RESUMEN
Bovine respiratory disease (BRD) remains a major health problem despite extensive use of vaccines during the post-weaning period. Apparent vaccine failure is attributed, in part, to primary vaccination during the period of greatest risk for BRD, providing inadequate time for onset of protective immunity. The current study investigated whether intranasal (IN) vaccination of 3-6â¯week old calves with a modified-live viral (MLV) vaccine induced sufficient immune memory to prevent respiratory disease and accelerate onset of protective immunity 5â¯months later. Vaccine groups included naïve controls, a single IN vaccination at 3-6â¯weeks of age, primary IN vaccination at 6â¯months, and either an IN or subcutaneous (SC) booster vaccination at 6â¯months (nâ¯=â¯10/group). All calves were challenged with BHV-1 four days after vaccination at 6â¯months of age. Primary IN vaccination at 6â¯months did not significantly reduce clinical disease but significantly (Pâ¯<â¯0.01) reduced virus shedding. A single IN vaccination at 3-6â¯weeks of age significantly (Pâ¯<â¯0.05) reduced weight loss but did not reduce fever or virus shedding. Both IN and SC booster vaccinations, significantly (Pâ¯<â¯0.01) reduced clinical disease but virus shedding was significantly (Pâ¯<â¯0.001) reduced only by IN booster vaccination. Reduction in virus shedding was significantly (Pâ¯<â¯0.01) greater following booster versus primary IN vaccination at 6â¯months. All vaccination regimes significantly (Pâ¯<â¯0.01) reduced secondary bacterial pneumonia and altered interferon responses relative to naïve controls. Only IN booster vaccination significantly (Pâ¯<â¯0.05) increased BHV-1 specific IgA in nasal secretions. These results confirm primary MLV IN vaccination at 3 to 6â¯weeks of age, when virus neutralizing maternal antibody was present, induced immune memory with a 5â¯month duration. This immune memory supported rapid onset of protective immunity four days after an IN booster vaccination.