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REVIEW
Occult hepatitis B virus infection and its clinical implications
Ke-Qin Hu Transplantation Institute and Division of Gastroenterology, Loma Linda University Medical Center and Jerry L. Pettis Memorial VA
Medical Center, Loma Linda, California, USA
INTRODUCTION
Hepatitis B virus (HBV) infection is one of the major global
human health problems. It is estimated that more than 350
million people world-wide, and 1.25 million people in the
United States are affected by HBV infection [1,2]. The spec-
trum of HBV-related disease ranges from acute hepatitis B,
asymptomatic HBV carrier, chronic hepatitis, to rarely
fulminant hepatitis [3,4]. Chronic HBV infection is also
associated with cirrhosis and hepatocellular carcinoma
(HCC).
Chronic HBV infection is characterized by persistence of
HBV surface antigen (HBsAg) and viraemia [3,4]. Early
studies revealed that clearance of HBsAg in patients with
HBV infection is associated with disappearance of viraemia
and remission of the disease. However, accumulated data
indicated that a low level of HBV-DNA remains detectable in
serum and liver tissue in some patients who cleared HBsAg
from either acute self-limited or chronic HBV infection, or
even after a successful anti-HBV treatment. Demonstration
of this clinical entity has resulted in introduction of the
concept of ‘occult, silent, or latent’ HBV infection, which
defines presence of HBV infection with undetectable HBsAg.
The prevalence, and clinical importance of this clinical entity
has been extensively reviewed through a special workshop
in September 1998 by a panel of Europe and USA scientists
[5]. However, tremendous progress has been made since
then. This review will summarize recent progress in the
concept of occult HBV infection, especially, its mechanisms
and clinical implications. Another detailed review on occult
HBV infection was published by Bre´chot et al. [6] whilst this
article was under review.
Abbreviations: aa, amino acid; anti-HBc, antibody to hepatitis B core
antigen; anti-HBe, antibody to hepatitis B e antigen; anti-HBs,
antibody to hepatitis B surface antigen; BMT, bone barrow trans-
plantation; CTLs, cytotoxic T lymphocytes; FHB, fulminant
hepatitis B; FHF, fulminant hepatic failure; HBV, hepatitis B virus;
HBV cccDNA, HBV covalently closed circular DNA; HCC, hepato-
cellular carcinoma; HBsAg, hepatitis B surface antigen; HBcAg,
hepatitis B core antigen; HBeAg, hepatitis B e antigen; HCV, hepa-
titis C virus; HDV, hepatitis D virus; IFN, interferon; Ig, immuno-
globulin; MHL, major hydrophilic loop; PBMC, peripheral blood
mononuclear cells; PCR, polymerase chain reaction; TNF, tumour
necrosis factor; WHV, woodchuck hepatitis virus.
Correspondence: Dr Ke-Qin Hu, Loma Linda University Medical Ctr.,
11234 Anderson Street, Rm 1405, Loma Linda, CA 92354, USA.
E-mail: khu@ahs.llumc.edu
Journal of Viral Hepatitis, 2002, 9, 243–257
� 2002 Blackwell Science Ltd
SUMMARY. Occult hepatitis B virus (HBV) infection is char-
acterized by presence of HBV infection with undetectable
hepatitis B surface antigen (HBsAg). Serum HBV level is
usually less than 104 copies/mL in these patients. Diagnosis
of occult HBV infection requires sensitive HBV-DNA PCR
assay. Several possibilities have been hypothesized as the
mechanisms of occult HBV infection. These include:
(i) mutations of HBV-DNA sequence; (ii) integration of
HBV-DNA into host’s chromosomes; (iii) infection of per-
ipheral blood mononuclear cells by HBV; (iv) formation of
HBV-containing immune complex; (v) altered host immune
response; and (vi) interference of HBV by other viruses. The
precise prevalence of occult HBV infection remains to be
defined. The clinical implications of occult HBV infection
involve different clinical aspects. First of all, occult HBV
infection harbours potential risk of HBV transmission
through blood transfusion, haemodialysis, and organ
transplantation. Second, it may serve as the cause of cryp-
togenic liver disease, contribute to acute exacerbation of
chronic hepatitis B, or even fulminant hepatitis. Third, it is
associated with development of hepatocellular carcinoma.
Fourth, it may affect disease progression and treatment
response of chronic hepatitis C. Most of the previous studies
utilized retrospective observation without control groups,
and lacked direct association of occult HBV infection with
specific pathological changes and disease progression. Highly
sensitive, quantitative, and functional molecular analyses of
HBV, combined with a well-designed prospective clinical
assessment will provide the best approach for the future
study of occult HBV infection.
Keywords: chronic hepatitis B, chronic hepatitis C, HBV
replication, hepatitis B virus, hepatitis C virus, hepatocellular
carcinoma, occult HBV infection.
CLINICAL SCENARIOS AND ANIMAL MODELS
OF OCCULT HBV INFECTION
Historical background
Although HBsAg is generally considered an essential sero-
logical marker for diagnosing ongoing HBV infection, and
anti-HBc IgG alone indicates previous HBV infection with
virus clearance and disease remission [7,8], exceptions have
been demonstrated more than two decades ago. In 1978,
Hoofnagle et al. reported that transfusion with blood con-
taining anti-HBc, but not HBsAg and anti-HBs, resulted in
HBV infection in the recipients [9]. This provided direct evi-
dence that patients with positive anti-HBc alone may har-
bour HBV and transmit infection. Subsequently, it was found
that HBV-DNA is also detectable in patients with negative
HBsAg, but positive anti-HBs, which has been a standard
marker indicating immunity to HBV infection [10,11]. These
early findings suggest that routine serological profiles are not
always reliable in determining status of HBV infection.
HBV polymerase chain reaction (PCR) is approximately
104-fold more sensitive than direct hybridization [12] for the
detection of HBV-DNA. Application of HBV PCR generated a
great deal of valuable information regarding occult HBV
infection. For instance, inoculation of serum from HBsAg-
negative, but HBV PCR-positive subjects to chimpanzees
induced acute hepatitis. After HBV PCR amplification and
cloning of HBV-DNA fragments, both human donor and
chimpanzee recipient showed comparable HBV-DNA
sequences [13]. These findings further confirm the func-
tional competence and infectivity of HBV present in HBsAg-
negative patients. It is now clear that HBsAg-negative (i.e.
occult) HBV infection is a genuine clinical entity.
