Detection of infectious bursal disease virus in experimentally infected chickens by in situ hybridization

In situ hybridization was used in a pathogenesis study of three vaccine pat hotypes (Delaware variant A, D78, and BursaVac) of infectious bursal diseas e virus (IBDV). Tissues were excised (bursa, thymus, spleen, proventriculus , and cecal tonsils), fixed in formalin, and paraffin embedded at 12, 24, 4 8, 72, and 120 hr postinoculation (HPI). With an antisense VP2 gene probe, viral nucleic acid was detected in bursas from both D78-;Ind BursaVac-infec ted chickens at 24, 48, 72, and 120 HPI. However, viral RNA was detected on ly in the Delaware variant A-infected birds at 72 HPI. Thymus and spleen we re positive in the D78-infected birds at 48 HPI and in the BursaVac-inocula ted group at 72 HPI. Viral nucleic acid was not present in detectable level s among any of the tissues tested at 12 HPI. However, by 24 hr, scattered p ositive lymphoid cells were visualized in the bursal follicles of chickens infected with D78 and BursaVac. In addition, low levels of viral nucleic ac ids were detected in the thymus and spleen among the D78- and BursaVac-infe cted birds. The sites of viral replication were consistent between the two vaccine-infected groups (D78 and BursaVac), whereas the chickens infected w ith Delaware variant A had limited IBDV replication in the bursa.