Occult Hepatitis B Virus: Implications in Endemic Region like Nigeria.

: The situation whereby there is appearance of HBV DNA in the absence of observable hepatitis B surface antigen (HBsAg) has been described as occult hepatitis B virus infection. The above situation was first mentioned in late 1970s and its prevalence depends on the differences in the distribution of HBV in each region or community, the study population, sensitivity and specificity of the adopted screening method. The sero-prevalence of HBsAg in Nigeria is estimated to range from 10-40% and this qualifies Nigeria as hyper- endemic area (Odemuyiwa et al ., 2001; Fasola et al ., 2008). There is also observable increase in detected HBV DNA among patients who were previously screened as negative for HBsAg (Adeniyi et al ., 2022). Furthermore, high prevalence of HBsAg has been reported in the population of screened blood donors (Ejele and Ojule, 2004) and also in patients attending clinics (Nwokedi et al , 2011; Adeniyi et al ., 2022). Occult HBV is clinically important as it can be contacted by means of blood transfusion and organ transplant among others (Kwak et al ., 2014). In Nigeria, there has being increase in prevalence of occult HBV despite ongoing immunization program (Opaleye et al ., 2015; Adeniyi et al ., 2002). When compared to similar viruses that could be transmitted during transfusion, occult HBV is more common (Comanor et al ., 2006).


Introduction
In the decades after World War II, various studies begin to differentiate the differences in critical hepatitis. Krugman et al. (1967) documented two types of hepatitis: serum hepatitis (SH) and infectious hepatitis. Hepatitis B, which was formally referred to as serum hepatitis, is transmitted through body fluids. Study to identify this virus was conducted by Prince et al. (1964) and also by Blumberg et al. (1965). Identification of variance in genetic makeup in various community or region was conducted using polyclonal antibodies to test for presence of proteins in human serum (Blumberg et al., 1965). The human serum contains antibodies that are capable of coagulating proteins different in genetic makeup from the blood recipients. In a study conducted, an indigenous Australia was tested to have a new antigen which was then named after Australia (Au) (Blumberg et al., 1965). One of the study conducted by Prince et al. (1964) identified an antigen denoted by SH similar to above Australia antigen (Au) and both are identical to antigen found on the surface of hepatitis B (HBsAg) (Prince et al., 1964;Krugman et al., 1967). The above studies have helped in serological detection of HBV and in different epidemiological studies. It is not new that HBV infection has affected over 300 million individual globally. The infection is presented in various clinical ways from asymptomatic status to people with various forms of liver problem resulted from HBV infection (Lee, 1997). One of the means of detecting HBV infection is by the presence of its surface antigen (HBsAg) in the circulation. There is now more sensitive molecular biology technique which has made it possible to identify presence of HBV in people without HBV www.cmhrj.com surface antigen, with presence or absence of antibody to the surface antigen (anti-HBs) and also HBV core antigen (anti-HBc) (Bréchot et al., 1981;Loriot et al., 1992). There is no doubt that many people around the world are now been exposed to HBV (Lavanchy et al., 2004) and of which majority of this population are ignorant of their HBV status. One could conclude that the course of HBV infection is complex, considering environmental factors, virus and the infected individual which all play important roles in determining the outcome of disease.

Classification of HBV
Hepatitis B virus belongs to Hepadnaviridae family  with two genera namely, orthohepadnaviruses (infecting mammals) and those common among birds, Avihepadnaviruses (Fauquet et al., 2005). All the viruses classified under this family have a significant similarity in genome organisation and replication. Asides from viruses in the family Spumaviridae, Hepadnaviruses is a DNA virus that infects animals and replicates using reverse transcription from viral RNA. Studies have shown more than 35% divergence in genetic makeup of orthohepadnaviruses when compared to more than 19% found in avihepadnaviruses (Guo et al., 2005). HBV has small genomic size of about 3-3.3kbp with unique arrangement which makes the virus stand out and these are criteria for designation of Hepadnaviridae into a distinct family (Howard, 1986). The understanding of the potent DNA sequence similarity and almost a complete lack of homology exhibited by members of this family formed the basis for assigning the viruses into two different genera: orthohepadnaviruses and avihepadnaviruses. On the other hand, the allocation of individual members of this group into above genera depends largely on the viral host preference. Human HBV are assigned into different eight groups of genotypes from A to H depending on their genetic variance which is put at 8% but less than 17%. The supposed number ninth group would have been group I but has less than 8% divergence (Olinger et al., 2008).

