The diagnosis of an outbreak of Mycoplasma bovis clinical mastitis in a multi-farm North Otago farming operation : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Science at Massey University, Manawatū, New Zealand

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Mycoplasma bovis (M. bovis) causes a multitude of disease syndromes in dairy cattle including clinical mastitis (CM), arthritis and pneumonia. The detection in July 2017 of M. bovis, for the first time in New Zealand (NZ), on a South Island dairy farm, prompted a national animal disease response. This descriptive study aims to describe the clinical and diagnostic test findings of an outbreak of M. bovis CM, on a large multi-farm dairy enterprise where there was a single hypothesised infection source and date. Samples were collected as part of surveillance activities on-farm and at slaughter, together with farmer-selected CM cows, to provide results from real-time polymerase chain reaction (qPCR) and enzyme-linked immunosorbent assay (ELISA) tests of bulk tank milk (BTM), individual cow serum ELISA tests, quarter milk samples (QMS), and palatine tonsils qPCR tests. Post-mortem sampling of the mammary glands of M. bovis CM cases was also performed. Positive BTM PCR, supported by BTM ELISA, confirmed infection in two of the four dairy herds in the enterprise and herd-level serology (serum ELISA) confirmed infection in a third herd. There was a common clinical presentation in infected herds of an unusually high incidence of apparent treatment failure (ATF) of non-systemically ill, multiple quarter CM cases, from some of which M. bovis was detected. Individual CM cases were found in the main to be QMS M. bovis qPCR positive, serum ELISA positive and palatine tonsil qPCR positive. In approximately 70% of M. bovis CM cases, M. bovis was found to be the sole pathogen. A smoothed function model between serum ELISA and time from first diagnosis of CM, from which M. bovis was detected, predicted that the average interval between clinical diagnosis and a serum ELISA test positive result was five days. The higher observed agreement between the serum ELISA and palatine tonsil qPCR, was for M. bovis CM cows sampled on-farm compared with cows sampled at slaughter. Gross lesions of fibrosis, caseous necrosis and cystic dilation in the udders of M. bovis positive CM cows were seen together with granulomatous and suppurative inflammatory patterns histologically. High immunoreactivity in immunohistochemistry for the M. bovis antigen was also present. From the key diagnostic test findings, M. bovis was likely to have been one of several pathogens which caused individual cases of CM on the farming enterprise, and in many cases may have been the sole cause of CM cases. The results of this study can raise awareness of and provide information to aid dairy farmers and veterinarians determine if M. bovis has a role in CM outbreaks with unexpectedly increased numbers of treatment failures and can inform the regulatory response for surveillance and testing of herds and individual cattle for M. bovis.
Mastitis, Dairy cattle, Diseases, Mycoplasma diseases in animals, New Zealand, North Otago