Epidemiology, clinical manifestations and diagnosis of hepatitis D virus infection

INTRODUCTION — 

Hepatitis D virus (HDV) infection is caused by a defective virus: the hepatitis D virus. HDV is often referred to as hepatitis delta virus or delta agent. However, the term HDV is preferred. Individuals with hepatitis D are always dually infected with HDV and hepatitis B virus (HBV). Although HDV can replicate autonomously, the simultaneous presence of HBV is required for complete virion assembly and secretion.

This topic review will provide general information concerning the virology, epidemiology, clinical features, and diagnosis of HDV infection. Issues related to treatment, prevention, and liver transplantation are discussed separately. (See "Treatment and prevention of hepatitis D virus infection" and "Liver transplantation in adults: Evaluation and prevention of Hepatitis D virus reinfection in liver transplant recipients".)

VIROLOGY

HDV virion — The hepatitis D virion comprises an RNA genome, a single HDV encoded antigen, and a lipoprotein envelope provided by hepatitis B virus (HBV) (figure 1).

●HDV genome – The HDV genome is a small RNA molecule (1676 to 1683 nucleotides in size) bearing some structural analogies with plant viroids and virusoids [1]. HDV RNA is a single-stranded circle, with a high degree of self-complementarity and G+C content causing the circle to collapse as a rod-like structure [1]. Significant sequence heterogeneity (as high as 40 percent) exists among the different HDV isolates that have been sequenced, and a classification into eight HDV genotypes has been proposed [2].

●Hepatitis D antigen – The only antigen associated with HDV, the hepatitis D antigen (HDAg), is a structural component of the virion. It consists of a protein encoded by an open reading frame present on the RNA strand complementary to the RNA genome (antigenomic strand) [1]. Approximately 70 molecules of HDAg are complexed with each molecule of HDV RNA to form the viral ribonucleoprotein.

Two forms of HDAg are co-expressed in infected individuals. Each HDAg has a different function as described below. (See 'Pathogenesis' below.)

The two HDAg molecules differ by 19 amino acids at the C-terminus; the molecular weights are approximately 24 and 27 kilodaltons (kd). Their synthesis arises via an RNA editing process during HDV replication [3]. (See 'Pathogenesis' below.)

•The stop codon UAG of the messenger RNA directing the synthesis of the HDAg causes the translation of HDAg to terminate, thereby giving rise to the small HDAg. During HDV RNA replication, this stop codon is deaminated on the antigenomic HDV RNA by a cellular enzyme (the adenosine deaminase acting on RNA 1, ADAR1) to UIC (in which "I" stands for inosine).

•During the next replication cycle, the "I" on the antigenomic RNA is read as a "G," leading to the replacement of the "A" with a "C" in the genomic HDV RNA. During the transcription of the messenger RNA directing the synthesis of the HDAg, the "C" will lead to the replacement of the UAG stop codon with a UGG codon, which directs the incorporation of tryptophan into the nascent HDAg. Translation of HDAg then proceeds until a new stop codon is reached, 19 amino acids downstream, thereby giving rise to the large HDAg.

●Lipoprotein envelope of HDV – The lipoprotein envelope of HDV is provided by the HBV and consists of the same proteins (large, middle, and small S) that are found in the hepatitis B virion; their relative proportion depends upon the level of HBV replication [4].

HDV life cycle — Individuals with hepatitis D are always dually infected with HDV and HBV. Although HDV can replicate autonomously [1], the simultaneous presence of HBV is typically required for complete virion assembly and secretion. Due to interference mechanisms that are not well understood, HBV replication is suppressed in most individuals with HDV infection.

HDV replicates at very high levels in hepatocytes [5]. The sodium taurocholate cotransporting polypeptide (NTCP), which is the receptor for HBV, has also been identified as the receptor for HDV [6,7]. (See "Characteristics of the hepatitis B virus and pathogenesis of infection", section on 'Replication cycle'.)

