Dogs can be kid magnets. So bringing therapy dogs in to a hospital to cheer up sick kids seems like a great idea.

But sick kids with weakened immune systems can be superbug magnets. And bringing an outwardly healthy therapy dog who might be carrying antibiotic-resistant bacteria into a children’s cancer ward can suddenly seem like a really bad idea.

At Johns Hopkins Hospital in Baltimore, Maryland, children receiving cancer treatment in the hospital’s oncology unit began to show traces of methicillin-resistant Staphylococcus aureus bacteria (MRSA) in 2016. Their doctors wondered where the superbug came from, and their suspicions settled on four therapy dogs who had been brought in to cheer up the children.

So the next time therapy dogs Pippi, Poppy, Badger, and Winnie dropped by for a visit, the doctors started an intervention study. They presented their findings last week in San Francisco at IDWeek 2018, an annual meeting of infectious disease professionals.

In the Johns Hopkins study, doctors studied 45 children who interacted with the four dogs—hugging, petting, feeding, or playing with them—over the course of 13 visits in 2016 and 2017.

The doctors discovered that kids who spend more time with the dogs had a six-times greater chance of coming away with MRSA than kids who spent less time with the dogs.

Among the kids who had no MRSA prior to the dogs’ visits, 10% showed exposure to MRSA after the dogs left. Doctors also found MRSA on close to 40% of samples taken from the dogs. Ironically, the dogs were probably superbug free when they first came to the hospital—researchers suspect the dogs picked up the bug from other patients during their visits.

“Our hypothesis is it’s really person-to-person transmission, but it happened through contact with the [dogs’] fur,” said Meghan Davis, PhD, DVM, assistant professor at Johns Hopkins Bloomberg School of Public Health and a coauthor of the study.

MRSA bacteria often live on the skin without causing symptoms. But they can become more dangerous if they enter the bloodstream, destroying heart valves or causing other serious issues, from sepsis to pneumonia to bloodstream infections. As many as 11,000 deaths a year in the US have been traced to the superbug. And it’s usually acquired in hospital and healthcare settings.

Although dogs are not normally colonized with Staphylococcus aureus, if a dog is exposed to a person who is colonized or who has an active infection, the dog can become infected or colonized as well.

During the study, researchers asked the dogs’ owners to start bathing the animals with a special shampoo before each visit. During the visits, researchers had the dogs patted down every 5 to 10 minutes with disinfecting wipes.

Those steps dramatically decreased the bacteria level on the dogs, said Kathryn Dalton, VMD, MPH, coauthor of the study, who also admitted that prior to the study, the hospital’s protocols regarding contact between patients and therapy dogs “wasn’t strictly enforced.”

The findings are good news for patients with suppressed immune systems in all hospitals with therapy-dog programs. Pet therapy can help people recover from a wide range of health problems: Studies have shown that therapy dogs can ease anxiety and sadness, lower blood pressure, and even reduce the amount of medications some patients need.

Dalton said she hopes further study will show that the new cleaning procedures can reduce any risk of superbug infection.

“I really had the opportunity to see how important these dogs were to the patients,” Dalton said. After visits with the dogs, the kids “would say how much this made their day.”

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