Caseous lymphadenitis (CLA) is a bacterial infection caused by Corynebacterium pseudotuberculosis (C. pseudotuberculosis) that affects sheep and goats, leading to abscess formation in their lymph nodes. The present study aimed to isolate and identify C. pseudotuberculosis from CLA in smallholder sheep and goats, and determine the resistance patterns, virulence, and resistance genes of the isolates. Additionally, genotypic and phylogenetic analysis of the isolates was conducted using ERIC-PCR and DNA sequencing techniques. A cross-sectional study examined 220 animals (130 sheep and 90 goats) from 39 smallholder flocks for clinical signs of CLA. Fifty-four (24.54%) animals showed CLA-compatible lesions, confirmed by C. pseudotuberculosis isolation and PCR identification. Sheep had a lower infection rate of CLA (18.46%) compared with goats (33.3%). Antimicrobial susceptibility testing of 54 C. pseudotuberculosis isolates to 24 antimicrobial drugs revealed that they were 100% resistant to bacitracin and florfenicol, while none of the isolates were resistant to norfloxacin. A high resistance rate was observed for penicillin and erythromycin (92.6% each). Interestingly, 16.7% of C. pseudotuberculosis isolates recovered from sheep showed vancomycin resistance. Molecular characterization of C. pseudotuberculosis isolates revealed that PLD, PIP, and FagA virulence genes were present in all examined isolates. However, the FagB, FagC, and FagD genes were detected in 24 (100%), 20 (83%), and 18 (75%) of the sheep isolates, and 26 (87%), 26 (87%), and 18 (60%) of the goat isolates, respectively. The β-lactam resistance gene was present in all isolates. Furthermore, 83% of the sheep isolates carried the aminoglycoside (aph(3″)-lb), chloramphenicol (cat1), and bacitracin (bcrA) resistance genes. Among the isolates recovered from goats, 73% were found to contain macrolides (ermX), sulfonamide (sul1), and bacitracin (bcrA) resistance genes. It is worrisome that the glycopeptide (vanA) resistance gene was detected in 8% of the sheep isolates as a first report. ERIC-PCR genotyping of 10 multi-drug-resistant C. pseudotuberculosis isolates showed a high similarity index of 83.6% between isolates from sheep and goats. Nucleotide sequence analysis of partial 16S rRNA sequences of C. pseudotuberculosis revealed 98.83% similarity with biovar Ovis of globally available reference sequences on the Genbank database. Overall, our findings might indicate that C. pseudotuberculosis infection in smallholders in Egypt might be underestimated despite the significant financial impact on animal husbandry and potential health hazards it poses. Moreover, this study highlights the importance of implementing a sustainable control strategy and increasing knowledge and awareness among smallholder breeders to mitigate the economic impact of CLA.
Keywords: C. pseudotuberculosis; ERIC-PCR; goats; multidrug-resistance; phylogenetic analysis; sheep; smallholders.