The steps in the HDV replication cycle can be summarized as follows:

●After entering the hepatocyte, HDV RNA localizes to the nucleus, where it is transcribed into its complementary RNA (antigenomic HDV RNA). Two forms of antigenomic HDV RNA exist: a 0.8 kilobase (kb) RNA, which is the messenger RNA being translated into the HDAg [8], and the full-length 1.7 kb RNA, which is the template directing the transcription back into the HDV genome [1]. The host RNA polymerase II appears to be involved in the transcription of the 0.8 kb mRNA in a process that is regulated by direct binding with the HDAg itself [9,10]. The transcription of the full-length genomic and antigenomic RNAs occurs by the cellular RNA polymerase II [11].

●HDV RNA replication is activated by the small HDAg through direct binding of the HDAg to the HDV RNA.

●The large HDAg suppresses HDV replication. In addition, it directs packaging of the HD virion through an interaction between the extra 19 amino acids at the C-terminal end and the small S protein (surface antigen of the hepatitis B virus [HBsAg]) of the helper HBV [3].

●Completion of the HD virion assembly and release is dependent on the simultaneous presence of HBV which provides the envelope. Without HBV, HDV cannot be packaged and exit the cells.

There have been some case reports of HDV infection occurring in the absence of hepatitis B coinfection in patients with hepatitis C virus (HCV) infection and Sjögren's disease [12,13]. However, these findings have not been confirmed in subsequent studies [14-16]. As an example, one study of 2123 plasma samples positive for anti-HCV antibody found that 1.9 percent of samples tested positive for HDV antibody (anti-HDV) but none were HDV RNA positive, suggesting that HCV may not have a supportive role in HDV spread [14].

However, divergent HDV-like viruses have been detected in fish, birds, amphibians, and invertebrates, without evidence of any HBV-like agent supporting infection with these viruses [17]. Another study found that HDV can be transmitted and propagated in experimental infections ex vivo and in vivo by different enveloped viruses unrelated to HBV, including HCV and flaviviruses such as Dengue and West Nile virus [18].

PATHOGENESIS — 

The detailed mechanisms by which HDV induces liver damage are unknown. However, the pathogenesis of hepatitis D-related liver disease appears to depend on the interplay of three major factors:

●HDV-associated factors, such as genotype [19] and the expression of specific HDV antigen (HDAg) species [20]

●Host-associated factors, such as the immune response

●Helper virus-associated factors, such as the hepatitis B virus (HBV) genotype and the level of HBV replication [21]

HDV is believed to cause direct cytopathic damage during acute infection, whereas immune-mediated damage predominates during chronic infection [1].

EPIDEMIOLOGY — 

Data on HDV epidemiology have mostly been gathered in chronic hepatitis B virus (HBV) carriers superinfected with HDV, in whom HDV infection has progressed to chronicity. HDV antibody (anti-HDV) is present in high titers in these patients, and the prevalence of chronic HDV infection can be reliably determined.

However, the incidence of acute hepatitis D may be underestimated [22]. Although the commercial availability of assays for the detection of anti-HDV has improved our understanding of the epidemiology of HDV infection, these assays have limitations. As an example, in acute hepatitis D, anti-HDV appears very late and may be missed if repeated testing is not performed. This is especially true in immunocompromised individuals (eg, patients with human immunodeficiency virus [HIV]) in whom a strong antibody response to HDV may be delayed or absent. Furthermore, after resolution of acute hepatitis D, anti-HDV may disappear with time. Thus, in some patients, recognition of past HDV infection may be impossible. More detailed discussion of diagnostic testing is presented below. (See 'Diagnosis of HDV infection' below.)

Prevalence of disease in patients with chronic HBV — The prevalence of HDV in patients with chronic HBV infection varies across studies. These variations are likely due to several factors, including lack of high-quality data because of under-testing of HDV in persons with chronic HBV infection, the variable accuracy of HDV screening tests, and the lack of population-based studies. Much of the data describing the epidemiology of HDV are based on studies conducted more than 20 years ago or in high-risk populations, and reflex testing of anti-HDV positive samples for HDV RNA was not performed. Thus, reliable, updated information is not available in many countries, impacting not only prevalence data in individual countries but also studies attempting to estimate global or regional prevalence.

