Genetics of udder health in dairy ruminants

Udder health troubles in dairy ruminants mainly include intra mammary infections, or mastitis, caused by a pathogen that enter the gland through the teat canal and develop in the gland cistern. Mastitis is a multifactorial disease; it’s origin, severity and outcome are highly variable and depend on environment, a wide variety of pathogens (mostly bacteria) and host. In dairy cattle, mastitis is especially severe for environmental causing pathogens such as enterobacteriaceae (Escherichi coli), and during the critical post partum period. Dairy sheep and goat experience less frequently acute mastitis (<5% on average) than cattle (20-40%), but essentially subclinical infections due to staphylococci and mycoplasma bacteria. Evidence of individual genetic variability of resistance to mastitis in dairy ruminants has been demonstrated for a long time. Resistance to mastitis is a complex trait that involves different aspects: avoid entry of the pathogen, limit its development in the udder, control pathogenic effect such as tissue damages, eliminate and recover from infection. Genetic basis of udder health has been established mainly on phenotypes that relate to the healthy or diseased status of animals, without any knowledge about mechanisms and genes underlying resistance. Some immune genes (MHC genes) and pathways, including recruitment and functionality of neutrophils and humoral or cellular adaptative immune response, have proved to play an important role. Understanding key mechanisms in the host’s response to intra mammary infections and their genetic component is, however, still a large field for investigation Because resistance to mastitis is partly genetically determined and can be characterized using simple measures, genetic improvement has been made possible as a complementary control method to treatment and prophylaxis. Regarding major economic importance of udder health for the dairy industry and its link with animal health and food safety, genetic selection has been implemented throughout the world and is expected to lead to reduced frequency of intrammamary infections and enhanced resistance to mastitis. This paper reviews the state of art of knowledge about genetic basis of udder health, including polygenic variation, QTL detection and candidate gene approach for health status indicators and immune response traits. In a second step, current selection strategies will be addressed with focus on actual results, questioning and prospects for the future.