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When Angela Theodosopoulos began studying Colorado's Mountain and Black-capped Chickadees four years ago, her original goal was to research hybridization among the species and the various parasites they each co-evolved with. But when the PhD candidate at the University of Colorado Boulder began analyzing the birds' blood, she made a worrisome discovery: Some of the chickadees were infected with a strain of introduced avian malaria.

According to a  in Biology Letters, Theodosopoulos' finding is the first record of this particular strain in a non-migratory bird species in North America. The parasite was found in two Mountain Chickadees and a single Black-capped Chickadee, but the low number of infections makes Theodosopoulos, lead author of the paper, believe that another avian species might be the main host. While the strain has the potential to be damaging to bird populations, Theodosopoulos stresses that more research is needed.

“I definitely would not panic at this point,” she says. “But it is important to think about it and to understand it, because it is a very invasive parasite, it can cause disease, and it has been associated with population declines in other places. We just don't know if that's going to be what happens here.”

Avian malaria is caused by a parasite that  and is spread via mosquitoes. Though some cases of avian malaria are mild or asymptomatic, infected birds can experience a loss of red blood cells, lethargy, a shortened life span, or in extreme cases, convulsions or death.

The strain found in Boulder is called SGS1, named for the species it was first identified in: a Sudan Golden Sparrow in 2002. Generally speaking, genetic variation in parasites leads to different strains of the disease, which tend to infect species in different geographic areas. But when a strain reaches birds that haven't encountered it before and are not adapted to fight it, “it can cause much harm to the population,” says Staffan Bensch, professor of animal ecology at Lund University in Sweden and co-author on the paper. Researchers can’t say yet how North American birds will respond to SGS1, but in London, SGS1 has been .

The fact that the parasite has infected non-migratory chickadees is concerning because it's “a sign that it is already spreading,” Theodosopoulos says. A migratory bird can be infected anywhere along its journey, but the chickadees had to have contracted the disease locally. Until now, the only recorded North American birds with SGS1 were captive birds in New York and one wild, migratory tree swallow in Canada. Theodosopoulos speculates mosquitoes may have spread the disease from infected birds in zoos, which could have contracted it before they were imported. 

“Many of them recover in captivity,” Bensch says. “But in the wild, such a bird is very easy prey for a raptor.”

Next, the researchers want to find out if SGS1 really is more prevalent in another local species—and which one. They think it could be House Sparrows, given the high infection rate of that species in Europe and their large populations in urban areas. Theodosopoulos hopes to discover if and when SGS1 infected House Sparrows in Colorado and, afterward, learn what other species it may have reached.

To do so, she partnered with Dr. Garth Spellman, curator of ornithology at the Denver Museum of Nature & Science. Spellman has a collection of tissue samples from House Sparrows from the Denver metropolitan area dating back 10 years. Incidentally, a paper published approximately a decade ago found no instances of SGS1 in House Sparrows in the Colorado Front Range, a mountain region including Denver and Boulder. So, Spellman and Theodosopoulos believe that if SGS1 has infected House Sparrows, it likely did so in the past 10 years.

“We can look at 20, 30 birds every year for the last decade to see if we can find SGS1,” Spellman says. “If so, we might be able to chronicle the year that it arrived in the Front Range.” Based on the prevalence of SGS1 in those samples, they could also estimate how common it is in House Sparrows today.

This future research will be crucial to understanding the extent of risk to North American birds. If SGS1 is found at a high rate in Colorado’s House Sparrows, Spellman believes that would be a sign that House Sparrows in metro areas nationwide are infected. Moreover, a high presence of SGS1 in urban birds might increase the chances of avian malaria spreading to birds outside cities.

“In Denver, it could be particularly important, because we have a number of grassland species, for example, that could be exposed right at that urban-wildland interface,” Spellman says. Already, . “If just one of those species is exposed to a novel pathogen, that could have dire consequences.”

For now, it’s a waiting game as scientists gather more information. If, theoretically, researchers find SGS1 in other places in the country, if dead birds test positive for SGS1, and if the birds that carry SGS1 start having negative health impacts while uninfected individuals don’t, then this strain would be something to really worry about, Theodosopoulos says. 

Even if SGS1 winds up not causing any major issues for North American birds, as the paper notes, the appearance of this strain is yet another reminder of the human impact on the environment and the need for diligence in a more globalized world. “We're changing the habitat; we’re moving species around,” Theodosopoulos says. “This certainly plays a role in the way that parasites and their diseases are able to spread.”