Global burden of diseae — Estimates of the global burden of chronic HDV infection vary widely, ranging from 12 million to 72 million people [23-25]. A 2024 report from the World Health Organization (WHO) suggests the prevalence of HDV infection among HBsAg-positive persons in the general population is 4.5 percent, which is closer to 12 million [26]. The prevalence is higher in those with chronic HBV and advanced liver disease, with the estimated prevalence of HDV being 25.8 and 19.8 percent in patients with cirrhosis or hepatocellular carcinoma, respectively [24].

Geographic distribution — HDV is not distributed uniformly across the globe, and the geographic distribution of HDV infection does not parallel that of HBV. As an example, there is a high prevalence of HDV in the Mediterranean Basin, Mongolia, Moldova, and some countries in Sub-Saharan Africa [25], whereas other areas, including many Asian countries, are relatively spared despite the high prevalence of HBV infection.

●The Mediterranean Basin – HDV infection is endemic in the Mediterranean Basin, where intrafamilial spread is thought to have been prominent in the past. The prevalence of HDV infection declined in the 1990s. In some countries, such as Italy, improvements in socioeconomic conditions, increased awareness of how infectious diseases are transmitted, and aggressive vaccination campaigns against HBV led to a dramatic decrease in the incidence of HBV infection and the spread of HDV infection among young adults through 1999 [27].

However, this decline appears to have stopped, and the prevalence of HDV has remained stable or even increased in the Mediterranean area [28,29] and in Central Europe [30]. Immigration from endemic countries has been suggested to be the cause for this trend [31,32] but is not the only reason. Injection drug use, sexual practices, and body modification procedures may also be involved. (See 'Risk factors in low prevalence countries' below.)

●Asia – The prevalence of HDV infection among persons who are HBsAg-positive in Asia is variable, depending on the geographic area. As an example, the prevalence is low in many countries, such as Japan [22]. However, other countries, especially Mongolia and those in Central Asia, are more heavily affected, with the prevalence among HBV carriers as high as 60 percent in some areas in Pakistan [33]. In a study evaluating 4103 HBsAg-positive sera from China collected between 2012 to 2019, the prevalence of anti-HDV was limited to high-risk persons who inject drugs and geographic hot spots in Mongolia and Xinjiang and was not detected in any of 2364 HBsAg-positive persons in other areas [16].

In Taiwan, studies in the early 1990s found that HDV infection was not infrequent and was predominantly sexually transmitted [34]. However, in a subsequent study that evaluated a hospital-based cohort of patients with chronic HBV infection, the prevalence was only 1.15 percent [35]. In a study from Shanghai, which evaluated 225 serum samples from persons who were HBsAg-positive, anti-HDV was detected in 4.9 percent; however, they only tested hospitalized patients with elevated transaminase levels [36].

●Western countries – HDV infection is uncommon in Western countries and is predominantly confined to high-risk groups [37-39]. (See 'Risk factors in low prevalence countries' below.)’

In the United States, the estimated prevalence of HDV infection in persons with chronic HDV also varies in different studies, ranging from 2.2 to 4.2 percent [37,40]. The study that reported a seroprevalence of 2.2 percent tested residual samples from 5251 patients who were positive for HBsAg in a large reference lab between 2023 and 2024 [40]. In this study, HDV RNA was only detected in 28 percent of the samples that were positive for anti-HDV, indicating a prevalence of active HDV infection of less than 1 percent.

Risk factors in low prevalence countries — In countries with a low HDV prevalence, infection is primarily found among groups with specific risk factors. These include persons who inject drugs, men who have sex with men, people who engage high-risk sexual behaviors (eg, condomless sex with multiple partners), persons who have received multiple blood transfusions, and persons who emigrated from countries with a high prevalence of HDV infection [41,42]. (See 'Geographic distribution' above.)

CLINICAL MANIFESTATIONS — 

Due to its dependence upon hepatitis B virus (HBV), HDV infection always occurs in association with HBV infection. The presentation depends in part upon whether the patient has acute infection (eg, HBV/HDV coinfection or acute HDV superinfection) or chronic HDV. In patients with chronic HDV, clinical manifestations can range from the asymptomatic carrier state to acute liver failure. (See 'Virology' above and 'Pathogenesis' above.)

