Infection of calves with parainfluenza virus (PI₃) and bovine leukemia virus (BLV) induced a clear immunologic response. A high titre of HI type antibodies (1:1280-1:2560) 2 weeks after the injection (p.i.) of PI₃ has been determined. The antibodies persisted at the level of 1:1280-1:2560 for one month and then steadily decreased. The Geometric Mean Titre (GMT) of antibodies for PI₃ virus in this group of calves after 2, 4 and 6 weeks and after 2, 3 and 4 months p.i. was 2250, 1810, 2560, 905, 240 and 323.8, respectively. In calves infected exclusively with PI₃ virus the positive HI titres were 1:80-1:160 two weeks p.i., but the highest titres (1:320-1:1280) were noted at 4 and 6 weeks after infection. Titres 1:20-1:80 were noted at the 4th month after infection. The GMT of antibodies in these calves was 95.1, 678, 1280, 269, 170 and 33.6, respectively. The level of antibodies for calves infected with PI₃ virus and BLV was two- or threefold higher than that in animals infected with PI₃ virus only. The HI titres were 23-fold higher in these animals at 2 weeks p.i. The simultaneous infection of calves with PI₃ virus and BLV significantly enhances immune response and the level of HI type antibodies for PI₃ virus.
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