The 'Red Queen' dilemma--running to stay in the same place: reflections on the evolutionary vector of HBV in humans

Origins and Evolution of the Primate Hepatitis B Virus

Recent interest in the origins and subsequent evolution of the hepatitis B virus (HBV) has strengthened with the discovery of ancient HBV sequences in fossilized remains of humans dating back to the Neolithic period around 7,000 years ago. Metagenomic analysis identified a number of African non-human primate HBV sequences in the oldest samples collected, indicating that human HBV may have at some stage, evolved in Africa following zoonotic transmissions from higher primates. Ancestral genotype A and D isolates were also discovered from the Bronze Age, not in Africa but rather Eurasia, implying a more complex evolutionary and migratory history for HBV than previously recognized. Most full-length ancient HBV sequences exhibited features of inter genotypic recombination, confirming the importance of recombination and the mutation rate of the error-prone viral replicase as drivers for successful HBV evolution. A model for the origin and evolution of HBV is proposed, which includes multiple cross-species transmissions and favors subsequent recombination events that result in a pathogen and can successfully transmit and cause persistent infection in the primate host.

A hepatitis B virus causes chronic infections in equids worldwide

Preclinical testing of novel therapeutics for chronic hepatitis B (CHB) requires suitable animal models. Equids host homologs of hepatitis C virus (HCV). Because coinfections of hepatitis B virus (HBV) and HCV occur in humans, we screened 2,917 specimens from equids from five continents for HBV. We discovered a distinct HBV species (Equid HBV, EqHBV) in 3.2% of donkeys and zebras by PCR and antibodies against EqHBV in 5.4% of donkeys and zebras. Molecular, histopathological, and biochemical analyses revealed that infection patterns of EqHBV resembled those of HBV in humans, including hepatotropism, moderate liver damage, evolutionary stasis, and potential horizontal virus transmission. Naturally infected donkeys showed chronic infections resembling CHB with high viral loads of up to 2.6 × 10⁹ mean copies per milliliter serum for >6 mo and weak antibody responses. Antibodies against Equid HCV were codetected in 26.5% of donkeys seropositive for EqHBV, corroborating susceptibility to both hepatitis viruses. Deltavirus pseudotypes carrying EqHBV surface proteins were unable to infect human cells via the HBV receptor NTCP (Na⁺/taurocholate cotransporting polypeptide), suggesting alternative viral entry mechanisms. Both HBV and EqHBV deltavirus pseudotypes infected primary horse hepatocytes in vitro, supporting a broad host range for EqHBV among equids and suggesting that horses might be suitable for EqHBV and HBV infections in vivo. Evolutionary analyses suggested that EqHBV originated in Africa several thousand years ago, commensurate with the domestication of donkeys. In sum, EqHBV naturally infects diverse equids and mimics HBV infection patterns. Equids provide a unique opportunity for preclinical testing of novel therapeutics for CHB and to investigate HBV/HCV interplay upon coinfection.

Chronic maternal hepatitis B virus infection and pregnancy outcome- a single center study in Kunming, China

Background

Chinese population has a high prevalence of chronic hepatitis B virus (HBV) infection, the impact of which on pregnancy outcome remains controversial. A single-center retrospective cohort study was performed in Kunming, a multi-ethnic city in south-western China to examine this issue.

Methods

The singleton pregnancies delivering at ≥28 weeks gestation under our care in 2005–2017 constituted the study cohort. Maternal characteristics and pregnancy outcome were compared between mothers with and without seropositivity for hepatitis B surface antigen (HBsAg) determined at routine antenatal screening.

Results

Among the 49,479 gravidae in the cohort, the 1624 (3.3%) HBsAg seropositive gravidae had a lower incidence of nulliparity (RR 0.963, 95% CI 0.935–0.992) and having received tertiary education (RR 0.829, 95% CI 0.784–0.827). There was no significant difference in the medical history, pregnancy complications, or labor or perinatal outcome, except that HBV carriers had significantly lower incidence of labor induction (RR 0.827, 95% CI 0.714–0.958) and of small-for-gestational age (SGA) infants (RR 0.854, 95% CI 0.734–0.994). On regression analysis, maternal HBV carriage was independently associated with spontaneous labor (aRR 1.231, 95% CI 1.044–1.451) and reduced SGA infants (aRR 0.842, 95% CI 0.712–0.997).

