Rapid identification of the pathogen responsible for an intramammary infection in a dairy cow can support herd management decisions. Polymerase chain reaction (PCR) has become available to identify mastitis pathogens in milk, offering a rapid and sensitive test. The performance of a commercial, real-time PCR assay was compared with traditional bacterial culture for the identification of the most-frequent pathogen in New Zealand, Streptococcus uberis, during three stages of lactation. Aseptically collected quarter-milk samples (n=6,848) were analysed by culture, and a subset (n=315) selected for PCR analysis based on S. uberis infection status. Using culture as the reference test, PCR had a relative sensitivity of 89.0%, and a specificity of 64.3% (kappa=0.49) for detecting S. uberis in milk collected at the first milking post-calving. The relative sensitivity and specificity for samples collected in mid-late lactation were 89.3% and 97.7%, respectively, indicating a greater agreement between the two tests later in lactation (kappa=0.88). Accurate diagnostic tests are necessary for mastitis management. In early lactation, when S. uberis prevalence is high, bacterial culture should be used. The PCR assay tested in this study is most useful after the first month of lactation.

NM Steele, JH Williamson, RA Laven, and JE Hillerton

Proceedings of the New Zealand Society of Animal Production, Volume 75, Dunedin, 20-23, 2015
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