Effect of Pathogen-Specific Clinical Mastitis on Milk Yield in Dairy Cows

Abstract

Our objective was to estimate the effects of the first occurrence of pathogen-specific clinical mastitis (CM) on milk yield in 3071 dairy cows in 2 New York State farms. The pathogens studied were Streptococcus spp., Staphylococcus aureus, Staphylococcus spp., Esche- richia coli, Klebsiella spp., Arcanobacterium pyogenes, other pathogens grouped together, and "no pathogen isolated."Datawerecollectedfrom October1999toJuly 2001. Milk samples were collected from cows showing signs of CM and were sent to the Quality Milk Produc- tionServiceslaboratoryatCornellUniversityformicro- biological culture. The SAS statistical procedure PROC MIXED, with an autoregressive covariance structure, was used to quantify the effect of CM and several other control variables (herd, calving season, parity, month of lactation, J-5 vaccination status, and other diseases) on weekly milk yield. Separate models were fitted for primipara and multipara, because of the different shapes of their lactation curves. To observe effects of mastitis,milkweightsweredividedintoseveralperiods both pre- and postdiagnosis, according to when they were measured in relation to disease occurrence. An- other category contained cows without the type of CM being modeled. Because all pathogens were modeled simultaneously, a control cow was one without CM. Among primipara, Staph. aureus, E. coli, Klebsiella spp., and "no pathogen isolated" caused the greatest losses. Milk yield generally began to drop 1 or 2 wk before diagnosis; the greatest loss occurred immedi- ately following diagnosis. Mastitic cows often never re- covered their potential yield. Among older cows, Strep- tococcus spp., Staph. aureus, A. pyogenes, E. coli, and Klebsiella spp. caused the most significant losses. Many multipara that developed CM were actually higher pro- ducers before diagnosis than their nonmastitic herd- mates. As in primipara, milk yield in multipara often