Clinical scenarios associated with occult HBV infection
Occult HBV infection can occur in different clinical scenar-
ios. Typically, seroclearance of HBsAg is followed by devel-
opment of anti-HBs with coexisting anti-HBc. If anti-HBs
remains undetectable, anti-HBc serves as the only marker
indicating past HBV infection [7,8]. Occult HBV infection is
most frequently seen in patients with anti-HBc as the only
HBV serological marker [14]. However, it is also reported in
patients with anti-HBs alone, or even in those without any
HBV serological marker [10,11,14,15]. Recovery from an
acute self-limited hepatitis B may be followed by occult
HBV infection, which could last greater than 2–3 decades
[16–19]. Also, occult HBV infection can be secondary to a
spontaneous or anti-HBV treatment- induced seroclearance
of HBsAg in patients with different stages of chronic hepatitis
B, including cirrhosis [20]. High prevalence of occult HBV
infection was also demonstrated in patients with HCC [21],
and those with chronic hepatitis C virus (HCV) infection
[15,22]. Occult HBV infection has been reported in children
after HBsAg clearance as well [23]. Serial HBV-DNA testinghas revealed that the incidence of occult HBV infection is
time-course dependent after HBsAg conversion, which
decreases gradually with prolonged observation [20].
It is estimated that in individuals with markers of HBV
infection, 10–20% present with a positive anti-HBc as the
only HBV maker (i.e. anti-HBc alone) [5]. The latter may be
seen in three different situations: (i) a resolving acute HBV
infection after HBsAg disappeared, but anti-HBs remains
undetectable (i.e. window period); (ii) a past HBV infection,
especially an infection which occurred long time ago and
anti-HBs has fallen to an undetectable level; (iii) a true occult
HBV infection with low level of HBV replication [3–9]. Dif-
ferentiation of these different presentations relies on a thor-
ough history, follow-up anti-HBc and anti-HBs tests, and
proper application of a sensitive HBV-DNA PCR assay.
Animal models
Recently, occult HBV infection was also demonstrated in
woodchucks infected by woodchuck hepatitis virus (WHV),
which is related to human HBV [24,25]. After spontaneous
recovery from acute WHV infection, a continuous and
lifelong occult WHV infection develops in these animal
models [24]. A low level of WHV replication continues in
both liver and lymphatic tissue. Its persistence is associated
with risk of offspring transmission, mild histological in-
flammation and development of HCC [25]. A delayed HBV-
DNA clearance was also reported in macaca mulatto (a
monkey species) after inoculation of HBV-containing plas-
ma [26].
MOLECULAR ANALYSIS AND MECHANISMS
OF OCCULT HBV INFECTION
Molecular analysis of HBV during occult HBV infection
Detection of HBV DNA. Using HBV PCR, HBV-DNA was
demonstrated in liver, peripheral blood mononuclear cells
(PBMCs) and serum in HBsAg-negative patients [12,27–31].
HBV viral level in serum is usually less than 104 copies/mL
in patients with occult HBV infection [32], which is signifi-
cantly lower than in those with HBsAg-positive HBV infec-
tion [28,30]. Furthermore, even using sensitive HBV PCR,
serial observation revealed that HBV viraemia tends to
fluctuate in patients with occult HBV infection [33]. The
frequency of intrahepatic HBV-DNA is usually higher than
that of serum HBV-DNA in patients with occult HBV infec-
tion [28,30]. This may be explained by the fact that the liver
serves as the central location that supports HBV replication
in these patients. However, occult HBV infection can also
present as positive serum HBV-DNA in the absence of int-
rahepatic HBV-DNA [20]. This may represent a transient
phase of occult HBV infection. Alternatively, an unidentified
location may support ongoing HBV replication in these
� 2002 Blackwell Science Ltd, Journal of Viral Hepatitis, 9, 243–257
244 K.-Q. Hu
patients. Most studies [14,15,33,34], but not all [31], con-
firmed detectable intrahepatic HBsAg and HBcAg in a vari-
able proportion of these patients. However, the frequency of
intrahepatic HBcAg is usually lower than that of HBsAg.
These findings indicate an active translation of HBV pro-
teins, at least in some patients with occult HBV infection.
HBV-cccDNA, HBV-RNA, and viral transcription and replica-
tion. HBV replication involves a reverse transcription
process, in which HBV covalently closed circular DNA (HBV-
cccDNA) is used as template to synthesize HBV transcripts,
including pregenomic RNA. Therefore, both HBV-cccDNA
and HBV-RNA serve as the best markers indicating ongoing
active HBV replication and transcription. Using HBV PCR
techniques, both HBV-RNA transcripts and HBV-cccDNA
were demonstrated in liver and PBMC in a variable portion
of patients with occult HBV infection [28,29]. In seven pa-
tients who received anti-HBV treatment and had cleared
HBsAg for 6–76 months, four patients had detectable HBV-
RNA fragments, including three with both HBV-S and C
RNA and 1 with only HBV-S RNA [29]. Three of the seven
had detectable HBV-cccDNA. However, only one of these
three patients had detectable HBV-RNA fragments from both
S and C genes. In two other patients with positive HBV-
cccDNA, one had only detectable HBV-S RNA, but not C
RNA, and other one had neither detectable HBV-S nor C
RNA. Using similar techniques, eight of 14 liver donors with
occult HBV infection and positive intrahepatic HBV-DNA
had detectable HBV-cccDNA. However, only three of five
these patients had detectable HBV-RNA, indicating low
level of HBV transcription. Five of these eight patients had
detectable HBV-DNA fragments from both S and C genes,
indicating possible presence of full-length HBV-DNA in these
patients. In the other three patients, two had only detectable
S gene, one with C gene, suggesting possible deletion
mutations or integration of HBV genome [31].
The reason(s) for the inconsistence of HBV-DNA with
HBV-cccDNA and HBV-RNA in these patients remain
unclear. Failure to detect HBV-cccDNA in patients with
positive intrahepatic HBV-DNA or RNA may represent a
technical limitation of the sensitivity (i.e. false negativity) or
a truly low rate of viral replication in occult HBV infection.
On the other hand, detection of HBV-cccDNA in the absence
of S or C gene sequence questions the specificity of the
technique (i.e. false positivity) used in these studies. In fact,
the specificity of PCR-based detection of HBV-cccDNA is
questioned by the unexpected detection of HBV-cccDNA in
PLC/PRF/5 cell line, which contains only integrated HBV-
DNA rather than HBV-cccDNA [29]. However, most studies
indicate that an active, but low level of intrahepatic HBV
replication and transcription are maintained at least in some
patients with occult HBV infection.
Mechanisms of occult HBV infection. Although occult HBV
infection is a well-demonstrated clinical entity, the mecha-
nisms by which these patients maintain a low, but stable
level of viral replication remain to be defined. Based on
available data, several possibilities have been proposed.
