Isolation, Identification and Pathogenic Characterization of Major Pathogens Associated with Calf Diarrhea on a Dairy Farm in Kunming

Purpose To identify the pathogen responsible for the group diarrhea in calves at a large-scale dairy farm in Kunming, and to provide a scientific basis for medication.

Methods Conventional polymerase chain reaction (PCR) was employed to detect typical pathogens in fecal samples from diseased calves. After ruling out non-infectious factors, the pathogens were isolated, purified, and identified. Antimicrobial susceptibility testing was conducted on the isolated strains, along with simultaneous detection of drug resistance and virulence genes.

Results All samples tested negative for suspected viral and parasitic pathogens. The isolated pathogens were exclusively identified as Escherichia coli and Salmonella spp. Antimicrobial susceptibility testing revealed that 93.75% of the isolates exhibited drug resistance. Molecular detection identified the quinolone resistance gene oqxAB, the tetracycline resistance gene tetB, and the colistin resistance gene mcr-1. Furthermore, the E. coli isolates harbored four virulence genes, including K88, while the Salmonella spp. strains carried ten virulence-associated genes, such as orgA.

Conclusion The major causative pathogens of calf diarrhea at this dairy farm are E. coli and Salmonella spp., both harboring antimicrobial resistance and multiple virulence genes. The E. coli isolates exhibit diverse serotypes, whereas all Salmonella isolates are identified as Salmonella Typhimurium.