Prevalence of clinical mastitis in 38 Waikato dairy herds in early lactation

Aim. To determine the prevalence of clinical mastitis in spring-calving dai ry herds in the Waikato Region of New Zealand and to identify factors assoc iated with variation in the prevalence of clinical mastitis between herds. Method. A total of 799 quarters from 595 dairy cows from 38 dairy herds wer e diagnosed by herd owners as having clinical mastitis between 8 July and 2 1 August 1997. Quarters diagnosed with clinical mastitis were sampled for b acterial culture and somatic cell count, and the presence of clots in the m ilk and the presence of udder oedema were assessed by a technician or veter inarian. Results. Clinical mastitis was diagnosed in an average (+/- s.e.m.) of 9.9% (+/- 0.8%, range 0.9-21.4%) of calved cows within the herds. Bacteria were not cultured from an average of 12.4 % (+/- 2.0%, range 0.0-45.5%) of cows and 22.3% (+/- 2.4%, range 0.0-54.0%) of quarters diagnosed as having clin ical mastitis. There were significant differences between herds in the prop ortion of cows diagnosed with mastitis and in the proportion of clinical ma stitis cases from which bacteria mere not cultured. A decreased prevalence of clinical mastitis (p < 0.001) was associated with an increased percentage of the herd treated with dry cow antibiotics. An i ncreased prevalence of clinical mastitis (p < 0.0001) was associated with b oth an increased percentage of cows treated in the previous season with lac tating cow antibiotics and an increased percentage of heifers in the herd. Herds that were fed supplements before or during lactation had a higher pre valence of clinical mastitis than herds that were nor fed supplements (p < 0.001). An increased proportion of quarters diagnosed with clinical mastiti s that did not culture bacteria was associated with an increased prevalence of clinical mastitis (p < 0.001). The proportion of quarters that the tech nician or veterinarian found with evidence of clinical mastitis (i.e. a som atic cell count > 500 000 cells/ml and the presence of either clots or udde r oedema) within a herd was inversely related to the proportion of quarters within a herd from which no bacteria were isolated. Conclusion. There was a large variation in the prevalence of clinical masti tis and in the proportion of clinical quarters from which no bacteria were grown between herds. Management factors such as the use of dry cow therapy, feeding regimes and heifer replacement rates all affected the prevalence o f clinical mastitis. Herd owners appear to differ in the sensitivity and sp ecificity of their diagnosis of clinical mastitis, with bacteria not isolat ed from up to 50% of quarters diagnosed with clinical mastitis in some herd s. Improvements in the specificity of herd owner diagnosis of clinical mast itis may reduce the use of antibiotics for mastitis during lactation and he nce may reduce the risk of antibiotic contamination of milk supplied for hu man consumption.