Mastitis in dairy cattle is an inflammatory condition of the mammary gland, primarily caused by bacterial infection. It is one of the most prevalent and economically significant diseases affecting dairy cattle worldwide. Mastitis can result in a variety of symptoms, including swelling, redness, heat, and pain in the udder, as well as changes in the appearance and composition of milk, such as clots or discoloration. Additionally, affected cows may experience systemic signs of illness, such as fever or reduced milk production. Mastitis negatively impacts milk yield, quality, and composition, leading to economic losses for dairy farmers due to decreased milk production, treatment costs, and potential culling of affected animals. Somatic cell score (SCS) serves as a pivotal indicator trait in the genetic enhancement of mastitis resistance within dairy cattle populations. SCS is calculated based on the concentration of somatic cells present in milk samples collected from individual cows. Somatic cells, including leukocytes and epithelial cells, are components of the cow's immune system and are present in milk primarily as a response to mammary gland infections, such as mastitis. SCS, reflecting the concentration of somatic cells in milk, is a robust proxy for mastitis susceptibility, as elevated levels indicate immune response to infection. The current study aimed to estimate the incidence rate of clinical mastitis (CM), and its genetic relationship with somatic cell score in Iranian Holstein dairy cows. For this purpose, information from 12,397 Holstein cows associated with the Ghiam Agricultural and Livestock Company of Isfahan province from 2014 to 2021 was utilized. Mastitis was defined as the binary trait differentiating between cows with at least one case reported during the defined period took (1) and cows without cases took (0). To follow a normal distribution, the somatic cell count SCC data were converted into a somatic cell score (SCS), which is the logarithmic transformation of the somatic cell count. According to the results of this study, the incidence rate of mastitis was 31.67 cases per 100 cows per year. The average daily milk yield and SCS in this study were 44.59 kg/day and 1.67, respectively. The significant genetic correlation of 0.62 between mastitis incidence and somatic cell score highlights a strong and robust positive relationship. These findings suggest that focusing on achieving low somatic cell scores through selective breeding effectively reduces the occurrence of mastitis. Genetic improvement strategies targeting lower SCS not only mitigate the incidence and severity of mastitis but also enhance overall udder health and milk quality. Consequently, integrating SCS into breeding programs facilitates sustainable advancements in dairy production, ensuring healthier and more productive herds. In conclusion, the high genetic relationship between somatic cell score and clinical mastitis has made somatic cell score a reliable indicator of clinical mastitis.