This section will review the clinical and laboratory findings associated with the different types of HDV infection (table 1). The approach to diagnosing HDV is discussed below. (See 'Diagnosis of HDV infection' below.)

Acute HBV/HDV coinfection — Acute HBV and acute HDV coinfection can occur in individuals susceptible to HBV infection (ie, hepatitis B surface antigen [HBsAg], hepatitis B core antibody, and hepatitis B surface antibody [anti-HBs]-all negative). Clinically, this entity is indistinguishable from classical acute HBV infection and is usually transient and self-limited. (See "Hepatitis B virus: Overview of management", section on 'Acute infection'.)

Most patients with acute HBV/HDV infection recover from both HBV and HDV infection, but acute HBV/HDV infection is associated with a higher risk of acute liver failure than acute HBV monoinfection. The rate of progression to chronic HDV infection is similar to that observed for HBV since the persistence of HDV infection is dependent upon persistence of HBV infection [43].

Acute HDV superinfection — HDV superinfection in a patient with chronic HBV may present as a severe acute hepatitis in a previously unrecognized HBV carrier or as an exacerbation of pre-existing chronic hepatitis B. Progression to chronic HDV infection occurs in almost all patients [44]; however, HBV replication is usually suppressed by HDV. Clinically, the presentation is often indistinguishable from those with acute HBV/HDV infection, but serologic testing may be able to separate the two entities (figure 2).

Chronic HDV infection — Once chronic HDV infection is established, it usually exacerbates the pre-existing liver disease due to HBV. In general, chronic HDV infection is associated with more rapid progression to cirrhosis and an increased risk of hepatocellular carcinoma [45].

●Cirrhosis – In patients with chronic HDV, the initial progression towards cirrhosis tends to be rapid [46,47] and is generally faster than among patients with HBV monoinfection [48]. Progression from compensated to decompensated cirrhosis is less accelerated but still more rapid than that of HBV monoinfection. This was illustrated in a report from Italy in which the estimated 5- and 10-year probability of survival (free of liver transplantation) in patients who had already developed clinically overt cirrhosis was 49 and 40 percent, respectively [48]. A more detailed discussion of the clinical manifestations of cirrhosis are presented elsewhere. (See "Cirrhosis in adults: Etiologies, clinical manifestations, and diagnosis" and "Cirrhosis in adults: Overview of complications, general management, and prognosis".)

●Hepatocellular carcinoma – There also appears to be an increased risk of hepatocellular carcinoma in patients with chronic HDV. In a meta-analysis that included 93 observational studies, the pooled odds ratio of hepatocellular carcinoma in persons with HBV/HDV coinfection compared to HBV monoinfection was 2.77 (95% CI 1.79-4.28) [49]. This association was seen even after adjusting for a higher prevalence of cirrhosis in those with HBV/HDV coinfection compared with HBV monoinfection; the association was particularly strong in prospective cohort studies. These results were subsequently supported by two additional meta-analyses [50,51].

However, the clinical course of chronic HDV can vary, and in some patients HDV-associated chronic liver disease may run an indolent course [49]. The clinical course is influenced by several factors, such as persistent HDV replication and HDV genotype. In a study of 299 patients with hepatitis D infection who were followed for up to 28 years, persistent HDV replication was associated with annual rates of development of cirrhosis and hepatocellular carcinoma of 4 and 2.8 percent, respectively [52]. In this study, the only predictor of liver-related mortality was persistent HDV replication. The importance of genotypes is discussed below. (See 'Importance of HDV genotypes' below.)

Importance of HDV genotypes — Studies have demonstrated that clinical outcomes may be related to the different HDV genotypes [19,53,54], which seem to cluster in distinct geographical areas. However, superinfection, or mixed infection with different genotypes, can occur, particularly in patients who are at high risk for multiple exposures. In such patients, a single genotype usually predominates, with the minor genotype representing only approximately 10 percent of the total viral population [55].

