For more than half a century, West Nile virus was someone else’s problem.
The mosquito-borne pathogen was first isolated from a feverish human in 1937 in northern Uganda’s West Nile district. It then lay low for a decade before emerging in an actual epidemic in Israel in 1951. With several Egyptian outbreaks in the early ’50s, researchers started to see the disease infect non-humans, particularly crows and horses. Mosquitoes of the Culex genus appeared to be its chief transmitter, or vector.
By the time the virus hit the United States, in 1999, it had taken on a more sinister character. Where before it mostly struck children, giving them feverish symptoms, it was now hitting older people. About one in 150 of those afflicted got central nervous system maladies like encephalitis and meningitis, and more people were dying.
Its arrival was like a sequel to Alfred Hitchcock’s The Birds. Birds started showing up dead in New York, and eight people in a 16-square-mile section of the borough Queens were diagnosed with encephalitis. By summer’s end, 62 people had acute cases. Only three years later, North America saw the largest outbreak ever recorded of West Nile meningoencephalitis, the form that invades the brain and swells the brain lining. The virus reached the Pacific Coast.
An infected human in Washington state was reported in 2006, after which only a few people were diagnosed each year, with the exception of 2009, which had 38 cases. But a curious trend appeared in the detection of infected mosquitoes, birds, and horses: Almost all were in Yakima County and its surrounding agricultural counties.
Enter Jeb Owen and David Crowder, Washington State University entomologists and, in the parlance of a Pacific Science Center exhibit, “disease detectives.” Owen is also a disease ecologist, tracking infection through the living landscape.
Until now, researchers have had a hard time charting the interplay of West Nile’s hosts, victims, and the worlds they inhabit. But one day, Owen and Wade Petersen, ’09 MS, are looking at several maps when something jumps out. There is a U.S. Department of Agriculture map of irrigated agriculture in the Pacific Northwest–Petersen is looking for water sources that could support juvenile mosquitoes. They also have a map of West Nile virus cases.
They are, in Owen’s words, “totally congruent.
“Right then I thought, ‘There’s something going on here with irrigated agriculture.’ This has been seen in other parts of the country. But paired with the history, it becomes something else.
“If you looked at all the patterns in the other states, you would have predicted the virus would quickly sweep the state. But it never did. It would flare up and then flame out. It never really became established, until the last few years, which was always perplexing.”
The species of mosquitos and birds that can be affected are found everywhere in the state, but they tend to carry the virus in just four counties.
“So there was something special about those counties that we were trying to explain,” says Owen.
Owen asks undergraduate Emily Martin ’13 to plot the acreage of irrigated land against the prevalence of infected mosquitoes. He also ponders three pools of data on mosquitoes, infected horses, and infected people. The data on infected people is notoriously unreliable, though, as the numbers are small, only one in five infected people show symptoms, and they could be getting infected outside the county where their illness is reported to health officials.
The datasets confirm that counties with more land dedicated to irrigated agriculture have more infected mosquitoes. But lots of different types of agriculture use irrigation. To get a more detailed look, Owen and Crowder study just what kinds of things are being grown throughout Washington, Oregon, and Idaho. They categorize areas into orchards and vineyards, natural habitat, and vegetable and forage crops. They also look at temperature, humidity, and the densities of sparrows, robins, and crows.
And there it is: clusters of orchards and vineyards, mostly around Yakima County, with infected mosquitoes and birds and consistently warm temperatures.
“All of that is this perfect storm of actors that allow the pathogen to get amped up quickly and get transmitted into people and horses,” says Owen.
Owen and Crowder speculate that a nexus of water, warm temperatures, orchards, and vineyards provide key resources for birds and mosquitoes to live near each other. The water is necessary for larval mosquitoes, the flowering plants support adult mosquitoes, and the fruiting crops attract and feed birds. The habitat becomes a focal point for the virus as female mosquitoes pick it up from bird blood and thrive on nectar long enough to transmit the pathogen when they feed again.
The findings, one of the most finely scaled looks at the interplay of land use and the virus’s activity in key hosts, were published last year in the journal PLOS ONE and are now part of an exhibit in the Pacific Science Center’s Portal to Current Research. The display, which runs through June, features a large photo of a smiling Owen among a group of fellow “disease detectives.” Visitors to the interactive exhibit can overlay different Washington state maps to see the connections between farmlands and the virus.
The notion of disease detectives is a “fun invitation to visitors,” says Mary Olson, the center’s current science project manager, with the West Nile story being a vehicle to describe epidemiology and how a pathogen moves.
“When we read his study,” she says, referring to Owen, “it was really intriguing and I think it worked really well with what we’re trying to show, the current science of what people are doing out there in the field.”
Owen says he likes how the exhibit reinforces the “One Health” concept, which aims to look at the dynamic human, animal, and environmental interactions underlying a disease, not just its human victims and possible treatment.
The One Health approach, a focus of WSU’s Paul G. Allen School for Global Animal Health, makes people aware of holistic concepts in medicine, says Owen, “and the exhibit shows that.”
“There continue to be these little mysteries in the system,” Owen says. “If we can understand them, it will help us to better understand where we have opportunities to exercise control and it will help us better predict where the disease will occur.”
Reprinted from Washington State Magazine, courtesy of Washington State University. To read the full story and other articles from the magazine, please visit wsm.wsu.edu.