Evaluating Carbapenem Resistance

Nontyphoidal serovars of Salmonella enterica, which are responsible for a significant proportion of foodborne illnesses in humans and animals, are acquired most often through ingestion of contaminated meat, milk, and egg products. Although these organisms typically cause self-limiting gastroenteritis, they can result in serious systemic infections, particularly in children, the elderly, or immunocompromised individuals. In this patient population, antibiotic resistance is of serious concern and may have fatal consequences. In a recent review in Veterinary Science, investigators evaluated carbapenem resistance in nontyphoidal serovars of S enterica.

Carbapenem resistance has generally been rare in nontyphoidal S enterica infections. Recently, horizontal gene transfer has been identified as an important mechanism of resistance, as well as co- and cross-selection of resistance across other antimicrobial classes.

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Several mechanisms of resistance were described in human infections, but of most interest and concern, various carbapenemase-encoding genes that originally segregated according to geographic derivation, showed evidence of worldwide spread through colonization of nosocomial bacteria via horizontal gene transfer. This is of great concern given the abundance of nosocomial organisms in hospitals and other health facilities.

In veterinary livestock medicine, carbapenem-resistant nontyphoidal S enterica infections are present, despite the lack of use of carbapenems in livestock medicine, suggesting reservoirs other than human nosocomial infections. Co- and cross-selection from other classes of antimicrobials, in particular third-generation (eg, extended-spectrum) cephalosporins, is likely responsible for the transmission of carbapenem resistance in these cases. The abundant use of long-acting cephalosporins in companion animal medicine is likely contributing to this. Widespread use of carbapenems and third-generation cephalosporins where more narrow-spectrum antimicrobials are sufficient, is thought to be inappropriate and should be avoided. Global surveillance and legislation limiting the use of these antimicrobials is recommended to avoid further transmission of resistance. Such legislation already exists in the European Union.

Clinical Impact

Carbapenem resistance is a serious and worldwide public health concern, as carbapenem is a valuable “last resort” antimicrobial. To minimize further spread of carbapenem resistance, carbapenems, as well as extended-spectrum cephalosporins, particularly long-acting, third-generation cephalosporin formulations (eg, cefovecin and cefpodoxime), should be reserved for exceptional cases.

Dr. Packer, an associate professor of neurology/neurosurgery at Colorado State University College of Veterinary Medicine and Biomedical Sciences in Fort Collins, is board certified in neurology by the American College of Veterinary Internal Medicine. She is active in clinical and didactic training of veterinary students and residents and has developed a comparative neuro-oncology research program at Colorado State University.