●Genotype 1 – In the Western world, where the predominant genotype is genotype 1, the clinical presentation can vary [54]. As an example, patients with genotype 1 infection with acute HDV infection have an increased risk of acute liver failure when compared with those with acute hepatitis B [56]. In addition, once chronic HDV infection is established, it usually exacerbates the preexisting liver disease due to HBV [44], and progression towards cirrhosis may be rapid [46,47]. However, HDV-associated chronic liver disease may also run an indolent course [57], and asymptomatic HDV carriers have been found in some geographical areas [58]. (See 'Geographic distribution' above.)

Many patients who are referred for evaluation of HDV were infected years ago; in such persons, HDV-related disease rapidly progressed to cirrhosis, but then subsequent disease progression was slow. This was illustrated in a report from Italy in which the estimated 5- and 10-year probability of survival free of liver transplantation in patients who had already developed clinically overt cirrhosis was 49 and 40 percent, respectively [48]. However, patients with active HBV and HDV replication are more likely to develop liver decompensation [21].

Whether superimposed HDV infection accelerates the development of hepatocellular carcinoma in patients with HBsAg-positive cirrhosis is controversial. However, a retrospective study involving 200 patients with compensated HBV-related cirrhosis, of whom 20 percent were HDV antibody (anti-HDV) positive, found that HDV infection increased the risk of hepatocellular carcinoma threefold and mortality twofold [59]. After adjustment for clinical and serological differences at baseline, the estimated five-year risk for developing hepatocellular carcinoma was 13, 4, and 2 percent for anti-HDV-positive/hepatitis B e antigen (HBeAg)-negative, anti-HDV-negative/HBeAg-negative, and anti-HDV-negative/HBeAg-positive patients, respectively. The corresponding figures for hepatic decompensation were 18, 8, and 14 percent, respectively. In addition, a recent meta-analysis found that patients with superimposed HDV infection had a higher risk of hepatocellular carcinoma compared to those with HBV monoinfection.

●Genotype 2 – In the Far East, where the predominant genotype is genotype 2, the risk of acute liver failure and rapidly progressive liver disease is low compared to other HDV genotypes [53,60].

●Genotype 3 – Severe outbreaks of acute hepatitis D with a high incidence of acute liver failure have been reported in the Yukpa Indians of Venezuela [61]; the Sierra Nevada de Santa Marta in Colombia [62]; and some remote areas of the Brazilian [63], Peruvian [19], and Amazon basin. Viral factors have been postulated to be related to the fulminant course in these outbreaks, as HDV isolates from Colombia and Peru belong to a distinct viral genotype denoted genotype 3 [19].

●Other genotypes – At least five additional HDV genotypes have been described [2,64]. Sequences previously assigned to genotype 2b are now classified as genotype 4, and African sequences seem to cluster into four additional genotypes, named from 5 to 8. The clinical features of these newer genotypes are less well characterized compared with genotypes 1 to 3. In a study from the United Kingdom, infection with genotype 5 was reported to be associated with a better prognosis (ie, less episodes of hepatic decompensation) than genotype 1 [65]. However, in another study from France, patients with European genotype 1 and African genotype 5 were both at high risk of developing cirrhosis [66].

DIAGNOSIS OF HDV INFECTION — 

Due to the dependence of HDV on hepatitis B virus (HBV), the presence of hepatitis B surface antigen (HBsAg) is necessary for the diagnosis of HDV infection. The additional presence of immunoglobulin (Ig)M antibody to hepatitis B core antigen (IgM anti-HBc) is necessary for the diagnosis of acute HBV/HDV coinfection (table 1).

Diagnostic approach — The diagnostic approach depends upon the clinical scenario.

Acute HBV and risk factors for HDV — In patients with acute HBV infection, testing for HDV coinfection should be performed in those who present with unusually severe or protracted hepatitis and those who have risk factors for HDV. (See 'Risk factors in low prevalence countries' above.)

Patients being evaluated for acute HBV/HDV coinfection should be tested for HBsAg, IgM anti-HBc, serum HDV RNA, and HDV antibody (anti-HDV).

●Those with acute HBV are positive for HBsAg and have high titers of IgM anti-HBc. (See "Hepatitis B virus: Screening and diagnosis in adults", section on 'Acute hepatitis'.)

