Antibiotic-Resistant Bugs Go Wild

18 October 2012 4:03 pm

Harvey Henkelmann/Wikimedia Commons

Bugs bunny. Researchers found MRSA in two Eastern cottontail rabbits.

One of the most notorious and hard-to-treat bacteria in humans has been found in wildlife, according to a new study in the Journal of Wildlife Diseases. The researchers isolated methicillin-resistant Staphylococcus aureus (MRSA) in two rabbits and a shorebird. Wild animals may act as an environmental reservoir for the disease from which humans could get infected.

S. aureus can cause skin infections or, if it gets into the bloodstream, life-threatening illness. Most infections are easy to manage with penicillin and related antibiotics, but MRSA, the resistant variety, is on the rise; also known as a "superbug," it kills an estimated 18,000 Americans a year. In most cases, people contract the bacterium from a hospital stay. Hospitals are breeding grounds for antibiotic-resistant organisms, because patients are treated with a wide variety of antimicrobial drugs, prompting pathogens to develop defenses.

It's been clear for more than a decade, however, that people can catch MRSA strains outside of the hospital as well; researchers call these "community-associated" strains. For instance, pigs on livestock farms have been found harboring the bug, likely because farmers give antibiotics to food animals as they grow, another way of encouraging resistance to evolve. Other studies have found MRSA in pets and zoo animals; they may have been infected by human caretakers.

Now it appears that even animals in the wild can be infected with MRSA. Researchers led by epidemiologist Tara Smith of the University of Iowa's College of Public Health in Iowa City took samples from 114 animals that came into the Wildlife Care Clinic, which rehabilitates injured or orphaned animals, at Iowa State University in Ames. Seven of the animals, or 6.1%, carried S. aureus that was sensitive to methicillin; these included owls, pigeons, a beaver, a heron, and a squirrel. Three animals, or 2.6%, carried MRSA: two Eastern cottontail rabbits and a lesser yellowlegs, a migratory shorebird. (For comparison's sake: An estimated 1.5% of Americans carry MRSA in their noses.)

A big question is how these species came to carry MRSA. "This is really, really hard to do—to understand the source, especially with something like migratory birds," says Jorge Ferreira, a veterinarian and epidemiologist working as a consultant in Switzerland who has studied the presence of MRSA on humans and their pets. Presumably, the infected animals have never received antibiotics, he notes, so they must have picked up the bugs directly from their environment.

Molecular typing of the isolates showed that the shorebird carried a hospital-associated strain of MRSA while the rabbits had community-associated strains. The rabbits' MRSA was also resistant to tetracycline, which Smith says is common in farm animals.

Perhaps most troubling of all was that one of the pigeons carried a Staphylococcus bacterium that, while still sensitive to methicillin, was resistant to the antibiotic vancomycin. "Vancomycin is used as a last resort in MRSA infections," says study co-author Shylo Wardyn, a research assistant in Smith's lab, and vancomycin-resistant staph strains are rare in humans.

Whether wild animals represent a reservoir of MRSA in the environment—that is, whether they could spread the superbugs to other animals and humans—is an open question, Smith says. The infections could be "spillover events" from humans, caused by hospital waste, sewage, or farming, that pose no wider threat. It's also not clear if the animals can rid themselves of the infection, if they can be infected multiple times, or if they have ever passed their infection back to humans.

Ferreira's work suggests that dogs and their owners can pass MRSA back and forth, and wildlife is a well-known source of other human infections, as with deer and Lyme disease, and mice and hantavirus. While such evidence is suggestive, there's a lot more work to be done to show whether humans can get infected with MRSA from wild animals.