Structure of HBV
There are three viral particles recognised with the use of electron microscopy and these are two spherical shaped structures of around 20nm in diameter and also the filaments shaped structures of about 22nm in width (Fig. 1). The two above are not infectious though they have viral surface antigen (HBsAg) but lack nucleic acids. The third one is HBV virion also known as Dane particles. The diameter is around 42 nm, has envelope that contains lipid embedded with HBsAg which surround the HBV core antigen (HBcAg) (Gerlich et al., 1980).

Genomic Organisation of HBV
The genome of HBV is described as partially double stranded DNA which is circle and is 3.2 kb pairs (Figure 2). At the 5prime end is attached, with covalent bond, the viral polymerase to the negative strand (Gerlich et al., 1980). According to work done earlier on HBV, the genome has four open reading frames (ORF) namely: S, C, P and X. They are in the same direction (which defined the plus and minus strands of HBV DNA), and these ORF help in translation of seven proteins. This means that a small genome contains a large amount of coding information. The P ORF runs almost the entire length of the genome thereby overlaps other segments like C and X and covers S portion of the genome. The genome also has some other characters like promoters, enhancer and polyadenylation signals. HBV transcripts are encoded by minus strand, according to Elders et al. (1985), and are capped and polyadenylated. The lengths of important transcripts are pre-C/C (3.5 kb), pre-S (2.4 kb), S (2.1 kb) and X (0.7kb) (Enders et al., 1985). Figure 1: Electron micrograph of circulating forms of HBV particles in the blood is shown at the top and a schematic drawing of Dane particle, the infectious HBV particle, is shown at the bottom with various structural features (Liang, 2009). Considering the 3.5kb transcript from preC/C mRNAs, it has two longer RNAs referred to as pre-core (pre-C) mRNAs and also a short one referred to as pregenome RNA (PgRNA) which is necessary for DNA synthesis (Ganem and Schneider, 2001). pre-C protein also referred to as HBeAg is important in regulating immune response to HBV infection (Chen et al., 2004). HBeAg is also important in predicting current infection. Pol gene is important in the production of DNA polymerase of the virus and has N-terminus necessary for terminal protein production which functions as primer for negative sense DNA production synthesis (Lin and Anderson, 2000). The section of C-terminal is important in production of reverse transcriptase while PreS/S genes are important in the production of viral envelope glycoproteins. S protein is usually the common envelope protein in the viral structures (Bruss and Ganem, 1991).

Genes and Proteins of HBV Virus
HBV surface envelope proteins (HBsAg) is encoded by S ORF, it is structurally and functionally segmented into pre-S1, pre-S2, and S portionsas seen in (Figure 2). Robinson et al., 1976 make us to understand that the pre S -S known in full term as "presurface surface" region of viral genetic makeup is responsible for production of surface antigen of the virus. HBsAg also known as 24-KD S protein is said to be the common type of surface antigen protein. Pre S2 or M protein is another HBV protein though the function is unknown. Pre S1 or L protein is important in attachment of HBV to the host cell and also helps in the release of the virus from the cell after virion assembly (Bruss and Ganem, 1991). HBcAg is produced by C ORF from translation of core region while HBeAg is produced from precore region. HBeAg as being described by Milich et al., 2003 as antigen yet to understand its function but it is believed that its presence indicate ongoing infection. The pol known as polymerase is produce in P ORF of the virus genome and is said to be large protein with different functions: production of negative sense strand and encapsidation took place in terminal protein segment; synthesis of genome is enhanced in reverse transcriptase domain denoted by RT; replication is facilitated by ribonuclease H domain which also involves in clearing of pregenomic RNA. The (HBxAg) a is 16.5-kd protein that is encoded by HBV X ORF and its function includes DNA repair, signal transduction, inhibition of protein degradation and transcriptional activation (Cross et al., 1993;Zhang et al., 2001;Bouchard et al., 2004;Hu et al., 2006). The HBxAg mechanism of action and biologic function in viral life-cycle remain largely unknown but it is believe to contribute to the oncogenic potential of HBV. Literature has also revealed the importance of HBxAg in productive HBV infection in vivo.

