ABSTRACT
Infectious bursal disease (IBD) causes high morbidity and high mortality among 3 – 6 weeks old chicks. Its occurrence even in vaccinated flocks is a global concern. Passive haemagglutination (PHA) test, a simple and rapid test has been adopted for IBD antibodies but its use for viral titres has not been reported. To modify the test for assessment of viral titres, serial dilutions of a reference IBDV sample were used to sensitize human group O+ red blood cells (RBC) before reacting them with known IBD positive serum. Effect of RBC concentrations (0.1%, 0.2% and 0.6%) on sensitivity of the Modified Passive Haemagglutination (MPHA) test for viral titres was tested. To assess immunogenicity of IBD vaccines used in Nigeria (NVRI, ABIC, GEORGIA, IZOVAC and JOVAC), viral titres of different batches of the vaccines were tested and they were used to vaccinate groups of chicks for sero-conversion abilities. Unvaccinated day old chicks hatched in Nigeria were also tested for IBDV maternal antibodies. To determine antibody titre that protects chicks against strains of IBDV circulating in Nigeria, 5 groups, each of 10 chicks, were vaccinated with different vaccines and by different methods. Five of the chicks from each group were tested for antibody titres while 5 were challenged with a Nigerian IBDV strain. Mortality rates and lesions in bursae of the chicks were recorded. Mortality rate of each group was plotted against its mean antibody titre so that a line of best fit was generated. Equation for the line was used to calculate 8 antibody titre that would give 0 % mortality as protective antibody titre against Nigerian IBDV strains. The reference IBDV gave MPHA viral titre of 2048. Mean viral titres of IBDV samples increased (p < 0.05) from 454.85 ± 315.32 with 0.6% RBC concentration to 2,396.57 ± 489.55 with 0.2% RBC concentration. RBC concentration of 0.1% could not give clear color difference from the control. Mean viral titres of the IBD vaccines were: 1,472.00 ± 748.55; 1,065.60 ± 780.03; 2,585.00 ± 926.92 ; 2,176.00 ± 1920 and 2,112.00 ± 1984.00 for NVRI, ABIC, GEORGIA, IZOVAC and JOVAC vaccine brands respectively, while mean antibody titres they provoked in vaccinated chicks were: 1,280.00 ± 174.88; 1,356.80 ± 241.51; 332.80 ± 51.20 ; 998.40 ± 196.27 and 448.00 ± 79.25. Each of the unvaccinated day old chicks hatched in Nigeria tested negative for IBDV maternal antibodies. The 5 groups of chicks which had 185.6, 256.00, 307.20, 243.20 and 0 antibody titres, respectively had 40 %, 0 % , 0% , 0% and 75% mortalities when challenged with the Nigerian IBDV strain. Equation for line of best fit of the graph of the mortality rates on the antibody titres was: Y = 75.00 - 0.25x. So, PHA antibody titre that would protect chicks against the Nigerian IBDV strain was 300. Seventeen of 18 chicks that survived the IBDV challenge had lesions in their bursae. The only survivor that did not have bursal lesion was from a group vaccinated with the Nigerian (NVRI) IBDV vaccine. It has been concluded that IBD vaccines used in Nigeria are immunogenic enough as all of them gave at least 300 PHA antibody titre.