Conclusions

Our 3.3% prevalence of maternal HBV infection was around the lower range determined in the Chinese population. The association with spontaneous labor and reduced SGA infants could have helped to promote the perpetuation of the infection through enhanced survival of the offspring infected at birth, thus explaining the high prevalence in the Chinese population.

HBV evolution and genetic variability: Impact on prevention, treatment and development of antivirals

Hepatitis B virus (HBV) poses a major global health burden with 260 million people being chronically infected and 890,000 dying annually from complications in the course of the infection. HBV is a small enveloped virus with a reverse-transcribed DNA genome that infects hepatocytes and can cause acute and chronic infections of the liver. HBV is endemic in humans and apes representing the prototype member of the viral family Hepadnaviridae and can be divided into 10 genotypes. Hepadnaviruses have been found in all vertebrate classes and constitute an ancient viral family that descended from non-enveloped progenitors more than 360 million years ago. The de novo emergence of the envelope protein gene was accompanied with the liver-tropism and resulted in a tight virus-host association. The oldest HBV genomes so far have been isolated from human remains of the Bronze Age and the Neolithic (~7000 years before present). Despite the remarkable stability of the hepadnaviral genome over geological eras, HBV is able to rapidly evolve within an infected individual under pressure of the immune response or during antiviral treatment. Treatment with currently available antivirals blocking intracellular replication of HBV allows controlling of high viremia and improving liver health during long-term therapy of patients with chronic hepatitis B (CHB), but they are not sufficient to cure the disease. New therapy options that cover all HBV genotypes and emerging viral variants will have to be developed soon. In addition to the antiviral treatment of chronically infected patients, continued efforts to expand the global coverage of the currently available HBV vaccine will be one of the key factors for controlling the rising global spread of HBV. Certain improvements of the vaccine (e.g. inclusion of PreS domains) could counteract known problems such as low or no responsiveness of certain risk groups and waning anti-HBs titers leading to occult infections, especially with HBV genotypes E or F. But even with an optimal vaccine and a cure for hepatitis B, global eradication of HBV would be difficult to achieve because of an existing viral reservoir in primates and bats carrying closely related hepadnaviruses with zoonotic potential.

The evolution and clinical impact of hepatitis B virus genome diversity

The global burden of hepatitis B virus (HBV) is enormous, with 257 million persons chronically infected, resulting in more than 880,000 deaths per year worldwide. HBV exists as nine different genotypes, which differ in disease progression, natural history and response to therapy. HBV is an ancient virus, with the latest reports greatly expanding the host range of the Hepadnaviridae (to include fish and reptiles) and casting new light on the origins and evolution of this viral family. Although there is an effective preventive vaccine, there is no cure for chronic hepatitis B, largely owing to the persistence of a viral minichromosome that is not targeted by current therapies. HBV persistence is also facilitated through aberrant host immune responses, possibly due to the diverse intra-host viral populations that can respond to host-mounted and therapeutic selection pressures. This Review summarizes current knowledge on the influence of HBV diversity on disease progression and treatment response and the potential effect on new HBV therapies in the pipeline. The mechanisms by which HBV diversity can occur both within the individual host and at a population level are also discussed.

Prisoners of war - host adaptation and its constraints on virus evolution

Recent discoveries of contemporary genotypes of hepatitis B virus and parvovirus B19 in ancient human remains demonstrate that little genetic change has occurred in these viruses over 4,500-6,000 years. Endogenous viral elements in host genomes provide separate evidence that viruses similar to many major contemporary groups circulated 100 million years ago or earlier. In this Opinion article, we argue that the extraordinary conservation of virus genome sequences is best explained by a niche-filling model in which fitness optimization is rapidly achieved in their specific hosts. Whereas short-term substitution rates reflect the accumulation of tolerated sequence changes within adapted genomes, longer-term rates increasingly resemble those of their hosts as the evolving niche moulds and effectively imprisons the virus in co-adapted virus-host relationships. Contrastingly, viruses that jump hosts undergo strong and stringent adaptive selection as they maximize their fit to their new niche. This adaptive capability may paradoxically create evolutionary stasis in long-term host relationships. While viruses can evolve and adapt rapidly, their hosts may ultimately shape their longer-term evolution.

