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WORCESTER, Mass. – One afternoon last winter, Julie Ellis unfurled a long, white tarp under a stand of trees near Coes Pond where hundreds of crows roost. Her mission: to collect as much bird poop as possible.

Back in the laboratory, Ellis’ colleagues combed through the feces. Testing its bacteria, they discovered something unusual – genes that make the crows resistant to antibiotics.

Drug-resistant infections are a fast-growing threat to human health, due largely to overuse of antibiotics in human medicine and livestock production, according to the Centers for Disease Control and Prevention. At least 2 million people each year in the United States alone are sickened by infections resistant to drugs.

Now new research, including the crow poop study conducted in four states, provides evidence that antibiotic resistance has spread beyond hospitals and farms to wildlife.

Some experts worry that contaminating wildlife with such genes may hasten the spread of drug resistance. Nevertheless, the consequences for human health remain poorly understood.

“We’ve documented human-derived drug resistance where it shouldn’t be – in wildlife and the environment. But we know very little about how this may impact public health. There just isn’t that smoking gun,” said Ellis, a research scientist at Tufts University’s veterinary school.

In addition to crows, resistance genes have been detected in the bacteria of gulls, houseflies, moths, foxes, frogs, sharks and whales, as well as in sand and coastal water samples from California and Washington.

The spread to wildlife is “an indicator of the wide-reaching scale of the problem. Microbes connect the planet,” said Lance Price, a professor of environmental and occupational health at George Washington University.

“The danger is that we enter a post-antibiotic era in which even our last-line drugs won’t work and routine infections become life-threatening,” he said.