Structure

HAV is a 27- to 28-nm spherical, nonenveloped virus¹¹ (Fig. 176-1) with a surface structure suggesting icosahedral symmetry.²⁹ High-resolution x-ray crystallography of HAV has not yet been achieved. However, good-quality cryoelectron microscopy has confirmed the icosahedral symmetry of the virion with typical picornavirus threefold and fivefold axes and 60 repeated pentamers comprising the capsid (Fig. 176-2; see Fig. 176-1). The cryoelectron microscopic views of HAV reveal the plateau surrounding the fivefold axis that is typical for picornaviruses, but not the pit or canyon believed to be the receptor-binding site of enteroviruses and rhinoviruses (see Fig. 176-2). The prominence at the threefold axis of symmetry is distinct from other picornaviruses and is believed to be the major antigenic site at which neutralizing antibodies are targeted.

Purification of virus from clinical samples or tissue culture yields three distinct populations of particles³⁰: (1) mature hepatitis A virions that band at 1.32 to 1.34 g/cm³ in cesium chloride and sediment at approximately 160 S (similar to enteroviruses and cardioviruses), (2) a lower-density fraction that bands at approximately 1.27 g/cm³ in cesium chloride, and (3) sediments at 70 to 80 S that may represent empty capsids or particles with incomplete genomes and a high-density fraction (1.4 g/cm³) that may represent particles with a more open virion structure that allows increased penetration and binding of cesium chloride to the viral particle. These high-density particles have been shown to contain RNA but tend to be less stable than mature virions.^31,32

HAV released from cells was shown to be enveloped in host-derived membranes that protect the virion from antibody-mediated neutralization. These enveloped viruses are fully infectious and circulate in the blood of infected humans. Antibodies to capsid restrict HAV replication after infection with the enveloped virus, suggesting a possible mechanism for postexposure prophylaxis.³³

Resistance to Physical and Chemical Agents

HAV is a relatively stable virus under a variety of environmental conditions. HAV is more resistant to heat than other picornaviruses and may not be completely inactivated (depending on the conditions) by exposure to 60° C for 10 to 12 hours.^34–36 Complete inactivation in food requires heating to higher than 85° C for at least 1 minute.³⁶ Outbreaks of hepatitis A have been reported after ingestion of steamed shellfish, suggesting that the internal temperature achieved by steaming sometimes may be insufficient to destroy the virus.³⁷ However, HAV can be reliably inactivated by autoclaving (121° C for 30 minutes).³⁸ Recent studies have shown that HAV can be inactivated in shellfish by a nonthermal method using high hydrostatic pressure.³⁹ The virus is resistant to most organic solvents and detergents and to a pH as low as 3.^38,40 HAV can be inactivated by many common disinfecting chemicals including hypochlorite (bleach) and quaternary ammonium formulations containing 23% HCl (found in many toilet bowl cleaners).³⁸ Currently licensed vaccines are inactivated by 1 : 4000 formalin at room temperature for at least 15 days to exceed complete inactivation by at least threefold. As a result of several outbreaks of hepatitis A in hemophiliacs who received factor VIII concentrates that had been treated by a solvent detergent method for inactivation of lipid-enveloped viruses, interest has focused on techniques capable of inactivating nonenveloped viruses without compromising the biologic activity of the product.⁴¹ Various manufacturers use different techniques to inactivate or remove HAV and human parvovirus B19. These include nanofiltration and other purification techniques, sensitive nucleic acid testing such as polymerase chain reaction (PCR) on minipools of source plasma, pasteurization at 60° C for 10 hours, and dry heat on lyophilized products.

