Hepatitis Viruses


            ‘Viral hepatitis’ refers to a primary infection or inflammation of the liver by any one of a heterogenous group of  hepatitis viruses which consists of types A, B, C, D, E and G. Hepatitis viruses are taxonomically unrelated. All the human hepatitis viruses are RNA viruses except for HBV, which is a DNA virus. The features common to them are their hepatotropism and ability to cause a similar icteric illness, ranging in severity.
            The most common viral hepatitis is hepatitis A, formerly called infectious hepatitis, caused by hepatitis A virus (HAV), a single stranded RNA virus usually transmitted by the faecal-oral route. Hepatitis B, formerly called serum hepatitis, is caused by the hepatitis B virus(HBV), a double stranded DNA virus usually transmitted via blood. Hepatitis C, formerly called non-A non-B (NANB) hepatitis. Hepatitis E, (HEV) transmitted by the faecal-route and formerly called non-A non-B non-C hepatitis. An especially severe form of the disease hepatitis D or delta hepatitis, is caused by the presence of both hepatitis D virus (HDV) and HBV.


Hepatitis A

            Type A hepatitis or infectious hepatitis usually ia transmitted by faecal-oral contamination of food, drink or shellfish that live in contaminated water and contain the virus in their digestive system. It occurs most often in children and young adults, especially in autumn and winter.the disease is caused by the Hepatitis A virus or HAV. HAV is included in the genus Hepatovirus of the family Picornaviridae.
It is a 27nm nonenveloped, icosahedral containing linear, single stranded positive sense RNA virus. It can be grown in some human and simian cell cultures and is the only human hepatitis virus which can be cultivated in vitro. It is one of the most stable viruses infecting humans. It can withstand heating at 600C for one hour and treatment with 20% ether, acid and many disinfectants. It can be inactivated by boiling for one minute. It survives prolonged storage at a temperature of 40C or below. Natural infection with HAV is seen only in humans.
            HAV transmission is by the faecal-oral route. After entering the body through the mouth, the viruses replicate on the intestinal epithelium on the gastrointestinal tract and spread through the blood to the liver, spleen and kidneys. It has an incubation period of 15-40 days and begins as an acute febrile illness. It is shed in faeces in the late incubation period. Occasionally viremia occurs, enter the liver, reproduce, enter the bile and are released in  the small intestine. Jaundice, yellowing of the skin  is common in hepatitis is caused by impaired liver function. The liver fails to rid of the yellow substance bilirubin, which is a product of the breakdown of haemoglobin from RBC. Other symptoms are malaise, nausea, diarrhoea, abdominal pain, and lack of apetite for a period of 2 days to 3 weeks, liver tenderness. Chronic infections are rare and recovery is slow, over a period of 4-6 weeks.
Laboratory diagnosis
  • Serum levels of both alanine and aspartate aminotransferase are markedly raised.
  • Viral particles can be demonstrated in faecal extracts by immunoelectron microscopy.
  • Faecal HAV may be detected by ELISA.
  • Detection of IgM by ELISA or RIA, which is detectable in the serum for 2-6 months after the onset of symptoms. It is followed by the appearance of IgG antibodies which persists for many years and is a useful indicator of immunity.
  • Trace amounts of HAV in food or water can be detected by PCR.
Prophylaxis – General prophylaxis consists of improved sanitary practices and prevention of faecal contamination of water and food. Gamma globulin injections and the killed HAV vaccine are used to provide temporary immunity.