●For those with HDV coinfection, serum HDV RNA is usually detectable at presentation; by contrast, anti-HDV is often negative at presentation. If HDV RNA testing is not available, follow-up testing for anti-HDV (total or IgM) should be performed to document anti-HDV seroconversion.

Markers of HBV replication may precede or follow those of HDV. In addition, occasional patients have already seroconverted to hepatitis B surface antibody (anti-HBs) if they present during the second phase of biphasic hepatitis. These patients should still be positive for high-titer IgM anti-HBc (table 1).

Persons with chronic HBV and hepatitis flare — HDV superinfection should be evaluated in patients with chronic HBV who have acute hepatitis of unclear etiology. Patients should be tested for serum HDV RNA and anti-HDV.

In persons with previously unrecognized chronic HBV, it can be difficult to determine if the person has HDV superinfection versus HDV coinfection; however, it is important to make this distinction because of the differences in prognosis (table 1). When evaluating the patient, clinicians should be aware that:

●HBsAg is present in both situations, but IgM anti-HBc should be negative in acute HDV superinfection (figure 3).

●HDV superinfection may cause transient suppression of HBV replication, resulting in very low and, rarely, undetectable levels of HBsAg.

●As in patients with acute HBV/HDV coinfection, patients with acute HDV superinfection usually have detectable HDV RNA in serum at the time of presentation. However, in contrast to acute coinfection, acute HDV superinfection is characterized by persistent detection of HDV RNA in serum and rapidly increasing titers of anti-HDV (total and IgM).

Persons with chronic HBV without hepatitis flare — We test all patients with chronic HBV for HDV . Screening is particularly important for persons with risk factors for HDV. In a study from Spain, routine testing for anti-HDV in HBsAg-positive persons increased the number of anti-HDV-positive persons who were identified by a factor of five [67]. Risk factors for HDV are described above. (See 'Risk factors in low prevalence countries' above.)

●Initial testing should be performed by testing for total anti-HDV antibody. (See 'Detection of anti-HDV antibody' below.)

●The presence of HDV infection should be confirmed by testing for serum HDV RNA. (See 'Serum HDV RNA' below.)

Patients with chronic HDV/HBV may also have certain characteristic findings on HBV testing. As an example, serum HBV DNA is usually present at low levels but may be undetectable. In addition, patients are often hepatitis B e antigen (HbeAg) negative and hepatitis B e antibody (anti-Hbe) positive. (See "Hepatitis B virus: Clinical manifestations and natural history", section on 'Phases of chronic HBV infection'.)

Identifying persons with chronic HDV/HBV is important since patients with coinfection may have more severe liver disease, and the presence of HDV can impact treatment decisions. In addition, the information can be used for counseling purposes so patients without HDV infection can reduce behaviors that might put them at risk for HDV superinfection. (See 'Risk factors in low prevalence countries' above and "Treatment and prevention of hepatitis D virus infection".)

Our approach to screening is consistent with European and Asian-Pacific guidelines, which recommend that persons with chronic HBV be evaluated for other causes of chronic liver disease, including HDV [68,69]. However, other guidelines differ. As an example, United States guidelines recommend testing only HBsAg-positive persons at high risk of HDV infection since there are no US Food and Drug Administration (FDA) approved HDV diagnostic tests or treatment and the reliability of tests in reference laboratories is variable [70].

Diagnostic tests

Serum HDV RNA — All patients testing positive for anti-HDV should be tested for the presence of serum HDV RNA to confirm active infection. HDV RNA is detected in serum by reverse transcriptase-polymerase chain reaction (RT-PCR)-based assays (table 1). In a systematic review and meta-analysis, the pooled proportion of anti-HDV-positive persons with a detectable HDV RNA was approximately 60 percent; the remaining ones most likely spontaneously cleared HDV [25].

RT-PCR assays are very sensitive and may detect viral loads as low as 10 genomes per mL [60,71-74]. However, the extensive sequence heterogeneity of different HDV isolates makes it difficult to choose suitable primers for the amplification of HDV RNA since only a few conserved regions exist in the HDV genome. In addition, the secondary and tertiary structure of the HDV RNA may hamper efficient amplification, even of highly conserved regions [74]. The best efficiency is obtained by amplifying the C-terminal half of the HDV antigen (HDAg)-encoding region [74]. Although automated assays are commercially available to allow quantification of HDV RNA in the serum of patients with HDV infection before and during treatment [75,76], none of them have been approved for clinical use by the FDA, and performance of these assays is variable. Furthermore, it appears that differences in sample extracting and processing procedures may also influence accuracy of HDV RNA testing.