Genome and Proteins

Although HAV had the characteristics of a picornavirus and indirect tests suggested an RNA genome, molecular cloning and sequence analysis demonstrated that the HAV genome is composed of single-stranded, positive-sense linear RNA of 7478 nucleotides (strain HM175) and a molecular weight of approximately 2.25 × 10⁶ Da, with an overall structure and gene order typical of picornaviruses.^17,19,42

The 5′ end of the genome does not have a cap structure but instead, as is typical for picornaviruses, has a small, covalently bound protein termed VPg.³⁹ The genome itself has a long 5′ untranslated region beginning with UU as found in all picornaviruses. This 5′ untranslated region folds to form a highly ordered secondary structure that is important for both replication and translation. These structured regions include an internal ribosome entry site (IRES) that directs the initiation of translation at either of two AUG codons at nucleotide positions 735-737 and 741-743 (see Fig. 176-3).⁴² The AUG codon initiates a single long open reading frame of 6681 nucleotides that encodes a polyprotein that is 2227 amino-acid residues in length. The coding region of picornaviruses has been arbitrarily divided into three parts, termed P1, P2, and P3, and the peptides that are ultimately cleaved from the translation products of these regions are referred to as 1A, 1B, 1C, 2A, 2B, 2C, and so forth, in order of translation from the 5′ to the 3′ end of the genome.⁴³ The HAV genome ends with a 3′ noncoding region of 63 nucleotides that is followed by a virus-encoded poly (A) tail.

A peculiar property of the HAV coding region is a highly deoptimized codon usage. The HAV genome is significantly biased against codons used abundantly by human cells. The accumulation of rare codons was hypothesized to result in the reduced translation rate and modulation of translation kinetics of HAV RNA, thus playing an important role in HAV replication and evolution.^44,45

The four capsid proteins of mature virus particles are coded by the first 2373 nucleotides (P1) and the nonstructural proteins by the remainder (P2 and P3). The four capsid polypeptides named by analogy with other picornaviruses are referred to in descending order of size as virion proteins (VP): VP1 = peptide 1D (molecular weight 32,800 Da), VP2 = 1B (24,800 Da), VP3 = 1C (27,300 Da), and VP4 = 1A (2500 Da).^22,46 A VP0 protein (1AB) that is the precursor to VP4 and VP2 can also be detected, especially from cell cultures where immature virions (provirions) may accumulate in large amounts.⁴⁷ The VP4 molecule is believed to be liberated during the maturation cleavage of VP0, which converts provirions to virions (see later), but VP4 has never been experimentally determined to be within the virion particle and, at just 23 amino acids, is approximately one third the size of the VP4 proteins of other picornaviruses.

Assembly of HAV particles proceeds through several steps (Fig. 176-3). Cleavage of the polyprotein by the 3C protease yields three capsid-related proteins, VP0, VP3, and VP1-2A (also known as PX), which constitute a monomer and subsequently assemble into pentameric subunits. Twelve copies of the pentamer then associate with viral RNA to form provirions or without viral RNA to form empty capsids (procapsids). The involvement of the VP1-2A precursor in assembly is unique to HAV, and it has been shown that the 2A extension is essential for proper processing and assembly of the pentameric subunit.^48–50 After assembly, 2A is removed from VP1 by cellular proteases,^51,52 and in the final maturation step, VP0 is cleaved to yield VP2 and VP4. The VP0 cleavage is dependent on the presence of viral RNA in the particle, and procapsids therefore fail to cleave VP0, but HAV procapsids are quite stable and seem to have the same antigenic structure as mature virions.

Antigenic Composition and Viral Diversity

Although a variety of genotypes of HAV have been identified by analysis of genome sequences (Fig. 176-4), there seems to be only one serotype.^53–55 This view is supported by the observation that IG prepared in developed countries and monovalent vaccines prepared from strains originating in Australia, Central America, or Europe protect travelers from disease equally well, irrespective of their destination.^56,57

Neutralization sites for HAV are located primarily on the structural proteins VP1 and VP3, with possibly a minor contribution from VP2.⁵³ Monoclonal antibodies in competition binding assays and the generation of neutralization escape mutants suggest that the dominant neutralization site is composed of overlapping epitopes on VP1 and VP3 that combine to form a conformational antigenic site at which neutralizing antibodies are targeted.^53,55,58

