Effect of pathogen-specific clinical mastitis on herd life in two New York State dairy herds

The objective of this study was to estimate the effects of clinical mastitis (CM) (both with and without specific pathogen identification) occurring in different stages of lactation on length of herd life in two New York State dairy farms. The 2,697 cows in the study were followed for one lactation (the first-occurring one on or after 1 October 1999), until it ended because of a new lactation, culling, or end of study (31 March 2001 in one farm; 31 July 2001 in the other). A Cox proportional hazards model with time-dependent covariates, in SAS((R)), was used to measure, within a lactation, the effect of the first occurrence of CM (without specific pathogen identification) occurring 1--7, 8--66, 67--100, 101--225, or >or=226 days in milk (DIM), on how long cows remained in the herd. For the first occurrence of CM due to Streptococcus spp., Staphylococcus aureus, Staphylococcus spp., Escherichia coli, Klebsiella spp., and 'no pathogen isolated', the intervals were before and after the median DIM of first occurrence of each pathogen. There were too few cases due to Arcanobacterium pyogenes, and 'other pathogens grouped together' to split into intervals, so they were modeled as binary variables, i.e. as they occurred. CM was modeled using time-dependent covariates, to account for its differing effects throughout lactation on culling. Other variables controlled for were herd, parity, calving season, and other significant diseases. In the dataset, the lactational incidence risk of the first occurrence of CM was 18.2%; 20.0% of the cows did not survive the lactation that was studied. The overall annual culling percentage for both herds during the study period (including all cows, whether eligible for the study or not) was 35.6%. For cows with CM without pathogen identification, their highest hazard ratio (HR) of culling occurred from 67 to 100 DIM. All of the pathogens modeled markedly reduced herd life. On average over the entire lactation, cows with Staphylococcus spp. CM had the highest HRs for culling, although there were no significant differences among pathogens (at p=0.0018 (reflecting 28 pairwise comparisons)). For early-occurring (before median DIM of first occurrence) S. aureus CM, the daily rate of change of the HR of culling increased over time. The HRs for culling were particularly high for late-occurring (after median DIM of first occurrence) E. coli and Klebsiella spp. CM early in the interval, but the daily rate of change of the hazard of culling for these two pathogens decreased sharply over time. Treating CM as time-dependent therefore allowed us to measure in greater detail, its varying effects (of when it occurred) on herd life.