The mosquito-transmitted disease, which leaves a trail of dead birds and can be fatal to humans, has gained a foothold in the United States. Still unknown is how widespread it could become.
West Nile virus appears to be settling in for the long haul.
Almost one year after the Western Hemisphere debut of the virus sent 62 people to hospitals, eventually killing 7 of them with severe brain inflammation, the bird-carried, mosquito-transmitted disease has struck again, infecting a 78-year-old man from Staten Island.
The man, who became ill last month before mosquito spraying began, recovered, as the vast majority of infected people do.
But despite widespread spraying, many scientists expect a scattering of such cases and a few deaths, mostly in the elderly or infirm, may well become the norm in years to come.
More slowly than expected, but steadily, the globe-hopping virus is dispersing from last year’s focal point in northern Queens County, part of New York City, leaving a trail of dead birds that so far extends north to Boston, west to Syracuse, N.Y., and south to Maryland.
According to virus experts, this pattern indicates that West Nile, which was first identified in Uganda in 1937 and flares up sporadically in Africa, Europe and the Middle East, appears ensconced in the New World.
Since last fall, dozens of scientists have been scrambling to understand West Nile’s origin and destiny as it travels this continent and tests new hosts, decimating some, like crows, leaving others, like pigeons, untouched, and occasionally infecting people. Thousands of hours of sleuthing, testing and analysis are only now offering preliminary clues. The big questions are largely unresolved.
How did the virus get here? Where did it come from? What is its main animal reservoir — the birds in which it can simmer in sufficient concentrations to be passed by mosquitoes from one host to another? How far will it spread?
Most important, will West Nile pose a major threat to public health in the United States? The answer to this question, at least so far, appears to be no.
Though the virus was first identified in Uganda, the source of the outbreak here was probably Israel; the American strain is almost indistinguishable from a virulent strain found in a goose on an Israeli farm in 1998.
House sparrows — ubiquitous, dust-colored, one-ounce crumb-snatchers — are probably a principal reservoir here; new tests at federal laboratories show they can harbor the virus for five days or more at levels high enough to infect mosquitoes that bite them.
Beyond that, little is known.
Given the lack of knowledge about what conditions set off the virus’ spread to people, the major response so far — spraying near spots where birds have died — is akin to shooting at an assailant in the dark, many scientists say. And while surveillance and spraying can help cut the risk that people will be infected, the virus probably is here to stay and will continue to spread.
The closest thing to an absolute fact in the investigation of the disease, scientists say, is that it is truly new in the Americas, arriving somewhere around New York City in the months before last summer’s outbreak.
Blood tests in people and birds showed the kind of immune response typical when a body is freshly exposed to an infection, according to federal health officials. Mapping of test results for captured house sparrows — which do not migrate — show a hump in the exposure rate in and around Queens and nowhere else.
And crows, the most vulnerable birds, are almost nowhere to be seen in northern Queens this summer. The population there crashed 90 percent, federal scientists say, while in other areas the birds were hit less hard.
West Nile virus is closely related to several other mosquito-borne viruses found around the world, including Japanese encephalitis and St. Louis encephalitis, which has a similar cycle in birds and mosquitoes and occasionally strikes people across the South and Midwest.
One big difference is that St. Louis encephalitis is “silent” in birds, generally not killing them, scientists say, so there is usually no warning before a human case. For West Nile, at least the American strain, birds, and particularly crows, offer a hint of possible trouble to come.
The disease has caused epidemics in the past. In West Nile outbreaks in Israel in the 1950s, South Africa in 1974 and Romania in 1996, hundreds and sometimes thousands of people became ill, with 5 percent to 10 percent of those cases proving fatal.
Over all, scientists say, human illness from the virus probably will remain rare because humans are likely to remain what biologists call “dead-end hosts.” They can be infected, but their immune systems almost always prevent the virus from multiplying enough to be passed back to mosquitoes and then to other hosts.