Classification of HAV into genotypes and subtypes was originally suggested using a 168-nucleotide fragment containing the VP1/P2A junction. Genotypes differ by at least 15% and subtype by 7% to 7.5% from each other in this region.²⁴ Seven genotypes, I to VII, identified using this approach were recently reclassified into six genotypes, I to VI, using the complete VP1 region. Genotypes I, II, and III are divided into subtypes A and B. The former genotype VII is classified as subtype IIB (2). These three genotypes infect humans, whereas genotypes IV to VI have a simian origin.^24,59,60 Genotypes have a particular geographic distribution.^24,61,62 Genotype I is most prevalent worldwide, with subtype IA being more common than IB, and is the most common in the United States.^24,61 Although IB usually constitutes only a small fraction of genotype I strains circulating in South and North America, Europe, Asia, and Africa,⁶⁰ it was reported among 35% to 70% of patients infected with HAV genotype I in Brazil.^63,64 Genotype 1B is predominant in the Middle East^65,66 and South Africa.⁶⁷ Genotype II isolates originally identified in France in 1979 and Sierra Leone in 1988²⁴ are not frequently reported. Subtype IIA was suggested to have West African origin.⁶⁸ Genotype III can be identified globally,⁶⁰ but it is endemic in Southeast and Central Asia.²⁴ An increase in genotype IIIA infections was recently reported in Korea and Russia.^26,69 In India, reported hepatitis A outbreaks have all been caused by genotype IIIA infections.^70–72 Genomic heterogeneity of HAV is used to track transmissions during outbreak investigations,⁶² with the VP1/P2A region being most frequently applied for genetic identification of HAV strains.^62,73 Although individual strains of HAV have differences at the molecular level that may be useful for epidemiologic studies, a high degree of identity in nucleic acid (up to 90%) and amino acid sequence (up to 98%) is generally seen between strains.^62,74

Biology of Hepatitis A Virus in Cell Culture

HAV was first propagated in marmoset liver explant cultures and a cloned line of fetal rhesus monkey kidney cells (FRhK-6) with a strain of virus (CR326) that had been adapted by multiple passages in Saguinus mystax and Saguinus labiatus marmosets.¹⁴ Many HAV strains have subsequently been isolated from clinical material, although the procedure may take several weeks. Until recently, only epithelial or fibroblast cells of primate origin had been shown to support growth of the virus.^15,16,75 However, in a systematic search for cells that would support HAV replication, growth was detected in cells of guinea pig, dolphin, and porcine origin.⁷⁶

The major characteristics of HAV in cell culture are slow growth and low yields relative to other picornaviruses. In addition, the virus remains largely cell associated, does not usually produce a cytopathic effect, and readily leads to persistently infected cell lines.⁷⁷ With adaptation, more rapid replication and higher yields can be obtained and cytopathic variants have been selected. These cell culture–adapted viruses are useful for virus titrations, neutralization, inactivation kinetics, and virus replication and have made the production of inactivated vaccines practical.^{14,16,78–80}

In one study, direct isolation of a wild-type strain of HAV was achieved through the use of a modified cell line.⁸¹ Detection of HAV in either patients’ or environmental samples is now done primarily through the use of PCR or other nucleic acid-based technology. The kinetics of viral replication and biosynthetic events has been studied in cells infected with cell culture–adapted strains of HAV and reveal a number of differences from most other picornaviruses. After attachment to cells, the uncoating of virus is delayed for more than 8 hours^82,83; this exceeds the duration of an entire growth cycle for many picornaviruses. The delayed uncoating seems to be related to the protracted maturation cleavage of VP0 to VP2 and VP4 because virions are uncoated more rapidly than provirions.⁸⁴ Accumulation of new viral RNA can be detected as early as 12 hours after infection of BS-C-1 cells with a fast-growing, cytopathic variant of strain HM175, but levels of viral replicative intermediates (double-stranded RNAs) remain much lower than in cells infected with other picornaviruses.⁸² As outlined previously, translation of the viral polyprotein is directed by the IRES within the 5′ untranslated region, but the IRES of HAV is relatively inefficient, with initiation approximately 1% of that seen for the IRES of encephalomyocarditis virus.^85–87