HDV genotyping can be performed by direct sequencing [19,53,54,77], but this test is not routinely recommended since the clinical relevance of HDV genotyping is uncertain. Furthermore, the phylogenetic classification of the various HDV genotypes is in debate [2]. (See 'Clinical manifestations' above.)

Sequence analysis of HDV RNA can also be used to detect common sources of infection, such as after intrafamilial or perinatal transmission [78,79]. However, it is unclear if HDV RNA levels in the blood can be used as a prognostic factor to determine disease progression [32].

Detection of anti-HDV antibody — Total (IgM and IgG) anti-HDV antibodies can be detected by enzyme immunoassays (EIAs) or radioimmunoassays (RIAs) (table 1).

●Total anti-HDV antibody usually appears after four weeks of acute infection in acute hepatitis D (figure 2). As a result, its clinical value is limited unless follow-up testing is performed [80,81]. A well-documented anti-HDV seroconversion may be the only way to diagnose acute HDV infection in the absence of other markers of HDV infection. However, limited data suggest anti-HDV is short-lived in patients who do not develop chronic HBV.

●High-titer anti-HDV of the IgG class is present in chronic HDV infection. It correlates well with ongoing HDV replication and may help in differentiating current from past HDV infection [82].

●Anti-HDV of the IgM class can be detected by EIAs or RIAs, but these assays are not available for clinical use in the United States [83]. IgM anti-HDV is transient and delayed if the course of acute hepatitis D is self-limited, but it may be the only serum marker of acute HDV infection [80]. In patients who progress to chronic HDV infection, which is usually the case in those with HDV superinfection, IgM anti-HDV is brisk and long-lasting. It should be remembered, however, that differentiating between HBV/HDV coinfection and HDV superinfection in an HBV carrier relies mainly on the detection of high-titer IgM anti-HBc in patients with coinfection.

●IgM anti-HDV is present in high titers during chronic HDV infection, and the titers correlate with the level of HDV replication and severity of liver disease [83], although HDV replication is best assessed by quantifying HDV RNA in serum. IgM anti-HDV gradually disappears from serum in patients who have persistent remission after interferon therapy and following liver transplantation [84].

Detection of serum HDAg — Serum HDV antigen (HDAg) can be detected by EIA or RIA. However, these assays are rarely used and are not approved for clinical diagnosis in the United States.

●In acute HDV infection, serum HDAg appears early, but its detection by EIA is short-lived and may not be present on follow-up testing (figure 2) [80,81,85,86], except in immunocompromised individuals [87].

●In chronic HDV infection, anti-HDV is present in high titers, but HDAg cannot be detected by EIA since it is complexed with anti-HDV. In this setting, serum HDAg is best detected by immunoblot assay [4], which is more sensitive [88]; however, this assay is technically difficult and time- and labor-consuming.

Tissue markers of HDV infection — Both HDAg and HDV RNA can be detected in liver tissues routinely processed for histopathologic evaluation.

●HDAg can be detected by direct immunofluorescence or immunohistochemistry. Although initially proposed as the "gold" standard for diagnosis of current HDV infection [89], as many as 50 percent of liver biopsy specimens from patients who have been infected for 10 or more years may be negative for HDAg, suggesting that the levels of HDV replication may decrease with time [60,90].

In patients who are negative for HDAg, the diagnosis of current HDV infection has to rely on the detection of HDV RNA or high-titer anti-HDV antibodies in the serum. (See 'Serum HDV RNA' above and 'Detection of anti-HDV antibody' above.)

●HDV RNA can be detected by in-situ hybridization. However, the techniques involved are time-consuming and tedious; they are not recommended for clinical use [91].

