Hepatitis Virus Nomenclature

To date, eight hepatitis viruses have been identified. FIGURE 10-2 is a flow diagram used in diagnostics to distinguish the viruses from one another. Originally, each hepatitis virus was named with a letter of the alphabet. Two newly identified members of the group are not named this way: transfusion transmission virus, or torque teno virus (TTV), and sentinel virus (SEN-V). Even though these hepatitis viruses cause similar diseases, they vary widely in their structure and molecular biology (TABLE 10-1).

For years, three main hepatitis viruses were associated with hepatitis: hepatitis A virus (HAV), the cause of infectious hepatitis; hepatitis B virus (HBV), the cause of serum hepatitis; and the hepatitis non-A, non-B viruses. Diagnostic tests were developed to identify HAV and HBV in the 1970s. A new “antigen” found in liver specimens of some patients infected with HBV led to the identification of hepatitis D virus (HDV) in 1977. HDV was only seen in a fraction of patients with HBV, and it was often associated with a more severe course of hepatitis. Hepatitis non-A, non-B viruses were considered the cause of all other unidentified viral causes of hepatitis. In 1989, hepatitis C virus (HCV), the agent that caused non-A, non-B hepatitis, was discovered using modern molecular techniques. Since that time, an additional non-A, non-B hepatitis virus has been discovered and named hepatitis E. Emerging or new viruses causing primary hepatitis are referred to as “non-A–E viruses.” They include hepatitis G virus (HGV), TTV, and SENV. Hepatitis A, B, and C viruses cause over 90% of acute primary viral hepatitis in the United States. TABLE 10-2 compares the various hepatitis viruses by mode of transmission, source or reservoir, whether the course of disease becomes a chronic infection, and how viral infections can be prevented.

The geographical distribution of hepatitis viruses A–E is shown in FIGURE 10-3. Hepatitis A commonly occurs in developing parts of the world where sewage disposal and food hygiene are unsatisfactory. HBV is spread through a parenteral mode of transmission (i.e., blood-to-blood transmission), such as through tattooing, body piercing, and injection drug abuse. Transmission can also occur sexually and by fomites (e.g., sharing contaminated razors). High to intermediate prevalence of HBV also occurs in developing countries, Western Europe, Russia, Central America, and the Caribbean. Variants of HBV have been described, and the current hepatitis B vaccine does not protect immunized individuals against the variant HBVs.

HCV is spread primarily by direct contact with human blood. The prevalence of hepatitis C is higher among blood donors in the Eastern Mediterranean, Western Pacific, and Southeast Asia compared to some countries in North America and Europe. Data from African countries are limited due to poor reporting.

Hepatitis D virus requires the presence of hepatitis B virus in order to replicate. In general, then, the global pattern of hepatitis D corresponds to the prevalence of hepatitis B virus infection; for example, in countries where chronic hepatitis B cases are low, there is a low prevalence of hepatitis D virus. Hepatitis E virus is spread by the fecal–oral mode of transmission, often through contaminated water. It is endemic and sometimes epidemic in the developing world.

Screening the Blood Supply for Viral Hepatitis Agents

The American Red Cross has been screening for HBsAg since 1971. In 1987, a test for anti-HBcAg was added to the screening protocol. From 1986 to 2003, the U.S. blood supply was also screened for elevated levels of ALT. In 1990, testing began for anti-HCV IgM and IgG, and in 1999 a nucleic acid test for HCV genomic RNA was added to blood donation screening in order to reduce the risk of disease transmission. Blood is not entirely risk-free, but it is very safe. The U.S. blood supply in particular is among the safest in the world.

Hepatitis A

Most hepatitis A outbreaks are sporadic and associated with contaminated food or water supplies. Shellfish may become contaminated with sewage. This becomes a problem because they are able to retain and concentrate viruses from the water. The major mode of transmission is fecal– oral. The average incubation period for hepatitis A virus (HAV) is 30 days. Adults experience signs and symptoms more often than children. Pregnant women are at increased risk for serious infection. Symptoms include:

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  Fatigue

  
  
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  Abdominal pain

  
  
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  Loss of appetite

  
  
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  Nausea and vomiting

  
  
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  Dark urine

  
  
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  Jaundice (in 70–80% of individuals older than 14 years of age; less likely in children)

Signs of jaundice are a yellow color in the skin, mucous membranes, or eyes (FIGURE 10-4). Jaundice occurs when the liver is not functioning properly. The yellow pigment is from bilirubin, a by-product of old red blood cells. You may have noticed that when you have a bruise sometimes the skin goes through a series of colors as it heals. When you saw yellow you were seeing bilirubin.

Hepatitis B

Hepatitis B is relatively rare in developed countries. In endemic areas, a major mode of spread occurs from mother to infant when the mother is a carrier. The blood from the infected mother enters the uninfected fetus. Other high-risk groups for hepatitis B are injection drug users, hemodialysis patients, individuals who have had multiple sex partners, institutionalized patients, and healthcare workers.

