replicate in human hepatocytes that were able to proliferate in these mice. in general, these results imply that nonhuman primate hbv variants may be transmittable to humans. at least they infect representatives of the families pongidae and hylobatidae. however, at present it remains to be elucidated if nonhuman primate hbv variants are transmittable to humans and whether they cause disease in humans. and till now, no report of hbv transmission from captive animals to humans currently exists. however the potential for zoonotic disease transmission exists where blood or body fluid exposure is common. such scenarios could include chronically infected animals kept as family pets, close contact with caretakers, in situations in which chimpanzees are slaughtered and used as bushmeat. within birds many cases about avihepadnaviruses transmission between or within species have been studied. dhbv: infects only certain duck and goose species, but does not even infect muscovy ducks or chickens, or infects them inefficiently. heron hbv: does not infect ducks and only infects primary duck hepatocytes very inefficiently. studies on different heron species which are geographically isolated populations suggest that lateral transmission, virus adaptation and environmental factors all play a role in hhbv spreading and evolution, but it is not clear whether hhbv infection occurs at all in free-living herons, due to the possibility of cross-species virus transmission. crane hbv: cranes are phylogenetically very distant from ducks and are more closely to herons and stocks, but chbv infects primary duck hepatocytes with efficiency similar to dhbv. between humans and birds there is no evidence for transmission of avian hepadnavirus between avian and human, but recombination of avihepadnavirus strains between cross-species avian hosts can offer potentials for the transmission between human and bird, considering that many birds are one of the main sources of food for human. factors affecting transmission species specificity the special replication strategy gives this genus a special character, but also the high replication capacity and high error rate of the viral reverse transcriptase, thus hbvs from this genus have the ability to adapt to the host's environment, and varies from different hepadnaviruses, thus exhibiting species specificities. proteins pre s1 regions of the l protein, exhibiting different host specificity, if proteolytically being removed from hbv particles results in a loss of infectivity. the s protein therefore is needed, but not sufficient for hbv entry. in contrast to the m protein, the l protein is essential for the viral infectivity. these special proteins plays different role in different viruses, thus determining the different host ranges, if this were strictly true, we would expect that divergence in pre-s would affect cross-species infection, which then can contribute the different transmission of different hepadnaviruses. replacing the special region of the envelope protein of one hepadnavirus with another hepadnavirus's. hhbv by dhbv:infect ducks and derived primary hepatocytes wmhbv by hbv:infect human hepatocytes subviral sphere particles: they are assumed to have the ability of absorbing virus-neutralizing antibodies to facilitate the virus spread and maintenance in the host. potentials co-infection a lot of studies have well characterized the horizontal transmission of hbv through parenteral routes and co-infections with different genotypes of hbv have also been reported, including genotypes a and d, genotypes a and g, and genotypes b and c. in asia, genotypes b and c account for almost all hbv infections. infection with genotype c may induce more severe liver diseases than infection with genotype b. recombination between genotypes b and c can occur as the result of co-infection of the two genotypes and recombinant strains may possess an enhanced disease-inducing capacity compared with genotype b. it has also been reported that the predominance of recombinant strains between genotypes b and c might be associated with the development of hepatocellular carcinoma in young carriers in taiwan. recombination though the hepadnaviruses have a very limited host range, with the potential cross-species transmission of hepadnavirus among primates and some families of birds, combined with recombinants, it might change the hepadnavirus host specificity. interspecies recombination between hepadnaviruses from cross-species hosts would provide a large variation of virus genome, which will change the pathogenicity and transmissibility, and enlarge the host range; the first evidence of potential recombination between chimpanzee hbv and human hbv genome was documented, recombination between human and non-human primate hbv strains, between gihbv strains from different genera of gibbons, and between birds hbv strains from different avian sub-families were confirmed. all those experiments provided data allowing us to better understand the background, virus hosts, and zoonotic transmission of the non-human primate hbvs.