Rama Nada. - Malik

Transcription

1 - 2 - Rama Nada - - Malik 1 P a g e

2 We talked about HAV in the previous lecture, now we ll continue the remaining types.. Hepatitis E It s similar to virus that infect swine, so its most likely infect humans through cross species transmission (from pigs to humans), usually the recent viruses that infect the humans have an animal origin, such as: HIV which is related to monkeys and flu which is related to birds and so on. Its called E ; as when it was discovered in early 80s it was thought to cause Enteric infection, it caused outbreak in that time, it was though to be HAV infection as it associated with similar symptoms, then they did a serological test for these viruses and found that they don t have hepatitis A antigens so the conclude that there is a hepatitis virus similar to HAV (associated with the same symptoms), transmitted through faecal oral route and cause jaundice. After they isolated it, they found it similar to enterovirus called noroviruses, so they classified it with norovirus under caliciviridae family, then after they combine larger number of sequences and undergo comparative genetic analysis they found that there were enough differences to classify it under another family called Hepeviridea. - Hepatitis E transmitted through faecal oral route. - The incubation period within few weeks in most cases (similar to HAV, but shorter than HBV and HCV). - The incubation period of HBV and HCV is few months nearly. - The incubation period of HEV vary from 2 weeks to 2 months, depending on some factors, such as: infectious dose, the immune status of the patient and so on. - HAV causes acute infections mostly, in these infections there is a complete clearance of symptoms spontaneously, and the patients don t need hospitalization. - Some patients with another underling liver injury or co morbidity end up with sever acute infection called fulminant Hepatitis, which will result in sever loss of normal liver function and sever complication, such as: hepatic encephalopathy and high percentage of mortality. 2 P a g e

3 - The fulminant hepatitis infections occur in very low percentage in both HAV and HEV (less than 1%), except in certain groups like in pregnant women (the severity and mortality reach 20%), why? The reason for the excessive mortality of hepatitis E in pregnancy is unknown, although a high viral load and abnormalities of progesterone signalling pathways have been suggested. - Its RNA virus, so the Dx will be done by molecular detection (RT- PCR); use reverse transcriptase to convert its RNA to DNA then amplify it using PCR. Another way to detect HEV is by serological test. - You don t need to screen the blood units before transfusion for HEV; you screen for HIV, HBV and HCV. - There is no specific treatment exists for acute hepatitis E. both interferon alpha and ribavirin have been used successfully to treat chronic HEV. - There are candidate recombinant vaccines in trials (not proven yet) - If you are asked in the exam if there is a vaccine for HEV, the answer is NO. Hepatitis B - HBV is the only DNA hepatitis virus, it belongs to Hepadnaviridae family. - HBV is the only human virus that belongs to the family Hepadnaviridae, while other animal viruses have been identified that they belong to the same family of viruses which can infect mammals and birds with characteristic tropism for hepatocytes. - HBV are enveloped, partially double stranded DNA (the 7th class of Baltimore classification), its DNA virus but its replication cycle involves RNA intermediate; so, it requires reverse transcriptase which lack the proofreading activity, for that reason you predict a high mutation rate, but in realty the mutation rate is intermediate between RNA viruses and DNA viruses, why? Because of the organization of HBV genome. The replication cycle of HBV comprises an error-prone reverse-transcription step, it replicates using overlapping reading frame; the same stretch of 3 P a g e

4 nucleotides expressed in more than one gene, what we mean by that? The viral DNA is around bp (depending on the genotype), (one of the smallest DNA viruses infecting humans) long and is circular in configuration. The long negative sense strand is complete, but there is a gap of variable length in the complementary positive sense strand. This incomplete strand is closed by the viral polymerase when virus replication starts. There are four overlapping open reading frames on the circular viral DNA coding for the core (encode the capsid), surface, polymerase and an X protein which is possibly an activator of transcription. An additional viral protein, the hepatitis B e antigen (HBeAg), is translated from the core open reading frame that encodes the HBcAg protein. HBeAg is secreted from infected cells into the bloodstream, particularly during active viral replication. It is therefore frequently used as a marker indicating significant viral activity. So,Hepatitis B virus (HBV) expresses two structural forms of the nucleoprotein (capsid antigens), the intracellular nucleocapsid (hepatitis core antigen [HBcAg]) which is transcribed from the beginning of Core gene, and the secreted nonparticulate form (hepatitis e antigen [HBeAg]) which is transcribed from precure and core. X protein works as a regulator for transcription (its role isn t proven) The presence of HBeAg in serum correlates with hepatitis B virus (HBV) infectivity, high e antigen in the serum indicates active replication and higher infectivity (high viral load). Very important What is the importance of overlapping reading frame? As we said previously, it decreases (restrict) the mutation rate, note that there is an overlapping between polymerase gene and the core gene. For example: if a mutation occurs in the c gene (the overlapping area) it 4 P a g e

