Characterization and Antimicrobial Resistance of Bacteria Causing Subclinical Mastitis in Dairy Cows in the Upper Cheliff Region, Northern Algeria

<b>Background/Objectives:</b> Subclinical mastitis is a common and economically significant infection in dairy cows. This study aimed to assess the prevalence and antimicrobial susceptibility of bacteria causing subclinical mastitis in dairy cows in the Upper Cheliff Region, Northern Algeria, and to investigate the effects of subclinical mastitis on milk production and reproductive performance. <b>Methods:</b> A total of 263 cows from 23 farms were screened for subclinical mastitis using the California Mastitis Test (CMT) and sampled for isolation and identification of bacteria by MALDI-TOF MS. Antimicrobial susceptibility testing (AST) was performed using the Vitek 2 system and disk diffusion method. Methicillin resistance in staphylococci and mammaliicocci was confirmed by <i>mec</i>A/<i>mec</i>C detection. <b>Results:</b> The results revealed a prevalence of subclinical mastitis of 58.9% at the cow-level and 31.1% at the quarter-level. The most prevalent microorganisms identified were <i>Enterococcus faecium</i> (<i>E. faecium</i>) (24.4%) and <i>Enterococcus faecalis</i> (<i>E. faecalis</i>) (20.5%), followed by non-<i>aureus</i> staphylococci (NAS) (16.9%), <i>Escherichia coli</i> (<i>E. coli</i>) (7.9%) and <i>Staphylococcus aureus</i> (<i>S. aureus</i>) (7.08%). Risk factors significantly associated with the prevalence of subclinical mastitis included parity, lower milk production (<12 L/day), poor animal cleanliness and a history of previous mastitis. Reproductive performance was significantly impaired in cows with subclinical mastitis, cows with longer calving-to-first-service intervals (130 vs. 102.7 days; <i>p</i> < 0.0001), more services per conception (2.5 vs. 1.9; <i>p</i> < 0.0001) and a lower pregnancy rate at first service (24.5% vs. 48.1%; <i>p</i> < 0.0001). Furthermore, 43.2% of cows with subclinical mastitis required three or more inseminations to achieve pregnancy. AST showed a low resistance rate for the antimicrobial agents most commonly used in human and veterinary medicine. Coagulase-negative staphylococci (CoNS) showed remarkable rates of methicillin-resistance (22.2%), as well as resistance towards fosfomycin (37.8%) and tetracycline (31.1%). A substantial proportion of <i>E. coli</i> isolates exhibited resistance to piperacillin (40%) and ciprofloxacin (15%). All <i>S. aureus</i> isolates were classified as MSSA without detection of <i>mec</i>A and <i>mec</i>C genes. <b>Conclusions:</b> The results of this study demonstrated that subclinical mastitis is prevalent in the Upper Cheliff region and is associated with reduced milk production and reproductive performance. The detection of pathogenic and resistant microorganisms in milk is alarming and requires effective management strategies to control subclinical mastitis and improve dairy farm productivity.