In the setting of acute HBV infection, HBsAg typically becomes detectable 4 to 8 weeks after infection. Shortly thereafter, IgM anti-HBc appears in the blood. Thus, the diagnosis of acute hepatitis B is generally made by the simultaneous detection of HBsAg and IgM anti-HBc. Rarely, acute hepatitis B may be diagnosed during the period when HBsAg titers have declined below detectable levels and anti-HBs have not yet appeared (Fourati and Pawlotsky, 2016). In this scenario, known as the "window period", the diagnosis of acute HBV infection is made based on the presence of positive IgM anti-HBc titers. These patients will thus have isolated anti-HBc as the only marker of acute hepatitis B infection.
The HBV DNA titers are extremely high early in acute infection, often reaching 200 million IU/mL (1billion copies/ml) (Whalley et al., 2001). At this stage, HBeAg is also usually detected with accompanied elevation of hepatic aminotransferase levels after HBsAg turns positive (Ganem, 2004).
The differential diagnosis of acute hepatitis B infection includes exacerbation of chronic hepatitis B, super-infection of hepatitis B carriers with hepatitis A, C, or D, and acute hepatotoxicity caused by drugs or other toxins (Lok and McMahon, 2009; Lok and McMahon, 2007). During exacerbations of chronic hepatitis B, anti-HBc IgM may become transiently positive, making differentiation from acute hepatitis B difficult. In such cases, previous test results or a history of recent exposure may assist in the diagnosis of acute hepatitis B. In the other conditions mentioned above, anti-HBc IgM is usually not present (Ganem, 2004). For the category of patient discussed in this case, the initial laboratory workup should include measurement of anti-HBc IgM. If this test is positive, the diagnosis of acute hepatitis B is likely, although an acute exacerbation of chronic hepatitis B cannot be ruled out without further information.
2.3.2 Chronic Immune Tolerant Hepatitis B Virus Infection
Immune tolerant hepatitis B infection refers to the clinical scenario of persistently normal hepatic aminotransferase levels in the presence of high titers of circulating HBeAg and HBV DNA. These patients with immune-tolerant hepatitis B usually have become infected early in life through vertical or early horizontal infection (Belongia et al., 2008; Kottilil et al., 2005). Such infection most often occurs in areas with high rates of endemic infection,
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low rates of maternal screening, and lack of widely available neonatal prophylaxis with HBV vaccine and hepatitis B immune globulin (McMahon, 2009). Clinically, it is important to determine if patients have evidence of immune tolerance, because such patients generally respond poorly to antiviral therapy, and their management differs from patients with more active hepatic inflammation (Dienstag, 2008).
2.3.3 Chronic Immune Active HBV Infection (Wild-Type)
Patients with chronic hepatitis B infection caused by wild-type virus (defined as a naturally-occurring strain without known mutations) have high titers of circulating HBeAg, almost always coupled with high titers of HBV DNA (defined as greater than 200,000 IU/ml). Indeed, prior to the widespread availability of HBV DNA assays, HBeAg was considered to be the principal marker of HBV replication and high infectivity (Hoofnagle, 1981). The laboratory profile of patients with chronic immune active (wild-type) hepatitis B typically shows the following:
• HBsAg positive longer than 6 months
• HBeAg positive, anti-HBe negative
• Serum HBV DNA greater than 200,000 IU/ml
• Intermitently or persistently elevated hepatic aminotransferase levels especially ALT
The natural history for patients with immune active chronic hepatitis B is substantially worse than that of patients in the inactive carrier state. Patients with chronic hepatitis B who become inactive carriers either through spontaneous or treatment-induced HBeAg seroconversion demonstrate improvement in clinical and biochemical evidence of liver disease (Hsu, 2002; McMahon, 2009). Furthermore, patients with HBeAg positive chronic hepatitis B have a greater than six-fold increased relative risk of developing hepatocellular carcinoma compared with HBeAg-negative inactive hepatitis B carriers (relative risk, 60.2 versus 9.6) (Yang et al., 2002). Patients with chronic immune-active HBV infection usually have intermittently elevated, or persistently elevated liver enzymes.
2.3.4 Chronic Immune Active HBV (Pre-core and Core Promoter Mutants)
Some individuals with chronic hepatitis B are infected with a mutant HBV variant that results in HBeAg negative chronic hepatitis. In such patients, viral mutations in the pre-core or pre-core promoter regions prevent HBeAg production in an otherwise normally
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replicating HBV. Thus, these patients typically have high serum HBV DNA levels, but negative HBeAg titers. The prevalence of pre-core or core promoter mutations is highest among persons from Southern Europe and Asia, with prevalence estimates of 60 to 70%
(Lin et al., 2002). Chronic HBeAg-negative hepatitis B is diagnosed in a patient with the following laboratory profile:
• HBsAg positive longer than 6 months
• HBeAg negative, anti-HBe positive
• Serum HBV DNA greater than 20,000 IU/ml
• Persistently elevated or intermittently normal hepatic aminotransferase levels especially ALT
Accurate diagnosis of chronic HBeAg-negative hepatitis B is imperative because the clinical management is markedly different from that of chronic inactive hepatitis B carriers (who are also HBeAg negative but have low serum HBV DNA titers).