Hepatitis B

          Type B hepatitis is the most widespread and the most important type of viral hepatitis. More than a third of the world’s population is estimated to be have been infected by hepatitis B virus, HBV. About a quarter of them become carriers. A quarter of these develops serious liver disease, including chronic hepatitis, cirrhosis and primary hepatic cancer. It can be transmitted by intravenous injections, by anal/oral sex practices, by contact with other virus-containing body secretions and by contaminated needles among intravenous drug users. Transmission via contaminated semen during artificial insemination has been documented.
Morphology and Genomic structure –(Diagram-Ananthanarayan-page no.510) HBV is a 42nm DNA virus with an outer envelope or Dane particle, a double shelled particle. The outer surface or the envelope contains hepatitis B surface antigen, HbsAg. It is made up of lipid, protein and carbohydrate. It encloses an inner icosahedral 27nm in diameter, nucleocapsid or core. It contains hepatitis B core antigen, HbcAg. Inside the core is the genome of HBV, consists of a 3.2 kb pair molecule of circular dsDNA. The plus strand is incomplete leaving 15-50% of the molecule single stranded, to which associated a viral DNA polymerase, which has both DNA-depentent DNA polymerase and RNA dependent reverse transcriptase functions. This polymerase can repair the gaps in the plus strand and render the genome fully double stranded. The minus strand is complete and contains four overlapping open reading frames coding for multiple proteins. A third antigen, hepatitis B e antigen, HbeAg is a soluble nonparticulate nucleocapsid protein.
            The four overlapping genes are gene S, gene C, gene P and gene X.
1.      The P gene comprises 80% of the genome and overlaps all the other three genes and codes for the DNA polymerase enzyme.
2.      S gene codes for the surface antigen or the envelope protein. It consists of the S region and two Pre-S regions, Pre-S2 and Pre-S1. The protein coded for the S region is S or major protein. When translation begins from the Pre-S2 region, the M or middle protein is formed. When the entire gene from Pre-S1 is translated, the L or large protein results.
3.      The C gene has two regions, C and Pre-C. When the C region is alone translated, the core antigen HbcAg is formed. It is assembled as the nucleocapsids. It is not secreted and does not circulate in blood, but can be demonstrated in the hepatocytes by immunofluorescence. HBc specific IgM and IgG appear in blood. IgG antibody persists in blood long after all other serological markers have disappeared. If translation begins from the Pre-C region, the resulting protein is HbeAg and is present incirculation. The presence of HbeAg in blood provides a readily detectable marker of HBV replication and high infectivity.
4.      The X gene codes for a small nonparticulate protein, HbxAg which has transactivating effects on both viral and some cellular genes. This leads to enhanced replication of HBV and some other viruses. HbxAg and its antibody are present in patients with severe chronic hepatitis and hepatocellular carcinoma.
(Diagram-Ananthanarayan-page no.511)
HBV is a relatively heat stable virus. It remains viable at room temperature for long periods. Heat at 600C for 10 hours reduces infectivity. It is susceptible to chemical agents.
Pathogenicity – HBV replicates within hepatocytes. Viral DNA exists in the hepatocyte nucleus in the free extrachromosomal state or integrated with the cell chromosome. The chimpanzee is susceptible to experimental infection and can be used as a laboratory model.
            There are 3 important modes of transmission of HBV infection : Parenteral, Perinatal and Sexual.
  • Parenteral Transmission : The virus is present in the blood, body fluids such as semen, vaginal secretions, menstrual discharge, saliva, colostrum and breast milk. Transmission of infection may result from accidental inoculation of minute amounts of blood or fluid containing HBV during medical, surgical or dental procedures. Needlestick injuries, use of contaminated needles and syringes, intravenous and precutaneous drug abuse, ear and nose piercing, tatooing and acupuncturing, sharing of shaving razor and kissing can transmit the infection.
  • Perinatal Transmission : HBV can be transmitted from carrier mothers to their babies during the perinatal period. Transmission usually occurs when maternal blood contaminates the mucus membranes of the new born during birth. Infection may also result from breast feeding and close postnatal contact between the infected parent and infant. Infection during this stage may lead to liver cirrhosis and hepatocellular carcinoma.
  • Sexual Transmission : Since the virus is present in the semen, vaginal secretions , transmitted by sexual contact.
The course of acute infection can be divided into 3 phases: preicteric, icteric and convalescent.
    1. Preicteric/prodromal phase : After an incubation period of 6 weeks to 6 months patient develops malaise, anorexia, weakness, myalgia, nausea, vomiting and pain in the right abdominal quadrant.
    2. Icteric phase : 2 days to 2 weeks following the initial symptoms patient develops jaundice, pale stools and dark urine, hepatocellular damage is detectable biochemically before the onset of jaundice and persists after it has resolved.
    3. Convalescent phase : This phase is long and drawn out with malaise and fatigue lasting for several weeks, mild symptoms may persist for more than one year.