Epidemiology of HBV
Death recorded as a result of HBV infection are put around 500,000 to over 1 million annually (Lee, 1997;Mahoney, 1999) and it accounts for 10 th most causes of mortality around the world. Depends on varying geographical area and different population subgroups, the prevalence of HBV infection are different (Yu et al., 2000;WHO, 2015 Belgium, Sweden, Iceland, US, Mexico and Australia are considered as having low HBV endemicity with HBsAg prevalence of < 2% (Lavanchy, 2004). It is not surprising that regions with a high prevalence of HBV infection are also implicated with high rates of HCC. HBV is responsible for about 60%-80% of the World"s primary liver cancer and this is common cause of mortality in Asia and Africa (Lemon et al., 2000;McGlynn et al., 2001). Literatures revealed that the risk of developing chronic HBV infection ranges from less than 5% in adult to 25% -30% in infants and children under 5 and to more than 85% in children born to HBeAg-positive mothers after acute exposure (Lok and McMahon, 2001). Chronic HBV infection is likely to develop in immunosuppressed patients after acute infection. Data have shown that more than half of the global population are living in highly endemic areas, while about 2 billion are estimated to be infected (WHO, 2015). Of the 2 billion people who have been infected globally, there are more than 3 million acute cases yearly and more than 350 million are chronic carriers of HBV (WHO, 2015; Goldstein et al., 2005) and approximately 15-40% of the infected patients will develop cirrhosis, hepatocellular carcinoma (HCC) or liver failure (Lok, 2001).

Genetic Diversity of HBV
Lack of proof reading activity by the viral polymerase (pol) and the use of RNA intermediate during its replication has resulted in HBV high genetic variability (Seeger and Mason, 2000;Ganem and Schneider, 2001). The classical serotype based classification of HBV strains has being replaced by HBV genotypes and subgenotypes (Datta, 2008). HBV has 10 genotypes and its genotypes/subgenotypes have been linked to different geographical locations (Okamoto et al., 1998). HBV/A is usually found in North America, Northern Europe, and Central Africa and is pandemic while HBV/B and HBV/C are found in the far East and Southeast Asia. HBV/D is prevalent in India, Mediterranean region and Middle East. Likewise, HBV/E and HBV/ F have been reported in Nigeria and other neighbouring countries. HBV/G has been found in the USA, Germany and France, HBV/H in Mexico and South America, HBV/ I in Vietnam and Laos and HBV/ J in the Ryukyu Islands in Japan (Norder et al., 1994;Magnius and Norder, 1995;Stuyver et al., 2000;Croagh et al., 2015).

Transmission of HBV
HBV infection spreads easily through contact with infected body fluids and thus said to be highly contagious. Human served as the only natural host of the virus and the possible means of transmission are through vehicles such as body fluids like blood, semen and saliva (Scott et al., 1980). Hepatitis B virus could be as high as 10 10 Dane particles per ml in body fluid of the carrier and it could be transmitted through sexual contact, sharing of sharp object with infected person and also during birth from infected mother to the child. Depend on the geographical regions; the risk of vertical transmission varies. Some of the routes of HBV transmission like mothers to child as stated above includes: transmission through placental in pregnancy, delivery, during care or breastfeeding (Hou et al., 2005). Many studies have shown rare cases of uterus transmission and this account for < 3% of perinatal infections (Tang et al., 1998). The HBeAg status of the infected mother determines vertical HBV transmission (Xu et al., 1985). More than 35% of HBV infections in US are traced to act of practising multiple sexual partners and more than 20% are found in people with gay relationship. This statistics demonstrated that sexual contact is most likely route of HBV transmission in such a low endemic areas (Wasley et al., 2008). There are other likely means of HBV transmission such as the practise of injecting drug common among gangsters, circumcision, use of infected blood products or whole blood and working in health care facility (Margolis et al., 1991).

