Etiologic mastitis agents of high somatic cell count dairy herds in Tennessee

Abstract

The objective of the study was to determine etiologic agent(s) of intramammary infections (IMI) in high somatic cell count (SCC) dairies in Tennessee. This was a prospective study utilizing 2,444 dairy cows from 20 high SCC dairy herds. Composite milk samples were aseptically collected from lactating cows in 20 dairy herds with a rolling herd average SCC >400,000 cells/mL. Milk samples were cultured and mastitis pathogens identified following procedures recommended by the National Mastitis Council. The average herd percentage (average of the 20 herd averages) of culture-negative cows was 49%. The average herd percentages of positive-culture cows were 28% for coagulase-negative staphylococci (CNS), 15% for Staphylococcus aureus, 10% for environmental streptococci (ES), and ?2% for gram-negative organisms. Among cows with individual SCC >1 million cells/mL, 30% of cultures yielded Staph. aureus, 23% CNS, 26% no growth, 26% ES, and 5% gram-negative organisms. Among cows with individual SCC >400,000 cells/mL, cultures yielded 26% CNS, 30% no growth, 28% Staph. aureus, 21% ES, and 4% gram-negative organisms. Staph. aureus was the most common major pathogen, or equally common major pathogen, in 75% of high SCC herds. Environmental streptococci were the most common major pathogen, or equally common major pathogen, in 45% of high SCC herds. Staph. aureus is the major pathogen, followed by ES, among Tennessee dairy herds with high SCC. Coagulase-negative staphylococci are common, but their significance needs to be determined. Gram-negative organisms do not appear to be significant pathogens of Tennessee dairy herds with persistently high SCC.