Tracing Ancient Human Migrations into Sahul Using Hepatitis B Virus Genomes

The entry point and timing of ancient human migration into continental Sahul (the combined landmass of Australia, New Guinea, and Tasmania) are subject to debate. Unique strains of hepatitis B virus (HBV) are endemic among modern-day Australian Aboriginals (HBV/C4) and Indigenous Melanesians (HBV/C3). We postulated that HBV genomes could be used to infer human population movements because the main HBV transmission route in endemic populations is via mother-to-child for genotypes B and C infections. Phylogenetic and phylogeographic analyses of HBV genomes inferred the origin of HBV/C4 to be >59 thousand years ago (ka) (95% HPD: 34-85 ka), and most likely to have occurred on the Sunda Shelf (southeast extension of the continental shelf of Southeast Asia). Our analysis further suggested an age of >51 ka (95% Highest Posterior Density (HPD): 36-67 ka) for the most recent common ancestor of HBV/C4 in Australia, correlating with the arrival time of anatomically modern humans into Australia, with the entry point suggested along a southern route via Timor. While we also inferred the origin of HBC/C3 to be on the Sunda Shelf, our analyses suggested that it was carried into Melanesia by Indigenous Melanesians who migrated through New Guinea north of the highlands. These findings reveal that HBV genomes can be used to infer ancient human population movements.

Evolutionary biology of human hepatitis viruses

Hepatitis viruses are major threats to human health. During the last decade, highly diverse viruses related to human hepatitis viruses were found in animals other than primates. Herein, we describe both surprising conservation and striking differences of the unique biological properties and infection patterns of human hepatitis viruses and their animal homologues, including transmission routes, liver tropism, oncogenesis, chronicity, pathogenesis and envelopment. We discuss the potential for translation of newly discovered hepatitis viruses into preclinical animal models for drug testing, studies on pathogenesis and vaccine development. Finally, we re-evaluate the evolutionary origins of human hepatitis viruses and discuss the past and present zoonotic potential of their animal homologues.

Insights From Deep Sequencing of the HBV Genome-Unique, Tiny, and Misunderstood

Hepatitis B virus (HBV) is a unique, tiny, partially double-stranded, reverse-transcribing DNA virus with proteins encoded by multiple overlapping reading frames. The substitution rate is surprisingly high for a DNA virus, but lower than that of other reverse transcribing organisms. More than 260 million people worldwide have chronic HBV infection, which causes 0.8 million deaths a year. Because of the high burden of disease, international health agencies have set the goal of eliminating HBV infection by 2030. Nonetheless, the intriguing HBV genome has not been well characterized. We summarize data on the HBV genome structure and replication cycle, explain and quantify diversity within and among infected individuals, and discuss advances that can be offered by application of next-generation sequencing technology. In-depth HBV genome analyses could increase our understanding of disease pathogenesis and allow us to better predict patient outcomes, optimize treatment, and develop new therapeutics.

Ancient hepatitis B viruses from the Bronze Age to the Medieval period

Hepatitis B virus (HBV) is a major cause of human hepatitis. There is considerable uncertainty about the timescale of its evolution and its association with humans. Here we present 12 full or partial ancient HBV genomes that are between approximately 0.8 and 4.5 thousand years old. The ancient sequences group either within or in a sister relationship with extant human or other ape HBV clades. Generally, the genome properties follow those of modern HBV. The root of the HBV tree is projected to between 8.6 and 20.9 thousand years ago, and we estimate a substitution rate of 8.04 × 10^-6-1.51 × 10^-5 nucleotide substitutions per site per year. In several cases, the geographical locations of the ancient genotypes do not match present-day distributions. Genotypes that today are typical of Africa and Asia, and a subgenotype from India, are shown to have an early Eurasian presence. The geographical and temporal patterns that we observe in ancient and modern HBV genotypes are compatible with well-documented human migrations during the Bronze and Iron Ages^1,2. We provide evidence for the creation of HBV genotype A via recombination, and for a long-term association of modern HBV genotypes with humans, including the discovery of a human genotype that is now extinct. These data expose a complexity of HBV evolution that is not evident when considering modern sequences alone.

Tracing hepatitis B virus (HBV) genotype B5 (formerly B6) evolutionary history in the circumpolar Arctic through phylogeographic modelling

Background

Indigenous populations of the circumpolar Arctic are considered to be endemically infected (>2% prevalence) with hepatitis B virus (HBV), with subgenotype B5 (formerly B6) unique to these populations. The distinctive properties of HBV/B5, including high nucleotide diversity yet no significant liver disease, suggest virus adaptation through long-term host-pathogen association.

