Cotton rats were used to study the replication and pathogenesis of bovine parainfluenza virus type 3 (bPIV3) and to test the efficacy of the F and HN glycoproteins in modulating infection. In vitro cultures of cotton rat lung cells supported the growth of bPIV3 as shown by virus recovery, immunofluorescence, immunoprecipitation, and syncytium induction. Intranasal (i.n.) inoculation of cotton rats with 10(7) PFU resulted in peak recovery of virus after 2 days (8 x 10(4) PFU/g of lung tissue) and significant bronchiolitis with lymphocyte infiltration 5 to 7 days postinfection. Immunohistochemical staining of lungs and trachea demonstrated that virus antigen-positive cells increased in frequency over the course of infection to a maximum on day 5. Serum antibody responses were evaluated by enzyme-linked immunosorbent assays (ELISA), hemagglutination inhibition (HAI), and serum neutralization (SN). Following a single i.n. inoculation, serum antibody levels were 1/40,960, 1/32, and 1/80, as detected by ELISA, HAI, and SN, respectively. When an intramuscular inoculation of 10(7) PFU was administered 10 days prior to the i.n. inoculation, a secondary response which resulted in an ELISA titer of 1/163,000, an HAI titer of 1/640, and an SN titer of 1/512 was induced. IN inoculation of recombinant adenoviruses type 5 containing the bPIV3 F or HN protein or a combination of the two viruses protected cotton rats from bPIV3 challenge. Protection was evaluated serologically by ELISA, HAI, and SN titers, histopathology, immunohistochemistry, and virus recovery.