HBV mutations in S region. Any mutation in the pre-S/S
region may cause alteration of HBsAg antigenicity and
inhibition of anti-HBs production. Although HBV nucleo-
tide sequencing revealed a wide range of HBV-DNA
mutations in pre-S/S region, studies have shown that most
mutations appear random and nonsense with undefined
implication. However, certain types of pre-S/S mutations
appear to be associated with occult HBV infection at least in
some patients. Amino acids (aa) 124–147 in the S region
comprise an ‘a’ determinant and contain a dominant B cell
epitope for anti-HBs response. A single aa substitution in this
region may mediate an escape from production of circulating
anti-HBs, resulting in breakthrough infection after HBV
vaccination or occult HBV infection [35–40]. In 33 patients
with positive anti-HBc and occult HBV infection, the
frequency of mutation in the major hydrophilic loop (MHL)
of S gene (aa 98–156) was 22.6/1000 aa, which was sig-
nificantly higher than that in the non-MHL of S gene in the
same group of patients (9.4/1000 aa) and HBsAg-positive
controls (n ¼ 36, 7.5/1000 aa for MHL, 12/1000 aa for
non-MHL) [32]. Although no single unique mutation was
identified, the increased frequency of mutations in the same
region may contribute to the immune-escape and patho-
genesis of the occult HBV infection at least in some of these
patients [32]. However, other studies showed that majority
of occult HBV infections are caused by wild-type HBV strains
[31]. A similar hypervariable region was also identified in a
domain of the HBV-DNA polymerase gene, which overlaps
with the S gene [32]. Thus, the impact of these mutations in
occult HBV infection remains to be determined.
Mutationsin the pre-S1 region may terminate the pro-
duction of HBV large S protein, reduce the formation of HBV
virions, and prevent interaction of HBV with hepatocytes.
However, the pre-S1 deletion is usually accompanied by
wild-type HBV infection in these patients [27]. The same pre-
S1 deletion is also present in patients with HBsAg-positive
HBV infection [41]. Thus, the precise relationship between
pre-S1 deletion mutations and development of occult HBV
infection remains to be defined. On the other hand, pre-S2
portion of HBV middle S protein possesses capability of
binding glutaraldehyde cross-linked human serum albumin
[42]. It remains unknown whether mutations in the pre-S2
region result in a diminished binding to glutaraldehyde
cross-linked human serum albumin, and reduced infectivity
of HBV, and eventually occult HBV infection. Recently, an
eight-nucleotide deletion in HBV core promoter/distal X
region was also demonstrated in patients with HCV and
occult HBV coinfection [15,22,43].
HBV integration. It is well known that the HBV-DNA sequence
can be integrated into the cellular chromosomal DNA during
� 2002 Blackwell Science Ltd, Journal of Viral Hepatitis, 9, 243–257
Occult HBV infection 245
both acute and chronic HBV infection [44–46]. Both integ-
rated and episomal HBV molecules were identified in patients
with occult HBV infection [44,46]. Although the presence of a
full-length genomic HBV DNA was demonstrated by South-
ern blot assay, or even cloned and sequenced in representa-
tive cases with occult HBV infection, the finding that different
regions of HBV genome can not be simultaneously coampli-
fied in a single patient with occult HBV infection questioned
the presence of a full-length HBV genome in these patients
[31]. This phenomenon may reflect integration of partially
deleted or fragmented HBV-DNA in these patients.
HBV-DNA integration may cause rearrangement of the
viral DNA sequence [46], which may alter the expression of
HBsAg and result in HBsAg-negative HBV infection. High
incidence of HBV integration has been associated with the
development of HCC during HBV infection, including occult
HBV infection [21,46]. Using Southern blot assay, early
studies confirmed that HBV integration occurs in patients
with occult HBV infection, especially those with HCC [46].
Using PCR amplification, a significantly higher frequency of
HBV-DNA integration (> 50%) was recently reported in
these patients [47–49]. In most of these patients, HBV
integration was more common in tumorous than in non-
tumorous tissue. It remains unclear so far whether other
factors, such as genetic instability, p53 gene mutation and
telomerase activity, play any contributory role in carcino-
genesis of HCC in patients with occult HBV infection,
although the related abnormality was reported in these
patients [48,49].
HBV infection of PBMCs. Studies have revealed a high fre-
quency of HBV-DNA in PBMCs during acute and chronic
HBV infection [50–52]. Further examination showed that
HBV was detectable in different subpopulations of PBMCs,
including monocytes as well as T and B cell subsets [51]. In
most patients, HBV-DNA presents in PMBCs in a full-length
and episomal form [50,52]. Some studies [50], but not all
[51,53], have identified HBV replicating intermediates in
PBMCs in a minority of these patients. Integrated HBV-DNA
was also occasionally detectable. The presence of HBV-DNA
in PBMCs has been associated with coexpression of HBV
proteins in these cells, serum HBeAg and HBV-DNA as well
as disease activity [50,52]. These findings suggest that HBV
infection of PBMCs is a common phenomenon during HBV
infection.
Identification of HBV-infected PBMC in HBsAg-negative
patients [53] supported the hypothesis that these cells may
serve as a reservoir to maintain HBV persistence in these
patients. Using both in situ hybridization and HBV PCR
techniques, it was found that detectable HBV-DNA in PBMC
could be maintained for as long as four years in some
patients after spontaneous or treatment-induced HBsAg
clearance [54]. As described above, active WHV replication
is also present in lymphatic tissue in woodchucks with occult
WHV infection [25]. These indicate a prolonged extrahepatic
harbouring of HBV or WHV during the occult infection in
patients as well as woodchucks.
The possible pathogenic role of HBV-infected PBMCs was
recently shown in patients undergoing liver transplantation
[55,56]. After liver transplantation for HBV-related liver dis-
ease, administration of high does of anti-HBs immunoglobulin
could maintain a negative HBsAg in serum and HBV-DNA in
liver, but PBMCs may remain harbouring HBV-DNA in these
patients [55]. The latter may serve as an HBV reservoir for
recurrent HBV infection. A direct comparison of HBV mutants
in a single patient with different proportions of HBV mutants
detected in liver and PBMCs indicated that after liver trans-
plantation the recurrent HBV infection is characterized by a
shifting of HBV strains from liver-derived predominance prior
to liver transplantation to a PBMC-derived predominance after
liver transplantation [56]. These findings indicate a direct role
of HBV-infected PBMC in recurrent HBV infection and selec-
tion of HBV mutant strains after liver transplantation.
HBV-containing immune complex. A few studies have dem-
onstrated that HBV particles persist in the blood after recov-
ery from acute self-limited hepatitis B, even in the presence of
anti-HBs [16–19]. The incidence of HBV-DNA positivity
could be as high as 91% and last longer than 2–3 decades in
these patients. In the early phase of acute hepatitis B, HBV
exists in both free and immunoglobulin (Ig)-bound (i.e.
immune complex) forms, which subsequently shifted to
Ig-bound predominance after seroconversion from HBsAg to
anti-HBs [18]. This indicates the persistence of serum HBV-
DNA as a form of immune complex with anti-HBs, which may
be attributed to occult HBV infection in these patients. It
remains unknown how the persistent circulating HBV-
containing immune complex is maintained in these patients.
