Treatment and management of mastitis accounts for the majority of antimicrobial usage on dairy farms in the United States today (USDA–APHIS–VS–CEAH–NAHMS, 2014). Mastitis is the most common and costly disease in the dairy industry. Selective non-treatment of mastitis for Gram-negative and culture negative cases of mastitis has been demonstrated as an effective mechanism to reduce antimicrobial usage on dairy farms without compromising animal welfare (Vasquez et al., 2017; Fuenzalida and Ruegg, 2019a; b). The overall objective of this dissertation is to evaluate the use of evidence-based strategies to reduce use of important antimicrobials for treatment of mastitis caused by Gram-positive pathogens. The first aim of this dissertation is to compare clinical and bacteriological outcomes of treatments of naturally occurring, non-severe clinical mastitis caused by Gram-positive pathogens. The second aim is to determine if nutritional supplementation with a commercially available postbiotic that is a fermentation product of Saccharomyces cerevisiae (SCFP) influences clinical and bacteriological outcomes of cows that receive an IMM challenge with Streptococcus uberis. The overall hypothesis of this dissertation is that evidence-based strategies can be used to reduce the usage of antimicrobials to treat clinical mastitis caused by Gram-positive pathogens on dairy farms. This dissertation begins in chapter 1 with a review of studies that evaluated the treatment of mastitis caused by Gram-positive pathogens in the United States. In chapter 2, several mechanisms to reduce antimicrobial usage are explored including shortening the duration of IMM therapy, treating with a lower class of antimicrobial, or not treating cows with clinical mastitis at all. Chapter 3 explores the susceptibility and AMR of Gram-positive bacteria identified as causing clinical mastitis on Michigan farms. Chapter 4 explores the clinical and immunological outcomes post-challenge with Streptococcus uberis. Together these four chapters demonstrate applied mechanisms that can be utilized to reduce antimicrobial usage on modern dairy farms.
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