The incidence and aetiology of clinical bovine mastitis on 14 farms in Northland, New Zealand.
Risk factors associated with the development of clinical mastitis (CM) in multiparous cows, defined as detection of abnormalities in the milk by farm staff in the 30days before and 90days after calving were studied using a retrospective longitudinal cohort study of 18,162 cows on 30 South Island commercial New Zealand dairy farms. Risk factors studied included age, breed, length of dry period, farm, herd size, yield and individual somatic cell count (ISCC) status 30-60days before the end of the previous lactation, rainfall at calving and number calving on the same day. A modified Cox Proportional hazards model with time varying effects for breed, age, length of dry period and ISCC was used to identify which factors were significantly associated with an increased hazard of CM after calving Rainfall at calving >10mm increased the hazard ratio (HR) by 1.14 (95%CI=1.01-1.30) for 30days before to 90days after calving. Milk production >1.5kgMS/cow/day in the 30-60days before the end of lactation increased the HR for CM by 1.36 (95%CI=1.21-1.52) for the same period. The effects of breed, age, length of dry period and ISCC 30-60days before the end of lactation varied with time around calving. The HR decreased as the proportion of Friesian genetics decreased. Compared to animals >75% Friesian the HR for animals that were <25% Friesian was 0.50 (95%CI=0.26-0.99) in the 30days before calving, 0.81 (95%CI=0.60-1.08) in the 20days after calving and 0.28 (95%CI=0.18-0.62) from 21 to 90days after calving. HR increased with age, with the largest effect seen 21-90days after calving. Compared to cows <4 years of age, cows that were 4-8 years of age had a HR of 2.18 (95%CI=1.71-2.76) and those >8 years old had a HR of 4.09 (95%CI=3.06-5.46) for this period. Dry periods >112days had a HR of 1.46 (95%CI=1.24-1.73) in the 20days after calving but a decreased HR (0.73, 95%CI=0.57-0.94) 21-90days after calving. Cows with ISCC >150,000 cells/ml 30-60days before the end of lactation had a HR of 1.60, (95%CI=1.39-1.84) 0-20days after calving and 1.96 (95%CI=1.67-2.27) 21-90days after calving. Neither number of animals calving per day, nor herd size was associated with an increased hazard of CM and there was a significant difference in the hazard across farms.