Efficacy of vaccination and antimicrobial treatment to eliminate chronic intramammary Staphylococcus aureus infections in dairy cattle

OBJECTIVE: To determine whether a combination of vaccination and extended intramammary antimicrobial treatment would eliminate chronic intramammary Staphylococcus aureus infections in lactating dairy cows. DESIGN: Randomized controlled clinical trial. ANIMALS: 50 dairy cows with chronic mastitis caused by S aureus. PROCEDURE: Cows were identified and paired within herd on the basis of days in milk, lactation number, milk production, and numbers of quarters infected. Treated cows (n=20) received 3 doses of a polyvalent S aureus bacterin on days 1, 15, and 21 of the study along with intramammary administration of pirlimycin in all 4 quarters once daily for 5 treatments (days 16 to 20). Control cows (n=23) received no treatment. Follow-up samples for bacteriologic culture were collected for at least 3 months after treatment to determine treatment success rates. RESULTS: Significantly more S aureus infections were eliminated from treated cows (8/20 40%]), compared with control cows (2/23 9%]). The proportion of infected quarters that yielded negative results throughout the follow-up period was also significantly higher in treated cows (13/28 46%]) than in control cows (2/41 5%]). CONCLUSIONS AND CLINICAL RELEVANCE: Results indicate that a combination of vaccination and antimicrobial treatment can be successful in eliminating some cases of chronic intramammary S aureus infections in dairy cattle. However, it is important to consider extended treatment protocols carefully because many cows are likely to remain infected with S aureus despite treatment and vaccination.