Diseases can rapidly evolve to become more—or less—virulent, according to songbird study

May 28, 2013

A novel disease in songbirds has rapidly evolved to become more harmful to its host on at least two separate occasions in just two decades, according to a new study. The research provides a real-life model to help understand how diseases that threaten humans can be expected to change in virulence as they emerge.

"Everybody who's had the flu has probably wondered at some point, 'Why do I feel so bad?'" said Dana Hawley of Virginia Tech, the lead author of the study to be published in PLOS Biology on May 28, 2013. "That's what we're studying: Why do pathogens cause harm to the very hosts they depend on? And why are some life-threatening, while others only give you the sniffles?"

Disease virulence is something of a paradox. In order to spread, have to reproduce in great numbers. But as their numbers increase inside a host's body, the host gets more and more ill. So a highly runs the risk of killing or debilitating its hosts before they get a chance to pass the bug along. It finds the right balance through evolution, and the new study shows it can happen in just a few years.

Hawley and her coauthors studied House Finch , a form of , or pinkeye, caused by the bacteria Mycoplasma gallisepticum. It first appeared around Washington, D.C., in the 1990s. The House Finch is native to the Southwest but has spread to towns and backyards across North America. The bacteria is not harmful to humans, which makes it a good model for studying the evolution of dangerous diseases such as , , and .

"There's an expectation that a very virulent disease like this one will become milder over time, to improve its ability to spread. Otherwise, it just kills the host and that's the end of it for the organism," said André Dhondt, director of Bird Population Studies at the and a coauthor of the study. "House Finch eye disease gave us an opportunity to test this—and we were surprised to see it actually become worse rather than milder."

The researchers used frozen bacterial samples taken from sick birds in California and the Eastern Seaboard at five dates between 1994 and 2010, as the pathogen was evolving and spreading. The samples came from an archive maintained by coauthor David Ley of North Carolina State University, who first isolated and identified the causative organism. The team experimentally infected wild-caught House Finches, allowing them to measure how sick the birds got with each sample. They kept the birds in cages as they fell ill and then recovered (none of the birds died from the disease).

Contrary to expectations, they found that in both regions the disease had evolved to become more virulent over time. Birds exposed to later disease strains developed more swollen eyes that took longer to heal. In another intriguing finding, it was a less-virulent strain that spread westward across the continent. Once established in California, the bacteria again began evolving higher .

In evolutionary terms, some strains of the bacteria were better adapted to spreading across the continent, while others were more suited to becoming established in one spot. "For the disease to disperse westward, a sick bird has to fly a little farther, and survive for longer, to pass on the infection. That will select for strains that make the birds less sick," Hawley said. "But when it gets established in a new location, there are lots of other potential hosts, especially around bird feeders. It can evolve toward being a nastier illness because it's getting transmitted more quickly."

House Finch eye disease was first observed in 1994 when bird watchers reported birds with weepy, inflamed eyes to Project FeederWatch, a citizen science study run by the Cornell Lab. Though the disease does not kill birds directly, it weakens them and makes them easy targets for predators. The quickly spread south along the Eastern Seaboard, north and west across the Great Plains, and down the West Coast. By 1998 the House Finch population in the eastern United States had dropped by half—a loss of an estimated 40 million .

Explore further: Recent studies warn surveillance of bird flu strains is needed

More information: Bird watchers can do their part to help House Finches and other backyard birds by washing their feeders in a 10 percent bleach solution twice a month. More tips for bird feeder maintenance can be found at www.allaboutbirds.org/clean-feeders

Related Stories

House finches 'avoid sick members of their own'

Nov 07, 2012

House finches avoid sick members of their own species, scientists said in a finding that could be useful for tracking the spread of diseases like bird flu that also affects humans.

Deadly bird parasite evolves at exceptionally fast rate

Feb 09, 2012

A new study of a devastating bird disease that spread from poultry to house finches in the mid-1990s reveals that the bacteria responsible for the disease evolves at an exceptionally fast rate. What's more, ...

Two new diseases could both spark global outbreaks

May 13, 2013

Two respiratory viruses in different parts of the world have captured the attention of global health officials—a novel coronavirus in the Middle East and a new bird flu spreading in China.

Recommended for you

Chimpanzees prefer firm, stable beds

7 hours ago

Chimpanzees may select a certain type of wood, Ugandan Ironwood, over other options for its firm, stable, and resilient properties to make their bed, according to a study published April 16, 2014 in the open-access ...

Offspring benefit from mum sending the right message

14 hours ago

(Phys.org) —Researchers have uncovered a previously unforeseen interaction between the sexes which reveals that offspring survival is affected by chemical signals emitted from the females' eggs.

Lemurs match scent of a friend to sound of her voice

Apr 15, 2014

Humans aren't alone in their ability to match a voice to a face—animals such as dogs, horses, crows and monkeys are able to recognize familiar individuals this way too, a growing body of research shows.

User comments : 0

More news stories

Revealing camouflaged bacteria

A research team at the Biozentrum of the University of Basel has discovered an protein family that plays a central role in the fight against the bacterial pathogen Salmonella within the cells. The so cal ...

Chimpanzees prefer firm, stable beds

Chimpanzees may select a certain type of wood, Ugandan Ironwood, over other options for its firm, stable, and resilient properties to make their bed, according to a study published April 16, 2014 in the open-access ...