The average incubation period for hepatitis B virus (HBV) is 80 days. About 30% of individuals have no signs or symptoms. When symptoms do occur, they are similar to those of hepatitis A, with the addition of joint pain. Chronic HBV infection occurs in 5–10% of individuals who have been infected by HBV after the age of 5. Chronic HBV infection can lead to liver cirrhosis and hepatocellular (liver) cancer (HCC). Genomic HBV DNA sequences can be detected in the chromosomes of hepatocellular tumors associated with HBV infection. Integration of HBV DNA occurs where there are breaks in the cellular DNA of liver cells. In humans, chronic HBV infection causes an ongoing inflammatory response, resulting in oxidative damage to the DNA, such as double-stranded breaks in the DNA of liver cells. Chronic liver disease is the cause of death in 15–25% of chronically HBV-infected individuals.

Hepatitis C

Prior to 1989, a number of cases of hepatitis after blood transfusions were found not to be caused by hepatitis A or B viruses. Investigators found it difficult to identify the cause of the non-A, non-B hepatitis. Individuals with an acute infection displayed the following symptoms:

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  Fever

  
  
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  Fatigue

  
  
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  Dark urine

  
  
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  Clay-colored stool

  
  
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  Abdominal pain

  
  
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  Poor appetite

  
  
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  Nausea

  
  
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  Vomiting

  
  
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  Joint pain

  
  
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  Jaundice

In 1989, a direct molecular biology approach by Michael Houghton and colleagues provided the tools to identify and characterize the cause of the non-A, non-B hepatitis. Their approach was to clone the non-A, non-B viral genome present in plasma from patients suffering from hepatitis. The new non-A, non-B causative agent of hepatitis was named hepatitis C virus (HCV). Amazingly, HCV was identified without the ability to grow, visualize, or detect the viral agent. Substantial progress has been made in the last decade of research to isolate, grow, and visualize HCV, which is now a major public health problem.

Individuals with chronic HCV infection are asymptomatic, but they may have chronic liver disease that can range from mild to severe, including cirrhosis and liver cancer. HCV infection is a “silent epidemic” because victims show few or no signs of disease for years, or even decades. Early assessments of the total number of infected individuals were underestimated. About 2.7 million people in the United States have chronic HCV infection. HCV infection is most prevalent among those born between 1945 and 1965, the majority of whom were likely infected during the 1970s and 1980s when rates were highest. Before 1992, when blood screening for HCV became available, blood transfusion was a leading means of HCV transmission. Today, CDC experts recommend HCV testing for the following high-risk individuals:

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  People born between 1945 and 1965.

  
  
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  People who have injected illegal drugs, including those who injected only once many years ago.

  
  
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  Recipients of clotting factor prepared before 1987.

  
  
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  Recipients of blood transfusions or solid organ transplants before July 1992.

  
  
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  Patients who received long-term hemodialysis treatment.

  
  
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  Healthcare workers with known HCV exposures after contact with needlesticks involving HCV-positive blood.

  
  
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  Recipients of blood or organs from a donor who later tested positive for HCV.

  
  
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  People who have tested positive for human immunodeficiency virus (HIV).

  
  
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  Patients with signs or symptoms of liver disease (e.g., elevated liver enzymes).

  
  
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  Children born to HCV-positive mothers (to avoid detecting material HCV antibodies, the children should not be tested before 18 months of age).

Unlike HBV infection, HCV infection is common in the developed world. About 4 million people in the United States (1.8% of the population) are infected with HCV. HCV is spread almost exclusively through blood contact (FIGURE 10-5). Sexual transmission is rare. Individuals who received blood transfusions or blood products, had long-term kidney dialysis, or had a solid organ transplant before 1992, an era when screening donated blood for HCV was not available, are at risk for having contracted HCV. About 80% of infected individuals have no signs or symptoms. If present, signs and symptoms are similar to other hepatitis virus infections. Between 55% and 85% of infected persons experience a chronic infection, resulting in liver disease. A chronic infection is one of long duration that is characterized by scarring of the liver, preventing it from functioning normally. It is usually insidious, progressing slowly without any signs or symptoms for several decades. Chronic HCV infection can lead to cirrhosis of the liver and HCC in 5–20% of persons. The average incubation period is 6–7 weeks.

Hepatitis D

Hepatitis D virus (HDV) is a defective virus that requires coinfection with HBV to assemble virus particles but not for genomic replication. HDV infections occur during two types of HBV infections: coinfection and superinfection. A coinfection refers to a simultaneous infection in the same host cells by separate pathogens, as by HBV and HDV. A superinfection occurs when a chronic HBV carrier is infected with HDV. Symptoms of hepatitis D are indistinguishable from those of hepatitis B. Less than 5% of cases of acute coinfection with HBD and HDV result in a chronic HDV infection. Sometimes HDV superinfection leads to the clearance of hepatitis b surface antigen (HbsAg), and HDV replication ceases, allowing the acute liver disease to resolve. In people in whom superinfection with HDV and HBV persists, liver disease is severe, and there is an increased risk of fulminant liver failure or progression to cirrhosis. The highest risk factor for HDV infection in the Eastern world is injection drug use: 39–90% of addicts test positive for HDV.

