West Nile virus made its US debut eight years ago in the heart of the East Coast, coming ashore in New York City and forging a chain of transmission among mosquitoes, birds, and people from Florida to New England. Football players and backyard gardeners cowered inside at night, prime time for spreading the potentially lethal germ. Disease trackers wondered if it would become a permanent threat in the East.
In 2002, the year when West Nile spread most dangerously in Massachusetts, three people died and 21 others fell ill. The next year, 18 people caught the virus.
"Then," said Dr. Alfred DeMaria, top disease investigator at the Massachusetts Department of Public Health, "the bottom dropped out of the market." This year, during the season that ended with last week's first frost, only three people contracted the illness in the state, and all recovered.
So where did West Nile go?
Look westward. The virus has established a firm foothold in an arc of states stretching from Minnesota through the Dakotas, Colorado, and out to California. Of the 3,195 human cases reported nationally this year, nearly 85 percent were recorded west of the Mississippi River.
"It certainly has found a home in the western US," said John Pape, an epidemiologist for the Department of Public Health and Environment in Colorado, where 544 people became ill with West Nile this year, the most in the nation. "I think it really relates to the ecology of the West being conducive for this virus to survive and circulate."
Specifically, specialists blame the complex interplay between the types of mosquitoes and birds that predominate in the West, the way people make a living in the region, and how water and land are used.
And, as much as anything, the story of West Nile in the United States may be the quintessential tale of an urban ill that ultimately invaded - and prospered - in rural swaths of the nation.
"You say, wow, in very many ways we thought of West Nile as an urban issue at first because it was in New York City, then we had Chicago in 2002, and then New Orleans," said Emily Zielinski-Gutiérrez, a specialist in mosquito-borne illnesses at the US Centers for Disease Control and Prevention branch in Fort Collins, Colo. "While we still see outbreaks in urban areas, the real impact has been in these agricultural and rural communities."
One reason is a bug called Culex tarsalis, a mosquito strain common in the West, rare in the East.
"Tarsalis, when it's flying around, will feed on whatever it comes across. If it comes across a bird first, it will feed on a bird. If it comes across a mammal, it will be happy to feed on a mammal," Pape said. And, as mosquitoes go, they're exceptionally prone both to contracting the virus and then spreading it.
Specialists such as Nicholas Komar, a CDC research biologist in Colorado, are investigating the role birds have played in spreading West Nile in the West. They know, for instance, that in rural sections of Colorado, where human infection rates are higher, house sparrows are common and serve as a potent reservoir. Conversely, in Denver, where people are less likely to contract the virus, there's a richer mix of birds, some that are good at harboring the virus, some that aren't.
So rampant transmission of the virus, Komar said, may depend on having the right mix of birds and the right mix of bugs.
Then there's the human factor, and how people earn a livelihood. "You have people engaged in an agricultural lifestyle," Zielinski-Gutiérrez said. "They're spending a lot of the hours of the day outdoors." And, she said, they spend time near fields irrigated with flood water, providing an ideal breeding ground for mosquitoes.
In a sense, the CDC researcher said, it's easier to explain why West Nile has thrived in the West than to understand why it has found a less welcoming home in the East. DeMaria, of the state, theorized that a dry summer may have been partially responsible.
But there's something else. Health authorities in New England the past few years have sounded alarms about another virus spread by bugs, Eastern equine encephalitis, the deadliest of all mosquito-borne germs. Maybe, DeMaria said, people really paid attention, and by heeding warnings about Eastern equine - wearing long-sleeved shirts and slathering on bug repellent - they also avoided West Nile.
Stephen Smith can be reached at firstname.lastname@example.org.