Methods

To investigate the origin and evolutionary spread of HBV/B5 into the circumpolar Arctic, fifty-seven partial and full genome sequences from Alaska, Canada and Greenland, having known location and sampling dates spanning 40 years, were phylogeographically investigated by Bayesian analysis (BEAST 2) using a reversible-jump-based substitution model and a clock rate estimated at 4.1 × 10⁻⁵ substitutions/site/year.

Results

Following an initial divergence from an Asian viral ancestor approximately 1954 years before present (YBP; 95% highest probability density interval [1188, 2901]), HBV/B5 coalescence occurred almost 1000 years later. Surprisingly, the HBV/B5 ancestor appears to locate first to Greenland in a rapid coastal route progression based on the landscape aware geographic model, with subsequent B5 evolution and spread westward. Bayesian skyline plot analysis demonstrated an HBV/B5 population expansion occurring approximately 400 YBP, coinciding with the disruption of the Neo-Eskimo Thule culture into more heterogeneous and regionally distinct Inuit populations throughout the North American Arctic.

Discussion

HBV/B5 origin and spread appears to occur coincident with the movement of Neo-Eskimo (Inuit) populations within the past 1000 years, further supporting the hypothesis of HBV/host co-expansion, and illustrating the concept of host-pathogen adaptation and balance.

Pregnancy and maternal chronic hepatitis B infection-Evidence of reproductive advantage?

PROBLEM

As multiparas have high prevalence of chronic hepatitis B virus (HBV) infection, we examined here the relationship between the number of pregnancies with HBV infection.

METHOD OF STUDY

Retrospective cohort study examining the prevalence of HBV infection by actual gravidity and parity in 104 242 gravidae managed during 1997-2013.

RESULTS

Infection rate increased from 8.5% to 10.6% for G1 to G≥6 and from 8.8% to 10.0% for P0 to P≥3 (P<.001). When stratified by parity, correlation with gravidity was maintained in the nulliparous gravidae. For the same gravidity, increasing parity was associated with higher rate of HBV infection for G2 and G3. Multiparas had higher HBV infection prevalence (all >10%) than nulliparas (<10%) for G2 to G≥4.

CONCLUSION

Prior pregnancies, especially successful ones, are associated with increased HBV infection in an endemic population, which could have enhanced reproduction and in the process facilitated its transmission to the following generations.

Overview of hepatitis B viral replication and genetic variability

Chronic infection with hepatitis B virus (HBV) greatly increases the risk for liver cirrhosis and hepatocellular carcinoma (HCC). HBV isolates worldwide can be divided into ten genotypes. Moreover, the immune clearance phase selects for mutations in different parts of the viral genome. The outcome of HBV infection is shaped by the complex interplay of the mode of transmission, host genetic factors, viral genotype and adaptive mutations, as well as environmental factors. Core promoter mutations and mutations abolishing hepatitis B e antigen (HBeAg) expression have been implicated in acute liver failure, while genotypes B, C, subgenotype A1, core promoter mutations, preS deletions, C-terminal truncation of envelope proteins, and spliced pregenomic RNA are associated with HCC development. Our efforts to treat and prevent HBV infection are hampered by the emergence of drug resistant mutants and vaccine escape mutants. This paper provides an overview of the HBV life cycle, followed by review of HBV genotypes and mutants in terms of their biological properties and clinical significance.

Bat hepadnaviruses and the origins of primate hepatitis B viruses

The origin of primate HBV (family Hepadnaviridae) is unknown. Hepadnaviruses are ancient pathogens and may have been associated with old mammalian lineages like bats for prolonged time. Indeed, the genetic diversity of bat hepadnaviruses exceeds that of extant hepadnaviruses in other host orders, suggesting a long evolution of hepadnaviruses in bats. Strikingly, a recently detected New World bat hepadnavirus is antigenically related to HBV and can infect human hepatocytes. Together with genetically diverse hepadnaviruses from New World rodents and a non-human primate, these viruses argue for a New World origin of ancestral orthohepadnaviruses. Multiple host switches of bat and primate viruses are evident and bats are likely sources of ancestral hepadnaviruses acquired by primates.

