Studies on epidemiology of infectious bursal disease and immune response to its vaccines in broiler chicken
Abstract
This study was conducted to understand the epidemiology of infectious bursal disease (IBD), to check
optimal age for IBDV vaccination and immune response offered by currently available intermediate plus vaccines
in broiler chicken flocks in Haryana state. The data related to IBD outbreaks for the period from January 2008 to
June 2016 was analyzed. A total of 1368 outbreaks of IBD occurred during this period with overall morbidity,
mortality and case fatality rate of 4.19%, 2.61% and 62.34% respectively. The disease was recorded throughout the
year with maximum outbreaks in August month and in July to September quarter. Approximately 1083 outbreaks
were in the 11-30 days age birds. Moreover the number of IBD outbreaks is increasing in the birds of 11-20 day of
age. Of these 918 were recorded in vaccinated flocks while 450 were in unvaccinated flocks. Case fatality rate was
higher in unvaccinated flocks. To check optimal age of vaccination in broiler chicken, serum samples were
collected from 18 hatcheries. Maternally derived antibody titres of all the serum samples collected from different
hatcheries and day of vaccination was calculated as per Deventer formula. Maternal antibody titres from the serum
samples of day old chicks of 18 hatcheries showed a wide variation and age of vaccination ranged from 7-19 day
of age. The non-uniformity in the MAb titres of day-old broiler chicks was expressed as percentage of coefficient
of variation (CV %) with a range of 28.2% to 75.3%. Due to wide range of MAb titre, twice vaccination is
advisable. As per our study however, calculating the age of vaccination for the chicks from individual hatchery is
advisable if possible. To evaluate the vaccine immune response of IBD vaccines, an experimental study was
conducted using two commercial intermediate plus vaccines P and Q. The level of MAbs ranged from 6834.6 to
9925.60 in day old chicks. In experimental group age of vaccination came 17 because maternal antibody titre was
uniform and high. The mean maternal antibody titre of chicks at 1 day age, 7 and 14 day age was 7874.21, 2449.4
and 1089.9. The mean MAb titre shows a steady decrease in the titre from day old age to 14th day of age. Both
vaccines P and Q provided protective immunity from 7 DPV till the end of experiment. Vaccine P showed static
increase in ELISA antibody titres from 7 DPV however vaccine Q showed quite higher level of antibody titres on
7 DPV itself which remain protective till the end of experiment in both. CD4+ cells increased at 7 DPV, decreased
at 14 and 21 DPV, again increased at 28 DPV. CD8+ cells increased at 7 DPV, decreased at 14 DPV, again
increased at 21 and 28 DPV. This could probably be due to immunosuppressive effect of IBD vaccines. Bursal
index was comparable with control in case of birds vaccinated with vaccine P at 7 DPV only. However it was
reduced significantly in birds vaccinated with vaccine Q at 7 DPV itself. Later the bursal index was significantly
lower in all the vaccinated groups vaccinated with either of the vaccine at 14, 21, 28 DPV immunosuppression.
Histopathology showed lymphoid depletion, medullary necrosis with cyst formation, fibrous connective tissue
proliferation in vaccinated groups which developed early and was comparatively more with vaccine Q.