On the other hand, HBV-containing immune complex was
not detectable in the sera from patients with occult HBV
infection secondary to a delayed HBsAg clearance from
chronic HBV infection [28], suggesting that HBV-containing
immune complex may not be involved in the maintenance of
persistent occult HBV infection in these patients.
Status of host immune responses. It is well known that the
outcomes of HBV infection depend on dynamic interaction
and balance of viral replication rate and host immune
response. Both humoural and cellular immune responses are
involved in virus elimination. An adequate multispecific
anti-HBV T cell response against HBV proteins results in
clearance of HBV infection, whereas, a defective immune
response causes persistence of HBV infection [57]. Theoret-
ically, a diminished host immune response may also be
involved in the development of occult HBV infection. It
is well known that immunosuppression contributes to
recurrent HBV infection after liver transplantation. Patients
who recovered from acute hepatitis B may maintain a very
low, but detectable serum HBV-DNA for a variable period of
time in the presence of adequate level of anti-HBV antibodies
� 2002 Blackwell Science Ltd, Journal of Viral Hepatitis, 9, 243–257
246 K.-Q. Hu
and anti-HBV-specific cytotoxic T lymphocytes (CTLs) [17].
These findings indicate certain limitation of host immune
response in eradicating HBV infection, at least in occult HBV
infection. Like human HBV infection, recovery of woodchuck
from acute WHV infection is preceded by astrong antiviral
cytokine (i.e. interferon (IFN)-c and tumour necrosis factor
(TNF)-a) and CD3 response [58]. The same study also
showed that an active intrahepatic antiviral cytokine re-
sponse, especially IFN-c, plays a central role in control of
occult WHV infection [58].
Virus interference. Studies on HBV and HCV coinfection
revealed a mutual viral interference between HBV and HCV.
Patients with acute HBV and HCV coinfection usually have a
delayed appearance, lower level, and shortened duration of
serum HBsAg. Human subjects and chimpanzee models with
chronic HBV and HCV coinfection tend to have a low level of
HBV-DNA and significantly enhanced HBsAg clearance [59].
Cotransfection studies revealed that HCV core protein may
be involved in the inhibition of HBV replication [60]. These
indicate that HCV coinfection may attenuate clinical pres-
entation and promote HBV clearance.
A high frequency of an eight-nucleotide deletion in HBV
core promoter/distal X region was recently reported in
patients with HCV and occult HBV coinfection [15,43,61].
Furthermore, this deletion mutation appears not to be
present in patients with active HBV infection alone or con-
trols [61]. Cotransfection of hepatoma cell line with HCV and
the HBV mutant strain isolated from patients with HCV and
occult HBV coinfection resulted in an increased and pro-
longed secretion of HCV-RNA to the culture supernatant.
This was not reproduced when the same cell line was
cotransfected with HCV and wild-type HBV [60]. Consistent
with this, patients with HCV genotype 1b and occult HBV
coinfection, and the eight-nucleotide deletion had signifi-
cantly higher serum HCV level than those with HCV infection
alone [15]. These provided both in vitro and in vivo evidence
that this eight-nucleotide deletion of core promoter/distal X
region may stimulate HCV replication in these patients.
However, it should be noted that approximately 30% of these
patients had variable coexisting wild-type HBV-DNA [15],
which should be able to support a normal, rather than a
diminished HBV replication in these patients. Furthermore,
besides this eight-nucleotide deletion, additional mutations
were also identified in some of these patients [15].
Other possible pathogenic mechanisms. Patients with HCC and
occult HBV infection have a high frequency of detectable HBV-
X DNA and RNA in liver tissue in the absence of full-length
HBV pregenomic RNA [62,63], indicating a selected accu-
mulation of X gene-related DNA and RNA molecules in the
absence of HBV replication in these patients. However, HBV-X
gene integration was demonstrated in both tumorous and
nontumorous tissue uncommonly, and usually coexisted with
other HBV genes in most patients [38,49]. In addition, trans-
acting function of HBV-X gene relies on the presence of X
protein rather than HBV-X DNA or RNA. Thus, it will be
critical to examine whether X protein is also over expressed in
HCC patients with occult HBV infection.
Based on HBV nucleotide sequencing, HBV can be classi-
fied into six different genotypes designated from A to F [64].
Several studies have revealed that HBV genotype may be
associated with disease activity, prognosis and, treatment
response [65–67]. It was recently reported that in patients
with occult HBV infection, 61% carried HBV genotype D,
while 53% of HBsAg-positive patients had genotype A
infection [32]. These indicate that HBV genotype may be
involved in maintaining occult HBV infection.
In summary, the maintenance of occult HBV infection
appears to be associated with interaction between viral and
host factors, and may involve different mechanisms. Muta-
tions or integration of HBV-DNA sequence may alter
expression of HBV proteins, resulting in undetectable HBsAg
and possible escape of host immune responses. Persistent
HBV infection may also be maintained by extrahepatic HBV
replication, such as HBV-infected PBMC. Antibody response
usually facilitates HBV clearance, but persistent circulating
HBV-containing immune complex may cause a status of
HBsAg-negative viraemia. An adequate cellular-mediated
immune response may terminate active HBV infection, but
probably fail to eradicate a low level of HBV infection.
Coinfection of HBV with other hepatotropic viruses may
result in virus interference and inhibition of HBV replication.
It remains to be determined whether HBV genotype and
mutations in HBV nonpre-S/S regions play any pathogenic
role in occult HBV infection.
CLINICAL IMPLICATIONS OF OCCULT
HBV INFECTION
Prevalence of occult HBV infection
Although studies on occult HBV infection have been exten-
sive, the precise prevalence of this clinical entity remains
very difficult to define for several reasons. First of all, the
clinical scenarios of occult HBV infection vary from conva-
lescence of acute self-limited hepatitis B, chronic asympto-
matic carrier status to delayed HBsAg clearance complicated
by cirrhosis or HCC. Second, most studies have involved
small sample sizes, heterogeneous populations, and lacked
controls. Third, although HBV-DNA PCR represents the most
sensitive test currently available for detecting HBV-DNA
molecules, its sensitivity is variable. In addition, a high rate
(10/19) of false positive HBV-DNA PCR was recently
reported in patients with only positive anti-HBc [68]. Thus, a
highly specific and sensitive HBV PCR amplification is
essential for determining the precise prevalence of occult
HBV infection. Fourth, it has been well demonstrated that
the prevalence of occult HBV infection is dependent on the
interval from HBsAg clearance to assessment of HBV-DNA.
� 2002 Blackwell Science Ltd, Journal of Viral Hepatitis, 9, 243–257
Occult HBV infection 247
The longer the interval from HBsAg clearance, the lower the
prevalence of occult HBV infection. In addition, HBV vira-
emia may fluctuate during occult HBV infection. All these
factors should be considered in determining the prevalence
of occult HBV infection.
