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Caused by: A virus.
Most common way people catch it: Mosquito bite.
Worst-case scenario: Death.
How common in the Northeast? The disease is relatively uncommon in people and most infections are mild. The range of the virus is expanding. Because it's mosquito-borne, many people can be exposed to the virus.
Most vulnerable groups: Elders and those with compromised immune systems.
Please note: Researchers are currently seeking answers to some important questions about this virus.
West Nile virus affects more than 70 species of domestic and wild birds (especially American crows jays, hawks, and owls) and mammals (especially people and horses). It's also been found in bats, chipmunks, raccoons, skunks, squirrels, domestic rabbits, mountain goats, and reindeer—and was recently identified as the cause of death of a few American alligators, although we don't yet know how susceptible reptiles are to this virus. Most mammal species do not become sick if infected with West Nile virus.
The virus causes a variety of symptoms, which usually appear in 3–14 days (although the precise incubation period is not yet known). Most common are a group of mild, flu-like symptoms including fever, rash, tiredness, and loss of appetite. A small percentage of people develop a more serious condition, an infection of the central nervous system that may cause encephalitis and meningitis, and in an even smaller group, paralysis or death.
Here's how the numbers are playing out so far: of all the people infected with West Nile virus, about 20–30% become sick. Many fewer, about 1 out of 150, develop a central nervous system infection. Of this unlucky group, about half will experience muscle weakness and about 10% will suffer paralysis—and about 10% will die. Fatalities have been highest among the elderly. Overall, the fatality rate for those infected with West Nile virus is about 1 out of 1500 people (that's less than 0.1% of the people who were infected).
The disease was first reported in the eastern United States in the summer of 1999, although it may have been present before then. It's now believed to be permanently established in the U.S. There's good evidence that birds spread the disease as they migrate.
People primarily catch West Nile virus from the bite of infected mosquitoes. The disease lives in the insects' salivary glands.
Mosquitoes are vectors of West Nile virus, which means that they can transmit the disease, but do not suffer from it themselves. The northern house mosquito, Culex pipiens, is an important vector of West Nile virus but we're not sure what role it may play in the transmission of the virus to mammals. In the Northeast, this is the most common mosquito around the home. There are other ways a person might be exposed to West Nile virus, such as from blood transfusions, organ transplants, and breast milk, or by handling infected birds (the virus may be present in feathers, feces, or dander).
Normally, the virus cycles between mosquitoes and birds. When an infected mosquito bites a bird, it transmits the virus to the bird. The virus circulates in the bird's blood for a few days. Uninfected mosquitoes that bite an infected bird may then pick up the virus, which reproduces in the mosquito's body. Once there's a certain amount of virus in the mosquito's body, it can transmit the virus to another animal.
Mosquitoes don't bite only birds, of course. When an infected mosquito bites a person, horse, or other mammal, it transmits the virus to that animal. The virus then multiplies and may cause illness. But the virus doesn't reproduce effectively in mammals, so later on, an uninfected mosquito biting an infected mammal probably can't pick up the virus. That's why mammals are currently considered "dead-end" hosts for the West Nile virus.
Some species of mosquito prefer to feed on birds, some tend to bite large mammals such as horses, and others bite people. Which mosquitoes find which animals tastiest? Do their preferences change during the season? Researchers are trying to answer these questions, which will help us better understand how, when, and where people are most likely to catch West Nile virus.
Right now, West Nile virus is infecting many birds and horses (in 2002, for example, almost five times as many horses were infected as people). Some wildlife biologists are concerned that West Nile virus could seriously affect the populations of some rare or endangered birds. When dogs and cats are infected, the virus doesn't appear to cause serious illness in them.
People infected with West Nile virus receive general supportive treatment. There is no vaccine for people yet, but specific medications and a vaccine are under development. There is a West Nile virus vaccine for horses produced by Fort Dodge under a conditional license, pending results from studies of its effectiveness and potential side effects. Other equine vaccines are under development. Horses vaccinated against other related viruses, such as eastern equine encephalitis, western equine encephalitis, and Venezuelan equine encephalitis, are not protected against West Nile virus.
Until we have a better understanding of which mosquitoes are most likely to transmit the virus to people, we can't offer a strategy that takes advantage of the particular habits of the vector. For example, some mosquitoes feed during the day while others are active from dusk to dawn. They're found in different habitats. And mosquitoes may not be the only vector of West Nile virus. Scientists are currently studying whether or not the bird louse can transmit the virus to people.
With all these unknowns, what should you do? Avoid mosquito bites and don't handle dead birds with bare hands (wrap them in layers of plastic instead). Wear loose-fitting clothing or mosquito repellent to reduce your chance of being bitten by an infected mosquito. Light colors are less likely to attract mosquitoes. Long sleeves aren't a bad idea, either. And stay tuned!
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