Effect of Maternally Derived Antibody on Sequential Infection with Highly Virulent Bursal Disease Virus in Newly Hatched Broilers.

Maternally derived antibodies (MDAs) are important for protection against very virulent infectious bursal disease virus (vvIBDV). In this study, 5-day-old commercial broilers with non-uniform MDA titers (with a coefficient of variation of 50%) were challenged with vvIBDV and given free contact with each other during a 2-week period. The chicks were assigned to four MDA-titer subgroups, GI-1 (very low MDA), GI-2 (low MDA), GI-3 (medium MDA), and GI-4 (high MDA). Transient symptoms of infection were observed in 35.7% of challenged birds. Body weight gain was significantly lower in GI-2, GI-3, and GI-4 birds than in an unchallenged control group. Seroconversion was observed in GI-1 birds and some GI-2 birds. The frequency of virus shedding via the cloaca in vvIBDV-challenged birds increased from 7.1% of GI birds at 5 days post inoculation (dpi) to 35.7% at 14 dpi. The timing of virus shedding was progressively later from GI-1 to GI-4. At 14 dpi, significant atrophy of the bursa of Fabricius (BF) was observed in GI birds compared with GII controls; atrophy was most severe in GI-1 birds and least severe in GI-4 birds. BF lesion scores decreased from GI-1 to GI-4. The proportion of birds with IBDV antigen in the BF at 14 dpi was higher in GI-2 and GI-3 than in GI-1 and GI-4, whereas the viral load in positive birds increased from GI-1 to GI-4. Our results indicate that high levels of MDAs would protect chicks from initial vvIBDV infection but that progressive decay of these MDAs would result in delayed infection by virus shedding in initially infected birds with low MDA titers, resulting in continuous circulation of the virus in a flock with non-uniform MDA titers.