Table 1 provides an overview of the prevalence of occult
HBV infection [14,21,22,32,33,48,49,69–76]. Clearly, the
incidence of occult HBV infection is consistent with the
baseline prevalence of overt HBV infection in a certain
region. However, one should keep in mind that as these
reports represent different clinical entity and geographical
distribution of occult HBV infection, direct comparison
remains impossible. Recently, Bre´chot et al. reviewed the
epidemiology of occult HBV infection extensively, which
provides additional valuable references [6].
Occult HBV infection and transmission of HBV infection
The risk of HBV transmission through blood transfusion of
HBsAg-negative individuals was recognized more than two
decades ago [9]. This was further confirmed by the findings
that cloned full-length HBV-DNA from patients with occult
HBV infection caused a typical HBV infection in chimpanzee
models [16]. In addition, vertical (i.e. perinatal) transmission
of HBV from a mother with occult HBV infection to offspring
was reported in both human subjects [77] and woodchuck
animal models [25].
Although in the developed countries, the risk of transfu-
sion-transmitted viral hepatitis has been extremely low, the
risk of transfusion-transmitted HBV infection (1/63 000)
remains much higher than that of transfusion-transmitted
HCV infection (1/103 000) [78]. It is possible that occult
HBV infection may be contributory to the relatively high risk
of transfusion-transmitted HBV infection.
HBV transmission and de novo infection from occult HBV
infection may take place during organ transplantation. It
was well documented that occult HBV infection may serve as
a sourceof infection in liver transplant recipients [79–82].
The origin of HBV infection can be from a liver donor with
HBsAg-negative, but anti-HBc-positive serologies. Although
serum HBV-DNA is not always detectable, HBV-DNA has
been identified in the liver from donors with occult HBV
infection [79–82]. Nucleotide sequencing of serum HBV-
DNA has shown a very high homologuey between donor
and the recipient [79]. These findings support that HBV
infection was from the donor with occult HBV infection. HBV
infection of the allograft may also recur in patients with
pretransplant occult HBV infection. The capability of PBMC
to maintain HBV replication in patients with occult HBV
infection [28] indicates that PBMC may serve as a reservoir
for recurrent HBV infection in these patients. This is
supported by the finding of shifting of predominance of
PBMC-derived HBV strains after liver transplantation in
these patients [56].
Table 1 Prevalence of occult HBV infection in relation to clinical entity and geographic distribution
Clinical entity
Anti-HBc (+) HBV DNA (+)/anti-HBc (+) HBV DNA (+)/all HBsAg (–)
[reference number] Ratio (%) Ratio (%) Ratio (%)
Subjects without liver disease
USA [69] 97/258 (37.6) 11/97 (11.3) 11/258 (4.3)
China [70] 214/2289 (9.3) 74/214 (34.6) 74/2289 (3.2)
Blood donors
USA [71] 158/26492 (0.6) 0/110 (0.0) N/A*
UK [72] 584/103867 (0.6) 0/103867 (0.0) N/A
Chronic hepatitis
USA [73] 8/8 (100.0) 4/8 (50.0) N/A
Germany [32] 357/357 (100.0) 33/357 (9.2) N/A
India [74] 0/203 (0.0) N/A 22/203 (10.8)
Spain [33] 22/101 (21.8) 6/22 (27.3) 19/101 (18.8)
France [14] N/A 15/30 (30.0) N/A
HCC
Mixed [21] 10/25 (40.0) 8/10 (80.0) 17/28 (60.7)
Europe [75] 89/301 (29.6) 28/44 (63.5) 135/405 (33.3)
Japan [49]� 16/26 (61.5) 0/26 (0.0) 3/26 (11.5)
Japan [48]� 13/16 (81.3) 6/13 (46.2) 8/16 (50.0)
HCV coinfection
Japan [76] 18/65 (27.7) 34/65 (52.3) 34/65 (52.3)
Italy [22] 46/200 (23.0) 46/46 (100.0) 66/200 (33.0)
*N/A, not available; �with HCV coinfection.
� 2002 Blackwell Science Ltd, Journal of Viral Hepatitis, 9, 243–257
248 K.-Q. Hu
Overall, the risk of acquiring HBV infection from liver
donors with occult HBV infection ranges from 25% to 94%
[81–83]. Besides serum HBsAg, HBV-DNA and HBV-RNA
may also become detectable in these patients. The precise
risk of recurrent HBV infection after liver transplant in a
patient with pretransplant occult HBV infection remains
unknown. However, these patients tend to have a shortened
survival than those who acquired HBV infection from donors
with occult HBV infection [79].
Clearly, occult HBV infection may alter the long-term
outcomes of liver transplantation. Thus, it is important to
rule out occult HBV infection both in organ donor and
recipient prior to a liver transplantation. A recent review has
well highlighted the diagnosis and management of occult
HBV infection in liver or other organ transplantation [83].
The prevalence of occult HBV infection is also high in
patients receiving haemodialysis, ranging from 14% to 19%
[84,85]. Both HBV-DNA and RNA are detectable in serum
and PBMC in these patients, indicating active virus replica-
tion [85]. The existence of anti-HBs suggests an insufficient
neutralization of HBV in these patients. The risk of acquiring
HBV infection seems much less from a kidney donor than
from a liver donor with occult HBV infection [80]. De novo
HBV infection was also reported in a patient with HBsAg-
negative and anti-HBs-positive serology after kidney trans-
plantation. Retrospectively, HBV-DNA was detected by HBV
PCR from pretransplantation serum specimen in this patient
[86].
The risk of a recipient of bone marrow transplantation
(BMT) to acquire HBV infection from a HBsAg-positive donor
is generally low, and only less than 50% would develop
chronic HBV infection after transplantation [87]. There is an
even lower risk of acquiring HBV infection from an HBsAg-
negative donor. However, in bone marrow recipients with
pretransplant occult HBV infection, reactivation of the latent
infection may sometimes occur and lead to fulminant hep-
atic failure (FHF) [88].
Occult HBV infection and HBV-related chronic
liver disease
Histological assessment of occult HBV infection secondary to
acute hepatitis B has been very limited. Bla¨ckberg et al.
assessed 16 patients who had acute self-limited hepatitis B
30 years prior to enrolment [19]. None had clinical evidence
of chronic liver disease, but all were positive for anti-HBc,
and 11/16 were positive for anti-HBs. Four of these patients
with positive anti-HBc, including three with positive anti-
HBs, underwent repeated liver biopsy 30 years after an
acute self-limited hepatitis B. Although they all had unde-
tectable HBV-DNA in serum and PBMC, 2/4 maintained
positive HBV-DNA in liver. The HBV strains detected in the
liver tissue were homogeneous to those originally infected
30 years ago. In these two patients with intrahepatic HBV-
DNA, both had histological evidence of mild inflammation,
and one had mild elevation of ALT. These findings suggest
that although occult HBV infection may cause mild and
chronic liver injury in some individuals, but the long-term
outcomes of this condition are usually benign in patients
with acute self-limited hepatitis B.
