A neonate develops yellowish eyes and skin after a month of life. When screened for hepatitis, the neonate is positive for hepatitis B infection, possibly by transmission from the mother. Hepatitis from the virus hepatitis B (HBV) is responsible for acute and chronic conditions in children and adults. The virus belongs to the family hepadnaviridae; hepa is related to liver cells and DNA because its genomic material is DNA.
Structure of HBV
- Small (approx. 3.2 kb) in size.
- The genome is partially double-stranded DNA as its positive strand is incomplete. The negative strand consists of 4 genes; S, C, P, and X. S gene codes HBsAg and HBeAg, P codes for DNA polymerase with reverse transcriptase activity, and X codes for X-protein that regulates transcription. Similarly, the C gene codes for capsid or core proteins.
- Under an electron microscope, three particles of HBV appears in the serum of patients; spherical (22 nm diameter), filamentous or tubular, and spherical, double-walled with 42 nm diameter, commonly known as Dane particle.
- Nucleocapsids surround the DNA. Proteins like HBV core protein (HBcAg) attached to the genome are also enclosed by nucleocapsid, whereas the virus’s envelope consists of HBsAg and HBeAg.
- HBsAg: This antigen consists of three glycoproteins, namely S, L, and M. The same gene encodes these glycoproteins. The S glycoprotein is the major component of HBsAg, which self-associates into 22-nm spherical particles released from the cells. The filamentous particles of HBsAg in the serum contains a large amount of S and only a tiny amount of L and M glycoprotein and other proteins and lipids. The glycoprotein L is essential for virion assembly and formation of filamentous particles, and retention of the structures in the cell.
- The genome of HBV is circular DNA, approximately 3,200 base pairs in length.
- It is capable of independent replication.
- The genetic arrangement of HBV has two distinct features; proteins encoded from overlapping frames and all regulatory sequences residing in protein-encoding sequences.
- The genome has four open reading frames (ORFs). These codes represent the genes that code nucleocapsid protein (core), an envelope protein (s), replicase protein (polymerase), and a protein involved in virus gene expression (X).
- The ORFs, as mentioned above, overlap; it is a distinct feature of the Hepatitis B virus that helps in encoding 50% (in mass) more unique protein than other DNA viruses.
- Unlike other viruses, the cis-acting regulatory elements are found inside the coding genes themselves instead of in the non-coding section.
Pathogenesis of Hepatitis B
- Resistant to extreme temperatures. It can be stored at 37℃ for 60 minutes, for seven days at 44℃, and at room temperature for six months. But are sensitive to high temperatures and are killed when heated to 60℃ for 10 hours and 100℃ for 1 minute.
- HBsAg is a very stable antigen. The same gene encodes the 3 glycoproteins (S, L, and M) of this antigen. S protein being the highest. These are responsible for attachment to host serum albumin, helping in the infection of hepatocytes.
- HBsAg helps in antigenic differentiation of the HBV as HBsAg contains group-specific antigen d or y and w or r. Combining these specific antigens gives rise to 4 subtypes of HBV; ADW, ADR, AYW, and AYR.
- And there is a total of eight genotypic variations (A, B, C, D, E, F, G, and H).
- Although the virus is DNA, formation of RNA with the help of reverse transcriptase mediates the replication. And replication occurs in the nucleus of the infected host cell.
Transmission of Hepatitis B
The transmission is generally in the following ways;
- Sexual intercourse with an infected person.
- Blood transfusion of an infected individual to healthy individuals.
- Organ transfusion without proper screening.
- From the infected mother to the fetus.
- Needle sharing and use of contaminated blades.
The incubation period of hepatitis B infection varies from a few weeks to 6 months.
- The virus enters through blood, and as it has an affinity for hepatocytes, it binds and replicates in those cells. The replication leads to the production of viral antigens that activate the cytotoxic T cells like CD8 lymphocytes, which causes various immunological reactions.
- The stage (acute or chronic) of infection depends upon the cytotoxic T-cell response of the individual. If the reaction is strong, the person can be free from the disease, and if the response is not strong, these individuals could be chronic carriers. The chronic case occurs more in children (neonates and infants) than in adults. Hepatocellular carcinoma is the result of persistent infection of the hepatocytes.
NOTE: When a patient tests positive for hepatitis for more than six months, the condition is chronic.
The replication of the hepatitis b virus has the following steps:
- Firstly, the infection begins with low-affinity attachment to heparan sulfate proteoglycans (HSP)– glycoprotein present in the basement membrane and secretary vesicles.
- After that, high-affinity attachment occurs to NTCP (sodium taurocholate co-transporting polypeptide) a carrier protein (glycoprotein) present in the basolateral membrane of the hepatocyte.
