Estimation of interdependence among quarters of the bovine udder with subclinical mastitis and implications for analysis.

Interdependency among udder quarters with subclinical mastitis was evaluated on 150 farms using a total of 35,828 udder quarters. The occurrence of high somatic cell count (SCC) (> 250,000 cells/ml) in 0, 3, and 4 quarters occurred at a higher rate than would be expected based on independence of the quarters. For all bacterial species, intramammary infection in 0, 2, 3, or 4 quarters of the same cow occurred at a higher rate than would be expected based on independence of the quarters. Intramammary infection and high SCC were found less often in front quarters than in rear quarters. High SCC and intramammary infection occurred more often in right front quarters than in left front quarters. High SCC in diagonal quarters occurred at a lower rate than expected. Corynebacterium bovis, Streptococcus agalactiae, and Staphylococcus aureus had the highest intraclass correlation within herd. Streptococcus uberis had a very low intraclass correlation within herd. The intraclass correlation within cow for the natural logarithm of SCC was 0.47. Corynebacterium bovis and Strep. agalactiae had the highest intraclass correlation within cow, and Streptococcus dysgalactiae had the lowest. Analytical methods were proposed to manage the problem of interdependence and its effect on the design or evaluation of field studies on subclinical mastitis.