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 puppies in a sample of roughly 150 dogs studied as part of the outbreak investigation were given antibiotics 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.

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“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.

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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 —

and many received more than one — 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.

Senior author Mark Laughlin, a veterinarian with CDC’s division of foodborne, waterborne and environmental diseases, said investigators were taken aback by the scale of antibiotic use in the industry.

“We were surprised to see the large number of different types of drugs and the large number of courses that the dogs were exposed to. These are pretty young animals on the whole,” he told STAT.

Initially the CDC thought it might be able to trace the infections to a single source — one breeder or commercial breeding facility where the bacterium had spread. But as the investigators learned more about the byzantine world of the breeding and distribution of dogs sold in pet stores, it became clear there wasn’t a single source.

In effect, the system was creating the problem. “These dogs were coming from a large variety of sources,” Laughlin said.

Price wasn’t impressed. “If your system requires a constant or regular dose of antibiotics to keep the animals healthy, your system’s broken. You’ve designed a system that makes sick animals,” he said.

Wellington agreed. “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,” he said. “These are lifesaving medicines that should only be used to treat sick animals or sick people.”

Campylobacter jejuni is a common infection; the CDC estimates that about 1.3 million cases occur every year in the U.S. Fortunately most people recover without needing medical care.

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.

The reality is that, although the outbreak appears to have ended, there could well be ongoing cases because the practices that led to puppies becoming infected with multidrug resistant drugs are still being used.

Laughlin said the CDC is working with veterinarian associations and the commercial dog industry, which he said was concerned and keen to make changes.

They must, Wellington insisted.

“This is one of the clearest examples I’ve seen where resistant bacteria are originating in animals from antibiotic overuse, and they’re passing directly to people and spreading rapidly,” he said. “So I think this is one of those situations where it’s incredibly clear that this is a problem we need to solve.”