Response of susceptible versus immune chicks to killed, live-modified, and wild infectious bursal disease virus vaccines.

Modified infectious bursal disease virus (IBDV) administered ocularly to either susceptible or passively immune chicks did not induce protection against bursal atrophy by wild IBDV, while intramuscular (IM) or intrabursal (IB) injection protected susceptible chickens. No protection was obtained in passively immune chickens vaccinated IM or IB. Susceptible and passively immune chickens vaccinated with a single dose of killed-virus suspension (KVS) did not become resistant to wild IBDV challenge. A killed-virus multiple emulsion (KVME) induced partial protection (3/5) against challenge 28 days after subcutaneous vaccination of three-day-old susceptible chickens. At 35 and 42 days old, all susceptible chickens were protected by KVME. Protection by passive antibodies was observed in unvaccinated, KVS-, and KVME-vaccinated chickens challenged at 10 and 17 days of age. Protection was only partial in passively immune chickens vaccinated with KVME after passive antibodies were no longer protective to unvaccinated and KVS-vaccinated chicks. Modified IBDV was detected two days post-IB inoculation only in the bursa of Fabricius of susceptible chickens, while wild IBDV was found also in the thymus, spleen, and blood. No evidence of virus was found in immune chickens inoculated IB with modified IBDV. The growth of wild IBDV was limited to the bursa of Fabricius in immune chickens. Furthermore, the intensity of fluorescence, as well as the number of positive bursa cells, was low when compared with fluorescence in the bursa of susceptible chickens infected with wild IBDV.