Clinical Chemistry 43: 1494-1499, 1997;
(Clinical Chemistry. 1997;43:1494-1499.)
© 1997 American Association for Clinical Chemistry, Inc.
Type A viral hepatitis: epidemiology, diagnosis, and prevention
Stanley M. Lemon
Division of Infectious Diseases, Department of Medicine, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7030.
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Abstract
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Hepatitis A virus (HAV) infection occurs worldwide and is an important
cause of acute viral hepatitis in the US. In this review, I cover the
epidemiology, course of infection, clinical manifestations, serological
responses, and prevention of this infection. Although most patients
completely recover from this disease, elderly patients have a
substantial mortality risk. Recently licensed vaccines are highly
efficacious.
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Introduction
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Five very different viruses make up the "classical"
etiological agents responsible for acute or chronic viral hepatitis in
humans. For the most part, these viruses share only a common tropism
for the liver, with the hepatocyte representing the dominant site of
viral replication and either acute or chronic forms of hepatitis
representing the major clinical manifestations associated with
infection. These viruses can be considered as two distinct groups,
based on several clinically and epidemiologically important
characteristics: those viruses that possess a lipid-containing outer
viral envelope (hepatitis B, C, and D viruses: HBV, HCV,
HDV)1
and those that do not (hepatitis A and E viruses:
HAV, HEV).
HAV and HEV, which lack a lipid envelope, are stable when they are
secreted from infected liver cells into the bile, and they gain entry
to the intestinal tract via this route. Thus, these viruses typically
spread by a fecal–oral mode of transmission, and they can cause
extensive common-source outbreaks of disease. However, neither of these
viruses causes persistent infection, and neither has been identified as
a cause of chronic viral hepatitis. In contrast, HBV, HCV, and HDV all
possess lipid envelopes and are likely to be rapidly inactivated by
bile. Thus, these viruses are not shed in feces in biologically
significant amounts. Their transmission occurs by several other routes,
most often involving virus shed from a mucosal surface or by direct
percutaneous exposures. In addition, HBV, HCV, and HDV may each cause
persistent infection, and each has been shown to be an important
etiological agent of chronic viral hepatitis and cirrhosis. Infection
with HBV or HCV may lead ultimately to the development of primary
hepatocellular carcinoma, often after many years of persistent
infection and chronic hepatitis. Although chronic viral hepatitis is
not associated with HAV infection, acute hepatitis due to HAV is
nonetheless an important public health problem, both in the US and
overseas (1)(2).
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important attributes of hav
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HAV is a positive-sense, single-stranded RNA virus classified
within the genus hepatovirus of the family Picornaviridae
(3)(4). This virus family also includes enteroviruses, such
as poliovirus and rhinoviruses, which are frequent causes of the common
cold. HAV is thus a positive-strand RNA virus, with genomic RNA that
can function as messenger in directing the translation of proteins. A
single large polyprotein is expressed from a large open-reading frame
that extends through most of the genomic RNA. Translation occurs in a
cap-independent fashion under control of a internal ribosome entry
segment located within the 5' untranslated RNA. The polyprotein
subsequently undergoes cleavage mediated by a viral protease
(3Cpro), resulting in the production of four capsid
proteins (VP1–4) and several nonstructural proteins.
Unlike other hepatitis viruses, HAV can be propagated in conventional
mammalian cell cultures with reasonable efficiency, usually without any
apparent cytopathic effects (3). African green monkey kidney
cells, or fetal rhesus kidney cells, are commonly used for culturing
the virus, although many different cell types are suitable. Cell
culture isolation of virus is not a useful approach to diagnosis,
however, because wild-type virus usually replicates poorly in cell
culture. More-efficient replication occurs after a number of passages,
when the virus has become adapted to growth in cell culture. Several
cell culture-adapted HAV variants have been shown to be highly
attenuated in their ability to cause disease in otherwise susceptible
primates, and such viruses have been used for production of both killed
(formalin-inactivated) and candidate live (attenuated) HAV vaccines
(3). The mutations responsible for both cell culture
adaptation and attenuation in primates have been reasonably well
characterized, and are found within the 5' nontranslated RNA as well as
within RNA encoding nonstructural proteins (4). Thus, these
phenotypes probably result from a change in the ability of the virus to
utilize cell type-specific host cell factors required for translation
and replication of the viral genome.
