Hepatitis B virus (HBV) infection is a global health problem with more than 240 million people chronically infected worldwide and 15 million of them co-infected with Hepatitis D virus (HDV) [1, 2]. Chronic HBV infection is an important cause of liver-related complications including cirrhosis, hepatic failure and hepatocellular carcinoma [3]. Based on the diversity in the complete HBV genomic sequence, 10 HBV genotypes, A to J, and numerous sub-genotypes have been described to date [4, 5].

Specific HBV genotypes have an impact on the clinical course of disease: for instance, genotype C infection is associated with more severe liver damage and faster progression to hepatocellular carcinoma in comparison to genotype B [6]. Furthermore, several studies showed that the different genotypes may vary in their ability to induce chronic hepatitis and in their treatment response to interferon [6]. Of note, HBeAg positive individuals infected with the two most prevalent European genotypes, A and D, show a different response to interferon: Genotype A infected individuals are more likely to have a sustained response to interferon treatment compared to those with genotype D [6, 7].

The geographic distribution of HBV genotypes is well characterized. In Europe the genotypes A (Northern- and Eastern Europe) and D (Mediterranean basin) are predominant, whereas genotypes B and C are most prevalent in Asia, and A, and E in sub-Saharan Africa [4, 8]. We hypothesized that Switzerland, which is located in the heart of Europe, could be the geographical transition zone between the regions where genotype D and A are predominant. The prevalence of Hepatitis B surface antigen (HBsAg) carriers in the Swiss general population is estimated to be 0.3 % [9], but the HBV genotype distribution and the proportion of patients co-infected with HDV is unknown. In this study we assessed the HBV genotype distribution of patients living in Switzerland by analyzing samples of a large reference laboratory in Switzerland and evaluated the association between genotypes and clinical, as well as virological characteristics, including HBeAg-positivity and serological evidence of HDV infection.

In this large and representative sample of HBV-infected patients living in Switzerland, we showed that HBV-related epidemiological and virological determinants were driven by migration patterns. The largest group of individuals originally originated from the Mediterranean basin and was predominantly infected with HBV genotype D. We found substantial differences in demographic and virological characteristics between groups of individuals of different regions of origin, including HBV genotype distribution and proportion of viral co-infections. The Swiss native population showed the typical genotype distribution of Northern European countries where patients are most commonly infected with genotype A. Serological evidence of HDV infection was surprisingly low, except for patients of sub-Saharan African origin, of which 16 % were anti-HDV positive. In line with other European studies, older patients were least likely to have a positive HBeAg.

Although only 23.8 % of the population in Switzerland consists of immigrants [16], patients originating from the Mediterranean basin, sub-Saharan Africa as well as Asia represented over 84 % of our study population. The overall HBV genotype distribution showed a typical Mediterranean pattern with a predominance of genotype D mainly due to the large proportion of patients from Portugal, Italy, Turkey and the Balkan region. The genotype distribution among the Swiss native population differed considerably from that of the overall study population and was comparable to patterns described for Germany, Belgium, Poland, the Czech Republic and Scandinavia [8]. As shown previously for Iceland, our study underlines the role played by migration patterns in shaping the epidemiological and virological landscape of HBV infection in countries with a low HBV prevalence in the domestic population [18]. Considering the strong link between HBV genotype and the natural history of disease as well as the better response to interferon treatment in genotype A compared to genotype D infected patients, a careful evaluation of the region of origin as well as the determination of HBV genotype for all patients might be warranted in countries like Switzerland.

Phylogenetic analyses showed that the Swiss native population was mainly infected with the sub-genotypes A2 and D3. Thus, Switzerland seems to be a geographical transition zone for HBV infection, between Italy, where the sub-genotype D3, is most prevalent [19] and northern European countries, where the sub-genotype A2 predominates [8]. The phylogenetic analyses of genotype D strains were particularly informative: Patients of Turkish origin were predominantly infected with the genotype D1, in line with a recent report on HBV infection in Turkey [20]; Furthermore, in our study, HBV D2 virus seemed to circulate primarily among patients who immigrated from the Balkan region which is consistent with the findings of Zehender et al. in Albania [21]. Interestingly four patients of Asian origin (two Tibetans, one Chinese and one of unknown origin) have clustered in subgenotype D4. This is unusual for HBV strains of this region and was not described before except for a report of C/D4 recombinants from Tibet where 96.8 % of the HBV strains analyzed showed a C/D4 recombinant pattern [22]. Due to the short fragment analyzed in our study one cannot exclude that the four samples would belong to the recombinant C/D4 genotype. Taken together, these results show that several homogeneous clusters of HBV-sub-genotypes have established in Switzerland, owing to migration patterns, and that the Swiss native HBV-infected population has a genotype distribution consistent with its geographical situation.

