Editor's note: the following information was presented by Rodrigo Paiva and colleagues, Department of Veterinary Diagnostic and Production Animal Medicine, Veterinary Diagnostic Laboratory, Iowa State University, during the 2024 annual conference of the American Association of Swine Veterinarians. 

Diarrhea due to enterotoxigenic Escherichia coli (ETEC) remains one of the main enteric challenges to US swine. The presence of different virulence factors has been associated with economic losses due to increased mortality and treatment costs and decreased weight gain.

Data from the Iowa State University Veterinary Diagnostic Laboratory (ISU-VDL) has shown a high prevalence of F4 (K88) and F18 isolated over the last 12 years (2010-2022). In addition, antibiotic susceptibility has changed during this period.

This study aimed to characterize changes over time in ETEC strains isolated at ISU-VDL based on fimbriae genotyping and antimicrobial susceptibility to provide comprehensive information to swine veterinarians.

A database containing 3,240 E. coli isolates recovered from ISU-VDL diagnostic cases associated with ETEC disease diagnostic code (Dx-code: representing confirmation of disease by a pathologist based on individual case history, pathological examination, and laboratory testing) was analyzed. Laboratory testing included bacteriological culture, genotypification based on fimbriae gene detection by PCR (F4, F18, F41, F5, AIDA, EAEA), and antimicrobial susceptibility testing.

E. coli F18 and F4

Isolates with the F18 and F4 fimbriae genes represented the most common adhesions: F18, 2,211 (68%); F4, 979 (30%). In contrast, 2% of the isolates contained other fimbrial genes: EAEA, F5, F41 and AIDA. An increased number of cases associated with F18 fimbria have been observed since 2019 (Figure 1).

The proportion of isolates susceptible to enrofloxacin dramatically decreased for F4 (from 2014 to 2022) and F18 (from 2017 to 2022), respectively (Figures 2 and 3).

There was no variation in the proportion of susceptibility to ceftiofur isolates. The proportion of isolates susceptible to florfenicol, gentamicin, neomycin, sulfadimethoxine, and trimethoprim-sulphamethoxazole decreased on F18 positive isolates since 2017 (Figure 3).

This study revealed important changes in the frequency of detection of different fimbrial genes and antimicrobial susceptibility patterns on E. coli isolated from ISU-VDL cases. Identification of the genetic characteristics of E. coli helps to understand how the pathogen has evolved to survive in the environment and in the host. Changes in antibiotic susceptibility patterns are a public health concern, and understanding these changes is important to help veterinarians in their decision-making on disease control.