Hepatitis E

Hepatitis E is endemic in the developing countries of Asia and Africa and rare in industrialized nations. Sporadic cases in the developed world almost always occur in travelers returning from endemic areas. The mode of transmission and symptoms of hepatitis E virus (HEV) are similar to HAV (i.e., fecal–oral transmission). Person-to-person transmission is rare. The average incubation period for HEV infection is 40 days. HEV epidemics are often associated with water supplies that are contaminated with fecal matter. Prevention strategies include the avoidance of drinking water and beverages with ice of unknown purity, uncooked shellfish, and uncooked vegetables that have not been peeled or prepared by the traveler.

Infection with HEV causes a more severe illness than HAV. It causes mortality in 1–3% of persons and in 15–25% of HEV-infected pregnant women. Even though hepatitis E is rarely diagnosed in industrialized nations, anti-HEV antibodies have been found in a significant proportion of healthy individuals (up to 28% in some areas). Sporadic cases have been reported in the United States, northern England, Italy, and France. Considerable evidence suggests that HEV may be a zoonotic pathogen (transmitted from animals to people). Human HEV, for which isolates are genetically similar to pig HEV strains, has been isolated from pigs in the United States. It has been shown that pig HEV can cross the species barrier and infect nonhuman primates. Human anti-HEV antibodies cross-react and bind to pig anti-HEV capsid proteins.

Hepatitis “Non-A–E” Viruses

As described earlier, hepatitis A–E viruses were not determined to be the causative agent of all cases of primary hepatitis. The term “hepatitis non-A–E viruses” was used to describe new and emerging viruses that caused primary hepatitis.

Pathogenesis: Chronic Hepatitis

Chronic hepatitis occurs when the inflammation of the liver is active, persists for more than 6 months, and is detectable by increased ALT levels in serum. Hepatitis B, C, D, and G viruses can cause chronic hepatitis infections. A patient who is viremic and has an abnormal ALT level should have a liver biopsy, which is the only way to assess the degree of inflammation of the liver and the stage of liver disease. Chronic liver damage may result in cirrhosis, which is characterized by the formation of fibrous tissues, nodules, and scarring that interfere with liver cell function and blood circulation (FIGURE 10-8). A late complication of cirrhosis is liver cancer (HCC; see Section 10.3), which takes many years to develop.

Virus Structure

Hepatitis viruses A–E have a spherical shape and icosahedral capsid symmetry 28–50 nm in diameter. Virus particles are either enveloped or nonenveloped. Transmission electron micrographs of HBV and HBC are shown in FIGURE 10-9. Hepatitis B, C, and D viruses are enveloped and relatively sensitive to many physical and chemical agents, whereas HAV is not enveloped and can remain infectious on inanimate surfaces in the environment for a month. Hepatitis A viruses can also retain their infectivity in the hostile environment of the gastrointestinal tract because they are acid and bile resistant.

Virus Replication

Hepatitis viruses enter the bloodstream and are carried to the liver, where they infect the hepatocytes of the liver.

As the viruses spread and replicate in the liver, hepatocytes may become damaged. Chronic liver damage results in cirrhosis, impairing liver function (FIGURE 10-10). The liver has many functions, such as playing a role in metabolism and the filtering of toxins and waste products in the blood. The liver produces bile, which aids in digestion.

Hepatitis B Virus

American physician and biochemist Baruch (“Barry”) Blumberg (1925–2011) and colleagues began characterizing and comparing serum proteins from hemophiliacs with that from nonhemophiliacs in the 1950s. They collected blood samples from native populations in remote parts of the world with the goal of learning about the genetics of infectious disease susceptibility. They reasoned that hemophiliacs who received multiple blood transfusions would be exposed to serum proteins they had not inherited genetically but instead may have “inherited” from their donors. As a result, the hemophiliacs would produce antibodies against the donor proteins. In 1963, Blumberg’s team discovered an antibody present in the serum of a New York hemophiliac that reacted with an antigen present in the blood of an Australian aborigine infected with hepatitis. Later, they determined that the new unknown Australia antigen was HBsAg. Further experiments by British virologist David M. S. Dane led to the discovery of the complete hepatitis B virus, known as the dane particle, which is now recognized as the agent responsible for serum hepatitis. Blumberg is a corecipient of the 1976 Nobel Prize in Physiology or Medicine (with Daniel Carleton Gajdusek) for this hepatitis B research.

