Development of a challenge model to experimentally induce Streptococcus uberis mastitis in bred dairy heifers
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A reproducible challenge model for inducing Streptococcus uberis mastitis in dairy heifers was developed. Heifers were assigned 2 quarters to receive an infusion of Strep. uberis NIRD-0140J. For challenge, a bacterial suspension was created to deliver 1000–2000 cfu in 1 mL saline. For successful challenge, 3 of 4 consecutive cultures had to be confirmed positive for Strep. uberis. Six of the 7 heifers (85.7%) were challenged successfully with the dose used (mean: 1080 cfu). Once confirmed, infected quarters were treated with nonlactating cow therapy. Results demonstrated that the challenge model developed was successful in routinely establishing experimental Strep. uberis mastitis in dairy heifers, and it was controlled (100% cure) by the administration of nonlactating cow therapy. At parturition, all heifers were free of mastitis and antimicrobial residues, and exhibited normal milk production and somatic cell counts. The challenge model will be instrumental in evaluating new Strep. uberis vaccine candidates.