A Retrospective Study on the Aetiology of Clinical Bovine Mastitis and Its Antibiotic Resistance Profiles in Western Australia Dairy Farms

Clinical data on antimicrobial profiles are useful for dairy udder health treatment programmes and represents a component of antimicrobial stewardship. The study aimed to determine the bacterial aetiology of clinical mastitis in dairy herds in Western Australia and to evaluate their antibiotic resistance profiles. This retrospective study utilised clinical antimicrobial profile data from two referral diagnostic centres within the region of Western Australia. A total of 545 mastitic samples were submitted for antimicrobial culture and testing over a period of 10 years (2008-2018). Of these, 406 showed bacterial growth and 139 no bacterial growth was observed. The most common isolates were <i>Streptococcus uberis</i> (25.3%), <i>Staphylococcus aureus</i> (17.2%), and <i>Escherichia coli</i> (9.4%). No growth was identified in 25.5% of the mastitis milk samples. The antimicrobial profiles revealed high susceptibilities towards cefuroxime (95.7%), clavulox (89.4%), and oxytetracycline (89%), whilst showing high resistance towards novobiovin (70%). From this study, it is concluded that there was a decline in the resistance trends towards the isolates of both <i>S. uberis</i> and <i>S. aureus</i> over the 10-year period and contagious mastitis had a higher occurrence. There is a need to consider surveillance programmes that determine the patterns of on-farm antimicrobial usage and further characterise the pathogens based on the presence of resistance antimicrobial genes. Data on antimicrobial surveillance represent an important component of antimicrobial stewardship.