Through research and clinical observations, it is known that three types of virus-associated particles are present in the blood of individuals suffering from hepatitis B. The most abundant hepatitis B–like particle present in highly viremic carriers of HBV is a spherical particle approximately 17–25 nm in diameter. The viruses appear as small spheres that contain HBsAg proteins (FIGURE 10-12). Less numerous are filaments approximately 17–20 nm in diameter and up to 200 nm in length (Figure 10-12). The filamentous particles contain small, medium, and large HBsAg proteins (labeled as SHBs, MHBs, and LHBs, respectively, in Figure 10-12), which are not infectious. The large amounts of free HBsAg present in the serum of hepatitis patients correlates with viremia, and in many cases indicate chronic infection.

Dane particles are infectious. The enveloped Dane particles measure 42 nm in diameter and contain small, medium, and large HBsAg proteins. In addition to the HBsAg proteins, the HBV has a DNA-based core that is 27 nm in diameter (Figure 10-12). The core that surrounds the DNA genome contains two proteins: HBcAg and HBeAg. Most of the HBeAg is secreted and is not incorporated into the particle. The viral DNA polymerase (reverse transcriptase), protein kinase C, and heat shock protein 90 are associated with the HBV RNA genome within the core of the particle. Hepatitis B viruses are amazingly stable in the environment. They are resistant to organic solvents, heat, and pH changes. HBV present in blood can withstand drying on a surface for at least a week.

HBV is a hepadnavirus (hepa from “hepatotropic” and dna from its “DNA” genome). The circular genome of HBV is composed of partially double-stranded DNA (dsDNA). The completed full-length strand is approximately 3.2 kb in length; the shorter strand is approximately 1.7 kb in length (FIGURE 10-13). One of the molecular biology hallmarks of hepadnaviruses is that they use a replication strategy common to retroviruses: they replicate through an RNA intermediate, sometimes referred to as a pregenomic RNA, by reverse transcription.

The host cell receptor used for the attachment and entry of HBV Dane particle into hepatocytes is unknown. A few potential receptors have been shown to interact with HBsAg, such as the transferrin receptor, human liver endonexin, and the asialoglycoprotein receptor molecule, but there is no convincing evidence of their connection to infectivity. Hepatitis B viruses replicate poorly in cell cultures, making it difficult to fully understand the early steps of viral infection. After fusion and entry into the host cell, the viral cores are released into the cytoplasm where the genome is uncoated and enters the nucleus through the nuclear pores.

In the nucleus, host enzymes repair the HBV relaxed circular DNA (rcDNA) by ligating the ends of the HBV genome. HBV DNA replication is completed and gaps in both DNA strands are repaired, resulting in a completed, covalently closed circular DNA (cccDNA) that is a plasmid-like dsDNA molecule. Subsequently, the HBV cccDNA is supercoiled and organized into a chromatin-like structure called an episome, or mini-chromosome. The mini-chromosome replicates independently of the host chromosome in contrast to being integrated into the cellular DNA, as is the case with retroviruses. This is because hepatitis B viruses do not possess integrase activity.

The mini-chromosome acts as a template for two classes of viral mRNAs: viral pregenomic RNAs and genomic RNAs that are transcribed by the host’s RNA polymerase II. The transcription factors identified in the HBV genome recognize at least four promoters, two enhancers, and several binding sites. Noncoding regions are not present on the genome. HBV DNA has a very compact coding organization that codes for four partially overlapping open reading frames (ORFs). The four ORFs are translated into seven known proteins (Figure 10-13). TABLE 10-4 lists the HBV ORFs and their gene products.

Following the replication of full-length HBV pregenomic +ssRNAs, it is complexed with the viral polymerase and protein kinase C protein into core particles. Host-cell heat shock proteins associate with the HBV reverse transcriptase, allowing it to fold into an active conformation. The active viral reverse transcriptase converts this pregenomic RNA into DNA inside of the Dane particles. Unlike retroviruses, the HBV reverse transcriptase activity occurs by protein priming as opposed to RNA priming. Precise replication can only occur in the specialized environment of the intact nucleocapsids.

ORF	Gene Products
1 (S)	Large, middle, and small hepatitis B surface (envelope) antigens (HBsAg)
2 (C)	Hepatitis B secreted precore antigen (HBeAg) and hepatitis B core (capsid) antigen (HBcAg)
3 (P)	Viral polymerase (DNA-dependent DNA polymerase, reverse transcriptase, with RNase H activity)
4 (X)	Transcriptional trans-activating protein kinase

The nucleocapsid cores reach the endoplasmic reticulum (ER), where they associate with the viral surface glyco-proteins and bud into the lumen of the ER and/or the Golgi apparatus out of the hepatocyte host cell (FIGURE 10-14). Empty envelopes containing viral surface proteins embedded in the host cell’s lipid bilayer are continually being shed along with mature, infectious Dane particles.

