Phenotypic and molecular epidemiologic evaluation of a Nocardia farcinica mastitis epizootic.

Nineteen putative Nocardia farcinica isolates epidemiologically associated with intramammary products containing neomycin were obtained from clinical cases in a Canada-wide mastitis epizootic. Epidemiologic investigations were unable to identify the mechanism for transmission. To evaluate the hypotheses generated (intrinsic versus extrinsic contamination) and to confirm the identity of N. farcinica, we compared these isolates phenotypically (biochemicals and antimicrobial susceptibility studies) and genotypically (16S rRNA gene sequencing analysis, chromosomal DNA and ribotyping profiles) with the type and reference strains of N. farcinica. Results of biochemical studies and 16S rRNA gene sequencing identified the isolates as N. farcinica. Results of chromosomal DNA and ribotyping profiles and antimicrobial resistance to amikacin indicated all were a unique clone of N. farcinica that differed from the control isolates. Our study suggests the epizootic was caused by transmission of a unique clone of N. farcinica through intrinsically contaminated dry cow intramammary products rather than an extrinsic source.