SUMMARY AND RECOMMENDATIONS

●Virology – Individuals with hepatitis D are always dually infected with hepatitis D virus (HDV) and hepatitis B virus (HBV). Although HDV can replicate autonomously, the simultaneous presence of HBV is required for complete virion assembly and secretion. Due to mechanisms that are not well understood, HBV replication is suppressed in most HDV-infected individuals.

●Epidemiology

•Prevalence – The prevalence of HDV in patients with chronic HBV infection varies across studies. Studies suggest the prevalence is 4 to 5 percent globally, with an estimated total of 12 million infected persons. (See 'Prevalence of disease in patients with chronic HBV' above.)

•Geographic distribution – HDV is not distributed uniformly across the globe, and the geographic distribution of HDV infection does not parallel that of HBV. As an example, there is a high prevalence in the Mediterranean Basin, Mongolia, Pakistan, Moldova, and some countries in Western and Middle Africa, whereas many Asian countries are relatively spared despite the high prevalence of HBV infection. (See 'Geographic distribution' above.)

•Risk factors for HDV – In low prevalence settings, HDV is predominantly found in groups with selected risk factors for HDV, such as persons who inject drugs, individuals who have received multiple transfusions in the past, men who have sex with men who engage in condomless sex with multiple partners, and persons who emigrated from countries with a high prevalence of HDV infection. (See 'Risk factors in low prevalence countries' above.)

●Clinical manifestations and disease course of acute HDV

•Persons susceptible to HBV – Persons susceptible to HBV may present with acute hepatitis during acute HBV/HDV coinfection. Clinically, this entity is indistinguishable from classical acute HBV infection, most patients will clear both HBV and HDV. (See 'Acute HBV/HDV coinfection' above.)

•Persons with chronic HBV – HDV superinfection occurs in patients with chronic HBV. Progression to chronic HDV infection occurs in almost all cases. The clinical features are indistinguishable from an exacerbation of their underlying HBV. In persons with unrecognized chronic HBV, infection may present as severe acute hepatitis. (See 'Acute HDV superinfection' above.)

●Clinical manifestation of chronic HDV – Patients with chronic HBV/HDV tend to have more active or advanced liver disease, although some may have an indolent course. The clinical course is influenced by several factors, such as persistent HDV replication and HDV genotype. (See 'Chronic HDV infection' above and 'Importance of HDV genotypes' above.)

●Diagnostic approach – The diagnostic approach depends upon the clinical scenario. Diagnostic tests for HDV typically include HDV antibody (anti-HDV) antibody and HDV RNA (table 1). (See 'Diagnostic approach' above and 'Diagnostic tests' above.)

•Persons with acute HBV infection and risk factors for HDV – In patients with acute HBV infection, testing for HDV coinfection should be performed in those who have risk factors for HDV and/or those who present with unusually severe or protracted hepatitis. (See 'Acute HBV and risk factors for HDV' above and "Hepatitis B virus: Screening and diagnosis in adults".)

Patients with acute HBV/HDV coinfection test positive for hepatitis B surface antigen (HBsAg), ant-HBc IgM (hepatitis B core antibody), anti-HDV (HDV antibody), and serum HDV RNA. If testing for HDV RNA is not available, follow-up testing for anti-HDV should be performed if the initial test is negative, as it may be delayed.

•Persons with chronic HBV and hepatitis flare – HDV superinfection should be evaluated in patients with chronic HBV who have acute hepatitis of unclear etiology. (See 'Persons with chronic HBV and hepatitis flare' above.)

Patients with HDV superinfection test positive for serum HDV RNA and anti-HDV. In persons with previously unrecognized chronic HBV, it can be difficult to determine if the person has HDV superinfection versus HDV coinfection, though anti-HBc IgM should be negative in the former and positive in the latter (table 1). Differentiating coinfection from superinfection is important because of the differences in prognosis.

•Persons with chronic HBV without hepatitis flare – We perform routine screening for HDV in all patients with chronic HBV. Initial testing should assess for total anti-HDV antibody; if positive, HDV infection should be confirmed by testing for serum HDV RNA. Identifying persons with chronic HDV/HBV is important because liver disease may progress more rapidly, and the presence of HDV can impact treatment decisions. (See 'Persons with chronic HBV without hepatitis flare' above.)