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House Finch Eye Disease: Outbreak, Then Understanding

By Rebecca Heisman
House Finch eye disease banner- Living Bird Winter 2017
House Finch by Bob Vuxinic.

From the Winter 2017 issue of Living Bird magazine. Subscribe now.

In February 1994, backyard bird watchers in Maryland noticed something strange. House Finches were showing up at feeders with eyes that were red and crusty, so swollen that they bulged out of the small birds’ heads.

In Ithaca, N.Y., volunteers at the Cornell Lab of Ornithology’s Project FeederWatch were sorting through data forms sent in by project participants, and they noticed a repeating theme among reports from the Baltimore/ Washington, D.C., area.

“It was the old handwritten report days, and we had a comments section for descriptions,” recalls former FeederWatch project manager Margaret Bark­er. “Our volunteers kept coming across mentions of House Finches with puffy eyes, some acting oddly at the feeders.”

After being alerted by the volunteers, Barker combed through the comments and found more sick finch reports— lots more, mostly from Maryland. But no one knew why the birds were getting sick, or what it meant.

Back in the early 1990s, the House Finch would have been a candidate for the title of most successful bird species in North America. Its populations were marching across the continent, and the little brown bird with the cheerful red head was a familiar sight at backyard bird feeders in the West and East.

Then a poultry pathogen made an unexpected cross-species leap, and House Finches debuted a new form of conjunctivitis. Ultimately, though, the outbreak for a beloved backyard bird species turned into the perfect opportunity for scientists to observe how disease spreads and evolves in a natural environment. Over the past two-and-a-half decades, Cornell University researchers and their colleagues have tracked the epidemic known as “House Finch eye disease” across the country. Their data are providing new insights that may be applicable to headline-mak­ing human epidemics such as Zika.

It all started with a Belgian ornithol­ogist newly arrived in America and a mass-mailing of thousands of paper forms, a pre-Internet appeal for citizen scientists to collect the data that the professionals couldn’t.

House Finch movement across North America
Early in the 20th century House Finches lived in their native western range. Around 1940 House Finches were released into the wild near New York City. From there an introduced eastern population marched west across North America. Maps from Birds of North America. [update 2/8/22: Birds of North America is now part of Birds of the World.] Click here for larger image.

How House Finches Moved Across North America

House Finch are native to the des­erts, grasslands, and chaparral of west­ern North America. Around 1940, an illicit trade in pet “Hollywood finches” sprang up, with illegally trapped birds shipped east out of California. When a passing birder recognized the songbirds in a Brooklyn pet-shop window and re­ported them to the Audubon Society, federal wildlife officials moved to shut down the illegal trade. Soon pet-shop owners released their inventory in or­der to avoid prosecution, shooing the birds out the door and into the wilds of Long Island.

At first, this new population of House Finches remained small and con­tained—the harsh winter of 1947–48 left fewer than 50 of the desert-adapt­ed birds clinging to survival in subur­ban New York City. But by the 1960s the eastern House Finch population was expanding rapidly, and by the mid-1990s they were taking over the con­tinent—the introduced eastern po­pulation radiated out from New York, the native western population spread eastward, and the two populations met in the Midwest. Then along came the mysterious new eye disease epidemic.

Like House Finches, André Dhondt is not a native of the eastern U.S. In spring 1994, the Belgian ornithologist left a 20-year career at the Universi­ty of Antwerp to come to the Cornell Lab of Ornithology. Dhondt was al­ready a successful population ecol­ogist when he arrived to take over as director of the Cornell Lab’s Bird Pop­ulation Studies program, having run a decades-long experiment in his native country on competition between Great Tits and Blue Tits. He was looking for a new challenge, but he had no way of knowing that his career was about to become inextricably intertwined with the new disease alarming backyard bird watchers.

…by the mid-1990s they were taking over the con­tinent. Then along came the mysterious new eye disease epidemic.

“As I arrived, there was a panic that this new disease, which at that time was unidentified, would spread to Neotropical migrants, they would take it back to South America, and everybody would die. I’m exaggerating,” he admits, “but there was general concern.”

With that concern came an un­precedented opportunity for scientists to track the spread of a new disease from almost the moment of its origin. Not only was House Finch eye disease easy to spot, eliminating the need to capture and test birds for diagnosis, but the Cornell Lab of Ornithology already had an army of thousands of backyard bird watchers across the U.S. and Canada submitting data to Project FeederWatch.

