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. Public health experts agree that this mosquito-borne infection won't go away.
"It's here to stay," says Richard Pollack, Ph.D. Pollack is a research fellow in infectious diseases at the Harvard School of Public Health.
The number of reported human cases reached a peak of 9,862 in 2003, according to figures from the U.S. Centers for Disease Control and Prevention (CDC). The cases that year included 264 deaths.
Since then, the number of human cases has fallen. Between 2004 and 2007, there were 2,500 to 4,500 cases a year. Human cases have been seen in every state except Alaska and Hawaii. As of December 11, there were 3,404 reported cases for 2007. They included 98 deaths.
Mild and Serious Cases
"West Nile virus is responsible for many human deaths and illnesses each year," Pollack says. 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.
In 2007, 33% of the cases reported in the United States were cases of serious illness involving the central nervous system. About 65% were confirmed as cases of West Nile fever. It is likely there were many more cases of West Nile fever, but they were not diagnosed or not reported.
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, Pollack says. 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, Pollack says.
"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 "to an incredible extent," Pollack says. "Some birds succumb to the infection and will 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 U.S. 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. "We do have a strong indication as to where it actually came from," Pollack says. "The strain that was isolated in New York was virtually identical to a strain that was isolated the previous year in Israel."
Pollack says the virus could 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.
His favorite theory is that a West Nile-infected mosquito hitched a ride inside an airplane.
Future Cycles
Future outbreaks in any particular area are likely to occur in cycles, Pollack says.
"When West Nile arrives at a new region you have a tremendous population of what we call naοve hosts. These birds have never seen this infection before and are susceptible," he says. "So it may infect lots of birds." The birds infect more mosquitoes. They in turn may spread the virus to people and other animals.
"But the survivors are then essentially immune. They will moderate the spread of the infection in the future," Pollack says. Eventually the immune birds in an area become old and die. They are replaced by other birds that are not immune. Then, another local outbreak becomes more likely, he says.
Prevention
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.
