Antimicrobial Susceptibility of Enterotoxigenic Escherichia coli from Diarrhoeic Neonatal Calves in Spain

SIMPLE SUMMARY: Neonatal calf diarrhoea, a worldwide concern for cattle production, can be caused by viral, bacterial and protozoan enteropathogens: the enterotoxigenic Escherichia coli (ETEC) is one of the most important. The use of antimicrobials for treating neonatal calf diarrhoea cases is still a common practice among veterinary surgeons, although its use is only justified in bacterial infections evolving towards a systemic disease. Since the indiscriminate use of antimicrobials for treating diarrhoeic calves increases the risk for the appearance of antimicrobial resistances, restrictions on the use of antimicrobials in veterinary practice were implemented. The aim of this study was to characterize the antimicrobial susceptibility of ETEC strains obtained from diarrhoeic calves. Our results are alarming since all ETEC strains were resistant to three or more families of antimicrobials; in addition, a high number of strains were resistant to most first-line antimicrobials used in veterinary practice. Only ceftiofur, cefoperazone, cefquinome and gentamicin presented efficacy against most ETEC strains. Thus, empirical treatment of calf scours caused by ETEC is usually ineffective. Our results reveal that performing antimicrobial susceptibility tests in each NCD outbreak is needed for establishing an effective treatment and avoiding the emergence of new resistance mechanisms. ABSTRACT: Enterotoxigenic Escherichia coli (ETEC) is one of the major pathogens involved in neonatal calf diarrhoea (NCD) causing high economic losses in dairy farms. Antibiotic treatment is common in cases of systemic illness caused by NCD, but antimicrobial susceptibility tests (AST) are usually not performed. Thus, the aim of this study was to characterize the antimicrobial susceptibility of ETEC strains obtained from calves with diarrhoea between 2018–2020. Faecal samples (n = 420) were analyzed to detect the typical ETEC virulence factors F5 and STa. Positive samples were cultured to identify and isolate ETEC strains (n = 41) and ASTs were performed. Our results are alarming since ETEC strains resistant to three or more families of antimicrobials were detected in all isolates. Only four antibiotics (ceftiofur, cefoperazone, cefquinome and gentamicin) presented efficacy against more than 90% of the ETEC strains, while the other ten antibiotics were effective against less than 40% of the strains. In addition, a high number of strains were resistant to most first-line antimicrobials used in veterinary practice. For this reason, when ETEC infection is suspected, an AST must always be performed to select the most appropriate antimicrobial in each case and to avoid the emergence of new resistance mechanisms.