Prevalence and antimicrobial resistance of bacteria associated with subclinical mastitis in sheep and goat in Igabi local government area, Kaduna state, Nigeria

Subclinical mastitis is an important constraint to small ruminant productivity and a public health concern due to its association with antimicrobial-resistant bacteria. However, information on the prevalence of subclinical mastitis and antimicrobial resistance patterns of associated bacterial pathogens in sheep and goat in Northern Nigeria, particularly Kaduna State, remains limited. A cross-sectional study was conducted between August-October 2024 in Igabi Local Government Area, Kaduna, Nigeria. Milk samples were collected from 488 lactating small ruminants (384 goats, 104 sheep) from farms, households, and markets. Subclinical mastitis was screened using the California Mastitis Test (CMT). Bacterial isolates were recovered from the milk samples and identified using standard cultural and biochemical methods, and antimicrobial susceptibility testing was performed using the Kirby-Bauer disk diffusion method. In addition to commonly used veterinary antimicrobials, imipenem was included to assess resistance to critically important antimicrobials of last resort. Overall, 41.0% (200/488) of animals were positive for subclinical mastitis, with a significantly higher prevalence in sheep (96.2%; 100/104) than in goats (26.0%; 100/384) (χ²=163.44, p < 0.0001). Bacterial isolates were identified from milk samples of all 488 animals using standard cultural and biochemical methods. A total of 705 bacterial isolates were recovered. The predominant isolates were Staphylococcus aureus (49.5%; 349/705) and Escherichia coli (48.0%; 338/705), while Streptococcus species accounted for 2.6% (18/705). The bacterial isolates showed the highest susceptibility to ciprofloxacin with 92.7–98.9% of Escherichia coli, 85.5–95.0% of Staphylococcus aureus, and 100% of Streptococcus species isolates being susceptible. High resistance was observed to tetracycline (82.6–100%), cefuroxime (88.6–100%), and imipenem (99.6–100%). All isolates exhibited MDR (100%), with MAR indices ranging from 0.20 to 1.00, predominantly 0.80–0.90. The high burden of subclinical mastitis and widespread multidrug resistance among small ruminants in Igabi LGA highlight significant animal health and public health risks. These findings underscore the need for routine mastitis surveillance and strengthened antimicrobial stewardship in small ruminant production systems.