The role of non-aureus Staphylococcus in small ruminant mastitis: A systemic review on etiological agents, risk factors, virulence determinants, and novel treatments

Clinical mastitis is a primary contributor to physical, chemical, and bacteriological changes in milk leading to significant economic loss and welfare impairment. While mastitis in small ruminants can be rooted in varied infectious agents and physical injuries, bacterial infections stand as the predominant cause. Staphylococci, Streptococci, and coliforms are identified as the primary bacterial pathogens responsible for mastitis. Mastitis-causing staphylococci are typically categorized into Staphylococcus aureus and non-aureus staphylococci (NAS). NAS encompasses mainly coagulase-negative Staphylococcus species (CNS) and selects coagulase-positive and coagulase-variable staphylococci. NAS species exhibit variability in their epidemiology, pathogenicity, virulence, ecological niche, host adaptation, and antimicrobial resistance profiles. Their unique interactions with the udder's microbiome composition may confer protection against other mastitis pathogens. In small ruminants, treatment for NAS mastitis faces challenges such as antibiotic resistance and limited availability of veterinary drugs. Emerging treatments include bacteriophages, probiotics, immunotherapy, phytochemicals, nanotechnology, and peptides. This review will discuss the importance of NAS as a cause of mastitis in small ruminants, the virulence factors of this group, and for greater scientific reach, the species-level interactions within the microbiome and its interplay with host genetics are crucial to elucidate the role of NAS in mastitis and udder health in small ruminants.