Pathogenesis of HBV Viral Infection
HBV of about 10 10 to 10 12 genome copies per ml found in body fluid is enough for possible transmission (Schildgen et al., 2006). On entering the host, the Dane particle locates the hepatocyte cells, its main target, for replication and persistence. There has been no evidence of HBV causing cytotoxic effect on the affected hepatocytes and no evidence of cytopathic change under normal conditions of infection (Thimme et al., 2003;Wieland et al., 2004). There are evidences that many people infected with HBV are usually asymptomatic and may have little injury of the liver, even when the virus is actively replicating (deFranchis et al., 1993). There are different schools of thought that believe the response of the host cells to viral antigen is responsible for liver injury. This observation is same with the fact that patient with low immunity tend to have minimal liver injury despite the high burden of the viral infection (Stevens et al., 1975). Consequently, confirmed by experimental data (Ando et al., 1994;Guidotti et al., 2000;Kakimi et al., 2001), generally, massive cytotoxic T Lymphocytes (CTL) and Natural Killer cells (NK) action result in the killing of infected hepatocytes and is essential for the elimination of the infection.

The Liver and Its Response to HBV Infection
Liver has remains primary site of HBV replication in humans. There has not been evidence that other cells in the body can be use to replicate the virus. In term of structure, blood enters from the hepatic artery, through miscroscopic lobules of the liver, and portal vein which is located in a structure known as portal triad then comes out through hepatic vein. The miscroscopic lobules has parenchymal cell where larger percentage of the liver mass are situated and is referred to as hepatocyte. Clearance of hepadnavirus infections leads to destruction of large numbers of hepatocytes causing jaundice (icterus) in some patients as a result of build up of bilirubin in the blood thereby giving a yellow colouration of the eyes and skin (Thorgeirsson and Grisham, 2003).

Immune Response to Hepatitis B Virus Infection
The process of HBV pathogenesis is not injurious to liver cell even after all the liver cells have been infected. The immune response to infection is usually slow as a result of minimal observable cells injury (Jilbert et al., 1992;Kajino et al., 1994;Thimme et al., 2003;Wieland et al., 2004). Due to much number of infected hepatocytes, there is massive attack by the immune response on the hepatocytes during the clearance stage of the infection (Summers et al., 2003). More than 90% population of immunocompetent adults are able to develop enough immune response during clearance stage to resolve the infection but that is not the case in less than 10% of adult and children below one year of age whom may not be able to clear the infection. Chronic hepatitis B is seen as a result continuous injury to liver cells which is a result from recruitment of immune mediated T-cell against hepatocytes with viral antigens (Ganem and Prince, 2004). It is usually lifelong and is believed to be as a result of dysfunction HBV-specific T-cell from exhausted T-cell thereby affecting the normal function of the cells to fight infection (Das et al., 2008). There is also observable reduction in viral load during the infection which could be due to production of antiviral cytokines which hinder viral replication (Meuleman et al., 2006).

Clinical Features of HBV Infection
There is dynamics in HBV infection and it clinical manifestation is determine by patients" age when infection occur (young children generally have milder disease than adults), the immune status, the infecting virus dose (higher doss appeared to result in shorter incubation period and generally more severe hepatitis) and also at what point the disease was observed (Robinson, 1995). At the onset of infection, the patient may develop fever like symptoms like feeling unwell, vomiting and headache. Jaundice may be noticed but more sometimes HBV infection produces no symptoms or jaundice (Hollinger and Liang, 2001). It has been stated earlier that most adult recovered after the episode of infection while few others may not recover but develop fulminant hepatitis and die (Robinson, 1995;WHO, 2015).

