Antibiotic-resistant bacteria that have infected more than 100 people and that have been linked to pet store puppies appear to have spread at least in part because healthy dogs were given antibiotics—a decision that all but surely fostered antibiotic resistance.

“This is shocking,” said Lance Price, head of George Washington University’s Antibiotic Resistance Action Center. “This is an important study that’s shining a light on something that we need to spend more time on.”

More than half of the treated dogs that fell ill in the outbreak were given the drugs not because they were sick, but to keep them from becoming so, according to a new study published Thursday. The technique, called prophylaxis, has been widely used in food animal production and is blamed for fueling antibiotic resistance.

“We just have to change how we’re thinking about antibiotics,” warned Matthew Wellington, antibiotics program director for U.S. PIRG, the Public Interest Research Group.

The outbreak of the bacteria, Campylobacter jejuni, which causes diarrheal disease, started in early 2016 and continued until February of this year. People from 18 states fell ill, including 29 pet store employees. The investigation, which began in August of 2017, discovered that puppies were the source of the problem.

Thursday’s study was published in Morbidity and Mortality Weekly Report, a journal produced by the Centers for Disease Control and Prevention. It revealed how many antibiotics the dogs had been given as well as the results of testing done on bacterial samples—known as isolates—from 10 of the sick people and eight of the puppies to see which drugs might kill the bacteria.

“Outbreak isolates were resistant by antibiotic susceptibility testing to all antibiotics commonly used to treat Campylobacter infections,” the authors reported.

“This outbreak demonstrates that puppies can be a source of multidrug-resistant Campylobacter infections in humans, warranting a closer look at antimicrobial use in the commercial dog industry.”

The outbreak involved six pet store chains, but the problem likely is a broader one, the study showed. Officials in four states visited 20 pet stores and collected antibiotic administration records for about 150 puppies. Of those, 95 percent had received at least one course of drugs before reaching the store or while at the store. Sixteen different types of antibiotics were used. And about half the treated dogs were not sick—they were given the drugs to prevent illness.

“Antibiotics should only be used to treat illness, not to compensate for poor practices—whether it’s trucking dogs long distances and having poor hygiene in the process along the way…” Wellington said. “These are lifesaving medicines that should only be used to treat sick animals or sick people.”

Both Wellington and Price have been vocal critics of misuse of antibiotics in food animal production. But use of the drugs in the commercial dog industry wasn’t on their radar.

Price was startled by the report. “For me, this is an indication that they need to be raising these animals differently. They’re creating this terrible distribution system for multidrug-resistant bacteria,” he said.

Republished with permission from STAT. This article originally appeared on September 20, 2018