Genetic aspects of somatic cell count and udder health in the Italian Valle del Belice dairy sheep

Mastitis is an inflammation of the udder, which leads to economic loss, mainly consisting of discarded milk, reduced milk production and quality, and increased health costs. Somatic cell count (SCC), and therefore somatic cell score (SCS), is widely used as indicator of mastitis. In this thesis, I focus on the genetic parameters of SCS as indicator of mastitis, and on the possibilities of using this trait for selection for mastitis resistance in the Valle del Belice dairy sheep. In Chapter 1, mastitis and SCS are defined and introduced. Chapter 2 deals with the estimation of genetic parameters for SCS and milk production traits in primiparous Valle del Belice ewes. Heritability estimates ranged from 0.09 to 0.14 for milk, fat and protein yields and contents. For SCS, the heritability of 0.14 was relatively high. SCS was genetically positively correlated to milk, fat and protein yields and contents. However, correlations were not extreme, so simultaneous improvement for milk yield and SCS seems possible. In Chapter 3, the level of SCC is included in a survival analysis to evaluate the effect of SCC on functional longevity. Results showed that an increase in SCC was associated with an increase in culling rate.Elevated SCC, therefore, play an indirect role in the culling decisions of Valle del Belice dairy sheep farmers, although, at present, farmers do not directly select for reduced SCC. In Chapter 4, the genetic parameters of the infection status and SCS, according to whether the samples were bacteria negative or positive are reported. Moreover, the impact of imperfect sensitivity and specificity on variance component estimates was investigated. The heritability was 0.10 for bacteria negative SCS, 0.03 for bacteria positive SCS, and 0.09 for infection status, on the liability scale. The genetic correlation between bacteria negative and bacteria positive SCS (0.62) suggests that they may be genetically different traits, confirming that SCSfrom healthy and infected an