Origins and Evolution of Hepatitis B Virus and Hepatitis D Virus

Members of the family Hepadnaviridae fall into two subgroups: mammalian and avian. The detection of endogenous avian hepadnavirus DNA integrated into the genomes of zebra finches has revealed a deep evolutionary origin of hepadnaviruses that was not previously recognized, dating back at least 40 million and possibly >80 million years ago. The nonprimate mammalian members of the Hepadnaviridae include the woodchuck hepatitis virus (WHV), the ground squirrel hepatitis virus, and arctic squirrel hepatitis virus, as well as a number of members of the recently described bat hepatitis virus. The identification of hepatitis B viruses (HBVs) in higher primates, such as chimpanzee, gorilla, orangutan, and gibbons that cluster with the human HBV, as well as a number of recombinant forms between humans and primates, further implies a more complex origin of this virus. We discuss the current theories of the origin and evolution of HBV and propose a model that includes cross-species transmissions and subsequent recombination events on a genetic backbone of genotype C HBV infection. The hepatitis delta virus (HDV) is a defective RNA virus requiring the presence of the HBV for the completion of its life cycle. The origins of this virus remain unknown, although some recent studies have suggested an ancient African radiation. The age of the association between HDV and HBV is also unknown.

Clinical relevance of hepatitis B virus variants

The hepatitis B virus (HBV) is a global public health problem with more than 240 million people chronically infected worldwide, who are at risk for end-stage liver disease and hepatocellular carcinoma. There are an estimated 600000 deaths annually from complications of HBV-related liver disease. Antiviral therapy with nucleos/tide analogs (NA) targeting the HBV polymerase (P) can inhibit disease progression by long-term suppression of HBV replication. However, treatment may fail with first generation NA therapy due to the emergence of drug-resistant mutants, as well as incomplete medication adherence. The HBV replicates via an error-prone reverse transcriptase leading to quasispecies. Due to overlapping open reading frames mutations within the HBV P can cause concomitant changes in the HBV surface gene (S) and vice versa. HBV quasispecies diversity is associated with response to antiviral therapy, disease severity and long-term clinical outcomes. Specific mutants have been associated with antiviral drug resistance, immune escape, liver fibrosis development and tumorgenesis. An understanding of HBV variants and their clinical relevance may be important for monitoring chronic hepatitis B disease progression and treatment response. In this review, we will discuss HBV molecular virology, mechanism of variant development, and their potential clinical impact.

Specific mutations in the C-terminus domain of HBV surface antigen significantly correlate with low level of serum HBV-DNA in patients with chronic HBV infection

BACKGROUND

To define HBsAg-mutations correlated with different serum HBV-DNA levels in HBV chronically-infected drug-naive patients.

METHODS

This study included 187 patients stratified into the following ranges of serum HBV-DNA:12-2000 IU/ml, 2000-100,000 IU/ml, and >100,000 IU/ml. HBsAg-mutations were associated with HBV-DNA levels by applying a Bayesian-Partitional-Model and Fisher-exact test. Mutant and wild-type HBV genotype-D genomes were expressed in Huh7 cells and HBsAg-production was determined in cell-supernatants at 3 days-post-transfection.

RESULTS

Specific HBsAg-mutations (M197T,-S204N-Y206C/H-F220L) were significantly correlated with serum HBV-DNA <2000 IU/ml (posterior-probability>90%, P < 0.05). The presence of Y206C/H and/or F220L was also associated with lower median (IQR) HBsAg-levels and lower median (IQR) transaminases (for HBsAg:250[115-840] IU/ml for Y206C/H and/or F220L versus 4300[640-11,838] IU/ml for wild-type, P = 0.023; for ALT:28[21-40] IU/ml versus 53[34-90] IU/ml, P < 0.001). These mutations were localized in the HBsAg C-terminus, known to be involved in virion and/or HBsAg secretion. The co-occurrence of Y206C + F220L was found significant by cluster-analysis, (P = 0.02). In addition, in an in-vitro model Y206C + F220L determined a 2.8-3.3 fold-reduction of HBsAg-amount released in supernatants compared to single mutants and wt (Y206C + F220L = 5,679 IU/ml; Y206H = 16,305 IU/ml; F220L = 18,368 IU/ml; Y206C = 18,680 IU/ml; wt = 14,280 IU/ml, P < 0.05).

