West Nile virus is known to infect more than 160 species of birds, even a partial list of which reads like the index of an Audubon field guide: chickadees, doves, eagles, finches, grackles, gulls, hawks, herons, kingfishers, owls, pelicans, sparrows, swans, turkeys, warblers, woodpeckers and wrens. Common birds such as sparrows and house finches also incubate the virus, and some researchers suggest that those birds may play an increasingly prominent role in urban epidemics.
Nor have other animals been spared. Veterinarians in Florida discovered last year that even alligators at a reptile farm had become infected (mosquitoes apparently can bite these thick-skinned reptiles on either their soft underbellies or around the eyes). Among the other mammals that the virus has been found to infect are bats, chipmunks, dogs, rabbits, reindeer and squirrels. West Nile virus infection last year afflicted some 14,000 horses, mostly in the Midwest.
Meanwhile, it remains unclear how serious a long-term threat the virus may be to human health—whether it will cause a lot of disease year after year, as some experts predict, or settle down and cause disease only rarely. Thomas Monath, chief scientific officer of Acambis—a British biopharmaceutical firm with a facility in Cambridge, Massachusetts, that hopes to begin testing a human West Nile vaccine in the United States this summer—said 2002’s heavy toll was probably only the beginning.Monath has impeccable credentials as an expert Cassandra in the field of arboviral disease. For 21 years he served at the CDC’s Division of Vector-Borne Infectious Diseases, and he literally wrote the book on one of the West Nile virus’s closest relatives, the St. Louis encephalitis virus. “The amplification of West Nile in 2003 could be worse than 2002,” he predicted, “and I think it could be a lot worse.”
It is part of American mythology that diseases spread by the bite of mosquitoes are scourges that happen somewhere else. Malaria continues to devastate Africa and tropical regions and claims one million to three million lives every year. Dengue, or “breakbone fever,” afflicts 50 million people worldwide and kills 24,000, mostly children. Yellow fever still plagues South America and Africa.
Those diseases are mostly strangers to our shores, but that wasn’t always the case. Yellow fever used to roar through New York, Philadelphia and New Orleans in the 18th and 19th centuries. American presidents fled the White House in summer partly to escape the seasonal yellow fever outbreaks that swept through Washington, D.C. But since the end of World War II, thanks to mosquito-control measures such as spraying pesticides and eliminating breeding sites, mosquito-borne diseases in the United States have largely been limited to outbreaks of generally rare viral illnesses that inflame brain tissue: St. Louis encephalitis (mostly in the South and Midwest), the eastern and western forms of equine encephalitis (which occasionally strikes humans) and La Crosse encephalitis (mostly in the Midwest).