Coinfection of Hepatitis B Virus and Hepatitis D Virus

As described earlier, HDV is a defective virus that requires the presence of hepatitis B “helper” virus for assembly. HDV is frequently associated with severe acute or chronic hepatitis. HDV infection occurs as a coinfection at the same time as an HBV infection or as a superinfection in individuals who are carriers of HBV. In the latter scenario, a superinfection is defined as an HDV infection following a previous infection by HBV. HDV probably makes use of the same host cellular receptor as HBV in order to attach to and enter hepatocytes.

HDV is a spherical enveloped particle that is approximately 36 nm in diameter. The envelope contains small, medium, and large HBsAgs. The internal nucleocapsid structure of HDV contains the –ssRNA circular genome and about 70 copies of delta antigen. HDV can only form particles in a host cell coinfected with HBV because HBV provides the HBsAg that is required for reinfection into another host cell. Once inside the host cell, HDV can replicate in the absence of HBV.

The genome is 1.7 kb in length, which is similar to plant viral satellites and viroids. It codes for two different types of the same protein (i.e., the delta antigen) that are 195 or 214 amino acids in length. About 70 molecules of delta antigen surround and stabilize the viral RNA genome. The large delta antigen contains four different functional domains: an RNA binding domain, a nuclear localization signal, a virus assembly signal, and a coiled-coil sequence that is responsible for oligomerization of the protein. The delta antigen can form either a homo-or heterodimer. The small form of the delta antigen is required for replication of the RNA genome, whereas the large form is required for the suppression of HDV RNA replication and the assembly and export of virions. The small delta antigen is missing the HDV assembly signal (FIGURE 10-15A).

The HDV genome has a high degree of intramolecular base pairing, giving it a double-stranded property similar to that found in plant viroids under natural conditions. When the envelope of the virus is removed inside of an infected cell, the viral genome is targeted to the nucleus. Unlike most RNA viruses, though, it does not encode its own RNA-dependent RNA polymerase for genome replication. Instead, HDV is dependent on the host’s RNA polymerase II and other host factors to replicate its genome. The HDV genomic RNA is a ribozyme that cleaves itself, and ribozyme activity is necessary for HDV replication. HDV genome replication is believed to occur via a rolling circle model similar to that of viroids. FIGURE 10-15B shows the structure of HDV RNAs; a model for the HDV replication cycle is shown in FIGURE 10-16. The HDV RNA –sense (negative-sense) genome acts as a template for synthesis of antigenomic RNA (+sense, or positive sense) and the viral mRNAs. The antigenomic RNA multimers are self-cleaved (step 2 in Figure 10-16) and recircularize, serving as a template for multimers of genomic RNAs, which are self-cleaved and circularized (step 4 in Figure 10-16). The single mRNA transcribed from the viral genome exits the nucleus to produce the small and large forms of the delta antigen (step 7 in Figure 10-16). The ribonucleoprotein complexes (RNPs) assemble and are exported to the ER located in the cytoplasm of the cell, where they associate with the HBsAgs (steps 8 and 9 in Figure 10-16). The HBsAgs are required for the packaging of the hepatitis D virions. Mature hepatitis D viruses are produced and secreted outside of the cell.

Hepatitis C Virus

Hepatitis C virus (HCV) is an icosahedral-shaped, enveloped, +ssRNA flavivirus that causes hepatitis non-A, non-B. Little is known about the ultrastructure of hepatitis C virus because it has been extremely difficult to visualize the virus particles directly from infected serum and tissues. The hypothetical structure of HCV is shown in FIGURE 10-17. It is the only flavivirus that is not transmitted by arthropod vectors (e.g., mosquitoes). Yellow fever virus is the prototype of viruses in the Flaviviridae family. Hepatitis C viruses are also similar to picorna-viruses (e.g., poliovirus), with the main exception that they are enveloped. Since the cloning of HCV nucleic acid in 1989, considerable progress has been made in characterizing the HCV genome and proteins.

The HCV genome is about 9.2 kb in length and contains an IRES and one long ORF that encodes a polyprotein precursor. The HCV +ssRNA is translated by host ribosomes by cap-independent translation. The polyprotein precursor is cleaved into structural and non-structural proteins by cellular and viral proteases. Hepatitis C virus NS5b gene encodes its own RNA-dependent RNA polymerase. The HCV genome has substantial sequence variation because NS5b lacks proofreading capability.

A better understanding of the HCV replication cycle, especially the entry process into host cells is needed. HCV entry requires clathrin and more than one receptor. CD81, an integral membrane protein belonging to a family of tetraspanins; scavenger receptor class B type 1 (SR-B1); and claudin 1 were identified as coreceptors based on their interaction with HCV structural E2 or E1 proteins. The lack of patient response to interferon therapy has been correlated with mutations in the E2 gene. The E1 and E2 proteins presumably self-assemble to form the virion (Figure 10-17).

