City-dwelling humans are having an impact on urban birds, according to new research that highlights the prevalence and severity of two distinct parasites in wild house finches.

Working in seven sites throughout Maricopa County in central Arizona, the researchers studied the effects of urbanization on the stress response system of the finches and the effects of an intestinal parasite and a virus on the birds. Although it is found across North America, the house finch (Haemorhous mexicanus) is native to the desert in the Southwest US.

"Several studies have measured parasite infection in urban animals, but surprisingly we are the first to measure whether wild birds living in a city were more or less infected by a parasite and a pathogen, as well as how these infections are linked to their physiological stress," said researcher Mathieu Giraudeau of the University of Zurich in Switzerland.

"We also capitalized on data gathered by the Central Arizona Phoenix-Long Term Ecological Research program to accurately measure the degree to which the landscapes at each study site were natural or disturbed by humans," Giraudeau added.

As the word's population continues to move to urban centers, the effects on the environment and wildlife are mounting. Natural habitats and ecosystems are being altered from their native states, and there in concern that as this continues to occur it will correlate with an increased spread of disease from wildlife to humans.

According to the researchers, as much as 75 percent of the world's emerging diseases can be transmitted from animals to humans.

"Much like the spread of human disease in populated areas, urban centers can foster increases in multiple disease types in wild animals," said senior study author Kevin McGraw, an associate professor at Arizona State University. "We are now investigating the mechanism underlying this observation -- are urban animals immuno-compromised and less able to fight off infections than rural ones? Or, do they acquire more disease because of increased contact with other, infected animals?"

McGraw and Giraudeau observed that the presence and seriousness of gastrointestinal parasitic infections were higher in more urbanized areas in central Arizona. This included areas where there was extensive cultivation of the land. House finches from areas where there was a lot of cultivation were also heavier, and heavier birds were more prone to intestinal infections, the researchers said.

"Our careful analyses of land-use characteristics reveal that decreases in natural habitat may be a driving force behind increases in avian parasite infections. The same may be true in other animals. Because disease is so critical to the survival of wild animals, this is a real concern," McGraw added. "We need to improve our understanding of how specific anthropogenic disturbances in cities are affecting the evolution of parasites and their hosts."

The authors published their study in the journal PLOS One.