Apr 1, 2005 (CIDRAP News) – A study of antibiotic-resistant Campylobacter levels on retail chicken products suggests that the pathogen lingers in chickens long after antibiotic use among the birds is stopped.
Researchers from Johns Hopkins University found that sizable percentages of retail chicken samples from two large companies had antibiotic-resistant Campylobacter on them even though the companies had stopped treating their flocks with the antibiotic in question a year earlier.
In addition, the researchers found that chicken samples from those two companies were more likely to carry antibiotic-resistant Campylobacter on them than were samples from two companies that marketed their products as completely antibiotic-free.
The study, which focused on fluoroquinolone (FQ)-resistant Campylobacter, was published online recently by Environmental Health Perspectives. The research was led by Lance Price, a doctoral candidate and fellow at Johns Hopkins University's Bloomberg School of Public Health Center for a Livable Future in Baltimore.
Investigators have been finding a link between consumption of FQ-treated poultry and cases of FQ-resistant Campylobacter infection in the United States, the authors note. Researchers have also reported an association between FQ use in poultry barns and the evolution of FQ-resistant bacteria in poultry.
Building on those themes, Price and colleagues selected two large conventional poultry producers that said they had ceased to treat their chickens with FQs, Tyson and Perdue Farms, and two antibiotic-free poultry producers, Bell & Evans and Eberly. They obtained three samples of each of the four brands from the same stores at the same time on seven or eight occasions over a period of 10 weeks. The samples were prepared and tested in identical ways. Researchers used the standard Food and Drug Administration methodology and a modified method that involved FQ-supplemented agar medium to identify resistant strains.
The Campylobacter isolates were confirmed and species were identified using a polymerase chain reaction (PCR) amplification/restriction protocol. Campylobacter was found on 84% of the samples tested, and when the standard FDA method was used, FQ-resistant strains were detected on 17%. But when the supplemented medium was used, FQ-resistant strains were found on 40% of the samples. In their genetic analysis, the researchers found that 19 of 21 resistant isolates were genetically distinct from the susceptible Campylobacter and would have been missed with the standard methodology.
The researchers report statistically significant differences in the rates of FQ-resistant Campylobacter carriage across the four brands. The rates were as follows: Tyson, 96%; Perdue, 43%; Bell & Evans, 13%; and Eberly, 5%. The difference between the latter two brands was not significant, the report says.
Because of the tight time frame of the study, researchers cautioned, it was limited in part by the inability to measure seasonal changes.
The authors say their findings suggest that previous use of this family of antibiotics can have lingering effects on the presence of Campylobacter in poultry houses. The results call into question the idea that drug-resistant populations quickly become susceptible again when the antimicrobial is withdrawn, the report says.
The study also highlights the importance of disinfecting facilities between flocks, the researchers write. The dirt floors common in many US poultry barns are cleaned only every 2 or 3 years, potentially creating a "long-term reservoir" for FQ-resistant Campylobacter. Likewise, processing plants could be a source of cross-contamination, the report says.
Because antimicrobial therapy can be critical for treating Campylobacter infections in people with weakened immune systems, the article says, FQ-resistant strains magnify the threat to those groups, making it more important to accurately measure those strains and identify factors contributing to their presence.
The FDA proposed withdrawing approval for fluoroquinolone use in poultry production in 2000, but the Bloomberg School of Public Health said in a news release that the effort has been stalled over legal objections from Bayer, which makes one of the drugs.
Tyson Foods has discontinued use of FQs among broiler chickens but still uses them in breeder operations, spokesman Gary Mickelson told CIDRAP News. He commented that the sample used in the study was small and that the researchers detected the presence of the bacteria but did not measure the amount present. He also said the researchers included Campylobacter species that may have a natural resistance to quinolone antibiotics even in the absence of exposure to them.
"We at Tyson Foods stand by the safety of our products and our efforts to operate responsibly," Mickelson wrote in an email today. Antibiotics are used in the growing operations only to protect bird health, he added.
Perdue Farms no longer uses FQs in any poultry and treats only ill or at-risk chickens with approved antibiotics, spokesman Joe Forsthoffer told CIDRAP News. Less than 1% of the company's chickens are treated with antibiotics, he said. Antibiotics aren't used to promote growth, nor are they administered continuously.
Perdue Farms could not comment on any conclusions about antibiotic resistance in the study, because the company didn't have information to link the poultry samples used back to specific lots, Forsthoffer said.
Price LB, Johnson E, Vailes R, et al. Fluoroquinolone-resistant Campylobacter isolates from conventional and antibiotic-free chicken products. Environ Health Perspect 2005 (in press, but published online Feb 2) [Abstract]
Mar 16 Bloomberg School of Public Health news release