One of the most common types of chronic mastitis is caused by the bacteria, Staphylococcus aureus. Often, it causes subclinical mastitis, where there is neither abnormal milk nor detectable change in the udder, but somatic cell count has increased. Some cows may flare-up with clinical mastitis, especially after calving. Although S. aureus is difficult to detect in some cows, culture of bulk tank milk is a good way to determine if present in the herd. Bulk tank samples should be cultured 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 2-3 percent of the cows are withheld from the bulk tank on any day because of clinical mastitis. Sample three consecutive bulk tanks. A culture of only one bulk tank sample is not a guarantee that contagious mastitis will be detected because S. aureus infected cows may shed the organism intermittently. Freeze the first two samples. Combine 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. A small fee probably will be charged, but it should be worthwhile to test bulk tank samples monthly. Some S. aureus herds have histories of gradually increasing SCC but with very few clinical cases. Use the California Mastitis Test (CMT) on cows with elevated DHI SCC to determine which quarters may be infected. In herds with acute mastitis problems, milk samples from clinical mastitis quarters should be collected aseptically and cultured. Early identification of the infection before the bacteria have an opportunity to invade deep within the udder and form abscesses is important. Mastitis caused by S. aureus bacteria is extremely difficult to control by treatment alone. Successful control is gained only through prevention of new infections and culling of infected cows.

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