Epidemic in the Northwest

Ten years after House Finch eye disease was discovered in the East, an epidemic breaks out on the opposite side of the content

By André A. Dhondt and Melanie Driscoll

To understand how House Finches are affected by the eye disease, we need more data from citizen-science participants, especially in the West. Photo by Raymond Belhumeur

Over the past decade, citizen-science participants have submitted more than 89,500 monthly reports to the House Finch Disease Survey, helping to track the spread of a new disease from an isolated area in the East to regions across the continent. This year?s data revealed unusually high numbers of House Finches with eye infections in the first widespread epidemic of House Finch eye disease recorded in the West.

The data show that the disease began spreading epidemically in the Northwest in early 2004, exactly 10 years after it began in the East. In February through April 2004, disease prevalence in the Northwest (British Columbia, Alberta, Washington, Idaho, Montana, Oregon, and Wyoming) was half again the highest levels previously observed there. By February 2004, disease prevalence had reached 15 percent in the Northwest. No similar increases were detected in California or the Southwest (see Figure 1).

House Finch eye disease is caused by a bacterium, Mycoplasma gallisepticum. Infected House Finches have red, crusty, swollen eyes; in severe cases the birds can lose their vision and become vulnerable to predators, starvation, and exposure. Three years after the disease arrived in areas where House Finches were abundant in the East, there were some 180 million fewer House Finches than expected if the disease had not occurred.

As the disease spread, we wondered whether it would cause a similar epidemic in the West. By 2000, numerous western participants joined the project to help us find out. Their data showed that about 5 percent of House Finches had eye infections that winter—a level much higher than that observed in eastern birds before they were hit by House Finch eye disease. However, there was a complicating factor: some of the reported eye infections may have been caused by a different disease, avian pox. More prevalent in the West than in the East, avian pox causes lesions around the eyes that can be mistaken for House Finch eye disease. To know whether House Finch eye disease was causing an epidemic in the West, we would need to see a spike in the number of eye infections beyond the background level assumed to be caused by pox.

In 2004, reports of House Finch eye infections spiked in the Northwest (British Columbia, Alberta, Washington, Idaho, Montana, Oregon, and Wyoming), indicating an epidemic. No similar increases occurred in California or in the Southwest (Nevada, Utah, Arizona, Colorado, and New Mexico). The graph includes only those months for which we had at least 25 participants in each region.

By 2002, we knew that House Finch eye disease had reached at least as far west as Missoula, Montana: laboratory tests on three sick House Finches there confirmed that Mycoplasma gallisepticum was the cause. The next year another eight cases turned up. In the larger picture, though, nothing seemed to happen: there was no increase in the prevalence of eye infections reported by our participants in the West.

However, in 2003–2004, disease prevalence in the Northwest increased rapidly. By February 2004, 15 percent of House Finches were being reported with eye infections—clearly higher than in preceding years. Why was there a two-year lag between the time House Finch eye disease was detected in Montana and the time when an epidemic broke out in the Northwest? In Montana and other inland areas where humans live farther apart, House Finch populations are more isolated from each other, which perhaps slowed the spread of disease. Another reason could be that House Finches are less susceptible to the disease in their native range in the West than in the East, where the finches descended from a small number of birds introduced during the 1940s. The makings of an epidemic also probably require an element of chance; in several midwestern states, the disease-causing bacterium had to arrive several times before it ultimately caused an epidemic.

Although further research will be required to determine whether any of these hypotheses are true, results from the House Finch Disease Survey already provide a hint. In the Northwest, the epidemic primarily occurred in British Columbia, Washington, and Oregon—the coastal states and provinces. Perhaps it is the coastal areas that provide the right environment for a full-blown epidemic. Incoming data from our participants will help us determine whether that?s the case. If so, House Finch eye disease should spread farther south, through California, but should not increase in the Southwest. If enough citizen scientists participate in the House Finch Disease Survey, we have a great opportunity to find out whether this will be true and to describe in detail how the new pathogen will continue to spread.

Join the House Finch Disease Survey! It?s free. For more information or to sign up, visit www.birds.cornell.edu/hofi or call (800) 843-2473 (United States) or (607) 254-2473 (Canada).

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu