Whole Dairy Herd Sampling to Detect Subclinical Intramammary Mycoplasma Bovis Infection After Clinical Mastitis Outbreaks

After clinical Mycoplasma bovis mastitis outbreaks in dairy herds, M. bovis can persist as subclinical intramammary infections. Identification and culling of sub-clinically infected cows may be warranted to reduce future pathogen transmission and disease. In this study, apparent cow-level prevalences of M. bovis intramammary infection within 4 milking herds immediately following outbreaks of clinical disease due to M. bovis were determined utilising PCR and culture. All clinically affected M. bovis cows had been culled from the herds prior to herd sampling. Composite milk samples were collected once from each cow (n = 2,258) using a routine milk recording sampling technique. These samples were pooled for PCR screening; positive pools were analysed in different sized pools as needed from large to small, until individual PCR-positive animals could be identified. Despite M. bovis seroprevalences of 76% (herd 1), 40% (herd 2), 20% (herd 3) and 16% (herd 4), apparent prevalences of intramammary infection in the main milking group based on PCR in herds 1 to 4 were 0.2% (1/497), 0.0% (0/475), 0.1% (1/816) and 0.0% (0/444), respectively. Due to the low apparent prevalences of subclinical intramammary mycoplasma infections in these herds and the high expense associated with milk sample collection and testing, the return on diagnostic investment was very limited, particularly considering that additional cows are likely to have been colonised with mycoplasma in other anatomical sites. The results of this study suggest that pursuing identification of cows with subclinical intramammary mycoplasma infections following resolution of clinical M. bovis disease outbreaks in dairy herds may be of minimal benefit in programs designed to control or eradicate M. bovis.