Hepatitis B Carriers

            About 5-10% of HBV infections result in chronic carrier state, which is defined as persistence of HBsAg in the circulation for more than 6 months. Carriers are of 2 types :
·         Super carriers : They have high titres of HBsAg, HBeAg and DNA polymerase in their blood, also HBV demonstrable in the blood. Very minute amount of serum or blood from such carrierscan transmit the infection.
·         Simple carriers : They are more common type with low level of  HBsAg, no HBeAg, HBV  and DNA polymerase in the blood. They transmit the infections when only large volumes of blood are transferred as in blood transfusion.
Laboratory diagnosis – Specific diagnosis rests on the serological demonstration of markers.  HBsAg is the first marker to appear in blood after infection. It remains throughout the icteric stage, lasts from 2 months to 6 months. When it is no longer detectable its antibody appears and remains for very long periods, it is protective. HBcAg is nondemonstrable as it is enclosed within the HBsAg coat, but its antibody appears in serum a week or two after the appearance of HBsAg, therefore it is the first antibody marker to be seen in the blood and remains life-long. Initially it is IgM, but after 6 months it is mainly IgG. HBeAg appears in blood soon after HBsAg, indicating active intrahepatic viral replication. The presence of  HBV, DNA polymerase in bloods indicates high infectivity. These antigens and antibodies can be detected by ELISA and RIA.
Prophylaxis – Screening of blood donors, use of sterile disposable syringes, practicing moral values. Medical personnel should wear gloves, gowns, masks, eyeglasses to prevent exposure to blood and body fluids.
Passive immunisation : Hepatitis B immunoglobulin (HBIG) is prepared from donors with high titres of anti-HBs. It is given intramuscularly preferably within 48 hours of exposure. A second dose is given 4 weeks after the first. It reduces the risk of the carrier state in babies born to infectoious mother, HBIG should be given not later than 12 hours after birth.
Active immunisation : For high risk individuals like health care personnels, patients requiring repeated transfusions of blood and blood products, dialysis patients., patients receiving prolonged inpatient treatments, parenteral drug users etc. Different types of vaccines : Plasma derived hepatitis vaccine, purified 22nm particles of HBsAg, prepared from plasma of symptomless carriers, seperated by ultracentrifugation and treating with proteinase,the product is immunogenic and safe. Recombinant yeast hepatitis B vaccine, produced by cloning the HBsAg gene in yeasts,products extracted and purified. The vaccine is safe and antigenic. Recombinant chinese hamster ovary cell hepatitis vaccine, commercially available, forst vaccine using mammalian cell expression system. Synthetic peptide vaccines, chemically synthesized polypeptide vaccines, safe and cheap. Hybrid virus vaccine, incorporating HBsAg  genes into vaccinia virus DNA, low cost, long shelf-life.