The initial proteolytic processing of HAV polyprotein is accomplished by the viral 3C protease, and assembly of viral particles proceeds via monomers and pentamers (see Fig. 176-3).⁴⁸ After infection of cell cultures with rapidly replicating and cytopathic variants of HAV, pentamers are first detected at 9 hours after infection and reach peak levels after approximately 18 hours even though the amount of viral RNA (and presumably viral translation) increases beyond this time.⁸⁸ These cells continue to produce virus for 2 to 3 days before cell death, whereas most HAV variants progress to a persistent infection with reduced levels of virus production over many weeks and subsequent cell passages.

Repeated passage in cell culture has been used to apply mutational pressure to HAV to alter the phenotype. For example, HAV variants that grow more rapidly or are resistant to neutralization by monoclonal antibodies have been selected.^80,89,90 Attenuated strains of HAV have been selected by multiple tissue culture passages, and cold adaptation has been achieved by passage at reduced temperature.^29,91 Some of the mutations responsible for these altered phenotypes have been identified by molecular cloning and sequencing of the mutant. Mutations within the 5′ untranslated region and mutations within the 2B and 2C coding regions of HAV RNA have been shown to enhance virus replication in vitro.^90,92,93 However, mutations within the VP1-2A and 2C proteins seem to be most important for attenuation of virulence.⁹⁰

Most viruses initiate infection by first binding to a specific cell surface receptor molecule or, in many cases, may require binding to both receptors and co-receptors to facilitate virus entry and uncoating. Identification of a specific receptor for HAV remained elusive for many years. However, Kaplan and colleagues⁹⁴ succeeded in the isolation of one specific receptor molecule, havcr-1, first in cells of simian origin and later in human cells.⁹⁵ This molecule is a novel mucin-like class I integral membrane glycoprotein of 451 amino acids, with the amino-terminal, cysteine-rich domain responsible for binding to HAV.⁹⁵ Because this molecule is expressed on cells from many tissues that are not susceptible to HAV infection, it is likely that specific co-receptors contribute to the organ tropism of HAV. However, a soluble form of havcr-1 has some ability to neutralize particles of HAV directly, which is consistent with roles in both attachment and uncoating of virus.⁹⁶

It remains possible that HAV may use other pathways for cell entry in addition to havcr-1. Recent studies demonstrated that the asialoglycoprotein receptor can also mediate infection of cells with HAV when the virus is first complexed with specific IgA, leading to the interesting hypothesis that IgA may play a role as both carrier and targeting molecule during infection and transmission, particularly in relapsing cases of HAV.⁹⁷

Host Range

Humans are considered to be the only important reservoir of HAV. However, the existence of extra human reservoirs of infection remains possible. In 1961, Hillis⁹⁸ described an outbreak of hepatitis A among chimpanzee handlers who apparently contracted the infection from the chimpanzees. Epidemiologic data suggested that the animals had become infected during captivity but before their importation into the United States. Interestingly, although epidemics of hepatitis were recognized in American primate handlers, the disease was rarely seen in Africa, presumably because most handlers were already immune. Widespread screening of nonhuman primates revealed antibodies to HAV in chimpanzees, gorillas, orangutans, gibbons, macaques, owl monkeys, pig tail monkeys, rhesus monkeys, and several species of South American tamarin monkeys.^99,100 It is unclear whether such primates may serve as reservoirs of infection or as transient hosts after exposure to HAV from human sources. However, both the PA21 and AGM-27 strains of HAV seem to be true simian viruses.^101,102 Interestingly, the AGM-27 virus produces attenuated disease in chimpanzees and was the subject of some study as a potential live attenuated vaccine.¹⁰³

