In 2008, for example, an elephant calf and 20 of its caretakers at the San Diego Zoo contracted MRSA skin infections. An investigation by the zoo and state health officials determined that the calf, which was eventually euthanized, had probably been infected by a keeper who unknowingly carried the bacteria. (The case was reported in The Morbidity and Mortality Weekly Report.)

Still, experts are not recommending routine testing of pets and their humans. Instead, they call for the same kinds of precautions that apply to other pathogens, especially frequent washing or sanitizing of hands before and after playing with a pet.

The first cases of MRSA in pets, about five years ago, appeared to be in therapy dogs and other animals exposed to patients or health care workers. Those animals are still thought to be at greatest risk, but the pattern might be changing.

In a study this summer in The American Journal of Infection Control, Elizabeth A. Scott and her colleagues at the Center for Hygiene and Health in Home and Community at Simmons College in Boston swabbed household surfaces like kitchen and bathtub drains, faucet handles, toilets, high chairs, trash cans and kitchen sponges at 35 randomly selected addresses to see what germs they would find. They found MRSA in nearly half of the homes they sampled.

When they tried to figure out what might make it more likely to have the bacteria at home, they ruled out many supposed risk factors, including working out at a gym, having children who attended day care, having a recent infection or recent antibiotic use, and even working in a health care facility.