Within months of the initial reports of the disease’s emergence, Dhondt spent all of his $5,000 in start-up money for his new lab on printing 50,000 paper forms to send to FeederWatch partici­pants, asking them for observations of House Finches. Then he waited.

“It was a perfect system,” says Dhondt. “Not for the birds, but for us.”

Eye Disease spread map and House Finch
From its origination in Maryland in 1994, House Finch eye disease spread across the northern United States to Washington state. Then the disease tracked south along the West Coast before beginning to curl back inland in the Southwest. Map from the Cornell Lab House Finch Eye Disease Research Project. Photo of House Finch by Kathie Johnston.

Dhondt’s Gamble Paid Off

Data for the newly christened House Finch Disease Survey soon poured in from citizen scientists across the country. Over the next 10 years, he amassed almost 300,000 monthly reports from 10,000 participants.

The culprit behind the new disease was identified from bacteria collected from infected birds’ lesions—Mycoplas­ma gallisepticum, a species of bacteria known to cause respiratory infections in chickens and turkeys. Genetic analysis suggested that M. gallisepticum jumped from poultry to wild songbirds unsuc­cessfully several times before a single strain of the bacteria finally adapted to persist in House Finches, possibly the result of some unlucky finch patient zero darting into a backyard flock of chickens to share their feed.

A year after it first emerged, the disease was reported in Ontario to the north, Virginia to the south, and Ohio to the west. In fall 1995, it was in Indi­ana, Michigan, and Illinois. By fall 1996, it had reached Wisconsin, Minnesota, and Iowa. In only two-and-a-half years, the conjunctivitis epidemic had spread throughout most of the eastern House Finch range.

In the first stages of disease an infected bird, like this male House Finch, may just look like it's keeping its eye closed. Photo by Marie Read.
In the first stages of disease an infected bird, like this male House Finch, may just look like it's keeping its eye closed. Photo by Marie Read.

Thanks to long-running volunteer bird surveys like the Christmas Bird Count, scientists had data on the host’s abundance both before and after the start of the epidemic—something almost unheard of in studies of wildlife diseases.

“In areas where there were a lot of birds around, it was very easy for the bacteria to hop from one House Finch to another. Infections were more rampant, and birds were dying at a higher rate,” explains Wesley Hochachka, Dhondt’s colleague at the Cornell Lab and part­ner on the House Finch eye disease re­search project. But after the initial peri­od when the disease hit and populations declined, disease occurrence tailed off and populations stabilized at smaller sizes. Mycoplasma simply couldn’t jump from bird to bird at feeders as effectively when population densities were lower. It was one of the first large-scale exam­ples of a disease regulating the popula­tion dynamics of its host.

The intersection of disease virulence and population dynamics opened up a host of new questions about bacterial mutations and pathogen evolution, but those were questions that citizen science alone couldn’t answer. And Dhondt and Hochachka—both population biolo­gists by training—had never worked on disease research before House Finches. “My main experience with disease was being sick, up until the point that I came to Cornell,” says Hochachka.

So they formed a multidisciplinary team of scientists from several institu­tions, from a mathematical modeler at Princeton to a veterinary microbiolo­gist at North Carolina State University, to tackle bigger inquiries into mycoplasma. The bigger team needed more money. Serendipitously, a major new funding source for disease research launched in 2000 when the National Science Foundation and the National Institutes of Health started a joint Ecol­ogy of Infectious Diseases program (lat­er expanded to Ecology and Evolution of Infectious Diseases). The goal was to fill the gap between studies of disease in humans and in wildlife, funding re­search on natural systems that could enhance our understanding of both.

The team submitted a proposal to use House Finch eye disease as a sys­tem for exploring how diseases evolve and spread. They were funded in the very first round of the new grants. With the new influx of funding, the House Finch researchers expanded their field­work across North America to monitor changes in disease prevalence and finch abundance over time, accumulating 15,000 observations of 4,500 banded birds from intensive field studies in New York, Wisconsin, and Georgia. They also began experiments with birds in captivity to study the course of the disease in controlled conditions, ob­serving how it passed from bird to bird, whether birds recovered, and how they fared if infected for a second time.

The new research program showed that wild birds could recover from the disease, and studies on captive birds confirmed that birds retained partial immunity for at least a year once they’d pulled through. Crucially, the House Finch research team built a mathe­matical model to generalize what they learned and apply it to other disease systems. Ultimately, people who can’t tell a finch from a robin may reap the benefits of the House Finch studies, because it’s giving science new insights into diseases that affect a species we all care about—humans.