Hepatitis C

           HCV is the commonest cause of post-transfusion hepatitis in the developed countries. The virus is 50-60nm with a linear single stranded positive sense, 9.4kb RNA genome, enclosed within a core, surrounded by an envelope, carrying glycoprotein spikes. It belongs to the family Flaviviridae. On the basis of the genomic differences, HCV has been classified into 11 genotypes and each with several subtypes. This makes the vaccine development difficult. It is difficult to grow in tissue culture. The virus shows considerable genetic and antigenic diversity. It can be inactivated by exposure to chloroform, ether and other organic solvents and by detergents
Pathogenicity/Epidemiology- HCV infection is seen only in humans. The source of infection is the large number of carriers. Infection is mainly by blood transfusion and other modes of contact with infected blood or blood products. Transplant recepients and immunocompromised patients are at high risk, less chances of sexual transmission, vertical transmission may take place. Transmission by saliva and tears cannot be excluded.
            The incubation period is 15- 160 days. When compared to Hepatitis B, Hepatitis C is generally less severe, has shorter preicteric period, milder symptoms, absent or less marked jaundice. About 85% of acute HCV infections become chronic, with some developing cirrhosis and hepatocellular carcinoma.
            The infection can be prevented by screening of blood donors, avoidance of unsterile needles for intravenous drug abuse, tattooing etc..
Treatment – Prolonged treatment with interferon alpha, either alone or in combination with antiviral agents like ribavirin.

Hepatitis D
            HDV is a defective satellite virus requiring HBV as helper virus for the survival and replication. It belongs to the genus Deltavirus. It is a spherical, 36nm particle with an outer coat composed of the hepatitis B surface antigen and HDAg nucleoprotein . The genome consists of a single small circular molecule of minus sense RNA of 1.7kb. It encodes HDAg, but HBsAg is encoded by the genome of HBV coinfecting the same cell.
Pathogenicity – HDV is transmitted principally by blood and blood products, also by sexual contact. Vertical transmission is also possible. Two types of HDV infections are possible :
·         Coinfection – In this, delta and HBV are transmitted together at the same time, most commonly results from parenteral transmission, more severe than the disease alone caused by HBV.
·         Superinfection – Delta infection occurs in a person already harboring HBV, leads to more serious and chronic illness. It leads to severe liver damage and elevated mortality.
GLaboratory Diagnosis – In patients with coinfection, shortly before the end of incubation period, HBsAg appears in the serum and towards the end of incubation period HDAg appears which can be detected by ELISA or immunoblotting and HDV RNA can be detected by hybridization to a radiolabelled RNA probe. In the later stages of acute disease, anti HD IgM appears followed by anti-HD Ig, also can be detected by ELISA.
Prophylaxis – requres all measures that apply to the prevention of HBV infection, including vaccination against HBV.

Hepatitis E
          HEV belongs to the family Caliciviridae. They are spherical, nonenveloped and 27-38nm in diameter, possess single stranded positive sense RNA genome of 7.6kb which is surrounded by icosahedral capsid with characteristic surface depressions. The virus is very labile.
Pathogenicity – It is primarily associated with ingestion of faecally contaminated drinking water (first reported in Delhi in 1955). Incubation period ranges from 2-8 weeks, occurs predominantly in the 15-40 years age group. Clinically the disease resembles Hepatitis A. Bilirubin levels tend to be higher and jaundice deeper and more prolonged. It may cause intrauterine and perinatal infections. The infection during pregnancy may cause a high rate of abortion and intrauterine death.
Laboratory diagnosis – immunoelectron microscopic examination of patient faeces for aggregated calcivirus-like particles using monoclonal antibodies, ELISA tests for IgM and IgG anti HEV can be done.
            It can be prevented by improved standards of sanitation and provision of chlorinated water throughout the developing world. No vaccine or effective antiviral drugs exist.

Hepatitis G
          HGV represents a newly discovered virus, cloned from the serum of a patient suffering from chronic hepatitis who had a history of parenteral exposure. The genome of HGV consists of 9.4kb molecule of ssRNA of positive polarity, structure resembles that of  HCV. It replicates in peripheral blood cells. The virus is transmitted by blood transfusion, haemodialysis, sexually and from mother to child.
            The virus is present worldwide. Majority of the individuals with HGV infection have no detectable evidence of liver disease. The infection is mainly detected by reverse transcriptase polymerase chain reaction (RT- PCR). The infection often subsides after several years and anti Hgenv antibody develops.
    

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