Subclinical mastitis (SCM) is a prevalent serious disease among dairy cows worldwide. It poses a significant challenge to the dairy industry, animal welfare, and a threat to public health. The present study aimed to investigate the molecular detection, prevalence, and antimicrobial resistance of Staphylococcus spp. and Streptococcus spp. isolated from raw composite milk samples obtained from SCM dairy cattle in Bangladesh. A total of 612 quarters milk samples obtained from 153 cows were analyzed for SCM using the California Mastitis Test. Bacterial isolation and identification were carried out and bacterial species were confirmed using molecular polymerase chain reaction methods. Antimicrobial susceptibility testing was performed using disc diffusion method. The findings revealed that the prevalence of SCM was 70.3% (26/37), 35.95% (55/153), and 23.04% (141/612) in the herd, cow, and quarter levels, respectively. Among the positive samples, 92.7% (51/55) were Staphylococcus spp. (S. aureus, S. chromogenes, and S. simulans) and the remaining isolates were 7.3% (4/55) Streptococcus spp. (Streptococcus agalactiae and Streptococcus dysgalactiae). The most prevalent species was S. chromogenes, accounting for 67.3% (37/55). Antimicrobial susceptibility testing showed that 65.5% of isolates were susceptible to cefoxitin, whereas, 89.1% were resistant to penicillin. Overall, 12 isolates (21.8%) out of 55 were resistant to more than three classes of antimicrobials and were defined as multidrug-resistant isolates. Methicillin-resistance gene was detected in 61.1% of the cefoxitin-resistant isolates. A multivariate logistic regression analysis identified five potential risk factors including the lack of post-milking teat disinfection (OR: 3.06), absence of immediate feeding after milking (OR: 9.81), poor udder hygiene (OR: 7.83), tick infestation (OR: 13.76), and absence of dry cow therapy (OR: 3.31). The findings of the current study underscore the urgent requirement for targeted interventions, considering the identified factors to effectively manage and control SCM in dairy cows.
Keywords: MRSA; Staphylococcus; Streptococcus; Subclinical mastitis; antimicrobial resistance; multidrug resistance.