Study reveals trends in drug-resistant Campylobacter infections across the U.S.

Campylobacter infections are the most common foodborne illnesses in the U.S., sickening an estimated 1.5 million people each year. A new study examined records of Campylobacter jejuni infections from 10 states, plotting regional, age-related, and drug-resistance trends from 2013 to 2019.

The study found that drug-resistant C. jejuni infections were highest in the 20-39 age group and that quinolone-resistant C. jejuni infections increased from 22.6% of those tested in 2013 to 33.54% in 2019. The researchers also identified regional differences in C. jejuni resistance to quinolones and six other classes of antibiotics. The new findings are reported in the International Journal of Antimicrobial Agents.

According to the U.S. Centers for Disease Control and Prevention, the main source of Campylobacter infections in humans is the consumption of contaminated poultry. Failing to follow proper sanitation and cooking methods is the primary cause of infection. Pigs, cows, sheep and other animals also harbor Campylobacter in their digestive tracts and may transmit it to people or other animals.

Most Campylobacter infections in humans are mild and resolve within about a week, but children under age five, adults 65 and older, and those with compromised immune systems may experience more serious infections that require hospitalization. More than 100 people die each year of Campylobacter infections in the U.S.

The new study relied on publicly available data collected by the National Antimicrobial Resistance Monitoring System, a U.S. public health service that tracks bacterial infections and antimicrobial resistance in foodborne and other intestinal bacteria. NARMS focuses its efforts on 10 states selected to reflect regional differences across the U.S.: California, Connecticut, Colorado, Georgia, Maryland, Minnesota, New Mexico, New York, Oregon and Tennessee.

"These sites are monitoring foodborne pathogens in the local population," said Csaba Varga, a professor of pathobiology at the University of Illinois Urbana-Champaign who led the new research. "They are tracking infections with Campylobacter, E. coli, Salmonella and other foodborne pathogens, and a subset of isolates are tested for their antimicrobial resistance status. We analyze this surveillance data because we are interested in emerging and long-term trends."

In addition to finding that people in the 20-39 age group were infected with drug-resistant C. jejuni at higher rates than all other age groups, the analysis also found that some states had significantly higher rates of drug-resistant C. jejuni infections than others. Of the 10 states examined, Connecticut had the highest rates of drug-resistant C. jejuni infections for all antibiotics except tetracyclines and quinolones. Tennessee and Georgia had higher percentages of drug-resistant infections for three of the seven antibiotics examined. New Mexico had the lowest levels of antibiotic-resistant C. jejuni infections overall.

These differences cannot be easily explained, Varga said. Differing climate conditions, the presence of poultry farms, variations in reporting methods or other factors may affect the rates of drug-resistant C. jejuni infections captured in the national data.

The study found that overall rates of tetracycline-resistant C. jejuni infections went down from 49.13% to 41.36% between 2013 and 2019, while rates of quinolone-resistant infections increased. Quinolones - a class of antibiotics that includes drugs like ciprofloxacin, or Cipro, and levofloxacin, also known as Levaquin - "are sometimes prescribed to treat campylobacteriosis that might impact the selection of quinolone-resistant strains," Varga said.

The findings are particularly interesting because most human cases of C. jejuni are not treated with antibiotics, he said.

When the Campylobacter infection in humans is so severe that it needs to be given drugs, the drug of choice is usually azithromycin, which is a macrolide antibiotic. In our study, we did not find a high resistance to macrolides, which is good news."

Csaba Varga, professor of pathobiology, University of Illinois Urbana-Champaign

The poultry industry also is reducing its use of antibiotics, Varga said. "We see quinolone resistance going up in Campylobacter infections, but quinolones are rarely used in poultry," he said.

A previous study from Varga's lab that tested samples from chickens and turkeys from 2013 to 2021 also found high levels of resistance to tetracyclines in chickens and high levels of resistance to tetracyclines and quinolones in turkeys.

That study and the new one point to the need for further research to understand the trends and the development of new approaches to curtail and contain the spread of antibiotic-resistant pathogens in the food supply and elsewhere, Varga said.

While the NARMS data are vital to understanding trends, they almost certainly undercount the number of foodborne infections, Varga said.

"This is just the tip of the iceberg," he said. "There is an underreporting of these cases because not everybody is getting to the doctor, depending on their symptoms, and not everybody is going to get tested."

Varga also is an affiliate of the Carl R. Woese Institute for Genomic Biology and of the Program in Ecology, Evolution and Conservation Biology the U. of I.