439 444 Wageningen Academic Publishers Wageningen, Netherlands

Clinical mastitis (CM) is one of the major diseases in dairy herds. It induces economic costs, mainly consisting of discarded milk, increased health care costs and reduced milk quality. Mastitis also contributes to consumer concerns regarding animal welfare and regarding the impact of use of antibiotics in animals on efficacy of antibiotics for human health and the possible development of antibiotic resistant bacteria. Decreasing the incidence of CM is thus of great interest of the farmer, the cow and the consumer, and could be achieved by either designing mastitis control programs, as these provide guidelines for udder health management, or by genetic selection. Although genetic selection is a slow process, it results in a steady change in the genetic composition of the dairy herd. This study provides insight in the use of patterns of peaks in somatic cell count (SCC) in genetic selection and mastitis control programs. Patterns of peaks in SCC were defined based on SCC recorded on consecutive test-day, and are based on biological understanding of pathogens and of the immune system of the cow. Results showed that selecting for lower lactation-average SCC caused a shift in the importance of the main mastitis-causing pathogen. Genetic selection against occurrence of SCC patterns, however, was more effective to decrease the natural susceptibility to all mastitis-causing pathogens, than selection for lower lactation-average SCC. Patterns of peaks in SCC are proven to be useful as basic tools for health management advice, as they can distinguish between cases of CM associated with either environmental or contagious pathogens, whereas the currently used primary traits were indicative for contagious, but not for environmental mastitis.