Frequency of reisolation of Staphylococcus aureus from multiple sequential milk samples

D. Scott Adams From Veterinary Medical Research and Development, Inc, PO Box 502, Pullman, WA 99163 (Adams) and Field Disease Investigation Unit, College of Veterinary Medicine, Washington State University, Pullman, WA 99164 (Hancock, Fox, McDonald).

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Dale Hancock From Veterinary Medical Research and Development, Inc, PO Box 502, Pullman, WA 99163 (Adams) and Field Disease Investigation Unit, College of Veterinary Medicine, Washington State University, Pullman, WA 99164 (Hancock, Fox, McDonald).

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Lawrence Fox From Veterinary Medical Research and Development, Inc, PO Box 502, Pullman, WA 99163 (Adams) and Field Disease Investigation Unit, College of Veterinary Medicine, Washington State University, Pullman, WA 99164 (Hancock, Fox, McDonald).

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John S. McDonald From Veterinary Medical Research and Development, Inc, PO Box 502, Pullman, WA 99163 (Adams) and Field Disease Investigation Unit, College of Veterinary Medicine, Washington State University, Pullman, WA 99164 (Hancock, Fox, McDonald).

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Summary

Bacterial cultures were performed on multiple sequential composite samples of milk from 1,172 cows in 9 dairy herds. If the initial diagnosis of Staphylococcus aureus infection was based on the first positive culture, an average of 37.8% of subsequent cultures on the same cows were negative for S aureus. However, if the initial diagnosis of S aureus infection was confirmed by 2 or 2 of 3 sequential positive cultures and any conversions from S aureus positive to negative were confirmed by 2 or 2 of 3 sequential negative cultures, then only 17.0% converted to a negative diagnosis. Conversion of cows from S aureus culture-positive to -negative varied between herds; 8.1 to 69% for single cultures and 0.0 to > 40% for confirmed cultures.

Summary

Bacterial cultures were performed on multiple sequential composite samples of milk from 1,172 cows in 9 dairy herds. If the initial diagnosis of Staphylococcus aureus infection was based on the first positive culture, an average of 37.8% of subsequent cultures on the same cows were negative for S aureus. However, if the initial diagnosis of S aureus infection was confirmed by 2 or 2 of 3 sequential positive cultures and any conversions from S aureus positive to negative were confirmed by 2 or 2 of 3 sequential negative cultures, then only 17.0% converted to a negative diagnosis. Conversion of cows from S aureus culture-positive to -negative varied between herds; 8.1 to 69% for single cultures and 0.0 to > 40% for confirmed cultures.

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