Patterns of HBV Infection
During HBV infection (primary infection), those who are susceptible (non-immune) can be either symptomatic or asymptomatic. While the asymptomatic is more common, symptomatic is less common, especially in young children. Both are usually self-limited in adult with clearance of virus from blood and liver and the development of lasting immunity to re-infection (Hoofnagle, 1981;Wright and Lau, 1985). It should be noted that some primary infections in healthy adults do not resolve as would be expected but develop into persistent infections. Persistent HBV infection can be presented as symptomatic or asymptomatic. Asymptomatic chronic HBV carriers are usually people with subclinical persistent infection, normal or nearly normal findings on liver biopsy and normal serum aminotransferase levels; chronic hepatitis B explain those with abnormal liver function and histologic; Liver cirrhosis explains is seen when new nodules and fibrosis develop acute liver injury. More than 15% of people with chronic hepatitis B develops cirrhosis (Ganem and Prince, 2004).

Primary Infection in HBV
Primary infection is characterised with production of IgM isotope against HBV core antigen (anti-HBc antibody) and at this time HBsAg is seen in circulation (Hoofnagle, 1981). The titre of virus in the blood will be at high level with detectable number of HBeAg (Ribeiro et al., 2002). After liver injury, the level of alanine aminotransferase (ALT) is only increase following established viral infection. This is an indication of time needed for the immune response mediated by T-cell to initiate liver injury. The clearance of hepatocyte of viral infection without necessarily causing liver injury shows the potency of non-cytolytic mechanisms" ability to achieve clearance (Ganem and Prince, 2004). Clearance of HBV infection is characterised by absence of HBsAg and HBeAg antigens with observable level of anti-HBs antibodies in circulation. There is also persistence of HBV DNA in the blood for long period or life (Prince et al., 2001).

Occult Hepatitis B Infection
This is use to describe the detection of HBV DNA in the absence of observable HBsAg and with presence or absence of anti-HBC or anti-HBs after incubation period (Allain, 2004).
There are still many things unclear about occult HBV infection especially its role in clinical and biological spectrum of infection caused by HBV. There is persistence of HBV DNA even after the clearance of HBsAg in the liver and serum (Feitelson et al., 1994;Mosley et al., 1995;Yuki et al., 2003). There is report of occult HBV infection among population of asymptomatic patience like blood donors, patience with good result of liver test and patients coming for general medical check up (Hennig et al., 2002;Adeniyi et al., 2022). Occult HBV can be found in different conditions including (1) people who was once infected with HBV with evidence of anti-HBs (2) people infected with escape mutant (3) apparently healthy HBV infected individual at nonproductive stage with presence of anti-HBc and absence of detectable anti-HBe (4) people who has chronic hepatitis or apparently healthy individual with no indication of HBV except HBV DNA (Allain, 2004).

Occult HBV in Recovered Infection
Replication of HBV DNA, at very low level with persistence detection over the years in the liver, peripheral blood, mononuclear cells or serum, continues in an individual who have recovered from HBV infection and produced neutralizing anti-HBs (Norborg et al., 2000;Rehermann et al., 1996). In such cases, the virus is either complexed with anti-HBs, or free in the circulation, or both forms might be present (Yotsuyanagi et al., 1998). Many authors have shown level of HBV DNA in OBI and put the viral load at range between 5 and 1000 IU/ml. An explanation to this might be as a result of cytotoxic T-lymphocyte (CTL) inadequate efficiency response resulted in clearing most, but not the entire virus produced (Rehermann et al., 1996).

HBsAg Mutants and Occult HBV
Mutation in genetic makeup of HBV results in variant different in phenotype from the initial virus (de Franchis et al., 2003). In some occasion mutation is conferred by pressure from the host immune response to infection given rise to change in sequence at S ORF as seen in vaccine-escape mutants. Likewise, changes can occur at P ORF as a result of pressure from taken some antiviral drugs. Over time there will be some stability in the HBV mutants and could be transmitted either by horizontal or vertical means (Weber, 2005). On the other hands, Carman et al. (1996) observed that immuneescape mutants result from pressure imposed as a result of humoral and cellular immune response to invading virus thereby making HBV invisible to detect. Someone who has been successfully vaccinated could still come down with infection by S-immune escape mutants like G145R and D145R (Dindoost et al., 2012).

