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It was found that M-PI3 virus could be propagated both in MDBK and FLK cells. TCID₅₀ in the MDBK cell line was 10⁶‧³⁵—10⁷‧⁵ per 0.2 ml in the course of 10 consecutive passages and haemagglutination titer ranged from 80 to 320 using a 0.75 per cent solution of goose erythrocytes. There was observed many mitotic divisions of cells revealed by Pappenheim’s staining. The M-PI3 virus passaged through FLK cells displayed changed biological properties compared with that propagated in MDBK cells. TCID₅₀ and haemagglutination titers of M-PI3 virus depended on the number of passages and infective dose. In the first four passages its TCID₅₀ titer was 10⁵‧³³—10⁶‧⁶⁰ per 0.2 ml, in the 5th — 10³‧³² and in the 6th — 10¹‧⁵. The virus was not found in the next four passages. Similar observations were done with respect to haemagglutination titer; in the first passage its titer was 320, in the 4th — 20 and later no virus was stated. By means of Pappenheim’s method the FLK cells formed syncytia containing 20—40 nuclei. The mean geometric titer of antibodies in sera taken from calves infected with BLV and M-PI3 viruses were 507 and in the group II i.e. infected only with M-PI3 virus — 226. The highest titer of HI antibodies in the group I was found at week 3 p.i. and in the group II at week 4 p.i.
The rate of infection with BLV was assessed in 2053 cattle from 17 farms enzootically infected with PI3. The animals were divided into 6 groups according to their age. The highest percentage (38.0 - 56.7) of the animals in all age groups had HI antibody titers from 1:80 to 1:320; the percentage of animals with the titers between 20 and 40 was 9.8 - 27.7. The HI titers decreased along with the age of the animals. The highest percentage of negative results (24.3 per cent) was noticed in calves aged between 6 months and 2 years. In 3-year-old animals the per cent of negative results was 11.9, in 4-5-year-old - 6.9, in cows 6-12-years-old between 5.6 and 2.2. The percentage of ani­mals whose HI titers ranged from 1:640 to 1:5120 was 7.3 - 50 with a gradual increase with age. The rate of seropositive animals for BLV also increased with the age of the animals and it was 28.3, 39.5, 48.0, 54.7, 61.0, 68.6 in reference to PI3, respectively; in sero-negative groups of the animals the findings were 16.0, 26.2,36.5,48.4, 54.1 and 67.0. The results revealed a close correlation be­tween the number of cattle infected with PI3 and the number of the animals infected with BLV
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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