Fong et al. evaluated ALT level and histological activity of
the liver biopsy in 11 patients after IFN treatment and
HBsAg clearance for 0.6–9 years [89]. They found that
although pretreatment ALT was elevated in all these pa-
tients, ALT became normal in nine patients and mild ALT
elevation remained in two other patients after HBsAg
clearance. Although histological activity of the liver was
improved, no patient achieved completely normal histology.
The spectrum of disease varied from chronic persistent
hepatitis (n ¼ 6), chronic active hepatitis (n ¼ 2) to inactive
(n ¼ 1) or active (n ¼ 2) cirrhosis. Thus, HBsAg clearance is
associated with an overall improvement of biochemical and
histology, but mild liver injury continues and is associated
with low level of detectable HBV-DNA in these patients.
The outcome of patients with chronic hepatitis B and
delayed HBsAg clearance remains controversial. While some
studies have shown a beneficial impact of HBsAg clearance
[89–92]; other studies suggested a higher incidence of dis-
ease progression and development of complication after
HBsAg clearance [93,94]. Detectable serum HBV-DNA can
be maintained a year after HBsAg clearance, but repeated
liver biopsy showed no histological activity in these patients
[90]. In addition, none of the 75 Taiwanese patients devel-
oped cirrhosis or HCC during a mean of 60-month follow-up
after HBsAg clearance from chronic HBV infection [92]. In
contrast, other studies from the same endemic area showed
as high as 32.7% (18/55) of the patients developed either
cirrhosis, HCC or liver failure after delayed HBsAg clearance
[93]. Similarly, four of seven Japanese patients with delayed
HBsAg clearance showed disease progression associated with
development of HCC [94]. Multiple factors may have been
contributed to the discrepancy of the results. First of all, the
latter two reports might have involved patients who had
extensive pre-existing liver injury or carcinogenic process,
since the mean interval from HBsAg clearance to develop-
ment of the complications is extraordinarily short in these
patients. Second, using clinical, rather than histological
criteria for diagnosing cirrhosis [93] may underestimate the
incidence of cirrhosis or even early HCC during enrolment.
Third, coinfection of other hepatotropic viruses (i.e. HCV)
may have served as a confounding factor. Thismay be
indicated by the fact that a high incidence of elevated ALT
after HBsAg clearance in these patients [93].
Spontaneous HBsAg clearance is also seen in approxi-
mately 5% of Chinese patients with decompensated cirrhosis
during a mean 9-year follow-up [95]. Most of these patients
had a negative HBeAg test and milder liver disease prior to
HBsAg clearance. Although a complication may develop
after the delayed HBsAg clearance, these patients tended to
have less ascites and overall better survival. This indicates
� 2002 Blackwell Science Ltd, Journal of Viral Hepatitis, 9, 243–257
Occult HBV infection 249
that a beneficial impact of delayed HBsAg clearance is pre-
sent even in patients with advanced liver disease.
The frequency of HBV-DNA in patients with cryptogenic
chronic liver disease varies depending on the baseline pre-
valence of HBV infection in certain geographical area, pop-
ulation studied, and techniques used to detect HBV-DNA.
For instance, the prevalence of serum HBV-DNA as assessed
by HBV PCR was 10.8% in patients with chronic hepatitis,
but without HBV and HCV markers in India [74]. A recent
report from Spain assessed prevalence of occult HBV or HCV
infection in 109 patients with unknown aetiology of elevated
liver enzymes, and negative HBsAg and anti-HCV [34]. It
was found that 19% of these patients had detectable HBV-
DNA, which is significantly higher than the incidence of
controls with overt HBV infection (9.9%). In 18 cases with
occult HBV infection, additional HBV serologies revealed five
with both anti-HBc and anti-HBs, one with anti-HBc alone,
three with anti-HBs alone, and nine with neither anti-HBc
nor anti-HBs. More importantly, histological evidence of
chronic hepatitis and cirrhosis was confirmed in 62% of
these 19 patients with occult HBV infection [34].
Chemin et al. examined 50 patients with ‘cryptogenic’
chronic hepatitis, and found that 15 (30%) of them had
detectable HBV-DNA, indicating an occult HBV infection
[14]. Among these 15 HBV-DNA-positive patients, 66% had
elevated liver enzymes and 53% had severe fibrosis or cir-
rhosis. During follow-up, repeated liver biopsy demonstrated
that 2/11 (18.2%) had progressed from chronic hepatitis to
cirrhosis. These findings indicate that occult HBV infection is
a common aetiology of ‘cryptogenic’ chronic hepatitis and a
progressive disease at least in some patients.
Taken together, the outcomes of the occult HBV infection
after a delayed HBsAg clearance vary significantly and may
depend on the type of underlying liver diseases, duration of
active HBV infection and extent of liver injury that had
occurred before HBsAg clearance, interval from HBsAg
clearance to the time of assessment, and presence of coin-
fection with other viruses. Although some studies indicated
that these patients appear to have more favourable short-
term outcomes than active chronic HBV infection, even in
patients who have advanced liver diseases, these were not
supported by other studies. Clearly, better designed, pros-
pective, and controlled clinical trials are required to deter-
mine the long-term outcomes of this clinical entity.
Occult HBV infection and reactivation
of chronic hepatitis B
Immunosuppresive or cytotoxic agents may suppress host
immune responses to HBV infection, promote viral replica-
tion and disease progression. Withdrawal of these agents
causes regain of host immune responses and development of
immune-mediated destruction of HBV-infected hepatocytes
and liver damage. Increased frequency of occult HBV infec-
tion has been reported in immunocompromised patients. For
instance, in HBsAg-negative Chinese patients with cancer,
the prevalence of positive serum HBV-DNA could be as high
as 11.6%, which was significantly higher in HBsAg-negative
healthy subjects (1.8%) [96]. Reactivation of chronic hepa-
titis B, or even FHF was reported in patients with occult HBV
infection who underwent cytotoxic therapy, autologous
stem cell transplantation, or allogeneic BMT for lymphoma,
aplastic anaemia or leukaemia [97–99]. However, cytotoxic
therapy-induced reactivation of chronic hepatitis B was
significantly less common and severe in patients with
pretreatment occult HBV infection than in those with pre-
treatment HBsAg-positive chronic hepatitis B [100].