Occult HBV and Anti-HBC
In time past presence of anti-HBc is important in determining non-B hepatitis but with the introduction of anti-HCV, anti-HBc test is no more important except in prevention of occult HBV transmission (Kleimman and Busch, 2001). The distribution of HBV in an area is an important factor in determining the HBV markers that would be found in anti-HBc positive individual (Allain, 2004). The presence of anti-HBc without observable anti-HBs is called anti-HBc-only and it could be explain in either two ways (Grob et al., 2000). The first case is seen in long time chronic HBV with no observable HBeAg and the level of HBsAg become undetectable as a result of its low level in the system (Allain et al., 2003). The second case is seen in a situation whereby an individual have recovered from HBV infection with short persistence of anti-HBs. The anti-HBs become undetectable but only anti-HBc is observed (Almeida-Neto et al., 2001).

Occult HBV without Serological Markers
Screening for anti-HBc in blood donors are important in avoiding transmission of HBV infection in the two circumstances mentioned above. It is now important to design blood safety strategy in lieu of rise in detection of HBV DNA in the absence of observable HBV blood markers (Allain, 2004). There have been situation of reported HBV DNA alone in hepatocellular carcinoma (HCC) or hepatitis (Berasain et al., 2000;Shiota et al., 2000). Some vaccinated children were reported to have no detectable anti-HBs with HBV DNA below 10 3 copies per ml. (Feitelson et al., 1994). In another study, a case was reported of a retrospective study of population who earlier received blood transfusion (Baginski et al., 1992).

Occult HBV Infection and Clinical Implication
The clinical implication of OBI has remained unknown over the years. People with Hepatocellular carcinoma or unclassified chronic liver disease were first to be detected of OBI (Brechot et al., 2001). This shows association between occult HBV (OBI) and liver disease, other things been equal like absence of HCV and abnormal alcohol intake. It is noteworthy to mention that people with OBI are usually asymptomatic and screening of large population is a prerequisite for its detection. Occult OBI has potential for reactivation and eventually causes hepatocyte inflammation in immunosupressed individual or people who are undergoing chemotherapy. OBI could also result in hepatocellular carcinoma in individual by its ability to influence hepatic inflammation and fibrosis (Giovanni et al., 2013). Blood ALT is usually normal in people with OBI and anti-HBs (Michalak et al., 1994). There are scanty report of ALT level and liver disease in blood donors with anti-HBc only. Studies have shown no observable change in ALT level or liver disease in OBI patients with either anti-HBs or ani-HBc only (Yotsuyanagi et al., 1998). In another study, more than 1.9% of patients with chronically elevated ALT levels were detected of HBV DNA and one of these patients was a previous blood donor (Berasain et al., 2000). This shows the importance of screening for OBI before blood transfusion.

Prevalence of Occult HBV Infection
Distribution of OBI in a population varies and depends on the level of endemicity of HBV infection in the study area, the type of assay adopted for the studies, and the population being studied. Many of the studies above have shown that the prevalence of OBI is higher in people who are at more risk of HBV infection and with liver disease than people with less risk of having HBV infection and without liver disease (Maria et al., 2015). OBI is predominant in regions with high prevalence of HBV infection (Chemin and Trepo, 2005;Adeniyi et al., 2022). HBV infection in endemic area is predominantly gotten perinatally or while growing up as a child. Prevalence of OBI is also depending on the sample being tested with higher detection seen using liver compared to serum sample (Chemin and Trepo, 2005).