Hepatitis E Virus

Hepatitis E virus (HEV) is transmitted by the fecal–oral route, often through contaminated water. Like HAV, it has a +ssRNA genome. It is a member of the Hepeviridae family. Hepatitis E is clinically indistinguishable from hepatitis A, but HEV particles are much less stable than those of HAV. Hepatitis E viruses are approximately 32–34 nm in diameter, nonenveloped, and icosahedral shaped. Its genome is approximately 7.2 kb in length and contains short 5′- and 3′-end noncoding regions. The 3′ end contains a poly(A) tail.

The genome has three ORFs, and all three coding frames are used to express different proteins. ORF1 codes for nonstructural proteins that possess methyltransferase, protease, RNA helicase, and RNA-dependent RNA polymerase (replicase) motifs. ORF2 does not overlap with ORF1 and is located at the 3′ end of the genome and presumably codes for the only capsid (structural) protein. ORF3 begins at the end of ORF1. It overlaps with ORF2 and codes for a small phosphoprotein of undefined function (FIGURE 10-18). Presumably, HEV replicates solely in the cytoplasm of the host cell. Overall, our knowledge of HEV replication is poor. The lack of a suitable cell culture system to propagate HEV is an obstacle in deciphering its molecular biology.

Hepatitis A

No specific treatment for hepatitis A infection is available, and management is supportive. Spread of hepatitis A virus infections can be reduced by handwashing and proper sanitary disposal of human feces. Prevention by passive immunization of individuals who likely have been exposed to or in close contact with a person with hepatitis A is achieved by intramuscular injection of human immunoglobulin that contains antibodies against HAV. In general, if administered before exposure or within 2 weeks after exposure, immunoglobulin is 85% effective in preventing hepatitis A.

Hepatitis A vaccines were developed in a manner similar for that of poliovirus vaccines. Formalin-inactivated, cell culture–produced, whole-virus vaccines such as Havrix, Vaqta, and Avaxim are now approved for use in much of the world. HAV is propagated in MRC-5 cells (a cell line of human lung fibroblast cells). The Centers for Disease Control and Prevention (CDC) recommends that all children 12–24 months of age should be vaccinated against HAV. At least two doses that are 6 months apart are needed for lasting protection. In addition to high-risk persons, those who travel to or work in areas with a high prevalence of hepatitis A (e.g., Mexico, Central or South America, Asia, Africa, and Eastern Europe) should be vaccinated. Live, attenuated vaccines similar to the Sabin-poliovirus oral vaccine have been developed and tested in animals and humans but are not yet available, as they have not been able to induce a satisfactory immune response when administered orally.

Hepatitis B

The basic aim of treating hepatitis B patients is to stop HBV replication and prevent end-stage liver disease. Six licensed drug therapies are used against hepatitis B infections: interferon α-2b, lamivudine, adefovir dipivoxil, entecavir, telbivudine, and tenofovir. Interferon is an immune modulator and is administered as an injection. About 46% of patients respond to inter-feron α-2b (intron A) in the first year of treatment. A response is the cessation of viral replication. Interferon α-2b treatment has three drawbacks: it has a large number of side effects, it is expensive, and it is not effective in a high percentage of patients.

Lamivudine (Epivir), adefovir dipivoxil (Hepsera), entecavir (Baraclude), telbivudine (Tyzeka), and tenofovir (Viread) are nucleoside analogs that are potent inhibitors of DNA replication. Telbivudine interferes with +strand synthesis of HBV DNA, thereby inhibiting HBV polymerase. Lamivudine was approved for treatment in 1999 and was first used to treat HIV patients; its mechanism of action is the inhibition of reverse transcriptase. Advantages of lamivudine are that it has few side effects compared to interferon α-2b and has been shown to dramatically inhibit hepatitis B virus DNA replication. The disadvantage is that replication rapidly returns when therapy is stopped. Lamivudine-resistant strains of HBV develop at a rate of 15–20% per year of therapy.

Adefovir dipivoxil has been used since 2002 as a first-line monotherapy, combination therapy, or a therapy in patients who have lamivudine-resistant infections. It can be administered orally, and patients tolerate it well.

Entecavir was approved for use in 2005. It cannot be used as monotherapy in patients with HBV/HIV infection who need treatment for HBV infection. Telbivudine was approved in 2006, but it has not been widely embraced for therapy against HBV. Tenovir was first approved as an HIV reverse transcriptase inhibitor and has similar activity against HBV. It was approved for use in hepatitis B patient treatment in 2008. To date, it has had superior results compared to adefovir and is expected to replace adefovir as a treatment for chronic hepatitis B.

Antibodies against HBsAg protect individuals from acute and chronic hepatitis B infection if used shortly after exposure to hepatitis B virus. Immunoglobulin is used as an adjunct to hepatitis B vaccine in preventing HBV transmission from an infected mother to fetus. If untreated, 70–90% of infants born to HBeAg-positive mothers become infected at birth and develop chronic hepatitis B. Hepatitis B immunoglobulin is also used for postexposure prophylaxis after accidental needlestick or other medical-related injuries in persons who work with infectious body fluids that contain HBsAg.