Occult HBV infection and fulminant hepatic failure
Viral hepatitis is one of the most common causes of FHF. It is
estimated that approximately 1% of hepatitis B cases are
complicated by FHF. The pathogenic role of occult HBV
infection in development of FHF is ill-defined. HBV-DNA was
detected in HBsAg-negative FHF in a significant proportion
of cases, which ranged from 0 to 47% [101–108]. This dif-
ference may reflect a geographical variation of baseline
prevalence of overt HBV infection. Intrahepatic expression of
HBV antigen [103] and recurrence of HBV infection after
liver transplantation for FHF [102] indicate possible HBV
replication in these patients. However, this does not neces-
sarily support a causal role for occult HBV infection in FHF
pathogenesis. Recently, a multicentre prospective study
indicated that occult HBV infection appears not associated
with acute liver failure in the US [108]. Fulminant hepatitis
B (FHB) was reported in a patient 22 months after BMT [99],
who had negative HBsAg, positive anti-HBs and intrahepatic
HBV-DNA prior to BMT. However, it is difficult to determine
whether this FHB is directly associated with occult HBV
infection or secondary to the immunosuppression after BMT.
Various HBV mutations have been identified in patients
with FHB secondary to an active HBV infection [109]. A
functional analysis showed that mutated HBV genomes
isolated from most FHF patients have comparable compet-
ence of replication and production of viral protein with the
wild-type HBV [110]. Inoculation of serum from patients
with infection of precore mutant HBV strain and FHB
reproduced HBV infection in chimpanzees [111], indicating
the infectivity of the mutant HBV. Although precore stop
codon mutation is confirmed in some patients with FHB, the
same mutation is also present in asymptomatic HBsAg car-
riers [109]. Mutations in pre-S2 region were also reported in
limited cases with FHB [109]. It remains unclear whether
occult HBV infection secondary to mutation in the S region
is associated with FHB. HBV-DNA sequencing of partial or
whole HBV genome from patients with FHB showed variable
mutations, but failed to confirm a distinct one specifically
associated with FHB [111–113]. On the other hand,
coinfection of other hepatitis viruses appears a common
phenomenon in these patients [114–116].
� 2002 Blackwell Science Ltd, Journal of Viral Hepatitis, 9, 243–257
250 K.-Q. Hu
Occult HBV infection and HCC
The association of overt HBV infection with HCC has been
well established. However, it remains controversial whether
occult HBV infection may also attribute to the pathogenesis
of HCC. The frequency of occult HBV infection varies signi-
ficantly in patients with HCC [21,117–120]. The prevalence
of anti-HBc/anti-HBs was 43% in these patients [21];
whereas, the prevalence of HBV-DNA varied from 5% to
80% [21,75,117–120]. Increased risk of developing HCC
was also reported in individuals with anti-HBs as the only
marker of past HBV infection [121].
HBV-DNA was detectable in both tumorous and nontu-
morous liver tissue in patients with HCC [21,117,118].
Intrahepatic HBV-DNA exists in episomal or integrated pat-
tern [21,117]. Using multiple sets of oligoprimers derived
from different regions of HBV genome, HBV-DNA can be
amplified simultaneously in nontumorous and tumorous
tissuein some [21,118,122], but not the majority of patients
[21,122]. This implies that in most patients, intrahepatic
HBV-DNA may present with deletions or truncations, which
may be associated with integration. In addition, the frag-
mented or integrated HBV genome was more frequently seen
in tumorous than in nontumorous tissue in patients with
HBsAg-negative HCC [21,62,122]. Although HBV subge-
nomic RNA was sometimes demonstrable, pregenomic HBV-
RNA was not detectable in patients with HBsAg-negative
HCC [62], indicating inactive HBV replication in these
patients.
It has also been well demonstrated that HCV infection is
associated with an increased risk of HCC [117,123]. Active
coinfection of HCV and HBV may significantly enhance the
risk for HCC in these patients [124,125]. Growing evidence
revealed a high frequency of occult HBV infection in patients
with HCV-related HCC. The prevalence of anti-HBc and/or
anti-HBs varies from 50% to 90% in patients with anti-HCV-
positive HCC. The prevalence of serum and intrahepatic
HBV-DNA ranges from 0% to 18%, and 13% to 80%,
respectively, in these patients [76,117,118,120,126–128].
The level of intrahepatic HBV-DNA is usually low in these
patients [129].
Likewise, the pathogenic role of occult HBV infection in
HCV-related HCC remains to be determined. Compared to
patients with HCV infection alone, patients with HCV and
occult HBV coinfection may develop HCC in a shorter
interval from HCV acquisition [128] and before cirrhosis is
established [126]. However, the estimated relative risk of
HCC was only twofold increased, which is less significant
than the presence of cirrhosis and history of heavy smoking
[63]. In 16 HCC patients with negative HBsAg and positive
anti-HCV, the frequency of HCV-RNA in tumorous and
nontumorous was 87.5% and 75.5%, respectively. Most of
these patients had ongoing HCV replication in both tissues
[48]. Although HBV integration was confirmed in only
12.5% of patients by Southern blot, it was 50.0% in
tumorous tissue and 43.8% in nontumorous tissue by PCR
amplification. Coexistence of HCV-RNA and integrated HBV-
DNA was seen in 43.8% of tumorous, but only 25.0% of
nontumorous tissue. These findings suggest that HBV
integration rather than active HCV replication is more likely
to contribute to the hepatocarcinogenesis in these patients.
Since most of these patients have only low level of HBV-DNA
and possible disruption of HBV genome by deletion or
truncation in liver [21,122], especially in tumorous tissue,
competent and active HBV replication appears unlikely.
Instead, HBV integration may cause rearrangement and
activation of the cellular genes and carcinogenesis in these
patients.
It should be noted that besides HBV, other environmental
and host factors may also be associated with the pathogen-
esis of HCC [48,49,130]. All these factors should be simul-
taneously considered when assessing possible carcinogenic
role of occult HBV infection.
Occult HBV infection and chronic HCV infection
Because of similar transmission modes, HBV and HCV
coinfection is a common clinical presentation. Early studies
have revealed that a high prevalence of anti-HCV antibody
was more associated with the presence of anti-HBc, rather
than HBsAg positivity [131,132]. The reported prevalence of
anti-HBc in patients with chronic HCV infection ranges from
50% to 55% [75,133,134]. Using PCR amplification, most
studies [15,22,61,135–138], but not all [139], demonstra-
ted the presence of HBV-DNA genome in 22% to 87% of the
patients with HBsAg-negative and HCV-RNA-positive tests.