2.3.5 Chronic Inactive Carrier of HBV
Chronic inactive hepatitis B infection is defined as persistent HBV infection of the liver without significant ongoing hepatic inflammation and necrosis (Keefe et al., 2008). Blood tests on such patients are typically show four characteristic features:
• HBsAg positive for longer than 6 months
• HBeAg negative, anti-HBe positive
• Serum HBV DNA less than 2,000 IU/ml
• Persistently normal hepatic aminotransferase levels
The "chronic inactive carrier" state may persist for decades. Patients who remain in this phase of infection have lower rates of disease progression and hepatocellular carcinoma (Liaw, 2008). Generally, antiviral treatment is not indicated for such patients, unless histologic or clinical signs of cirrhosis are present (Keefe, 2008).
2.3.6 Occult Hepatitis B Virus Infection
Occult hepatitis B virus (HBV) infection (OBI) is simply defined as serologically undetectable hepatitis B surface antigen (HBsAg-ve), despite the presence of circulating HBV DNA (Allain, 2009; De-Mitri, 2010). OBI was reported for the first time almost 30 years ago in a case report of HBV infection through blood transfusion by an antibody to
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hepatitis B core antigen (anti-HBc) only positive donor (Tabor et al., 1979). The residual risk of HBV transfusion transmission is mainly related to blood donations negative for HBsAg that have been collected either during the ]pre-seroconversion “window period”
(WP), defined as the time between infection and detection of a viral antigen or antibody marker, or during the late stages of infection (Candotti and Allain, 2009). Allain (2004) reported OBI in several clinical contexts including: (1) recovery from past infection indicated by the presence of hepatitis B surface antibody (anti-HBs); (2) chronic hepatitis with surface gene escape mutants that are not recognized by current assays; (3) chronic carriage without any marker of HBV infection other than HBV DNA (referred to as
“seronegative”); and (4) most commonly in endemic areas, chronic carriage stage with HBsAg too low to be detected and recognized by the presence of anti-HBc as the only serological marker (referred to as “anti-HBc alone” or “isolated anti-HBc”) (Allain, 2004).
A true OBI remains HBsAg-negative during the entire course (Gerlich et al., 2010). A 2008 international workshop on occult hepatitis B virus (HBV) infection (OBI), endorsed by the European Association for the Study of the Liver (EASL) (Raimondo et al., 2008), as well as The Taormina Consensus Conference in 2008, defined “OBI” as the “presence of HBV DNA in the liver of individuals testing HBsAg-negative with currently available assays” (Raimondo et al., 2008) and introduced a cutoff value for serum HBV DNA (< 200 IU/mL). Therefore, cases whose serum HBV DNA levels are comparable to those with different serologically evident (overt) HBV infection are generally due to infection with HBV escape mutants and should be labeled as “false” OBI (Raimondo et al., 2010). As confirmed by Hollinger and Sood (Hollinger and Sood, 2010), this definition implies that infectious viral clones may be present. However, the detection of HBV DNA does not always correspond to infectivity or to the number of HBV progeny viruses released from hepatocytes; therefore, the authors suggested a more comprehensive term “occult hepatitis B (OHB)” (Hollinger and Sood, 2010) rather than OBI. Moreover, nosocomial sources should be carefully excluded before speculating that blood donors with OBI were involved in HBV viral disease transmission (Prati etal., 2006).
Possible Mechanisms of OBI
Both host and viral factors are important in suppressing viral replication and keeping the infection under control (Hollinger, 2008; Hollinger and Sood, 2010). The majority of OBI cases is secondary to overt HBV infection and represents a residual low viremia level
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suppressed by strong immune response together with histological derangements occurring during acute or chronic HBV infection (Ozaslan and Purnak, 2009). It was previously suggested that long-term maintenance of an active anti-viral T cell response several years after clinical recovery from acute hepatitis B could be important, not only for protection against reinfection, but also for keeping the persisting virus under control where detection of minute amounts of virus in some recovered subjects was confirmed. Several mechanisms have been postulated for OBI. These include: The low level of viral replication was a result of the presence of defective interfering particles or of mutations in transcription control regions or the polymerase domain leading to decrease in HBV DNA replication and HBsAg expression (Gutierrez et al., 2004; Jeantet et al., 2004;Fang et al., 2009). Humoral and cellular immune pressure on the HBV envelope proteins are major mechanisms generating OBI. Amino acid substitutions are significantly concentrated in the immunologically active parts of the Pre-S/S proteins affecting both cellular CD8 T-cell epitopes and B-cell neutralizing major hydrophilic region epitopes (Candotti et al., 2008).
Escape mutation is one mechanism which also leads to decreased reactivity in HBsAg detection assays (van Hemert et al., 2008; El Chaar, et al., 2010). Masking of HbsAg by HbsAg-anti-HBs immune complexes is another postulated mechanism for the development of OBI (Hu, 2002; Zhang et al., 2007). Also, co-infection with hepatitis delta virus or hepatitis C virus (HCV) which results in down-regulation of HBV replication and a reduction in HBsAg synthesis has been reported. Hollinger and Sood (2010) and Sagnelli et al (2000) showed an inhibitory effect of HCV on HBV replication. This inhibitory activity of HCV on HBV replication has also been reported by other investigators in a follow-up study of 6 years duration, where it was shown that the rate of HBsAg clearance is 2.5 times higher in HBsAg/anti-HCV-positive cases than in those with HBV infection alone; it was suggested that HCV is the most important hepatotropic virus that enhances HBsAg clearance in chronic hepatitis B (Sheen et al., 1994). HCV RNA was a significant predictor for OBI, with an increased frequency of HBV DNA in those who were HBsAg-negative and HCV RNA positive (Said et al., 2009).