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Diagnosis of intramammary infections at dry-off based on sampling strategy, epidemiology of pathogens, and agreement beyond chance

Correspondence: ¹Corresponding Author: Päivi J. Rajala-Schultz, 1920 Coffey Road, Columbus, OH 43210. paivi.rajala-schultz{at}cvm.osu.edu

Isolation of pathogens from duplicate or multiple milk samples is currently considered the gold standard in diagnosis of bovine intramammary infections (IMI). However, in large field studies and especially in normal dairy production conditions, collection of single samples is often the most practical option to determine the causal agents of mastitis in a herd. The objective of the present study was to determine how well results between the first and the second sample in pairs of duplicate and successive quarter milk samples agree, using 5 different IMI definitions, based on the number of colony forming units (CFU) per milliliter of milk and epidemiology of the pathogens isolated. Agreement between microbiologic results from the first and the second sample of a pair was assessed by calculating the percentage of agreement and kappa coefficient. Milk samples collected at dry-off from 561 Holstein cows in 4 Ohio dairy herds were included in the analyses. Results of the study indicate that the agreement between the first and the second sample of a duplicate pair was high when criteria to call a sample positive was adjusted for the number of CFU/ml of milk by considering the epidemiology of different mastitis organisms. This finding suggests that an IMI can be accurately diagnosed with single samples. For contagious pathogens (Staphylococcus aureus) a cutoff of 100 CFU/ml and a cutoff of 1,000 CFU/ml for major environmental and minor pathogens will serve as a sensible approach to diagnose bovine IMI with single milk samples.

Key Words: Colony forming units • cows • diagnosis • intramammary infections • kappa coefficient • milk