Clinical impact of Mycoplasma bovis on New Zealand cattle farms : a thesis presented in partial fulfilment of the requirements for the degree of MVS in Veterinary Science at Massey University, Palmerston North, New Zealand. PERMANENT EMBARGO.

Abstract

Mycoplasma bovis was diagnosed for the first time in New Zealand in July 2017 after an outbreak of non-responsive mastitis and arthritis in a dairy herd in North Otago. Subsequent tracing by Ministry for Primary Industries (MPI) found a network of infected farms in multiple regions around New Zealand, predominantly in the South Island, and a decision was made to eradicate M. bovis through testing and slaughtering. This thesis summarises a herd-level observational case study of 69 infected properties (IPs) New Zealand-wide. The survey was conducted in 2018/19 with the aim of determining the clinical and subclinical impact of M. bovis on cattle farms, including dairy herds and calf/drystock properties. Data were collected on the incidence of clinical disease known to be caused by M. bovis such as refractory mastitis, arthritis, pneumonia and otitis media, as well as morbidity and mortality rates. The association between the subclinical effects of M. bovis on milk production, somatic cell count and antimicrobial usage was also examined on a seasonal basis. Data was then compared with data from seasons before and after the introduction of M. bovis as a within-farm control. In addition, factors that may have had an effect on disease severity were investigated. Farms were surveyed through questionnaires, veterinary sales records, milk production data and laboratory testing results.

Clinical disease attributable to M. bovis was not widespread. On drystock properties, little clinical disease caused by M. bovis was confirmed, although many infected calf-rearing enterprises experienced respiratory and gastrointestinal disease secondary to other pathogens. Clinical disease was also rare on dairy farms, but when present it was severe. Two multi-platform dairy herds showed severe non-responsive multiple quarter mastitis, both in dry and lactating cows. One of these affected herds also displayed severe arthritis in milking cows and experienced an uncommon presentation of disease in calves characterised by prematurity, neurological signs and failure to thrive accompanied by arthritis.

Compared to milk production prior to exposure to M. bovis, milk production on infected dairy farms did not significantly decrease after exposure. Bulk tank somatic cell count levels and restricted veterinary medicine (RVM) sales across all farms did not show a significant increase after M. bovis infection. The results of this survey could be built upon using longitudinal studies with larger sample size to further assess the impact of M. bovis infection on subclinical disease in New Zealand cattle at both the cow and herd level.