- Likewise, the internalization occurs by endocytosis, and the uncoating of the viral envelope facilitates the translocation of capsid with relaxed circular DNA (rcDNA) into the nucleus of the host.
- Similarly, rcDNA repairs into covalently closed circular DNA (cccDNA).
- The cccDNA acts as template for the transcription of mRNA.
- The mRNA translates the viral protein.
- The viral protein (core, surface, and polymerase protein) and capsid encloses pre-genomic RNA assembles in the cytoplasm.
- The pg-RNA converts into DNA by reverse transcriptase in the viral capsid and may return back into the nucleus to repeat the replication process.
- Then. the complete viral particles releases from the infected host cell.
Clinical signs and symptoms
The clinical signs and symptoms of HBV depend on various factors like; age, immune system, and level of HBV in the body.
Acute HBV infection: Symptoms of HBV are as follows:
During the pre-icteric stage
During the icteric phase
- Jaundice with liver tenderness
- Dark-colored urine
- Although the symptoms may be similar to HAV infections, the severity is high in HBV infections, leading to fatal.
NOTE: Icteric- of, relating to, or affected with jaundice.
Chronic Hepatitis B infections: Most chronic hepatitis cases are asymptomatic, but if symptoms develop, it is similar to acute hepatitis and can be a sign of liver damage. It may lead to
- liver cirrhosis.
- and liver cancer.
Diagnosis of Hepatitis B
Sample collection and Transport
Blood serum is the desired sample to diagnose hepatitis B infection. For obtaining serum, Collect the blood in vials without anticoagulant. Allow the blood to clot and centrifuge to get the serum. Transport the sample as soon as possible.
- Direct microscopy
Electron microscopy can detect the antigen-specific to HBV
Hepatitis B panel tests are necessary for a definitive diagnosis of the infection. The panel includes three serological/immunological tests; detection of HBsAg, anti-HBs, and detection of anti-HBc.
- HBsAg: It is the surface antigen of hepatitis B. It is usually present in the serum of the patients during the incubation period and is detected during the acute phase. The presence of HBsAg even after six months indicates chronic infection. But the presence of this antigen alone is not the definitive diagnosis for chronic conditions.
- HBsAb/anti-HBs: The antibody is not detected during the acute phase. But is found during the chronic phase of the infection. It works in neutralizing the HBsAg by binding and forming an immune complex to a large number of HBsAg present in the blood.
- HBcAb/anti-HBc: It is the antibody against the core antigen of the hepatitis b virus. Both IgM and IgG are present as total antibodies. It persists for life and appears shortly after HBsAb. The presence of HBcAb does not differentiate between acute and chronic infection.
- Other antigens: Detection of HBeAg and HBcAg, is done during the convalescent period of the infection using immunofluorescence techniques.
- Molecular Diagnosis
The genome and DNA polymerase activity is detectable during the incubation period of the infection. HBV PCR demonstration is helpful during the treatment of chronic stage and in diagnosing causes of liver failure.
- Other Tests
Liver function tests that help in determining abnormalities in the liver are also carried out to diagnose hepatitis. High levels of ALT and AST are found during the infection. Similarly, levels of serum bilirubin also indicate the intensity of jaundice.
Treatment of Hepatitis B
No specific antiviral therapy is available to treat acute hepatitis B infection. Treatment of symptoms is the only measure in treating acute disease. The antiviral therapy is recommended for chronic cases. The treatment with interferons and nucleoside analogs is the standard method used. These do not cure HBV but only prevent morbidity and complications related to HBV infections.
Interferon: Interferon-alpha is the most common interferon used in treating hepatitis. The interferon prevents the progression of acute infection to chronic and neutralization of viremia.
Nucleoside analogs: Nucleoside analogs such as lamivudine, adefovir, and telbivudine are widely used in antiviral therapy. These analogs halt the replication of HBV. These are bio-available and highly effective antiviral therapy available.
Prevention and Control
The preferable method to prevent and control HBV infections is both vaccines and immunoglobulins .
- Immunoglobulin: The passive immunization just before or after exposure to the virus with HBIg. It is obtained from the patients who have recovered from the infection and contains a high titer of HBsAb. It is recommended to those in recent contact with infected people, who are to be in household contact with the infected people, infants born with HBsAg positive mothers, and those in sexual contact with acutely infected patients.
- Vaccines: Plasma-derived and recombinant DNA HBV vaccines are the two types of vaccines available with HBsAg that generates anti-HBs. Recommended vaccine schedule for adults is at 0,1 and 6 months, and infants are at birth, at 1-2 months, and at 6-18 months.
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- Maepa, M., Roelofse, I., Ely, A., & Arbuthnot, P. (2015). Progress and Prospects of Anti-HBV Gene Therapy Development. International Journal Of Molecular Sciences, 16(8), 17589-17610. https://doi.org/10.3390/ijms160817589.
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