Response to experimentally induced infection with bovine respiratory syncytial virus following intranasal vaccination of seropositive and seronegative calves.

OBJECTIVE: To determine whether a combination modified-live bovine respiratory syncytial virus (BRSV) vaccine can stimulate protective immunity in young BRSV-seropositive calves following intranasal (IN) administration.
DESIGN: Controlled challenge study. ANIMALS: 66 Holstein bull calves, 3 to 8 days old. PROCEDURES: In experiment 1, BRSV-seropositive and -seronegative calves were vaccinated IN with a commercially available combination modified-live virus vaccine formulated for SC administration; calves underwent BRSV challenge 4.5 months later. In experiment 2, BRSV-seronegative calves were vaccinated IN or SC (to examine the effect of route of administration) with the same combination vaccine that instead had a 1/100 dose of BRSV (to examine the effect of dose); calves underwent BRSV challenge 21 days later.
RESULTS: In experiment 1, BRSV challenge resulted in severe respiratory tract disease with low arterial partial pressures of oxygen and lung lesions in most calves from all groups. Maximum change in rectal temperature was significantly greater in seropositive IN vaccinated calves, compared with seronegative IN vaccinated and seropositive control calves. Number of days of BRSV shedding was significantly lower in seronegative IN vaccinated calves than in seropositive IN vaccinated and seropositive control calves. In experiment 2, maximum change in rectal temperature was significantly greater in seronegative control calves, compared with seronegative IN and SC vaccinated calves. Shedding of BRSV was significantly reduced in seronegative IN and SC vaccinated calves, compared with control calves; also, lung lesions were reduced in seronegative IN and SC vaccinated calves. CONCLUSIONS AND CLINICAL RELEVANCE: Maternal antibodies may inhibit priming of protective responses by IN delivered BRSV vaccines.