HAV strains recovered from humans in different regions of the world
demonstrate negligible antigenic diversity, leading to the conclusion
that only a single serotype of HAV exists, despite the considerable
genetic heterogeneity at the nucleotide level. The antigenic structure
of the virus is relatively simple, with a restricted number of
overlapping epitopes combining to form a single dominant antigenic site
that interacts with virus-neutralizing antibodies (4). These
epitopes are highly conformational, and are formed by amino acid
residues contributed from more than one capsid protein. Thus, for the
most part, antigenicity depends on the assembly of the major capsid
proteins into capsid or smaller capsid precursors. Individually
expressed capsid proteins do not share antigenic characteristics with
the native virus particle. Empty capsids, however, appear to be
antigenically indistinguishable from infectious, RNA-containing
virions.
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epidemiology
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HAV is present in a worldwide distribution, the highest prevalence
of infection occurring in regions where low standards of sanitation
promote transmission of the virus (1). Risk factors
associated with the acquisition of HAV within the US have been
determined in the Sentinel Counties Study carried out by the Centers
for Disease Control and Prevention over several years (5).
Frequently reported risk factors include having lived in the same
household with a patient with hepatitis (~24% of all patients);
homosexual activity that might lead to fecal–oral spread of virus
through oral–anal contact (~11%); and close contact with young
children attending day-care centers (~18%). Several other studies
also suggest that preschool day-care centers may at times be important
foci for transmission within the US (6). International
travel to regions where hepatitis A is endemic poses a substantial risk
for acquiring hepatitis A, but this was reported by only ~4% of
patients with acute hepatitis A in the Sentinel Counties Study. In
recent years, illicit use of parenteral drugs has been reported as a
risk factor by only 2% of patients, but in other studies
(7) this has been a much more prevalent risk factor.
Significantly, ~40% of patients studied in the Sentinel Counties
Study have not reported any apparent risk factor (discounting the
~14% who report having lived in the same household with a child
under 5 years of age).
Most cases of hepatitis A can be explained by fecal–oral transmission
of the virus. However, the occasional association of hepatitis A with
intravenous drug use is interesting. This was first noted in Sweden
(7) and later confirmed in the US. It has been suggested
that this association may largely reflect general living conditions and
poor sanitation, but this may not be the entire story. Acute infection
with HAV is associated with a substantial viremia that persists for
several weeks (8). Needle-borne transmission of the virus is
certainly a possibility if individuals share nonsterile needles during
the prodromal phase of the illness. In addition, HAV is a very stable
virus, capable of withstanding substantial heat and drying. In 1992,
several outbreaks of hepatitis A were noted among hemophilic patients
receiving factor VIII concentrates that had been prepared by a
solvent-detergent inactivation process (which does not reduce the
infectivity of nonenveloped viruses).
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virologic events in acute hepatitis a
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Figure 1
shows the course of events in a New World owl monkey after intravenous
inoculation with ~104 infectious units of a
cell-culture-adapted HAV strain that could be readily isolated in cell
culture and measured quantitatively by a modified viral plaque assay
(8). About 4 weeks after inoculation, the animal developed
relatively mild chemical hepatitis A, as evidenced by increasing serum
alanine aminotransferase (ALT) activities. This was associated with
evidence of inflammatory changes in the liver parenchyma. Concordant
with the onset of hepatitis was the appearance of antibody, which could
be detected either by an ELISA or by a more biologically relevant
neutralizing antibody assay that measures the amount of antibody
capable of inhibiting the infectivity of the virus particle (see
below).
Preceding the onset of chemical hepatitis and the appearance of
antibody, there was the typical shedding of infectious virus in feces.