Deinhardt¹⁰⁴ pioneered the use of nonhuman primates for hepatitis A studies in both chimpanzees and tamarins (Saguinus species). He showed that in chimpanzees, liver function test abnormalities developed on inoculation with known infectious material of human origin. Since that time, the chimpanzee, several species of tamarins, and the owl monkey have all been shown to be susceptible to HAV infection and to develop hepatitis.¹⁰⁴ These animals have been valuable tools in the study of hepatitis A pathogenesis and for the development of vaccines. Recently, it was shown that guinea pigs express a receptor similar to the human HAV receptor and that both guinea pig cells and guinea pigs can be infected with HAV. However, the animals did not have evidence of hepatitis.^76,105

Person to Person

Person-to-person transmission by the fecal-oral route remains the primary means of HAV transmission throughout most of the world.^117,118 Transmission can occur among close contacts, particularly in households and extended family settings.¹¹⁹ In the United States prior to the introduction of hepatitis A vaccine, young children had the highest rates of infection. Further, these children often serve as a source of HAV infection for others because HAV infection is often asymptomatic and standards of hygiene are generally lower in young children compared with adults, as in daycare facilities.^119–121

Foodborne and Waterborne

HAV can remain infectious in the environment for long periods of time,¹²² allowing for common source outbreaks and sporadic cases to occur from exposure to fecally contaminated food or water. Many uncooked foods have been recognized as the source of outbreaks. Cooked foods also can transmit HAV if the cooking is inadequate to kill the virus or if the food is contaminated after cooking, as commonly occurs in outbreaks associated with infected food handlers.^{123–125,126} Contaminated shellfish were responsible for a large outbreak in Shanghai, China, in 1988.^127,128 Although outbreaks continue to be reported,^{129–130,131} shellfish-associated HAV outbreaks in the United States and other developed countries have become increasingly uncommon. In Italy, detection of HAV in samples of shellfish has decreased, believed to be the result of hepatitis A vaccination programs in that country.¹³² Waterborne outbreaks of hepatitis A are also now uncommon in developed countries. Molecular techniques allow for identification of common source foodborne hepatitis A outbreaks around the globe.^133,134

Bloodborne

Transfusion-related hepatitis A is rare because HAV does not result in chronic infection, and, in the developed world, blood donors have been screened for many years for elevated aminotransferase levels. However, transmission by transfusion of blood or blood derivatives collected from donors during the viremic phase of their infection has been reported, including outbreaks in Europe and the United States among patients who received factor VIII and IX concentrates prepared using solvent-detergent treatment to inactivate lipid-containing viruses.^{41,113,135,136–138} HAV is resistant to solvent-detergent treatment, and contamination presumably occurred from plasma donors with hepatitis A who donated during the incubation period. The degree to which transfusion-related transmission occurs in developing countries where screening for blood and blood products may be limited is unknown.

Vertical

Two published case reports describe intrauterine transmission of HAV during the first trimester, resulting in fetal meconium peritonitis.^139,140 After delivery, both infants were found to have a perforated ileum. The risk of transmission from pregnant women who develop hepatitis A in the third trimester of pregnancy to newborns appears to be low.¹⁴¹ However, newborns who acquire infection in this manner are usually asymptomatic, and an outbreak among hospital staff related to exposure to such an infant has been reported.¹⁴²

