Technical note: Development of a challenge model for Streptococcus uberis mastitis in dairy heifers

A challenge model for experimentally inducing Streptococcus uberis mastitis in bred dairy heifers was developed. Qualifying heifers (n = 7) exhibited antibody titers of < 1:10,000 against Strep. uberis antigens and were free of intramammary infections (IMI). Two contralateral quarters of each heifer were assigned to receive an infusion of Strep. uberis (1,000 to 2,000 cfu); remaining quarters served as unchallenged controls. For a successful challenge and infection, 3 of 4 consecutive mammary secretion samples had to culture positive for Strep. uberis. Six of the 7 heifers were challenged successfully in both infused quarters with a mean dose of 1,080 cfu; once confirmed, infections were treated with a one-time infusion of nonlactating cow therapy. Before challenge, mammary secretion leukocyte counts averaged 8.4 × 106/mL in all quarters. At 24 h after challenge, leukocyte count increased to 18.4 × 106/mL in challenged quarters, peaking on d 5 at 24.3 × 106/mL; unchallenged quarters remained at ≤ 10.4 × 106/mL, but increased to 15.2 × 106/mL on d 7 and then decreased. Before challenge, macrophages predominated (81%) in mammary secretions followed by lymphocytes (15.3%) and neutrophils (3.7%). By 24 h after challenge, neutrophils increased in challenged quarters and predominated for the duration of the trial (65.3 to 70%), whereas macrophages predominated in unchallenged control quarters (65.2 to 75.2%). The challenge model was successful in establishing Strep. uberis IMI in 85.7% of animals, and IMI were controlled (100% cure) by administering nonlactating cow therapy. All heifers calved free of IMI and antimicrobial residues, with milk production similar to that of herd mates and with somatic cell counts (SCC) < 200,000 cells/mL.