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A model of hepatitis B surface antigen has been derived, based on extensive sequence analysis and biochemical data. The surface antigen sequences of the human, woodchuck, ground squirrel and duck hepadnaviruses were examined using hydrophobicity, hydrophobic moments, flexibility and secondary structure prediction. The helix phase diagram, which is a modified version of Eisenberg’s hydrophobic moment plots and which specifically addresses the problem of transmembrane helices, was used to examine the predicted helices. In this model four transmembrane helices are predicted. The N and C termini and the second hydrophilic region, which bears the major B-cell antigenic determinants, are external. It is suggested that the transmembrane helices may pack to form a channel through the membrane and may also be involved in the mechanisms of cell entry. A significant difference between the duck hepadnavirus and the mammalian HBsAg sequences was found, hence care must be taken when extrapolating data between the duck and the human surface antigen.

Emerging virus diseases are a major threat to human and veterinary public health. With new examples occurring approximately one each year, the majority are viruses originating from an animal host. Of the many factors responsible, changes to local ecosystems that perturb the balance between pathogen and principal host species is one of the major drivers, together with increasing urbanization of mankind and changes in human behavior. Many emerging viruses have RNA genomes and as such are capable of rapid mutation and selection of new variants in the face of environmental changes in host numbers and available target species. This review summarizes recent work on aspects of virus emergence and the current understanding of the molecular and immunological basis whereby viruses may cross between species and become established in new ecological niches. Emergence is hard to predict, although mathematical modeling and spatial epidemiology have done much to improve the prediction of where emergence may occur. However, much needs to be done to ensure adequate surveillance is maintained of animal species known to present the greatest risk thus increasing general alertness among physicians, veterinarians and those responsible for formulating public health policy.

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Hepatitis B (HB) breakthrough infections, identified by the presence of HB core (c) antibody, were found in 32 of 358 Gambian children vaccinated with plasma-derived HB vaccine. Over 2 years, 15 of these children lost their HBc antibodies. These children had significantly higher HB surface antibody levels before infection than those who retained HBc antibodies. One child, who responded well to the vaccine, had HB viral DNA detected in the presence of HBs antibodies. The S gene sequence of this DNA showed nucleotide changes that resulted in an amino acid substitution at residue 141 (lysine to glutamic acid) of the surface antigen. This finding suggests the child was infected with a variant virus that was not neutralized by antibodies resulting from HB vaccination.