Comparison of 3-, 5-, and 7-day post-treatment evaluation periods for measuring therapeutic response to tilmicosin treatment of bovine respiratory disease

Abstract

Three-hundred steer calves with clinical signs of bovine respiratory disease (BRD) and a rectal temperature of 104øF (40øC) or greater were enrolled in a 56-day study to evaluate the effect of a 3-, 5- or 7-day post-treatment evaluation (PTE) period on therapeutic response to tilmicosin. Trial calves (average weight 557 lb; 253 kg) were purchased from livestock auction markets in Colorado, Kansas, Oklahoma and Wyoming, and delivered to the trial site in Wellington, Colorado. Upon randomization to a treatment group, tilmicosin was administered at 4.55 mg/lb (10 mg/kg) of body weight. Three treatment groups were defined by the number of days (3, 5 and 7) post-treatment during which they were not eligible for retreatment for BRD. The 7-day PTE group had a significantly higher treatment success rate compared to the 3-day PTE group (P=0.05). The 3-day PTE group had a higher (P<0.05) first relapse rate than the 7-day PTE group, and a higher (P<0.01) second relapse rate than either the 5- or 7-day PTE groups. There were a total of five BRD-associated mortalities in the study; all occurred within the first eight days of the study. Results from this trial suggest that a 3-day PTE period following tilmicosin administration could result in an overestimation of treatment failure rates compared to a 7-PTE period.