CONCLUSIONS

Specific HBsAg-mutations (compartmentalized in the HBsAg C-terminus) correlated with low-serum HBV-DNA and HBsAg-levels. These findings can be important to understand mechanisms underlying low HBV replicative potential including the inactive-carrier state.

Genotypes and genetic variability of hepatitis B virus

Sequence heterogeneity is a feature of hepatitis B virus (HBV), the prototype member of the family Hepadnaviridae. Based on an intergroup divergence of greater than 7.5% across the complete genome, HBV has been classified phylogenetically into 9 genotypes, A-I, with a putative 10th genotype 'J', isolated from a single individual. With between approximately 4 and 8% intergroup nucleotide divergence across the complete genome and good bootstrap support, genotypes A-D, F, H, and I are classified further into subgenotypes. There is a broad and highly statistically significant correlation between serological subtypes and genotypes, and in some cases, serological subtypes can be used to differentiate subgenotypes. The genotypes, and certain subgenotypes, have distinct geographical distributions and are important in both the clinical manifestation of infection and response to antiviral therapy. HBV genotypes/subgenotypes and genetic variability of HBV are useful in epidemiological and transmission studies, tracing human migrations, and in predicting the risk for the development of severe liver disease and response to antiviral therapy. Moreover, knowledge of the genotype/subgenotype is important in implementing preventative strategies. Thus, it is crucial that new strains are correctly assigned to their respective genotype/subgenotype and consistent, unambiguous, and generally accepted nomenclature is utilized.

Clinical application of estimating hepatitis B virus quasispecies complexity by massive sequencing: correlation between natural evolution and on-treatment evolution

AIM

To evaluate HBV quasispecies (QA) complexity in the preCore/Core regions in relation to HBeAg status, and explore QA changes under natural evolution and nucleoside analogue (NUC) treatment.

METHODS

Ultra-deep pyrosequencing of HBV preCore/Core regions in 30 sequential samples (baseline [diagnosis], treatment-free, and treatment-nonresponse) from 10 retrospectively selected patients grouped according to HBeAg status over time: HBeAg+ (N = 4), HBeAg- (N = 2), and fluctuating HBeAg (transient seroreversion/seroconversion pattern) (N = 4). QA complexity was defined by Shannon entropy, mutation frequency, nucleotide diversity, and mutation frequency of amino acids (MfAA) in preCore and Core.

RESULTS

The QA was less complex in HBeAg+ than in HBeAg- or fluctuating HBeAg. High complexity in preCore was associated with decreased viral replication (preCore MfAA negatively correlated with HBV-DNA, p = 0.005). QA complexity in the treatment-free period negatively correlated with values seen during treatment. Specific variants were mainly selected in the Core region in HBeAg- and fluctuating HBeAg patients, suggesting higher immune pressure than in HBeAg+.

CONCLUSIONS

The negative correlation between QA natural evolution and on-treatment evolution indicates the importance of pre-treatment QA study to predict QA changes in NUC nonresponders. Study of QA complexity could be useful for managing HBV infection.

Possible origins and evolution of the hepatitis B virus (HBV)

All members of the family Hepadnaviridae are primarily viruses which contain double-stranded DNA genomes that are replicated via reverse transcription of a pregenomic RNA template. There are two subgroups within this family: mammalian and avian. The avian member's include the duck hepatitis B virus (DHBV), heron hepatitis B virus, Ross goose hepatitis B virus, stork hepatitis B virus and the recently identified parrot hepatitis B virus. More recently, the detection of endogenous avian hepadnavirus DNA integrated into the genomes of zebra finches has revealed a deep evolutionary origin of hepadnaviruses that was not previously recognised, dating back over 40 million years ago. The non-primate mammalian members of the Hepadnaviridae include the woodchuck hepatitis virus (WHV), the ground squirrel hepatitis virus and arctic squirrel virus, as well as the recently described bat hepatitis virus. The identification of hepatitis B virus (HBV) in higher primates such as chimpanzee, gorilla, orangutan, and gibbons that cluster with the human genotypes further implies a more complex origin of this virus. By studying the molecular epidemiology of HBV in indigenous and relict populations in Asia-Pacific we propose a model for the origin and evolution of HBV that involves multiple cross-species transmissions and subsequent recombination events on a background of genotype C HBV infection.