Microplasma drawing and House Finch
The illustration shows Mycoplasma gallisepticum (yellow cells in illustration) at the microscopic point of attack. M. gallisepticum are among the smallest known self-replicating bacteria, and here they nestle in among the cilia of a House Finch’s epithelial cells and attach to the cells’ surface, like barnacles attaching to a freighter ship. From there, the M. gallisepticum cells begin reproducing en masse, eventually disrupting normal cell function and inducing an inflammatory response. Illustration by Virginia Greene with scientific input from Dr. David Ley, North Carolina State University. Photo shows an infected male House Finch; credit: Raymond Belhumeur.

A Microscopic Arms Race

According to the world health organization, human infectious diseases are spreading faster and emerg­ing more quickly than ever before, with new diseases identified at the rate of one per year since the 1970s. In just the last five years, more than 1,000 epidemics have affected humans around the world.

In 2002—almost exactly 10 years after House Finch eye disease first ap­peared in Maryland—a smaller epidem­ic was detected among backyard birds in Missoula, Montana. M. gallisepticum was now in the western range of the House Finch, probably introduced by sick finches from the East that were in­fected with a relatively mild strain and were still healthy enough to fly from town to town. The disease’s spread of­fered new insights into how pathogens evolve over time.

“Like Alice in Wonderland, we’ve got a ‘Red Queen’ situation here where the bacteria are running as fast as they can to keep up.”
~Wesley Hochachka

In both the East and the West, M. gallisepticum’s virulence increased after emergence, likely spurred by the immunity that House Finches develop once exposed. Because finches’ immune sys­tems became better and better at dealing with the disease, M. gallisepticum had to become more virulent over time.

“One of the parts of the story that’s recently become apparent is that we’ve got an arms race happening, where the finches are able to develop immunity to the bacteria and it seems like the only way that the bacteria can then work around the immunity is to evolve to cause more severe disease,” explains Hochachka. “Like Alice in Wonderland, we’ve got a ‘Red Queen’ situation here where the bacteria are running as fast as they can to keep up.”

The birds, he says, are essentially able to vaccinate themselves through their own immune response, but because the pathogen is able to evolve rapidly in re­turn, the prevalence of the disease is re­maining relatively stable over time.

“The human implication, if we were to extrapolate, is that if you’re vaccinat­ing a human population against some sort of a pathogen and you don’t do it completely enough to eradicate it, you may end up with enough little living laboratories among the hosts for the bacteria to live and evolve,” Hochachka says. “What might pop out in the end is a more virulent pathogen, able to cause more severe disease.”

“The current outbreak of Zika is due to some kind of a mutation that occurred somewhere in Asia that suddenly made Zika much more virulent, and now it’s spreading very rapidly,” says Samuel Scheiner, the NSF program officer who manages the Cornell Lab’s grant for the finch research. “The idea is that you can use things like the House Finch project as a way to study those processes, and then you can apply that knowledge to things like the Zika outbreak.”

This interdisciplinary research proj­ect is helping to train new generations of disease ecologists who are comfortable blurring the boundaries between popu­lation biology, epidemiology, and veter­inary science. Dhondt’s House Finch re­search is currently on its third round of funding from the Ecology and Evolution of Infectious Diseases program, and scientists who began their careers as graduate students in Dhondt’s laborato­ry have now moved on to head disease ecology labs of their own at institutions such as Virginia Tech.

As for Dhondt, even after 20 years he still isn’t out of questions for more House Finch experiments. “The next step is to see to what extent other patho­gens in the system play a role,” he says, nodding to the idea that other patho­gens could partner up with M. gallisep­ticum to double-team a finch’s immune system. “That’s why we’ve started look­ing at avian malaria.”

And what would he have thought, on his way to America two decades ago, if he’d known that finches with crusty eyes were going to change the course of his career? “I would have thought it was real­ly cool,” he says. “I came to Cornell when I was 50, a midlife crisis kind of thing, and I had to decide what I would do here. I was going to do something new. I didn’t know exactly what, but this happened.

“And one thing you learn in life is, if you have a chance to do something challenging, do it.”

Rebecca Heisman is a freelance science writer and the communications assistant for ornithology journals The Auk and The Condor. She lives in Walla Walla, Washington.

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