Worldwide Disease Patterns

With more than 100,000 deaths globally caused by hepatitis A, it is one of the leading causes of death among vaccine-preventable diseases.¹⁴³ Although hepatitis A occurs worldwide, major geographic differences exist in endemicity and resulting epidemiologic features (Fig. 176-6).¹⁴⁴ The degree of endemicity is closely related to hygienic and sanitary conditions and other indicators of the level of development. In less developed areas, especially when there is limited access to clean water and inadequate disposal of human feces, HAV infects most people early in life, when infection is rarely clinically apparent (Fig. 176-7). When high standards of hygiene and sanitation apply, the majority of adults remain susceptible. Distinct patterns of HAV infection can be described, each characterized by particular age-specific anti-HAV prevalence and hepatitis A incidence and prevailing environmental (hygienic and sanitary) and socioeconomic conditions (see Fig. 176-7).^117,144

In areas of high endemicity, represented by the least developed countries (i.e., parts of Africa and Asia), poor hygienic and sanitary conditions allow HAV to spread readily (see Figs. 176-6 and 176-7). Infection is nearly universal in early childhood, when asymptomatic infection predominates, and essentially the entire population is infected before reaching adolescence, as demonstrated by the age-specific prevalence of anti-HAV (see Fig. 176-7).^145,146 Because most adults are immune, reported disease rates in this population are low and few outbreaks occur. However, susceptible adults in these areas are at high risk for hepatitis A. Recent surveillance data have demonstrated increasing numbers of cases among adults, and small seroprevalence studies have identified greater susceptibility of some children into adolescence, in some highly endemic countries.^147–149 Differences in susceptibility to hepatitis A in low and middle income countries can be defined by socioeconomic status, where more affluent children living in more hygienic conditions are more likely to escape HAV infection as infants and young children, leaving them susceptible to HAV infection as adolescents and adults.¹⁵⁰ In past years, some ethnic or geographically defined groups within highly developed countries also experienced high endemicity, including Native American, aboriginal populations in Australia, and Roma populations in Slovakia.^151–153

In areas of moderate endemicity, HAV is not transmitted as readily because of better sanitary and living conditions, and the predominant age at infection is older than in areas of high endemicity (see Figs. 176-6 and 176-7).^144,154 Paradoxically, the overall incidence and average age of reported cases are often higher than in highly endemic areas because high levels of virus circulate in a population that includes many susceptible older children, adolescents, and young adults who are more likely to develop symptoms with HAV infection.¹⁵⁵ Large common source food- and water-associated outbreaks can occur because of the relatively high rate of virus transmission and large number of susceptible persons, especially among those of higher socioeconomic level. Such an outbreak occurred in Shanghai in 1988, with more than 300,000 cases associated with consumption of clams harvested from water contaminated with human sewage.¹²⁷ Nevertheless, person-to-person transmission in community-wide epidemics continues to account for much of the disease in these countries.

Shifts in age-specific prevalence patterns that reflect a transition from high to intermediate endemicity are occurring in many parts of the world (see Fig. 176-6). A feature of this transitional pattern is striking variations in hepatitis A epidemiology among countries and within countries and cities, with some areas displaying a pattern typical of high endemicity and others of intermediate endemicity.^{117,156–162,163–167} Considerable hepatitis A–related morbidity and associated costs occur with this transition, even in developing countries.^168,169 For example, hepatitis A was the etiology of the fulminant hepatitis of two thirds of children presenting to two hospitals in Argentina during a 15-year period, and, in one of these hospitals performing liver transplantations, one third of liver transplantations among children were performed for fulminant hepatitis A.¹⁶⁸

In high-income, developed countries, including the United States, Canada, and much of western Europe, the endemicity of HAV infection is low or very low (see Fig. 176-6). Few children are infected, the incidence of disease is generally low, and disease is usually sporadic. Community-wide and child care center outbreaks may occur, although such outbreaks are becoming increasingly rare because of routine hepatitis A vaccination of children.^{117,138,152,170–174} Population-based seroprevalence surveys show a gradual increase in the prevalence of anti-HAV with increasing age, primarily reflecting declining incidence, changing endemicity, and resultant lower childhood infection rates over time. In countries where hepatitis A vaccine has been introduced, such as the United States, anti-HAV prevalence is increasing among children as a result of vaccination.¹⁷⁵ In low and very low endemicity countries, most cases occur in defined risk groups such as travelers returning from endemic areas, men who have sex with men, and users of injection drugs.^{138,174,176–181} The risk of hepatitis A among children and other contacts of family members who are residing in low endemic countries and travel to their high or intermediate endemic country of origin is documented.^182,183,184

