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Virginia Cooperative Extension -
 Knowledge for the CommonWealth

Determining Type of Mastitis in Herd

Dairy Pipeline: April 1998

by G. M. Jones, Extension Dairy Scientist
Extension Dairy Scientist, Milk Quality and Milking Management
Virginia Tech
(540) 231-4764 email:

Culture bulk tank milk and milk from individual cows to determine the type of mastitis infection in the herd. Bulk tank milk should be tested in herds where DHI somatic cell count (SCC) score is 3.3 or above, or the average actual SCC is above 250,000, or more than 0.5 percent of the cows are withheld from the bulk tank on any day because of clinical mastitis. Bulk tank culturing is easy, economical, and is an important aid in determining the microbiological cause of mastitis in the herd. Sampling only one bulk tank load is not a complete guarantee that contagious mastitis will be detected because Staphylococcus aureus infected cows may shed the organism intermittently. Sample three consecutive bulk tanks. Freeze the first two samples. Comingle these two with the third for culturing. Check with your State Lab to see if they will conduct cultures and test for bacteria count, lab pasteurized count, preliminary incubation count, and somatic cell count. In herds with clinical mastitis problems or high SCC, milk samples from clinical mastitis quarters or cows with high SCC should be collected aseptically and cultured by the State Lab or the College of Veterinary Medicine to decide which cows should be: held out of the bulk tank, considered for culling, dried off early, or segregated from low SCC, especially at milking. When collecting samples for culturing, be sure to sample cows from different lactations, including those in first lactation who have had elevated SCC, as well as cows in other stages. Include cows who have had clinical mastitis but samples should be collected before any treatment and frozen for later testing. The SCC profile/report can be used to identify cows with highest SCC and the California Mastitis Test (CMT) used to identify which quarters have high SCC.

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