Early hepatitis B vaccines were prepared by harvesting the 17- to 25-nm particles from the plasma of individuals with chronic HBV infections. The hepatitis B virions were purified and inactivated by heat, formal-dehyde, urea, or pepsin. Plasma-derived vaccines have been available since 1982. Subsequently, vaccine manufacturers have used genetic engineering to express HBsAg in Saccharomyces cerevisiae, a yeast, and mammalian cells. This has led to the development of recombinant DNA vaccines.

Recombinant DNA vaccines were produced by inserting the S gene that codes for the HBsAg into a plasmid DNA. The plasmids containing the S gene were transformed into yeast or transfected into mammalian cells in culture. The HBsAg was expressed and purified by chromatography and filtration (FIGURE 10-20). Under these conditions, purified HBsAg self-assembles into the spherical particles closely resembling the 17- to 25-nm particles found in the serum of people with chronic hepatitis B. The vaccine is not an infectious particle, making this a very safe vaccine.

The CDC recommends that all newborns in the United States receive a birth dose of the recombinant (noninfectious) hepatitis B vaccine before leaving the hospital unless a physician provides a written order to defer the birth dose. A second dose is required at 1–2 months of age and a third dose at 6–18 months of age. HBV can cause liver cancer. An added benefit of the hepatitis B vaccine is that it prevents hepatitis B–related liver cancer. This vaccine was the first to prevent a cancer. From 1990 to 2002, the incidence of hepatitis B decreased 67% in the United States. Hepatitis B vaccination programs have been implemented with considerable success. Unfortunately, the recombinant hepatitis B vaccine is expensive to purchase, which restricts its availability in developing nations. For this reason, oral hepatitis B vaccine candidates are being produced and delivered in plants such as potatoes and soybeans. A number of candidates are suitable for early stage clinical trials.

Hepatitis C

An estimated 2.7–3.9 million people in the United States have chronic hepatitis C and about 275,000 Canadians. Chronic HCV infection affects 170 million people worldwide. HCV infections become chronic in up to 85% of patients. No vaccine is available to prevent HCV infection.

The HCV genotype determines the duration of treatment and expected efficacy of antiviral activity (see Section 10.9 on genetic diversity and genotypes of HCV). Prior infection with HCV does not protect against later infection with the same or different genotypes of the virus. This is because infected individuals typically have an ineffective immune response toward HCV due to changes in cells caused by HCV during infection. No pre- or postexposure immunoglobulin is available for treatment.

Until 2011, the mainstay for the treatment of acute and chronic HCV was pegylated interferon α-2a or pegylated interferon α-2b and/or ribavirin. Pegylation involves the attachment of a polyethylene glycol molecule to the active interferon molecule. Pegylation improves the half-life of interferon, limiting its degradation in the body, thereby making it long acting. interferon is a glycoprotein produced by the body as part of the innate, or nonspecific, immune system response toward viral infection. Therefore, interferon treatment may prevent HCV replication and the production of new infectious virions in infected individuals. Interferon is administered to the patient as weekly injections. Ribavirin is an oral nucleoside analog that inhibits genome replication of a number of viruses. Response to therapy is measured by determining the levels of serum transaminases and the detection of HCV RNA genomes in serum.

In order to prescribe a treatment plan with the highest probability of success, a person must have his or her particular HCV genotype and subtype identified. Once the genotype is identified, it need not be tested again; genotypes do not change during the course of infection. Knowing the genotype predicts the likelihood of treatment outcome and determines the duration of treatment. The combination of ribavirin/pegylated interferon α-2b treatment over a 6- to 12-month period increased overall cure rates to an average of 40% when treating patients with genotype 1 or 4 and an average of 80% for genotypes 2 and 3. However, the therapy is challenging because of the side effects caused by interferon and ribavirin. Ribavirin is teratogenic. It interferes with normal embryonic development and therefore cannot be taken by patients who are pregnant. The main side effect of ribavirin is anemia. Genotype-5 patients respond similarly to those with genotype 1 toward combination treatment. Patients with genotype 6 have an intermediate response level, between that seen with genotype 1 and genotype 2 or 3. The response to standard treatment is not well established for genotype variants 7–11. Patients needed to be monitored closely for complications or symptoms of adverse reactions by combination therapy. About 50% of patients cannot tolerate the side effects of interferon and ribavirin.

Thus, it was clear that there was a need for more promising orally administered antiviral drugs that interrupted HCV replication and that caused fewer side effects. In 2005, the development of a tissue-culture system to propagate HCV paved the way for researchers to develop new antivirals (VIRUS FILE 10-2).

