Hepatitis A virus
The Picornaviridae are small, nonenveloped, single-stranded RNA viruses. Human pathogens include species in the genera Rhinovirus (human rhinoviruses) and Enterovirus (poliovirus, coxsackieviruses, echoviruses, and human enteroviruses). Although HAV shares some major characteristics with other genera of the picornavirus family, it is sufficiently different that it is classified as the only species in the genus Hepatovirus. There are naturally occurring strains that infect nonhuman primates (three genotypes) as well as four genotypes that comprise the human-infectious viruses 148. The strains belonging to each genotype have 
85% nucleotide identity. Most human strains belong to either genotype I or III. The prototypic laboratory strains HM175, originally isolated in Melbourne, Australia, and CR326, from Costa Rica, are closely related genotype I strains 148. HAV, unlike other members of the Picornaviridae family, is stable at pH 1 and resistant to heat (56°C for 30 min) and shows no cross-hybridization with enteroviruses, rhinoviruses, or other picornaviruses. Details of these characteristics are amply referenced in standard texts 115.
Genome Organization
The organization of the HAV genome is similar to that of the other picornaviruses 115. The positive-sense (i.e., translatable), single-stranded RNA is 7.5 kb in length and consists of a 5' noncoding region (NCR) of 734 to 740 nucleotides, a coding region of 2,225 to 2,227 nucleotides, and a 3' noncoding region of 40 to 80 nucleotides 115. The secondary structure of the 5' NCR is important in translation initiation. The Picornaviridae RNA genomes lack the cap assembly found at the 5' end of mRNA species that normally guides the ribosomal complex to the translation start site. An internal ribosome entry site formed by the 5' NCR functions to initiate translation in the picornaviruses, including HAV. The 5' NCR of HCV, another single-stranded, positive-sense, uncapped RNA genome, also includes an internal ribosome entry site.
Proteins
Although HAV was first successfully adapted to cell culture 20 years ago 201, its protein constituents have not been completely defined 115. Infected cells contain only low titers of virus, and consequently protein chemistry has been limited. The P1 region encodes the three major proteins of the viral capsid, VP1, VP2, and VP3. A fourth viral capsid protein (VP4), essential for virion formation 199, is not detected in mature viral particles. Each of the capsid proteins is cleaved from the precursor polyprotein by the viral protease 3C, encoded in the P3 region. The native conformation of the capsid proteins VP1 and VP3 forms a single, dominant, serologic epitope on the viral capsid and elicits a neutralizing antibody response. Nonstructural proteins encoded in the P2 and P3 regions are predicted to function in RNA synthesis and virion formation. VPg (virion protein, genome linked), also encoded in the P3 region, is covalently linked to the 5' genome terminus and involved in initiation of RNA synthesis.
Life Cycle
Available data as to the exact fate of virions immediately after oral intake are sketchy (see Fig. 1). In experimental infection of owl monkeys with human HAV, viral antigen was detectable by immunofluorescence in the stomach, small intestine, and large intestine not only after the initial oral inoculation but also later in the course of the disease 20. The ability to detect viral antigens in intestinal crypt cells using immunofluorescence suggests that viral replication can occur in the intestine 20. Virions presumably reach the liver in the portal blood (or after systemic circulation) and are taken up by hepatocytes. An attachment receptor for HAV in nonliver primate cells has been characterized 21, 77, 131; however, the relationship of this mucin-like class I integral membrane glycoprotein to hepatocyte uptake of virus is not clear. Once HAV has replicated in the liver and been released into bile (see below), the enterohepatic cycle of gastrointestinal uptake and transfer to the liver could continue until neutralizing or other antibodies interrupted the cycle.
Replication
Current evidence indicates that HAV replication is probably exclusive to hepatocytes and gastrointestinal epithelial cells in vivo, although cell culture infection and replication in nonhepatocyte cell lines are well documented 115. Virus-encoded proteins replicate the RNA genome via a negative-strand intermediate and are themselves synthesized from the genomic positive strand. Intact virions contain the RNA genome, the covalently linked VPg protein, and a capsid of the coat proteins VP1, VP2, and VP3 with icosahedral symmetry. Virus particles appear in bile and blood, presumably being released across the apical hepatocyte membrane into the biliary canaliculus and across the basolateral membrane into the bloodstream. The mechanism of viral release and secretion is unknown but clearly is not dependent on cell destruction, since high viral titers are present in stool before there is any evidence of hepatocyte necrosis 51, 139, 245.
Detection
HAV was first visualized after aggregation of fecal material with serum containing specific homologous antibodies 78. The fecal material was collected from Joliet prison volunteers 34 inoculated with the MS-1 strain of hepatitis virus characterized by Krugman and colleagues 138. The technique of immune electron microscopy of stool was then used to assay for specific anti-HAV antibodies in convalescent-phase sera after episodes of naturally occurring hepatitis and to investigate the transmission of virus 67, 70. HAV can now be detected by a variety of immunologic and molecular techniques, including radioimmunoassay (RIA), DNA-RNA hybridization 245, and reverse transcriptase PCR (RT-PCR) amplification. RT-PCR amplification was used to identify specific viral strains implicated in parenteral transmission of virus 7, 159.
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