Diagnosis of Hepatitis B Virus
The use of different serological markers of HBV and by inclusion of other screening method to exclude other viral agent like hepatitis A and C viruses helps in diagnosing HBV. www.cmhrj.com Vaccine-induced immunity could be detected using serological tests and to differentiate acute, self-limited infections from chronic HBV infections. Measurement of HBV viral load and antiviral agents" potency can be accomplished by Nucleic acid testing for HBV-DNA (Mel et al., 2005). Blood is usually preferable sample for the detection of HBV infection. Assay for viral DNA, though rigorous, can be used to determine the presence of circulating virus. A viral load around 2,000IU/ml is considered enough to cause episode of hepatitis and also damage to liver cells (Chu et al., 2002). In occult HBV, viral load is usually low and may not be detected even with PCR diagnostic technique (Raimondo et al., 2008).

Serologic Diagnosis of HBV
The use of enzymes immunoassays (EIAs) is good in accomplishing detection of HBV using their antigen and corresponding antibodies (Table 1) like HBeAg and anti-HBe, HBsAg and anti-HBs, anti-HBc and HBcAg, though not freely found in circulation (Servoss et al., 2005). HBsAg are found in excess in serum (Martin et al., 1993), this first marker indicate presence of HBV infection and seen within 7 days from first exposure. Presence of anti-HBs in the circulation indicate HBV infection resolution and gives lasting immunity to the individual (Servoss and Friedman, 2006). Presence of IgM anti-HBc is important in diagnosing acute HBV infection during window period. IgG anti-HBc soon replaces IgM anti-HBc whose presence indicate resolved HBV infection (with observable HBsAg) (Servoss and Friedman, 2006). When the virus is finally cleared from the system, HBeAg level decreased and then replaced with anti-HBe. The level of HBV DNA is markedly reduced during this period. Chronic HBV infection is indicated by the presence of HBeAg in serum (Servoss and Friedman, 2006). There is also presence of HBsAg exceeding 6 months with more than 10 5 HBV DNA copies/ml and elevated serum ALT level (Lok et al., 2001). There is varying appearance of HBV serological markers during HBV infection depends on either the infection is an acute or chronic HBV infection (Table 2.) (Gitlin N, 1997;Mahoney, 1999;Cabrerizo et al., 2000).

Molecular Diagnosis of HBV
Molecular detection of HBV infection has greatly improved diagnosis. It could be use to achieve both qualitative and quantitative diagnosis of HBV. The quantitative diagnosis of HBV has achieved much success including: the detection of acute HBV infection, differentiation of ongoing from resolving HBV infection and also monitor patience response to antiviral treatment (Servoss and Friedman, 2006). HBV DNA could be assayed both qualitatively and quantitatively. The use of signal or target amplification method could help to quantify HBV DNA in the serum. Signal amplification, like branched DNA technique, are very specific but not as sensitive to detect low level of HBV DNA. On the other hands, target amplification, like PCR technique, are very sensitive and could detect HBV DNA as low as 10 copies per ml (Loeb et al., 1991). Recent advance in PCR technique is real-time PCR which could detect and quantify HBV DNA simultaneously in real time. It is very sensitive and could detect viral genome as low as 10 copies per ml (Sum et al., 2005).

Prevention and Control
We have basically three main strategies put in place to prevent HBV infection and this includes passive immunoprophylaxis, active immunization and behaviour modification. An individual could gain immunity as a result of exposure to antibodies (monoclonal and polyclonal) thereby prevents infection (i.e passive immunoprophylaxis). An active immunization involves inoculation of an individual with all or part of live or killed microorganism to activate the host cells to produce immune response to infection. Behavioural modification involves changes in sexual activities, adequate screening of blood and blood products and many more. Even though neonates and children are more at risk of infection in early stage of their growing up, active and passive immunisation would be of great benefit in preventing infection (Hou et al., 2005).

Conclusion
Occult HBV poses much concern especially in HBV endemic region, like Nigeria, where its spread as a result of lack of appropriate molecular diagnostic techniques in routine laboratory blood screening before transfusion or even among general population is being increasingly observed. It is important to screen for HBV DNA, HBcAg, anti-HBsAg, HbeAg, anti-HbeAg in individual. In addition to the use of ELIZA diagnostic technique, blood donors and the general population should be screened for the presence of occult HBV. This will assist in preventing circulation of Occult HBV in the population in addition to effort being put in place by immunization program.