Epidemiology in the United States

Hepatitis A epidemiology in the United States can be divided into two time periods, before and after implementation of national recommendations for use of hepatitis A vaccine.

Prevaccine Era

In the prevaccine era, hepatitis A incidence was primarily cyclic, with peaks every 10 to 15 years (Fig. 176-8). Throughout the 1980s and early 1990s in the United States, approximately 25,000 to 35,000 hepatitis A cases were reported annually to the Centers for Disease Control and Prevention,¹⁸⁵ but incidence models indicated that the actual number of infections occurring during that period was likely 10 times greater with an estimated 271,000 infections per year.¹²¹

The highest hepatitis A rates were among children 5 to 14 years of age, with approximately one third of cases occurring among children younger than 15 years.¹⁸⁶ Furthermore, because young children are more likely to have unrecognized or asymptomatic infection than older individuals, incidence models estimated that more than half of HAV infections occurred among children younger than 10 years old, the majority of which were in children younger than 5 years old (Figs. 176-9 and 176-10).¹²¹

In the prevaccine era, hepatitis A incidence differed among racial and ethnic groups, with rates among Native Americans and Alaska Natives that were more than five times those in other racial/ethnic groups, and rates among Hispanics that were approximately three times higher than those among non-Hispanics (Fig. 176-11).¹⁸⁷

Hepatitis A incidence also exhibited striking regional variation with the highest rates and majority of cases consistently occurring in a limited number of states and counties in the western and southwestern United States. During 1987-1997, cases among residents of 11 primarily western states, representing 22% of the U.S. population, on average accounted for 50% of reported cases.¹⁸⁸ An additional 18% of cases occurred among residents of six additional states.

In the prevaccine era, most cases of hepatitis A in the United States occurred in the context of community-wide epidemics in which infection was transmitted from person to person in households and extended family settings.^118,189 In these outbreaks, no single risk group accounted for the majority of cases, and infections among children, many of whom were asymptomatic, played an important role in sustaining transmission. For cases in which a risk factor could be determined, the most frequently reported source of infection was household or sexual contact with another person with hepatitis A, accounting for 12% to 25% of cases. Cyclic outbreaks also occurred in injection and noninjection drug users and among men who have sex with men; up to 15% of nationally reported cases occurred among persons reporting one or more of these behaviors. Other potential sources of infection such as international travel and foodborne outbreaks accounted for a small proportion of cases. For approximately 50% of reported cases, no source of infection could be identified.

Results of the Third National Health and Nutrition Examination Survey, conducted from 1988 to 1994, indicated that approximately one third of the U.S. population had serologic evidence of previous HAV infection.¹⁹⁰ Anti-HAV prevalence was related directly to age, ranging from 9% among children 6 to 11 years of age to 75% among persons older than 70 years of age and was related inversely to income. Anti-HAV prevalence was highest among Mexican Americans (70%) compared with non-Hispanic blacks (39%) and whites (23%).

Variation by Age, Race or Ethnicity, and Region

Hepatitis A incidence rates among children have decreased more sharply than among adults after the implementation of routine vaccination of children, and since 2002, rates have been similar among adults and children (see Fig. 176-10).¹⁹²

Previous disparities in rates across racial/ethnic groups have largely disappeared (see Fig. 176-11).¹⁹² By 2000, hepatitis A incidence among Native Americans and Alaska Natives had decreased by 97% compared with the beginning of the decade and by 2010 was the lowest ever recorded (0.2/100,000).¹⁹² The disparity in incidence between Hispanics and non-Hispanics has also nearly disappeared.¹⁹² The geographic variations that characterized hepatitis A incidence in the past have also essentially disappeared, presumably due, at least in part, to the implementation of vaccination recommendations.^188,193 Since 2002, the county-level hepatitis A rates in the western states have been similar to those in other regions of the country.

