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

Abstract

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 2,000cfu); 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 1,080cfu; 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 24h 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 24h 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.