Impact of intramammary infections in dairy heifers on future udder health, milk production, and culling.

Dairy heifers represent the future of a dairy herd, and are expected to freshen with a healthy and well-developed udder, capable of producing an optimal amount of high quality milk. A high proportion of heifers have infected mammary quarters at calving, with coagulase-negative staphylococci (CNS) being the most common cause. Staphylococcus aureus and environmental pathogens are also found. The aim of this paper is to summarize how intramammary infections during (late) gestation and early lactation impair the development of the mammary gland and negatively affect future udder health and milk production. Heifers calving with either subclinical or clinical mastitis are also at a higher risk to be culled in first lactation. The magnitude of the effect is most likely related to the virulence of the causative pathogen, the persistence of the infection when milk production has started, and the time of onset of infection. Histological changes in udder tissue from quarters infected with S. aureus are more pronounced than those in udder tissue from CNS-infected quarters. The longer the infections exist and the longer they persist into lactation, the larger the impact on heifers' future udder health and milk production will be. In general, CNS infections are cleared early in lactation and some studies show that CNS do not have a large impact on future milk production and udder health. Future research should elucidate to what extent pathogen-specific as well as host-related factors affect the persistence of IMI in early lactating heifers.