Infectious bursal disease virus (IBDV) contains two genome segments (segment A/segment B) that can reassort among the viruses. Reassortant IBDVs have been identified in several countries including the United States. These reassortant viruses usually include at least one genome segment from a very virulent (vv)IBDV strain. In vivo virulence of six reassortant IBDV from the United States was assessed relative to the virulence of three frequently described IBDV pathotypes: vvIBDV (rB strain), classic virulent (cv)IBDV (STC strain), and subclinical (sc)IBDV (Del-E strain). Morbidity and mortality in 4-wk-old specific-pathogen-free (SPF) leghorns indicated that reassortant IBDV with a vv genome segment A and non-vv segment B were less pathogenic than the vv/vv rB strain but more pathogenic than the cv/cv STC strain. The sc/vv IBDV strain D6337 (sc/vv) was comparable to the STC strain in pathogenicity. Viruses with a serotype 2 (ser2) genome segment A, regardless of the type of genome segment B, did not cause clinical disease in SPF chickens or turkeys. None of the reassorted viruses caused morbidity, mortality, or gross lesions in SPF turkeys. Histopathologic lesions in the bursa of turkeys were not observed in any group except those challenged with the serotype 2 OH strain, which had a mild lymphocytic depletion. No mortality was observed in maternally immune broilers inoculated with any of the IBDV pathotypes at 1, 2, 3, and 4 wk of age. No bursal lesions were observed in any of the broiler chicken groups at 1 wk of age except for the D2712 (ser2/cv)-inoculated birds that had mild lymphocyte depletion. Based on evaluation of bursal lesion scores and IBDV reverse transcriptase-PCR on broilers challenged at 2 wk of age, the K669 (vv/ser2) virus broke through the maternal immunity while the STC, Del-E, rB, D2712 (ser2/cv), and 7741 (vv/cv) viruses did not. All viruses broke through maternal immunity in the broilers at 3 wk of age except the Del-E scIBDV and D2712 (ser2/cv) reassortant IBDVs. At 4 wk of age, maternal antibodies were very low and bursal lesions were observed in all broilers challenged with the viruses. The data indicate that genome reassortant IBDVs are less pathogenic than is the rB (vv/vv) IBDV. However, the reassortant viruses with a vv genome segment A can still cause morbidity and mortality in SPF chickens, and they were able to break through maternal immunity produced via use of commercial classic and variant vaccines at an early age. This suggests that current breeder vaccination programs may not adequately protect against the reassortant vv/ser2 and vv/cv IBDV strains.
You have requested a machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Neither BioOne nor the owners and publishers of the content make, and they explicitly disclaim, any express or implied representations or warranties of any kind, including, without limitation, representations and warranties as to the functionality of the translation feature or the accuracy or completeness of the translations.
Translations are not retained in our system. Your use of this feature and the translations is subject to all use restrictions contained in the Terms and Conditions of Use of the BioOne website.
Vol. 60 • No. 4