Infectious virus may be identified in the feces as early as 4 days
after the intravenous injection of infectious material in susceptible
primates (7 days in animals fed wild-type virus orally), and continues
to increase in magnitude until just before the onset of chemical
evidence of liver disease. Fecal shedding of virus then declines,
reflecting a developing immune response to the virus that is not only
antibody-mediated but also involves the stimulation of CD8+
virus-specific cytotoxic T cells (9). The viremia lasts
almost as long as the fecal shedding of virus. In a group of six
monkeys infected with this virus (a strain that may have been partially
attenuated with respect to its ability to replicate in the liver), the
average duration of detectable viremia was ~3 weeks (8).
The magnitude of the viremia is perhaps 2–3 log10 less
than the fecal shedding of virus but is still typically substantial.
Importantly, the viremia is maximal during the prodromal phase, prior
to the development of clinical, chemical, or serological manifestations
of the infection.
The shedding of the virus in feces comes predominantly from replication
of the virus within the liver. However, quite solid data now indicate
that the virus also replicates within epithelial cells of the distal
ileum. These multifunctional cells are highly polarized. Based on
studies with cultured colonic carcinoma cells (Caco-2 cells), the entry
and release of HAV from intestinal epithelial cells appear to be
restricted to their apical (i.e., lumenal) surface (C Blank and SM
Lemon, unpublished data). Thus, it remains unclear how the virus
reaches the liver in the initial stages of the infection. However,
virus replicated within hepatocytes (which are also highly polarized)
is similarly secreted across the apical hepatocyte membrane into the
bile, and thence into the gastrointestinal tract. It is likely that the
viremia also derives from replication of virus within the hepatocyte,
but with spread into the circulation rather than secretion into the
bile.
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clinical manifestations of hav infection
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The onset of hepatitis A is characteristically abrupt. Early
symptoms include malaise, fatigue, nausea, vomiting, and discomfort in
the right upper quadrant of the torso. Although fever occurs in about
half of all patients and may be impressive in early stages of the
disease, it is usually absent by the time the patient seeks medical
attention. The first specific evidence that the illness relates to an
intrahepatic process may be any of the following: the onset of dark
urine, light clay-colored stool, or icterus of the conjunctival
covering of the sclera. The urine typically acquires the color of cola
soda and is characteristically frothy. Icterus may be noted when the
serum bilirubin exceeds 25–30 mg/L. These signs typically appear
during the first few days of illness, but are usually absent at the
onset. Diarrhea occurs in about one-half of all infected children but
is uncommon in adults.
The most striking laboratory findings include increases of serum
aminotransferase activities, serum bilirubin (both total and direct),
and serum alkaline phosphatase activity. Serum ALT is usually increased
more than serum aspartate aminotransferase. These enzyme activities may
be minimally increased, or in severe cases may be >100-fold the upper
reference limit. Serum aminotransferase activities usually increase
before serum bilirubin does. Alkaline phosphatase activity also rises
relatively late in the course of the infection, and tends to parallel
serum bilirubin increases. Other laboratory findings include
nonspecific increases of acute-phase reactants, immunoglobulins, and
the erythrocyte sedimentation rate. Concentrations of total hemolytic
complement may be depressed in acute hepatitis A, possibly reflecting
the presence of circulating immune complexes as described by some
workers. Rheumatoid factor (IgM anti-IgG) is often present.
The entire acute illness may last from 1 to several weeks. Resolution
is marked by a return in the patient's general sense of well-being and
appropriate declines in abnormal serum chemistries. Typically, the
aminotransferase activities may begin to resolve before the serum
bilirubin. However, aminotransferase activities may remain abnormal for
several months after acute HAV infection, although usually at
relatively low values. Abnormal ALT activity may persist after the
serum bilirubin has returned to normal. However, it is probably safe to
say that abnormalities in serum chemistries are seldom present >6
months after the acute infection and virtually never after 1 year.
Extrahepatic manifestations of hepatitis A are uncommon. Urticarial
rash and acute arthritis, such as may occur with acute hepatitis B, are
not found in acute hepatitis A. Concomitant meningoencephalitis has
been reported in several cases, possibly reflecting the fact that HAV
is a picornavirus and thus relatively closely related to the
enteroviruses, common causes of viral meningitis and
meningoencephalitis in humans. Acute renal failure has been
occasionally reported in association with acute hepatis A.
Children younger than 2 years rarely manifest symptoms and signs of
acute viral hepatitis when they are infected. However, the severity of
disease progressively increases with the age at the time of infection.
