
Newly hatched mosquitoes are feeding on a larger population of infected birds. And these mosquitoes also bite humans and other animals. Now you have a local outbreak.
Over time, many of the birds in the region develop immunity to West Nile virus. So there are fewer infected birds. The outbreak largely subsides.
But eventually the immune birds become old and die. They are replaced by other birds that are not immune. Mosquitoes carrying West Nile virus have a new bird population to feed on, starting a new cycle.
Public health officials across the country have been dealing with West Nile, in part, by monitoring bird and mosquito populations. They test a proportion of the dead birds found in an area to detect the virus. They also may sample and test mosquitoes.
Officials also are making more intense efforts to eliminate standing water or treat areas that can breed mosquitoes. These can include anything from storm drains and ponds to backyard breeding grounds such as open garbage cans and birdbaths.
In many areas, officials have used insecticides to kill mosquito larvae. If an outbreak has occurred, sometimes they have sprayed to kill adult mosquitoes.
For advice on what you can do at home to protect yourself from mosquito-borne illnesses, read our tips on staying safe.