10,000 birdwatchers monitor sick house finches to give scientists insight into avian flu and West Nile virus
The day after Christmas, Susan Williams, a 34-year-old birdwatcher in Loveland, Ohio, noticed a house finch at her backyard feeder—a lethargic brown female with swollen, pink eyes encrusted with dried ooze. Williams reached under the bird’s belly, and the bird stepped up like a parrot, mistaking her finger for a perch. She examined it for a moment, and then placed it gingerly back among the seeds. Later, when the finch was gone, she cleaned the feeder with bleach.
“Since I first noticed the problem, it’s increased tenfold,” Williams said, “at least in my yard.”
Each month for the past four years—ever since house finches with conjunctivitis began turning up at Williams’s feeder—she has submitted a log of her observations to the Cornell Lab of Ornithology. She is one of more than 10,000 volunteers, dubbed “citizen-scientists,” participating in the House Finch Disease Survey, a project established a dozen years ago to monitor the emergence of a novel pathogen.
A collection of some 100,000 monthly reports now tells an exceptionally complete story about how an epidemic spread through a single species in the Northeast—a tale that, according to a recent study, may provide insights into the movement of diseases such as avian influenza and West Nile virus.
“Avian influenza is largely asymptomatic in birds. I don’t know the last time you saw a bird sneeze,” said Parviez Hosseini of Princeton University, the lead author of the study, which appeared in December in the journal Ecology. “House finch conjunctivitis is interesting because it’s an easily observable disease.”
The infection appears to be highly host-specific, meaning it sticks almost exclusively to house finches—plump, sparrow-sized birds with heads ranging in color from brown to yellow to a bright, blushing red. By contrast, avian influenza and West Nile virus infect all kinds of birds, as well as other animals, skipping from species to species.
“The question is: What are the processes by which these diseases spread? Are they similar?” said Marmaduke Kilpatrick, an avian-disease expert at the Consortium for Conservation Medicine and the lead author of a recent study analyzing the global spread of avian influenza in The Proceedings of the National Academy of Sciences, or PNAS. “You can certainly say this is a neat way to address the problem.”
The plight of the house finches began in 1994 outside Washington D.C., where testing revealed that dozens of blind birds had contracted a novel strain of Mycoplasma gallisepticum or MG—a bacterium known to cause mild infections in domestic poultry such as chickens and turkeys.
As far as ecologists were aware, MG had never before crossed over to a wild bird species. Without an immune defense in place, the house finches were especially hard hit, catching the attention of amateur bird-watchers accustomed to seeing the gregarious birds at their backyard feeders.
Native to the West Coast, house finches were introduced to Long Island in the 1940s and expanded throughout the Northeast. Well before the birds began contracting conjunctivitis, researchers at Cornell had been collecting data on their whereabouts as an invasive species. When the first infections appeared, ornithologist André Dhondt acted swiftly, instructing volunteers already submitting observations to begin reporting the presence or absence of eye disease.
The resulting data set tracks the dispersal of affected birds by county. The researchers found that a general model of diffusion—known to describe the spread of mammalian diseases, such as rabies in foxes—fit the house finch data well, especially at its outset. In other words, random movement proved to be the best predictor of how the pathogen would travel.
Very quickly, however, factors such as migration and elevation came into play. Outbreaks extended along migratory routes with pockets of infection corresponding to the lower elevations finches prefer.
“There is a huge difference in seasonal infection rates,” said Andrew Dobson, an infectious disease expert at Princeton and an author of the study. Dramatic outbreaks in the fall have been linked to the prevalence of juvenile birds with naive immune systems at that time of year; a second, smaller spike occurs in late winter and early spring when birds gather most heavily around feeders.
Dobson explained that asymptomatic birds seem to spread the bacterium along migratory pathways, contributing to the seasonal outbreaks. “We know this is also true of avian influenza,” he said.
Within a few weeks of the publication of the study in Ecology, the paper by Kilpatrick and five other scientists appeared in PNAS, suggesting that the trafficking of domestic poultry was largely responsible for the spread of avian influenza in Asia. For 20 of the 23 affected European countries, however, the authors pointed to the migration of wild birds as the most likely entry point for the virus. Based on their findings, the authors predict avian influenza may enter the Western Hemisphere through the poultry trade, and then reach the mainland United States via wild birds migrating from neighboring countries. “One of the things that people got pretty upset about was that we were blaming wild birds for something,” said Kilpatrick. “Conservationists want to blame poultry rather than wild birds.”
The fear is that birds will be slaughtered in futile attempts to curb the spread of disease. The authors of both studies adamantly oppose such culling, emphasizing that wild birds should be disturbed as little as possible.
The disruption of migratory routes—through habitat destruction or shooting at a flock—often brings different species into new contact, creating opportune settings for pathogens to jump hosts. For this reason, most ecologists maintain that understanding and protecting established migratory pathways is essential. Indeed, analyses of disease spread can only be as good as the original data on which they are based.
“We’d love to have county by county data,” said Kilpatrick whose study used records from the Food and Agriculture Organization of the United Nations. “But each country doesn’t report exactly where its infected birds are found.”
A project is under way to involve amateur bird-watchers in surveillance efforts for more serious diseases.
Peter Marra, a researcher at the Smithsonian Institute’s Migratory Bird Center, described a project he is designing, in conjunction with the Air National Guard Flight Safety Division, called dBird. Through a Website, bird-watchers will be able to report the locations of dead birds they find in the field. Scientists will screen the data for red flags—such as three dead swans in close proximity—and retrieve suspicious carcasses for testing.
Given time, such efforts may provide the raw material necessary for more thorough analyses of how serious avian diseases spread.
Well over a decade after the first outbreak, conjunctivitis continues to persist at a steady rate in Northeastern house finches and has reached the native Western birds. Since the spread of MG slows at the population thins, the infection will never wipe out the species. Some birds, moreover, recover from conjunctivitis, developing resistance to subsequent infection. However, the number of house finches in the Northeast has consistently remained 60 percent lower than it was before the emergence of the disease.
“We know that 95 percent of infectious and emerging diseases in humans have a zoonotic origin,” Dobson said. “So it’s vital we understand more about infectious diseases in wild populations.”