Relationships between milk culture results and composite milk somatic cell counts in Norwegian dairy cattle

Associations between test-day composite milk somatic cell counts (CMSCC) and results from quarter milk cultures for various pathogens associated with mastitis, including Staphylococcus aureus, Streptococcus spp., coagulase-negative staphylococci (CNS), were investigated. S. aureus was dichotomized according to sparse (≤1,500 colony forming units/mL of milk) or rich (>1,500 colony forming units/mL of milk) growth of the bacteria. Quarter milk samples were obtained on between 1 and 4 occasions from 2,714 cows in 354 Norwegian dairy herds, resulting in a total of 3,396 samples. Cows included in the study were randomly selected, without regard to current or previous udder health status. Measures of test-day CMSCC were obtained every second month, and related to 3528 microbiological diagnoses at the cow level. Mixed linear regression models incorporating a compound symmetry covariance structure accounting for repeated test-day CMSCC within cow, and a random effect variable on herd level, was used to quantify the relationship between a positive milk culture and the natural logarithm of test-day CMSCC (LnCMSCC). The material was stratified in time periods before 151 d in milk (DIM) and after 150 DIM. A positive diagnosis for any category of mastitis pathogen was significantly associated with elevated CMSCC. Pathogen positive cows sampled for microbiological diagnosis during the first 150 DIM had higher levels of CMSCC throughout lactation than cows with a positive diagnosis after 150 DIM. Streptococcus spp.-positive milk cultures were associated with steadily elevated values for CMSCC throughout lactation both when sampled before and after 150 DIM. Cows diagnosed with rich growth of S. aureus after 150 DIM experienced a characteristic and sharp increase in CMSCC, but this effect was not observed in cows with a positive diagnosis for rich growth of S. aureus during the first 150 DIM. A considerable increase in CMSCC in cows positive for CNS during the first part of the lactation period was also observed. The practicability of using CMSCC in a diagnostic test to identify cows with a positive milk culture for mastitis pathogens was also assessed. The sensitivity, specificity, and positive predictive values of the tests were regarded as low when sampling for milk culture was conducted, irrespective of cow level characteristics.