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Hemorrhagic enteritis virus (HEV) belongs to the Adenoviridae family, a subgroup of adenoviruses (Ads) that infect avian species. In this article, the complete DNA sequence and the genome organization of the virus are described. The full-length of the genome was found to be 26,263 bp, shorter than the DNA of any other Ad described so far. The G + C content of the genome is 34.93%. There are short terminal repeats (39 bp), as described for other Ads. Genes were identified by comparison of the DNA and predicted amino acid sequences with published sequences of other Ads. The organization of the genome in respect to late genes (52K, IIIa, penton base, core protein, hexon, endopeptidase, 100K, pVIII, and fiber), early region 2 genes (polymerase, terminal protein, and DNA binding protein), and intermediate gene IVa2 was found to be similar to that of other human and avian Ad genomes. No sequences similar to E1 and E4 regions were found. Very low similarity to ovine E3 region was found. Open reading frames were identified with no similarity to any published Ad sequence.

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A very virulent strain of infectious bursal disease virus (IBDVks) was isolated from the bursae of Fabricius of IBDV-affected broiler chickens. Following 43 serial passages in specific pathogen-free embryonated eggs, an attenuated strain was established (IBDVmb). Dosages of IBDVmb in the range 10 2 to 10 4 embryo infective dose of 50% were found to be safe and protective for commercial chicks. Chickens vaccinated with live vaccine containing IBDVmb responded with precipitating and type-specific neutralizing antibodies, and were immune to subsequent challenge with a very virulent IBDV. IBDVmb has been used as an attenuated vaccine throughout the world since 1993. A comparison of the full sequences of the virulent and attenuated strains (IBDVks and IBDVmb, respectively) revealed seven nucleotides that were different, four of them leading to changes in the amino-acid sequence. Comparison of the protein sequencse of these strains and published sequences of very virulent and attenuated phenotypes lead us to sugget that the novel difference responsible for virulence of the Israeli strains are: residue 272 (VP2, very conserved site) and residue 527 (VP4), both in segment A, and in segment B (VP1) residues 96 and 161 (both conserved). Our study strengthens the possibility that more than one protein is involved in IBDV attenuation. In all reports, including ours, virulence was reduced without affecting antigenicity of the neutralizing epitopes in VP2. This could have practical implications for attenuated-vaccine development.

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