Drug-Resistant Shiga Toxin-Producing Escherichia coli Cases Increasing

A recent study of Shiga toxin-producing Escherichia coli (STEC) strains recovered from patients in Michigan during 2010 to 2014 has reported a high frequency of STEC antimicrobial resistance to drugs commonly used in human and veterinary medicine.

Sanjana Mukherjee, MS, from Michigan State University, East Lansing, Michigan, and colleagues published the results of their study online ahead of print in the September 2017 issue of Emerging Infectious Diseases, [NP1] the Centers for Disease Control and Prevention’s monthly peer-reviewed public health journal.

“Because Michigan is not part of the Centers for Disease Control and Prevention Foodborne Diseases Active Surveillance Network and resistance in STEC has not been widely researched, data about the prevalence and impact of resistance are lacking,” the authors write.

“This study detected a high frequency of STEC resistance to antimicrobial drugs commonly used in human and veterinary medicine, particularly for non-O157 serotypes, which have increased in frequency.”

STEC strains are important enteric pathogens worldwide, causing diarrhea with or without blood, and hemolytic uremic syndrome. These food-borne pathogens are an important public health concern worldwide, and, according to CDC estimates, account for approximately 265,000 cases of food-borne illness each year in the United States. Cattle are the most important natural reservoir of STEC, and humans become infected by eating foods contaminated with cattle feces.

Although most infections are associated with O157 strains, the authors stress that infections with non-O157 strains have been increasingly reported.

The emergence of antimicrobial-resistant bacterial pathogens is another significant public health problem. However, although antimicrobial drug resistance among STEC has been documented, it is probably underestimated.

Therefore, Dr. Mukherjee and colleagues performed a study to examine the prevalence of resistant STEC infections and determine the effects of resistance on disease.

They analyzed 358 STEC isolates from the Michigan Department of Health and Human Services (MDHHS) Reference Laboratory. These isolates were collected between 2010 and 2014, and 14 were associated with STEC outbreaks.

The investigators examined one strain from each outbreak and found 31 (8.8%) drug-resistant strains (23 of which were non-O157, while 8 were O157). They identified ampicillin resistance most frequently (7.4%), followed by trimethoprim/sulfamethoxazole (SXT) (4.0%), and ciprofloxacin (0.3%).

With respect to the O157 isolates, resistance rates to ciprofloxacin and SXT in Michigan were higher than those found nationally, but they were not significantly higher. Nevertheless, the higher resistance frequencies in Michigan indicate that selection pressures vary according to location and source, the authors write.

To assess how this antimicrobial resistance might affect the patient, Dr. Mukherjee and colleagues also performed a predictive analysis, using logistic regression with hospitalization as the dependent variable. This showed that hospitalized patients were more likely to have antimicrobial-resistant STEC infections, and less likely to have non-O157 STEC infections, suggesting that resistant infections or O157 infections may cause more severe disease. The analysis also showed that patients over 18 years of age, women, and patients with bloody diarrhea were also more likely to be hospitalized.

The investigators emphasize that drug resistance in STEC should be monitored because resistant strains can be transmitted from food animals to humans and because resistance genes can be horizontally transferred from STEC to other pathogens.

“Routine monitoring can uncover new treatment approaches and guide development of strategies for controlling emergence and spread of resistance in STEC and other E. coli pathotypes,” the authors conclude.

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.