A new study led by researchers with the Centers for Disease Control and Prevention (CDC) indicates that strains of extensively drug-resistant (XDR) bacteria linked to pet store puppies have been circulating for a decade and continue to cause illness.
The study, published this week in JAMA Network Open, identified 168 patients who had XDR Campylobacter jejuni infections with epidemiologic or molecular links to pet store puppies from 2011 to 2020. Analysis of bacterial isolates from the patients found resistance to seven classes of antibiotics, including antibiotics that are recommended for treatment of Campylobacter infections.
The study summarizes a two-part investigation that the CDC began in 2017 following reports of a multistate outbreak of antibiotic-resistant Campylobacter infections that originally began in Florida. That investigation identified 118 cases in 18 states through February 2018, along with evidence indicating that puppies purchased from six national pet store chains were the source of the outbreak. Reports of ongoing cases led to a follow-up investigation in 2019.
The study authors say the findings suggest clinicians should ask about puppy exposure when treating people with Campylobacter infections, especially those that don't respond to routine antibiotics. They also call on the commercial dog industry to take action to prevent the spread for these XDR strains from humans to people.
Infections date back to 2011
Campylobacter infection is the most common cause of diarrheal illness in the United States, according to the CDC, affecting 1.5 million people each year. More than 90% of infections are caused by C jejuni, and most are associated with consumption of undercooked poultry, followed by international travel and exposure to animal feces. But the XDR infections identified by the CDC in these investigations are associated only with dogs.
The first CDC-led investigation began in August 2017 following reports of six patients in Florida with C jejuni infections who had reported contact with puppies sold by a national pet store chain based in Ohio. State and local public health investigators interviewed patients with confirmed infections, asking them about outcomes and exposures prior to illness, and conducted whole-genome sequencing on fecal samples from 211 puppies from 33 pet stores. They also conducted trace-back investigations of puppies whose fecal samples contained C jejuni isolates that were highly related to patient isolates or had epidemiologic links to infected patients.
That process was than repeated for the second investigation, which began in January 2019. In both investigations, enhanced surveillance was conducted to detect any additional human cases caused by related C jejuni strains. Investigators also examined Campylobacter isolates uploaded to PulseNet—a national laboratory network that connects foodborne illness cases—prior to 2016 to identify any that may be genetically related to confirmed cases.
Of the 168 cases identified in the two investigations as having a molecular or epidemiologic link to pet store puppies, 137 had illness onset from Jan 8, 2016, to Feb 20, 2020, and 31 cases occurred before January 2016, dating back to Feb 2, 2011. Thirty-one patients out of 126 with available information were hospitalized, and none died. Several patients, however, had prolonged hospital stays after multiple antibiotic courses failed to resolve the infection.
Overall, 117 of 121 patients (97%) reported contact with a dog in the week before symptoms began, and 69 of 78 (88%) with additional information reported contact with a pet store puppy. No patients reported contact with puppies from an animal shelter or a rescue organization.
Molecular analysis of 168 Campylobacter isolates from humans and 23 dog isolates revealed that they were closely related genetically and clustered into three clades, suggesting likely transmission from dogs to humans.
Antibiotic susceptibility testing showed that 88% of the isolates were XDR, with high levels of resistance to aminoglycosides, ketolides, lincosamides, macrolides, phenicols, quinolones, and tetracylclines. By comparison, only 1.3% of Campylobacter isolates submitted from 2011 through 2019 to the National Antimicrobial Resistance Monitoring System (NARMS)—which tracks resistance in intestinal bacteria from food, animals, and people—were found to be XDR.
No single breeder, distributor, or transportation company was found to be the sole source of infected puppies.
Inappropriate antibiotic use, management practices cited
The connection to pet stores and commercial breeders may be linked to inappropriate use of antibiotics and other management practices in the commercial dog industry.
The authors note that, in the initial investigation, which was described in a 2018 Morbidity and Mortality Weekly Report article, CDC investigators reported that 142 of 149 puppies from 20 pet stores had received at least one or more antibiotic course before arriving at the stores. More than half of the puppies received antibiotics to prevent illness.
"Use of antibiotics and other management practices in the commercial dog industry might have selected for extensively drug-resistant strains and facilitated spread among dogs from 1 or more breeding facilities to many stores," the authors of the new study wrote.
As veterinary and public health consultant Gail Hansen, DVM, explains, puppy mills and commercial dog suppliers sometimes give antibiotics to dogs either just before they are shipped or just after they arrive at pet stores to thwart diarrhea and respiratory diseases.
"It is not best practices, and in fact is a terrible practice, but is still done," she said.
The fact that cases continued to occur after the initial outbreak and subsequent investigation also suggests that the recommendations made by the CDC to reduce illness among dogs, pet store employees, and customers haven't solved the problem. The agency recommended that employees and customers wash their hands after touching puppies, that pet stores separate human eating areas from animal areas, and that employees use gloves when cleaning dog cages.
The authors say that while commercial dog breeders could implement measures to curb unnecessary antibiotic use and improve hygiene and infection control from the breeding facilities to the pet stores, a larger effort may be needed.
"This study highlights an ongoing problem within the companion animal sector that will require a collaborative solution," they wrote. "These results indicate that public health officials, the commercial dog industry, animal welfare advocates, regulatory officials, physicians, and veterinarians should adopt a One Health approach to prevent the development and slow the spread of antibiotic resistance."