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Immunodiagnosis of viral hepatitides A to E and non-A to -E

Primary viral infection of liver cells is caused by several distinct hepatotropic viruses transmitted by oral and parenteral routes. While the clinical, epidemiological, pathological, and immunological aspects of viral hepatitides have many common attributes and subtle differences (39), the etiologies of liver disorders cannot be established on the basis of clinical signs and symptoms or abnormal liver function tests (e.g., elevated alanine aminotransferase [ALT]). Following the discovery of Australia antigen (8), which was subsequently termed hepatitis B surface antigen (HBsAg), the identification and characterization of hepatitis B virus (HBV), hepatitis A virus (HAV), hepatitis D virus (HDV), hepatitis C virus (HCV), and hepatitis E virus (HEV) occurred successively between 1972 and 1992 (46, 60, 61, 63). The virological and immunological properties of these five well-established agents of viral hepatitis are summarized in Table 1. The rapid evolution of the field of viral hepatitis is reviewed periodically at the Triennial International Symposia on Viral Hepatitis, which originated at the University of California, San Francisco, in 1972 (83). The proceedings of the last symposium, which was held in Tokyo in 1993, provide recent and exhaustive information about the hepatitis viruses (56). More than 25,000 papers and 40 books and monographs published during the past 2 decades reflect the pace of advances in the field of viral hepatitis, and yet the progress continues, with recently published reports of non-A, -B, -C, -D, and -E viruses, i.e., hepatitis F virus (HFV), hepatitis G virus (HGV), and three novel hepatitis agents termed GB-A, GB-B, and GB-C viruses (21, 65, 69). HGV and the GB agents may be closely related to one another. GB agents are representatives of two new genera of the family Flaviviridae and are distantly related to HCV (54). The recognition of a variety of immunological and virological markers in patients’ sera enables the establishment of sensitive and specific diagnostic assays for discriminating between various forms of viral hepatitides. While infections with HAV, HEV, and HFV resolve in less than 3 months, HBV, HCV, HDV, and HGV tend to establish persistent infections, which are defined by the presence of serological markers for periods exceeding 6 months. The direct detection of viral genomes by hybridization with cloned DNA or RNA probes has been superseded by the recent advent of PCR for amplification of specific viral gene sequences. However, enzyme immunoassays (EIA) for viral antigens and antibodies formed during the natural history of the host response to hepatotropic viral infection serve as the most practical approach to establishing an immunodiagnosis of viral hepatitis. The purpose of this brief review is to present succinctly a composite picture of hepatitis virology and serology relevant to the laboratory diagnosis of viral hepatitis and liver diseases.