A higher incidence of HBV-DNA was more commonly seen
in patients with anti-HBc-positive (46%) than those with
anti-HBc-negative serology (20%) [22]. The frequency of
detectable HBV-DNA was significantly higher in patients
with chronic HCV-related liver disease than in those with
nonHCV-related liver diseases [22]. In the former group,
approximately 62% (45 of 73) [22] and 19% [140] had also
detectable serum and liver HBV-DNA, respectively. These
findings indicate an active HBV replication and release of
HBV virion into the circulation in these patients. Therefore,
concomitant HCV and occult HBV coinfection is a frequently
encountered clinical entity in patient with chronic HCV
infection. However, follow-up HBV-DNA PCR has shown a
variable prevalence of occult HBV infection in these patients
[138], indicating a fluctuating HBV viraemia during occult
HBV infection.
The clinical impact of HCV and occult HBV coinfection
remains controversial. Some studies [22], but not all [138],
have shown that patients with HCV and occult HBV coin-
fection had a significantly higher incidence of cirrhosis than
those with HCV infection alone [22]. Consistent with this, a
relatively low incidence of HCV and occult HBV coinfection
was seen in patients with noncirrhotic chronic hepatitis C
[136]. Occult HBV coinfection was also associated with a
� 2002 Blackwell Science Ltd, Journal of Viral Hepatitis, 9, 243–257
Occult HBV infection 251
higher level of ALT and histological activity [15,61]. How-
ever, other studies suggested that the incidence of liver-
related complications in these patients appears comparable
to that in patients with HCV infection alone [134,138].
Patients with active HBV and HCV coinfection tend not to
respond to IFN treatment [59]. Such decreased IFN response
rate was also demonstrated in patients with HCV and occult
HBV coinfection by some [15,22,140], but not all [141]
studies. In patients with HCV and occult HBV coinfection,
IFN therapy may facilitate HCV clearance while HBV-DNA
remains detectable [140]. HCV-RNA quantitative assay
revealed a significantly higher HCV level in patients with
occult HBV coinfection than in those with HCV infection
alone [15]. Although this same study showed that HCV
genotype 1b was a more frequent coinfection in these
patients, this is not supported by other studies.
Data remain lacking on long-term dynamic interaction of
HBV and HCV virological markers from early active HBV
and HCV coinfection to HCV and occult HBV coinfection,
disease progression and complication in these patients.
Indeed, most previous studies were unable to define acqui-
sition models, the sequential order and duration of the
infection [15,22,61], which may affect outcomes in these
patients [59].
Human hepatoma cell line was successfully transfected by
full-length HBV-DNA genome cloned from patients with
HCV and occult HBV coinfection [61], indicating replicating
capacity of HBV in these patients. However, it remains
unclear whether HBV-cccDNA is frequently detectable in
these patients. It appears that only a small portion of these
patients might have the entire HBV genome in liver and
serum as determined by different HBV oligoprimers
[22,136]. The high frequency of fragmented HBV genome in
liver tissue indicates possible HBV-DNA integration into the
cellular DNA in these patients. Likewise, the presence of
circulating fragmented HBV-DNA may be secondary to HBV
subgenomic expression of the integrated HBV-DNA. Unlike
patients with HCC [62,63], HBV-X DNA sequences can only
be identified in 22% serum [136] and 14% liver tissue [22]
in patients with chronic hepatitis or cirrhosis. These findings
appear not to support an important pathogenic role of X
gene over expression in these patients as assumed in patients
with HCC. As discussed above, several independent studies
identified an eight-nucleotide deletion mutation in the core
promoter/distal X region in patients with HCV and occult
HBV coinfection [15,61]. Further investigation is required to
determine whether this mutation contributesto the patho-
genesis and disease progression in these patients.
CONCLUSION AND PERSPECTIVES
In conclusion, extensive studies have demonstrated that
occult HBV infection represents a special form of HBV
infection with clinical relevance. Perhaps, the most convin-
cing clinical impact of occult HBV infection is the risk of HBV
transmission through transfusion, organ transplantation,
and perinatal route. Although exacerbation of the disease
have been well documented in patients with occult HBV
infection during cytotoxic or immunosuppressive therapy,
the frequency of this complication is unknown. It is also well
demonstrated that an increased incidence of HCC is fre-
quently seen in patients with occult HBV infection, especially
those with HCV coinfection. It remains uncertain whether
occult HBV infection plays any direct pathogenic role in
fulminant hepatitis, and cryptogenic liver diseases, and
whether it alters natural history and anti-HCV treatment
response in patients with chronic hepatitis C.
In order to understand the precise pathogenic role and
clinical implications of occult HBV infection, we have to
better define the epidemiology of this entity. However, more
reliable results will rely on uniform definitions and diag-
nostic criteria of occult HBV infection, and standardized
techniques for HBV-DNA detection. For definition, the term
‘occult, latent, or silent HBV infection’ is sometimes confu-
sing and even misleading. In the most situations, occult HBV
infection means absence of HBsAg and existence of a low
level of HBV infection, but not necessarily silent or latent. To
this author’s opinion, the term ‘HBsAg-negative HBV infec-
tion’ may better define this clinical entity. Even in the group
of HBsAg-negative HBV infection, patients may carry dif-
ferent HBV serological markers. For instance, the incidence
of detectable HBV-DNA is much higher in patients with anti-
HBc than in those without any HBV markers. Since fre-
quency of HBV-DNA may vary from time to time and depend
on the interval from HBsAg clearance to the time of
assessment in these patients, the diagnosis criteria should be
adjusted by these factors. In addition, the prevalence of
HBsAg-negative HBV infection should also be adjusted by
the baseline prevalence of overt HBV infection in the same
geographical area. Being the most sensitive technique, the
specificity of HBV-DNA PCR is always a concern. It is
important to rule out false positive HBV-DNA PCR in these
patients. In order to make the results comparable among
studies, a standardized PCR technique should be employed.
The pattern (coinfection vs. sequential superinfection),
mode (perinatal vs. adulthood), and duration of viral
acquisition, which had not been well addressed in the
previous studies, may affect clinical impact of HBsAg-
negative HBV infection. Other factors that may attribute to
the pathogenesis of occult HBV infection include the
presence of other viral infection, comorbid conditions (e.g.
age, gender, body weight and other medical conditions),
and alcohol consumption. All these factors should be
simultaneously considered when assessing the impact of
occult HBV infection in these patients. Improved tech-
niques for analysing HBV-RNA, HBV-cccDNA, HBV integ-
ration and HBV genotyping will provide a comprehensive
approach to understand the molecular mechanism of
HBsAg-negative HBV infection. Application of animal
models and in vitro expression system will provide a
� 2002 Blackwell Science Ltd, Journal of Viral Hepatitis, 9, 243–257
252 K.-Q. Hu
valuable alternative approach to long-term and dynamic
observation of this entity. Our future clinical studies should
also focus on long-term, prospective and controlled multi-
centre trials combined with virological, immunological,
histological, and prognostic aspects.
ACKNOWLEDGEMENTS
This work was supported by an institutional grant from the
Division of Gastroenterology/Hepatology and Transplanta-
tion Institute, Loma Linda University School of Medicine.
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