Outbreak of <i>Serratia marcescens</i> mastitis linked to teat spray contamination in a New Zealand dairy herd

CASE HISTORY: in three of four cows. Herd records also showed an increase in the monthly percentage of cows treated for CM from 0.8% in December 2023 to 2.9% in January 2024 and 2.1% in February 2024. CLINICAL FINDINGS: A herd investigation was conducted in February 2024. The farmer was using a chlorhexidine-based teat spray system. Cows were selected for sampling on the basis of a very high individual somatic cell count and/or farmer-identified persistent CM, and quarter milk samples were collected aseptically from selected cows after screening using the rapid mastitis test. Additional samples of diluted teat spray, undiluted teat spray, and bore water were also collected for culture. The diluted teat spray was stored in a container in which the withdrawal tube passed through an old teat liner without a water-tight seal, leaving a visible gap around the opening and creating a risk of contamination. LABORATORY FINDINGS: was also identified in the diluted teat spray sample, but not in the bore water or undiluted teat spray samples. DIAGNOSIS: mastitis, most likely associated with contamination of the in-shed diluted teat spray system. CLINICAL RELEVANCE: Following replacement of the contaminated teat spray and storage container, no new cases were identified. This case highlights the importance of milk sampling, accurate pathogen identification, and prompt investigation of hygiene-related sources when atypical mastitis pathogens are detected.