5 may be silent for the protein which translated from C gene, but it also result in early stop codon and nonsense mutation in the polymerase region (as there is overlapping between them) which result in defective protein, so the virus isn t infectious. The surface antigen (HBsAg) gene is transcribed to produce three messenger RNAs (mrnas): L, M and S. The S mrna is the, the product of the M mrna is medium in length and the protein from the L mrna is the longest. The importance of these surface antigens is to recognize the receptors on hepatocytes (sodium taurocholate cotransporting polypeptide NTCP ) and bind to them. The presence of hepatitis B surface antigen (HBsAg) in the serum for a period more than 6 month indicates the chronicity of the infection. the antibodies and the antigens for hepatitis B surface protein will not be found in the blood at the same time; the presence of high concentration of antigen indicates that the immune response was not efficient enough to eliminate the infection and the surface antigens. On the other hand, the presence of high concentration of antibodies indicates that the immune response was efficient enough to eliminate the infection and the surface antigens. (important) - An effective vaccine for HBV is available, given in three doses over 6 months, this vaccine doesn t contain the complete virus, only the surface antigens (recombinant), these antigens are neutralized by the antibodies, so they won t be able to bind the hepatocytes. - The effectiveness of the vaccine is from 90% to 95%, this indicates that there is some genotypes of HBV resistance to the vaccine - The presence of hepatitis B antibodies in the blood indicates either the vaccine or previous infection, you can discriminate if it is previous infection or vaccine by the presence of antibodies against the core protein which indicates previous infection as the vaccine doesn t contain core protein so there will not be antibodies against them. - This vaccine provides protection against HBV and HDV infections, as HDV will not enter hepatocytes without acquiring envelop 5 P a g e

6 contains HBV surface antigen from an infected cell (simply the patient can t get HDV infection without being infected by HBV). Note** the surface proteins are impeded in the viral envelop. - In children the immune system is weak so the time from infection till the development of immune response might take months, that s why they infection will be asymptomatic and chronic. - In adults, the majority clear the infection and a minority develops chronic infection, as the immune system is stronger than it in children the infection will be symptomatic and acute. - HBV is currently classified into at least 8 genotypes designated with capital letters (A-H) with pairwise inter-genotypic distance of more than 8%, the most common genotype in Jordan is HBV-D. - HBV genotypes are further divided into subtypes designated with Arabic numerals with molecular divergence 4-8% - The percutaneous transmission is the major route for HBV infection, other major routes of transmission include sexual spread and mother to child transmission (MTCT) vertically. - In endemic areas, MTCT represents a frequent mode of spread with its subsequent high prevalence of chronicity (discussed earlier) Laboratory diagnosis: HBV DNA is the first detectable HBV marker in an acute infection followed shortly by HBsAg and both are present for some weeks before the onset of symptoms. In patients who have rapid viral clearance, antigenemia is of short duration and may no longer be detectable at the onset of symptoms. If successive serum samples are examined, the development of anti-hbs will confirm recent primary infection, although there is considerable variation in the timing of the appearance of this antibody. HBV DNA may remain positive after disappearance of HBsAg. This creates what called window period when an individual is HBsAg negative but remains infectious. In such cases, detection of IgM anti-hbc indicates that a primary infection has occurred recently. Hepatitis D 6 P a g e

7 - Hepatitis D virus (HDV) is a deficient RNA virus and can only infect simultaneously with HBV (co-infection) or as a superinfection of an individual chronically infected with HBV, that s mean for HDV to establish an infection there MUST be hepatocytes infected by HBV. - HDV bind to the same receptors as HBV. - Prevention: Hepatitis B vaccine (discussed previously). - Epidemiology: There are an estimated 25 million HDV infected individuals in the world, in Mediterranean countries HDV infection is endemic among those with hepatitis B. in nonendemic areas HDV infection is confined to persons exposed frequently to blood and blood products, primarily injection drug users and haemophiliacs. Under the microscope: the diameter of HDV virion (complete virus particle) is 42 nm, in the blood you find particles with 22 nm in dimeter in huge amount, these are pure hepatitis B surface antigens (due to excessive production from the infected cells), these particles aggregate and form numerous spherical particles in the blood devoid from nucleic acid (so the are not infectious), the antibodies which is formed by the immune system will bind these particles as they are found in large amount so the probability for the immune system to recognize HDV and neutralize its antigen decrease due to the high number of Hepatitis B surface antigen in the blood, this is a decoy mechanism for HDV to evade the immune system. **if you take a sample of 100 people and inject them with injection contaminated with HBV, HCV and HIV, 30% will be infected by HBV, 3% will be infected by HCV and 0.3% will be infected by HIV. Hepatitis C: The doctor didn t have time to explain it, I don t know what is exactly required this information may be helpful. Hepatitis C virus (HCV), discovered in 1989, was the long sought-after and highly elusive causative agent of post-transfusion non-a, non-b hepatitis. This hugely important discovery allowed rapid development of serological screening assays for HCV infection and their adoption for blood donor screening. Consequently, transmission of HCV by blood 7 P a g e

8 transfusion has been virtually eliminated, as has the occurrence of transfusion-associated hepatitis. Infection with HCV is widespread throughout the world and is particularly associated with risk groups for parenteral (blood-borne) exposure. Amongst these, drug users sharing needles are numerically the most significant in Western countries. HCV infection is frequently persistent, and leads to the development of significant liver disease, such as cirrhosis and hepatocellular carcinoma (HCC), but only after a long asymptomatic carrier phase. - HCV genome is positive sense SS RNA. - There is no effective vaccine for HCV. Direct detection methods are based upon either the detection of HCV RNA sequences by the polymerase chain reaction (PCR) or other nucleic acid amplification methods, or of viral antigens by ELISA. Reverse transcriptase PCR is the most commonly used method for direct detection of HCV. Sorry for any mistake Best of luck 8 P a g e