Resistant E. coli endemic in humans, not animals

San Francisco-Farm animals often are implicated as the source of drug-resistant Escherichia coli and other pathogens in humans. In some cases, human outbreaks have been traced back to infected farm animals, but a recent study of such outbreaks in Iowa exonerates food herds as the source of resistant E. coli that are responsible for urinary tract infections.

"In recent years, there has been significant resistance to TMP-SMZ," Menard said in a presentation at the 46th annual Interscience Conference on Antimicrobial Agents and Chemotherapy here. "Traditionally, even the most virulent strains of E. coli have been susceptible. But TMP-SMZ is no longer available for first-line therapy in many locations."

James Johnson, MD, Minneapolis VA Medical Center, Patricia Winokur, MD, University of Iowa, Iowa City, and Menard conducted a study aimed at tracing the emergence of CGA. The researchers randomly selected 147 TMP-SMZ-resistant E. coli isolates and 150 susceptible isolates from the EIEIO collection. The specimens had been collected from humans and cattle or swine throughout the state between 1998 and 2000. All isolates were screened for CGA.

Screening identified 22 CGA isolates, 7% of the total screened. CGA accounted for 18 TMP-SMZ-resistant isolates (12%) and four susceptible isolates (3%; p=.002). While CGA was found in 22 human isolates (15%), it was not found in any animal isolates (p<.001). CGA also accounted for 18 (25%) of the drug-resistant human isolates and only four (5%) of the susceptible human isolates (p=.001).

"Cattle and swine do not seem to constitute a significant reservoir for transmission to humans," Menard said. "We are probably talking about human-to-human spread of CGA, which is already endemic in Iowa."