Randomized noninferiority study evaluating the efficacy of a postmilking teat disinfectant for the prevention of naturally occurring intramammary infections

The objective of this study was to complete a positive-control, natural exposure, noninferiority design field study to test the efficacy of a novel glycolic acid-based postmilking teat disinfectant as compared with a previously proven iodine-based postmilking teat disinfectant (positive control). The primary outcome of interest was the effect of treatment on incidence of new intramammary infections. Secondary outcomes included the effect of treatment on prevalence of infection, somatic cell count, and teat condition. After blocking by parity, approximately 300 early- to mid-lactation cows on a large Wisconsin dairy farm were randomly assigned to 1 of 2 groups. For a 12-wk period between May and August 2014, the 2 groups were dipped after each milking with either the experimental (EX) or positive control (PC) product. Individual quarters were sampled to establish bacteriological infection status at the beginning of the study, and every 2 wk thereafter, by use of a 2-stage process evaluating somatic cell count (SCC), and then culturing milk samples only when SCC exceeded a parity-specific threshold. Teat condition scoring was completed at the beginning of the study and on wk 4, 8, and 12. Mixed logistic regression was used to evaluate the effect of treatment on dichotomous outcome measures including the odds of acquiring a new infection during a given 2-wk sampling interval (incidence), the odds for presence of infection at sampling (prevalence), and odds for a normal teat skin condition score. Mixed linear regression was used to evaluate the effect of treatment on somatic cell count. For the noninferiority analysis, the upper bound of the 95% confidence interval for the difference in new infection rate between the 2 treatments (EX - PC), had to be to the left of the critical value d (0.035) to conclude that EX was noninferior relative to PC with respect to risk for new infections. Results showed that the incidence of new infections was not different for quarters dipped with EX (3.2%) as compared with PC (4.2%). Similarly, the prevalence of infection tended to be lower for quarters dipped with EX (3.92%) as compared with PC (5.03%). No overall difference was found between treatments when evaluating somatic cell count measures and teat condition scores. Because the upper bound of the 95% confidence interval of the new IMI rate difference was smaller than the predefined noninferiority limit, it was concluded that the experimental product was not inferior compared with the positive control. As such, the glycolic acid-based teat disinfectant evaluated in this study can be considered an effective postmilking teat disinfectant, as well as safe, in so far as the product was not irritating to teat skin and did not negatively affect skin condition measures, as compared with the positive control group.