In studies done in American military populations, ~70% of soldiers
who became infected with HAV developed specific clinical signs of
hepatitis A, including jaundice (10). Hepatitis A in adults
is usually an illness of several weeks' duration, with perhaps a few
months of extended convalescence. It is, however, occasionally a fatal
disease, particularly in older persons. Fulminant hepatitis and death
due to hepatitis A occur almost exclusively in individuals who are
infected after age 50 (Fig. 2
).Overall, ~70–80 deaths from hepatitis A infection are reported in
the US each year. Severe manifestations of hepatitis A infection are
more likely in individuals with underlying alcoholic liver disease or
with chronic hepatitis caused by other viral agents.
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Figure 2. Age distribution of reported deaths from (n = 689,
upper panel) and reported cases of (n = 235 153,
lower panel) hepatitis A within the US, 1983–1991
(2).
Bars represent reported deaths or cases (left
ordinate), lines represent incidence rates (right
ordinate).
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serological measures of hav infection
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The development of antibody to HAV, as measured by a solid-phase
competitive inhibition blocking assay or a virus neutralization assay,
coincides with a marked diminution in the quantity of viremia and
fecal shedding of virus (8)(11). Detection of this
acute-phase antibody response is the mainstay of diagnosis. IgM
antibodies to the virus are present in >99% of individuals at the
time of their initial presentation. The IgM anti-HAV response usually
peaks within the first month of illness and declines to nondetectable
amounts within 12 (usually 6) months. Virus-specific IgM antibody
usually is detected by very sensitive and specific solid-phase
antibody-capture immunoassays and typically is found against a
background of nonspecific increases in IgM.
Total antibody to HAV, on the other hand, is measured in a separate
competitive inhibition (blocking) ELISA. Total antibody to HAV consists
predominantly of IgG antibody, except immediately after acute HAV
infection, when IgM and IgA antibodies represent a greater proportion
of the virus-specific antibody response. Quantitative estimates of the
IgG anti-HAV content of sera are commonly based on a comparison with a
World Health Organization anti-HAV Reference Immunoglobulin Reagent,
and are reported in terms of mIU/mL. Like tests for IgM anti-HAV, tests
for total anti-HAV are almost always positive at the onset of acute
hepatitis A. Thus, a negative test for total antibodies to hepatitis A
effectively excludes a diagnosis of acute HAV infection. However, a
positive competitive inhibition test is unable to distinguish between
acute (recent) or long past infection. Because of its high positive and
negative predictive value, IgM anti-HAV is the test of choice when
confronted with a patient who might have acute HAV infection.
Competitive inhibition immunoassays for total anti-HAV antibodies
usually remain positive for life following acute infection and are a
good marker of immunity. Commercially available immunoassays for total
anti-HAV antibodies are relatively insensitive, however, and may not
detect the protective antibody responses present after a single dose of
inactivated HAV vaccine (12). The minimal protective amount
of antibody is <10 mIU/mL; unfortunately, 100 mIU/mL is the
approximate minimum amount of antibody detectable in commercial
antibody tests. Some investigators have successfully circumvented this
problem by modifying the commercial test and increasing the volume of
serum added to the competitive inhibition reaction.
Commercially available tests for IgM anti-HAV and for total anti-HAV
are not influenced by passive administration of immune globulin given
in normal doses. Thus, the administration of immune globulin does not
complicate subsequent efforts at serological diagnosis.
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prevention
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Formalin-killed whole-virus vaccines are highly effective in
prevention of hepatitis A (13)(14). Two such vaccines have
been recently licensed within the US: Havrix (SmithKline Beecham) and
Vaqta (Merck & Co.) (15). Both of these vaccines contain
inactivated whole-virus particles and empty capsids produced in
infected cell cultures, and both are adsorbed to alum in an effort to
enhance immunogenicity. However, these vaccines vary with respect to
the methods by which the amount of viral antigen in each vaccine dose
is determined, as well as the relative purity of the viral proteins
within the vaccine. Nonetheless, both vaccines appear to be comparably
immunogenic. They are generally given as a single primary immunization,
followed by a booster dose 6–12 months later. Pediatric formulations
contain about one-half the antigenic mass of the adult formulation and
are recommended for individuals younger than 17–18 years. Thus far, no
significant clinical differences have been documented with respect to
the adverse events reported after immunization with these two vaccines.
