The effect of repeated episodes of bacteria-specific clinical mastitis on mortality and culling in Holstein dairy cows

The objective of this study was to estimate the effect of a first and repeated cases of bacteria-specific clinical mastitis (CM) on the risk of mortality and culling in Holstein dairy cows. The pathogens studied were Streptococcus spp., Staphylococcus aureus, Staphylococcus spp., Escherichia coli, Klebsiella spp., Trueperella pyogenes, others, and no growth on aerobic culture. A total of 50,166 lactations were analyzed from 5 large, high-milk-producing dairy herds in New York State from 2003/2004 to 2011. Generalized linear mixed models with a Poisson error distribution were used to study the effects of parity, month of lactation, CM, calving diseases, pregnancy status, current season, and economic values on the risk of mortality and culling. Among first-lactation cows, the presence of a first CM case generally exposed cows to a greater risk of mortality in the current month (compared with the absence of a first case). This was especially acute with a first case of Klebsiella spp., where cows were 4.5 times more at risk [95% confidence interval (CI): 2.7-7.6] of mortality, and with a first case of E. coli were 3.3 times more at risk (95% CI: 2.5-4.5). In first-parity cows, the risk of culling generally increased with a case of bacteria-specific CM. This was observed among cows with a first case of T. pyogenes [relative risk = 10.4 (95% CI: 8.4-12.8)], a first case of Klebsiella spp. [relative risk = 6.7 (95% CI: 5.5-8.1)], a first case of Staph. aureus [relative risk = 4.8 (95% CI: 2.7-8.4)], a first case of E. coli [relative risk = 3.1 (95% CI: 2.7-3.6)], and a third case of Klebsiella spp. [relative risk = 5.0 (95% CI: 3.1-8.0)]. In general, the presence of a first or second/third case resulted in cows in parity >= 2 with a greater risk of mortality. This was greatest for cows with a first case of Klebsiella spp. [relative risk = 3.7 (95% CI: 3.3-4.3)], followed by a second/third case of Klebsiella spp. [relative risk = 3.2 (95% CI: 2.5-4.0)], a first case of E. coli [relative risk = 3.0 (95% CI: 2.7-3.3)], and a first case of other CM [relative risk = 1.8 (95% CI: 1.6-2.0)]. Among cows of parity >= 2, the risk of culling was greater for cows as they progressed through lactations [i.e., cows in parity 4+ were 2.1 (95% CI: 2.0-2.2) times more likely to be culled compared with cows in lactation 2 (the baseline)]. The risk of culling dependent on the cow's characteristics can be easily calculated from the parameter estimates in the provided tables.