Genetic factors affecting susceptibility to udder pathogens.

Many studies have identified genetic factors underlying resistance or susceptibility to mastitis in dairy cows and heifers. Some authors focused on polygenic variation while others searched for genes and/or quantitative trait loci with major effects on mastitis. Classical traits related to mastitis include somatic cell counts, electrical conductivity and clinical cases of the disease. With the development of automatic milking devices and '-omics' technologies, new traits are considered, such as acute phase proteins, immunological assays, and milk flow patterns, and new biological pathways are discovered, for example the role of mammary epithelium and the nervous system. The usefulness of these traits for the identification of resistant cows is discussed in relation to the biological mechanisms underlying the development of the disease. In addition, the utility of these traits for genetic improvement is reviewed. Finally, the problem of durability in resistance is addressed, including co-evolution and the cost of resistance.