Subclinical staphylococcal intramammary infections: Within-herd prevalence and effects on milk yield and somatic cell counts in Thuringian dairy herds

Basic data for calculating the economic losses of subclinical staphylococcal intramammary infections are the reduction in milk yield and the within-herd prevalences. This study aimed to determine these parameters in selected herds. Quarter foremilk samples were taken from all lactating cows without clinical mastitis of 34 Thuringian dairy herds twice with an interval of five to nine months. A total of 81 567 samples from 14 157 cows were cultured and screened for Staphylococci, Streptococci and Enterobacteriaceae. For statistical analysis a multifactorial variance analysis which included the factors farm, quarter, days in lactation and number of lactation was used. Least square means of the within-herd prevalence were 3.14% for Staphylococcus (S.) aureus and 6.64% for Coagulase negative staphylococci (CNS). The highest frequency of S. aureus-infections was detected at 201-250 days in milk. The risk of S. aureus-infections increased with increasing lactation number, whereas the frequency of CNS-infections decreased with lactation number (p < 0.001). Compared to not infected cows, S. aureus infected cows showed no differences in milk yield or milk components, but had a higher somatic cell count (SCC) (219 000 cells/ml, p < 0.001). The SCC by CNS infected cows was 89 000 cells/ml (p = 0.049). High SCCs were associated with low milk yield. Subclinical intramammary infections with S. aureus and CNS result in a higher SCC. There is a direct association between SCC and milk yield.