Dynamics of somatic cell counts and intramammary infections across the dry period.

The objectives of this research were to study the relationship between somatic cell count (SCC) and intramammary infection (IMI) across the dry period and the risk of subclinical mastitis at the first dairy herd improvement (DHI) test of the subsequent lactation. A secondary objective was to determine SCC test characteristics for diagnosis of IMI at both the cow and quarter levels. A total of 218 cows from a university herd were enrolled at dry-off. Duplicate quarter milk samples were collected from all quarters at dry-off, calving and on the day of the first DHI test. Somatic cell count status across the dry period was defined based on the comparison of quarter SCC from dry-off and the post-calving sampling periods and comparison of composite SCC from DHI samples from the last test and first test of the following lactation. Of new IMI detected from post-calving milk samples (n=45), 46.7, 26.7 and 11% were caused by CNS, Streptococci and Gram-negative bacteria, respectively. Of cured IMI at post-calving (n=91), 61.5, 23.1 and 9.9% had CNS, Streptococci and Coryneforms isolated from dry-off milk samples. The most frequent microorganisms related to cured IMI were CNS (33%). Of chronically infected quarters across the dry period (n=10), only one had the same species of pathogen isolated from dry-off and post-calving samples. The sensitivity of a SCC threshold of 200,000 cells/mL for detection of subclinical IMI was 0.64, 0.69 and 0.65 for milk samples obtained at dry-off, post-calving and first DHI test, respectively. The specificity was 0.66, 0.84 and 0.93 for milk samples obtained at dry-off, post-calving and first DHI test, respectively. Quarters with SCC> or =200,000 cells/mL at both dry-off and post-calving sampling periods were 20.4 times more likely to be subclinically infected by a major pathogen (rather than being uninfected) and 5.6 times more likely to be subclinically infected by a minor pathogen (rather than being uninfected) at the first DHI test than quarters with SCC<200,000 cells/mL at both periods. Cows with SCC greater than 200,000 cells/mL at both the last and the first DHI test between lactations produced 9.1 kg less milk on the first DHI test day than the average milk production of cows with SCC less than 200,000 cells/mL at both periods.