It is striking that the New World has been spared West Nile virus for so long. The flow of people
across the Atlantic and Pacific was not enough to carry the virus to the Americas. Scientists cannot
say exactly how West Nile virus finally landed in New York in 1999, but they have a few clues. The
New World strain of West Nile virus is most closely related to viruses that caused an outbreak in
birds in Israel in 1998. It’s possible that pet smugglers brought infected birds from the Near East to
New York.
On its own, a single infected bird could not have triggered a nationwide epidemic. The viruses
needed a new vector to spread. It just so happens that West Nile viruses can survive inside 62 species
of mosquitoes that live in the United States. The birds of America turned out to be good hosts as well.
All told, 150 American bird species have been found to carry West Nile virus. A few species, such as
robins, blue jays, and house finches, turned out to be particularly good incubators.
Moving from bird to mosquito to bird, West Nile virus spread across the entire United States in just
4 years. And along the way, people became ill with West Nile virus as well. About 85 percent of
infections in the United States cause no symptoms. The other 15 percent of infected people develop
fevers, rashes, and headaches, and 38 percent of them have to go to a hospital, where they stay for
about 5 days on average. About 1 in 150 infected people end up developing encephalitis. Between
1999 and 2008, U.S. doctors recorded 28,961 cases of West Nile virus. Of those victims, 1,131 died.
Once West Nile virus arrived in the United States, it settled into a regular cycle, a cycle set by the
natural history of birds and mosquitoes. In the spring, robins and other birds produce new generations
of chicks that are helpless targets for virus-carrying mosquitoes. By the summer, many birds are
positively brimming with West Nile virus, raising the fraction of mosquitoes that carry it. It’s at that
time of year that most human cases of West Nile virus emerge. When the temperature falls, mosquitoes
die, and the viruses can no longer spread. It’s not clear how the virus survives North American
winters. It’s possible that they survive in low levels among mosquitoes in the south, where the
winters aren’t so harsh. It’s also possible that mosquitoes infect own their eggs with West Nile virus.
When infected eggs hatch the next spring, the new generation is ready to start infecting birds all over
again.
West Nile virus has fit so successfully into the ecology of the United States that it’s probably going
to be impossible to eradicate. Unfortunately, doctors have no vaccine to prevent West Nile virus and
no drugs to treat an infection. If you get sick, you can only hope that you are among the majority who
suffer a fever and then recover. And in the future, West Nile virus may become even more entrenched
in its new home. Jonathan Soverow of Beth Israel Deaconess Medical Center and his colleagues
examined sixteen thousand cases of West Nile virus that occurred between 2001 and 2005, noting the
weather at the time of each outbreak. They found that epidemics tended to occur when there was
heavy rainfall, high humidity, and warm temperatures. Warm, rainy, muggy weather makes mosquitoes
reproduce faster and makes their breeding season longer. It also speeds up the growth of the viruses
inside the mosquitoes.
Unfortunately, we can expect more of that sort of weather in the future. Carbon dioxide and other
heat-trapping gases are raising the average temperature in the United States, and climate scientists
project that the temperature will continue to rise much higher in decades to come. Now that West Nile