In 2011, Victrelis (boceprevir) and Incivek (telaprevir) were enthusiastically approved by the FDA as two new treatment options for HCV infection. Boceprevir binds to the active site of the NS3 HCV protease. Telaprevir targets the NS3/4a protease of HCV. Targeting the protease activity cripples HCV replication. The viral proteases are responsible for processing the nonstructural polyprotein located in the N-terminus of the HCV polyprotein. Inhibition of the proteases results in unprocessed HCV polyprotein, which ultimately affects the viral proteins involved in genome replication. Both of the drugs increase the already difficult side effects of standard therapy, such as anemia. However, the drugs do not need to be taken for a full 48-week course of treatment.

Because the drugs target HCV specifically, they are a major improvement over current hepatitis C therapy. Survival rates for people infected with genotype 1 increased from 40% in 2001 to an 80% average in 2011 with the availability of the new drugs that are specific HCV inhibitors. In 2013, two new drugs were approved for chronic hepatitis C: sofosbuvir, an inhibitor of HCV RNA polymerase, and simeprevir, an inhibitor of HCV NS5a replication complex. They were approved for use in combination with or without ribavirin and pegylated interferon. This was a major breakthrough for hepatitis C patients who could not tolerate treatment with interferon due to its toxic effects.

Subsequently, four additional hepatitis C drugs, including the first combination drugs available in pill form that did not require ribavirin or interferon administration, became available during 2014–2015 with the approval of the combination HCV drugs Harvoni and Technivie (TABLE 10-5). The hepatitis C treatment regimens approved in the United States and Canada and by the European Association for the Study of the Liver (EASL) are listed in TABLE 10-6. The interferon-free rows in Table 10-6 are shaded in green. The EASL was formed in 1966 as a small group of 70 hepatologists from 15 European countries to share best medical practices to treat liver disease. Today, the EASL has international influence dedicated to liver disease treatment and research. It has over 4,000 members from all over the world and convenes at the International Liver Congress, where more than 11,000 experts meet and discuss the latest treatment regimens and scientific research. Its mission is educational for physicians and scientists. EASL also facilitates multicenter controlled clinical trials and supports young investigators to ensure that the liver remains at the forefront of research.

The newer drugs that specifically targeted HCV replication resulted in higher than 90% survival rates with less severe side effects because regimens did not require the additional use of interferon and/or ribavirin. This means that HCV is a curable disease. The barrier to treatment is cost. The newer HCV drugs are expensive.

As of 2014, Harvoni cost $1,125 per pill, or $94,500 for a 12-week course. Sovaldi is a once-daily pill taken with Olysio. The two-drug combination cost about $150,000 for a full course of treatment in 2014. A study by Liverpool University showed that the cost to manufacture a 12-week course of Olysio could be as low as $100–$250. Some insurance companies have decided to limit the availability of the medicine for specific patients (e.g., compensating the “sickest” patients). Access to HCV treatment in low- and middle-income countries is extremely limited because of the high costs of treatment and complexity of patient management. Of the 20 countries with the largest HCV epidemics, 12 are classified as low or lower-middle income. TABLE 10-7 lists income classification by country and most prevalent HCV genotypes.

Year	Treatment Regimen	Duration	Genotype
2001	Peg-IFN + RBV	48 weeks	1, 4
2001	Peg-IFN + RBV	24 weeks	2, 3
2011	Peg-IFN + RBV + BOC	24–48 weeks	1
2011	Peg-IFN + RBV + TVR	24–48 weeks	1
2013	Peg-IFN + RBV + SMV	12 weeks	1
2013	Peg-IFN + RBV +SOF	12 weeks	1
2013	Peg-IFN + RBV +SOF	12 weeks	4
2013	RBV + SOF	12 weeks	2
2013	RBV + SOF	24 weeks	3
2014	SOF + LDV	12 weeks	1, 4, 5, 6
2014	SOF + SMV	12 weeks	1,4
2014	HVI	12 weeks	1
2015	SOF + DCV	12 weeks	1–6
2015	RBV + TCV	12 weeks	4
2015	DCV + TCV	12 weeks	1
Abbreviations: Peg-IFN: pegylated interferon; RBV: ribavirin; BOC: boceprevir; TVR: telaprevir; SMV: simeprevir; SOF: sofosbuvir; DCV: daclatasvir; TCV: Technivie; LDV: ledipasvir; HVI: Harvoni. Only gold members can continue reading. Log In or Register to continue Share this: Click to share on Twitter (Opens in new window) Click to share on Facebook (Opens in new window) Like this: Like Loading... Related Related posts: Rabies Poxviruses Epidemiology Host Resistance to Viral Infections Stay updated, free articles. Join our Telegram channel Join Tags: Understanding Viruses_gate Jul 29, 2017 | Posted by admin in MICROBIOLOGY | Comments Off on Hepatitis Viruses Full access? Get Clinical Tree Get Clinical Tree app for offline access Get Clinical Tree app for offline access %d