In general, these are very safe vaccines, with reactogenicity similar
to that of hepatitis B vaccines. However, a few reports of anaphylaxis
after administration of inactivated HAV vaccines mandate that vaccine
should be administered only in settings where epinephrine is
immediately available. Guillain–Barré syndrome has occurred in
several immunized persons, but its relation to the immunization is
uncertain.
Immunization should be a priority for individuals at increased risk of
acquiring hepatitis A, or who are at increased risk of fatal, fulminant
disease if they contract hepatitis A (2)(15). The former
include children in communities where hepatitis A is traditionally
endemic, as well as travellers going abroad to developing nations where
HAV can be highly endemic. Other high-risk persons include male
homosexuals and persons using illicit parenteral drugs
(5)(7). Persons with underlying chronic liver disease from
any cause, particularly if they older than 45–50, are at increased
risk of fulminant hepatitis A and should be immunized. Immunization of
foodhandlers could prevent common-source outbreaks of hepatitis A, but
the cost-effectiveness of such a strategy is not known (2).
Given the large proportion of cases with no defined risk factor for
hepatitis A, universal immunization likely would be required to
successfully control hepatitis A. The high costs and limited
availability of the vaccine precludes such a recommendation at present,
however.
Immunization with inactivated HAV vaccines results in demonstrable
titers of IgG anti-HAV, and occasionally low amounts of IgM antibody.
The titlers of vaccine-induced antibody are usually much lower than
those present after natural infection, yet these lower antibody amounts
confer good protection against disease (and probably infection as
well). The minimal protective antibody amount has not been defined, but
several lines of evidence suggest that it is <10 mIU/mL. The most
sensitive method for detection of vaccine-induced antibodies is a
radioimmunoprecipitation assay using metabolically labeled virus
(5). Differences in the relative titers of neutralizing and
immunoprecipitating antibodies in the vaccinee's serum vs that in sera
from naturally infected persons suggest that the vaccine-induced
antibodies are of much lower affinity. This does not seem to be due to
formalin-induced alterations in the antigenicity of the virus (which
have not been demonstrated), but more likely reflects the method of
presentation and the antigenic mass.
Limited evidence indicates that immunization with inactivated HAV
vaccines may delay the appearance of IgM antibody to the virus, should
an individual subsequently become infected with HAV (certainly an
unusual situation). When considering this possibility in an immunized
person, therefore, it may be useful to obtain a late serological
specimen for IgM antibody, or to look for increases in total antibody
concentrations. Preliminary results with immunoassays that utilize
nonstructural HAV proteins as antigen suggest that it may be possible
to develop assays capable of effectively discriminating between
antibody response to immunization vs infection.
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Footnotes
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Present address and address for correspondence: Department of Microbiology and Immunology, University of Texas–Galveston, 301 University Blvd., Galveston, TX 77555-1019. Fax 409-772-3757.
1 Nonstandard abbreviations: HAV, HBV, HCV, HDV, HEV, hepatitis A, B, C, D, and E viruses, respectively; ALT, alanine aminotransferase. 
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References
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Hadler SC. Global impact of hepatitis A infection: changing patterns. Hollinger FB Lemon SM Margolis HS eds. Viral hepatitis and liver disease 1991:14-19 Williams & Wilkins Baltimore. .
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Lemon SM. HAV: current concepts of the molecular virology, immunobiology and approaches to vaccine development. Rev Med Virol 1992;2:73-87.
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Robertson BH, Lemon SM. Hepatitis A and E viruses. In:
Collier L, ed. Topley and Wilson's microbiology and microbial
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Top
Abstract
Introduction
) and serum
(
), measured by radioimmunofocus assay and reported in terms of
radioimmunofocus forming units (rfu) per gram or milliliter. Also shown
is the virus titer in liver tissue at the time of peak hepatitis (
).
The solid line represents serum ALT activity. Adapted from
data in Lemon et al. (