Hepatitis D co-infection is clinically important because it causes more severe liver disease and accelerated progression to cirrhosis leading to higher rates of hepatic decompensation and mortality compared to HBV mono-infection [23]. The prevalence of HBV/HDV co-infection varies widely across European countries, ranging from 2 % in Great Britain to 46 % in eastern Turkey [24, 25]. The overall decreasing prevalence of HDV-co-infection observed in recent years has been attributed to the decline of chronic HBV carriers following the introduction of vaccination programs [2]. In Switzerland, we found 4.4 % of the chronic HBV-infected patients to be anti-HDV positive, which was slightly lower than the 5.9 % estimated in a recent survey completed by physicians treating HBV-infected patients in 2008 [26]. In contrast to the low overall rate of patients with positive anti-HDV antibodies in our study population, 16 % of the patients from sub-Saharan Africa in our study were anti-HDV positive, in line with reported estimates from West Africa [27].

Seventy percent of our study population was HBeAg-negative. The increasing predominance of HBeAg-negative HBV infections was highlighted in a recent French study [28] and a report from Italy showed that the proportion of HBeAg-negative infections increased from 41 % between 1975 and 1985 to 90 % in the 1990′s [29]. In line with other European studies, older patients and those originating from the Mediterranean basin were less likely to have a positive HBeAg [30]. A possible explanation for the significantly lower rate of HBeAg-positive patients among the Europe and Mediterranean group compared to the Swiss group in our study could be the already previously described short duration of the immune reactive HBeAg positive phase in the Mediterranean population with subsequent HBeAg loss in combination with the high prevalence of precore mutations in this population [30].

This is one of the largest studies evaluating epidemiological and virological patterns of HBV infection in a low prevalence country and the first study assessing the HBV genotype distribution as well as the HDV seroprevalence of HBV-infected patients in Switzerland. The detailed virological and phylogenetic analyses allowed us to describe the main determinants of HBV infection in Switzerland, a country with a highly diverse HBV landscape due to its geographical situation as well as its migration patterns. Our study had several limitations. The individual patient data were collected retrospectively, resulting in partially incomplete information on several key variables such as HIV-coinfection. This is of importance as the epidemiological and clinical determinants of HBV infection are linked to the presence of HIV-coinfection [31]. It also underlines the importance of conducting prospective, large cohort studies dedicated to the study of HBV infection. In addition we did not know the region of origin of 70 patients, representing 16.6 % of the study population. However, as the genotype distribution in this group was similar to that of the overall study population, this should not have had a major impact on the overall results. Instead of using full genome phylogenetic analyses (gold standard) to determine HBV sub-genotypes, we analyzed a fragment of the polymerase/HBsAg using the geno2pheno tool. However, to verify the accuracy of our phylogenetic analysis we inserted reference sequences for every HBV subgenotype. Finally, only serological analyses for HDV status were available, therefore the proportion of patients with replicating HDV infection in our cohort was unknown.

We showed that the molecular epidemiology of HBV infection in low prevalence countries like Switzerland is mainly driven by migration and not by the genotype distribution of the native population. The predominance of genotype D in the overall population can be explained by immigration of patients infected with HBV sub-genotypes D1 and D2 from Turkey and the Balkan region, respectively. The HBV genotype distribution in native Swiss individuals mainly included genotypes A2 and D3, which are most prevalent in neighboring countries. In light of these results and considering the impact of HBV genotypes on the course of disease and treatment response, the region of origin and, ideally genotypes, should be assessed in all patients in order to improve management. To address the most important research questions regarding the impact of HBV genotypes on clinical outcomes and treatment response, large, prospective cohorts of HBV-infected patients need to be established.