Metaphylaxis with tildipirosin did not inhibit effectiveness of an experimental monovalent vaccine of live, attenuated Mannheimia haemolytica administered intranasally to dairy calves

Abstract

The objective of this study was to evaluate the effect of tildipirosin on the efficacy of a concurrently administered experimental vaccine against Mannheimia haemolytica. The experimental vaccine was developed for intranasal (IN) administration and contained the same live, attenuated, streptomycin-dependent Mannheimia haemolytica, at the same titer as in commercial product(s). Eighty-eight 14-week-old Holstein or Holstein-cross male calves from a single source were randomly assigned (using a 2 x 2 factorial design) to 1 of 4 treatment groups (22 head per group) as follows: 1) experimental vaccine (VAX) (IN); 2) VAX (IN) + tildipirosin subcutaneously (SC); 3) placebo vaccine (PLBO) (IN) as negative control; and 4) PLBO (IN) + tildipirosin (SC). Seventy days after enrollment, 84 calves were challenged with virulent M. haemolytica. Clinical assessment of each calf was recorded daily after challenge. There was no significant (P > 0.05) effect of any experimental treatment on clinical signs. Seven days after challenge, the calves were euthanized, a percent lung lesion score (LLS) was assigned to each, and samples of lung were submitted for isolation of M. haemolytica. There was no significant (P = 0.51) effect of tildipirosin on LLS for calves vaccinated with VAX or those vaccinated with PLBO; however, VAX resulted in significantly (P = 0.046) lower LLS. Under the conditions of this study, concurrent administration of tildipirosin and IN live, attenuated, streptomycin-dependent M. haemolytica vaccine with the same antigen as used in this study did not interfere with the efficacy of the vaccine.