Early-lactation extended pirlimycin therapy against naturally acquired Staphylococcus aureus intramammary infections in heifers: A randomized controlled trial.

The primary objective of the current study was to evaluate cure rate following an early-lactation extended intramammary pirlimycin treatment on heifers naturally infected by Staphylococcus aureus. The secondary objective was to assess Petrifilm Staph Express (3M Microbiology, St. Paul, MN) count plate characteristics when used in a protocol for early-lactation detection of infected quarters in heifers. Milk samples were collected from heifers (n = 946) in the first few days following calving (mean = 5 d). Heifers with laboratory-confirmed S. aureus intramammary infection (n = 72) were randomly allocated into 2 groups. The treatment group (n = 54 quarters from 38 heifers) received an intramammary infusion of 50 mg of pirlimycin once per day for 8 consecutive days in infected quarters. The control group (n = 44 quarters from 34 heifers) did not receive any treatment. Treatment success was defined as having negative culture results for S. aureus in all 3 post-treatment quarter milk samples collected on d 17, 24, and 31 post-treatment. Treatment group mammary quarters showed a statistically significant higher cure rate (64.8%) compared with the control group (34.1%). A total of 38% of quarters identified as S. aureus-positive using the Petrifilm Staph Express count plate were in fact identified as non-aureus staphylococci on routine laboratory-based bacteriological culture. The current study demonstrates that a higher cure rate for S. aureus IMI can be achieved in dairy heifers if an extended treatment protocol is put in place soon after calving. Use of Petrifilm Staph Express count plate for identification of S. aureus-infected heifers could lead to unnecessary treatments because of false-positive results.