Potential Sources of Infection

Currently, risk factor and exposure data for hepatitis A cases are often lacking in the United States. However, on the basis of the data from enhanced disease surveillance during 2005-2007, the most commonly reported exposure to HAV in the United States was international travel (accounting for 46% of cases), followed by household or sexual contact with hepatitis A cases (Fig. 176-12).¹⁹⁴ Cases occurring among children and employees of child care centers and members of their households, which previously accounted for a substantial proportion of reports, were uncommon (8%), as were cases associated with common source food or water exposure (7%), illicit drug use (4%), and among men who had sex with men (MSM) (4%).¹⁹⁴ It remains to be seen if cyclic outbreaks will continue to occur among MSM and users of injection and noninjection drugs.^{118,177,195–199} Historically during outbreak years, these exposures could account for 10% of nationally reported cases and, with the large decreases in incidence among children and their adult contacts, can now account for an even larger proportion of cases. Approximately half of hepatitis A cases reported through routine surveillance do not have a recognized source of infection,^192,193 even with enhanced surveillance; one third will not have a source identified¹⁹⁴ but may be contacts with persons, especially children, with asymptomatic infection.

Outbreaks of hepatitis A in child care centers, which historically were a common occurrence, particularly in larger centers and in those that cared for children in diapers,^200–202 are now rare,¹⁹⁴ at least in part because of the routine vaccination of children against hepatitis A. However, the potential for transmission in these settings remains. Outbreaks in child care centers have occasionally been the source of more extensive transmission within a community,^201,203,204 but in most cases disease in child care centers reflects disease transmission from the community. Similarly, hepatitis A cases among older children in schools usually reflect disease that has been acquired in the community, although multiple cases among children in a school may indicate a common source outbreak.²⁰⁵ Historically, HAV infection was endemic in institutions for the developmentally disabled, but with smaller facilities and improved conditions, the incidence and prevalence of infection have decreased and outbreaks are rarely reported in the United States.²⁰⁶

Incubation Period

Determination of the incubation period of disease is imprecise because the early symptoms of hepatitis are often vague and nonspecific. Jaundice may not be noticed by the patient, so the most useful marker of the onset of the disease is a change in urine color, which is almost always recognized by the patient and is the most common reason for seeking medical attention. The range of incubation is between 15 and 50 days, with a mean of approximately 28 days. Although HAV can be transmitted orally or parenterally, the incubation period is independent of the route of inoculation.²⁶⁷ Experiments in primates and observations in humans suggest that the incubation period is dependent on the infectious dose.²⁶⁸

Site of Viral Replication

HAV is generally transmitted by the fecal-oral route. Because the virus is acid resistant, it probably passes through the stomach, replicates lower in the intestine,^269–271 and is then transported to the liver, which is the major site of replication.^269,272,273 Evidence of replication in the oropharynx has been obtained in chimpanzees, and HAV has been identified in human saliva.^115,116 HAV, like many other picornaviruses, is highly organ specific with little evidence of significant replication outside the liver. Virus is shed from infected liver cells into the hepatic sinusoids and canaliculi, passes into the intestine, and is excreted in feces. In humans and nonhuman primates, HAV has been detected in the liver, bile, and feces.^274,275 The first indirect evidence that virus may replicate in the gut was the detection of co-proantibodies in the feces,^276,277 followed by the demonstration of hepatitis A antigen in duodenal lining cells.²⁶⁹ Nonetheless, the major pathology is restricted to the liver.