Structure

HAV has a 27-nm, naked, icosahedral capsid surrounding a positive-sense, single-stranded RNA genome consisting of approximately 7470 nucleotides (Figure 63-1). The HAV genome has a VPg protein attached to the 5′ end and a polyadenylate sequence attached to the 3′ end. The capsid is even more stable than other picornaviruses to acid and other treatments (Box 63-1). There is only one serotype of HAV.

Figure 63-1 The picornavirus structure of hepatitis A virus. The icosahedral capsid is made up of four viral polypeptides (VP1 to VP4). Inside the capsid is a single-stranded, positive-sense ribonucleic acid (ssRNA) that has a genomic viral protein (VPg) on the 5′ end.

Replication

HAV replicates like other picornaviruses (see Chapter 54). It interacts specifically with the HAV cell receptor 1 glycoprotein (HAVCR-1, also known as T-cell immunoglobuluin and mucin domain protein [TIM-1]) expressed on liver cells and T cells. The structure of HAVCR-1 can vary for different individuals, and specific forms correlate with severity of disease. Unlike other picornaviruses, however, HAV is not cytolytic and is released by exocytosis. Laboratory isolates of HAV have been adapted to grow in primary and continuous monkey kidney cell lines, but clinical isolates are difficult to grow in cell culture.

Pathogenesis

HAV is ingested and probably enters the bloodstream through the epithelial lining of the oropharynx or the intestines to reach its target, the parenchymal cells of the liver (Figure 63-2). The virus replicates in hepatocytes and Kupffer cells. Virus is produced in these cells and is released into the bile and from there into the stool. Virus is shed in large quantity into the stool approximately 10 days before symptoms of jaundice appear or antibody can be detected.

Figure 63-2 Spread of hepatitis A virus within the body.

HAV replicates slowly in the liver without producing apparent cytopathic effects. Although interferon limits viral replication, natural killer cells and cytotoxic T cells are required to eliminate infected cells. Antibody, complement, and antibody-dependent cellular cytotoxicity also facilitate clearance of the virus and induction of immunopathology. Icterus, resulting from damage to the liver, occurs when cell-mediated immune responses and antibody to the virus can be detected. Antibody protection against reinfection is lifelong.

The liver pathology caused by HAV infection is indistinguishable histologically from that caused by HBV. It is most likely caused by immunopathology and not virus-induced cytopathology. However, unlike HBV, HAV cannot initiate a chronic infection and is not associated with hepatic cancer.

Epidemiology

Approximately 40% of acute cases of hepatitis are caused by HAV (Box 63-2). The virus spreads readily in a community because most infected people are contagious 10 to 14 days before symptoms occur, and 90% of infected children and 25% to 50% of infected adults have inapparent but productive infections.

The virus is released into stool in high concentrations and is spread via the fecal-oral route. Virus is spread in contaminated water, in food, and by dirty hands. HAV is resistant to detergents, acid (pH of 1), and temperatures as high as 60° C, and it can survive for many months in fresh water and salt water. Raw or improperly treated sewage can taint the water supply and contaminate shellfish. Shellfish, especially clams, oysters, and mussels, are important sources of the virus because they are efficient filter feeders and can therefore concentrate the viral particles, even from dilute solutions. This is exemplified by an epidemic of HAV that occurred in Shanghai, China in 1988, when 300,000 people were infected with the virus as the result of eating clams obtained from a polluted river.

HAV outbreaks usually originate from a common source (e.g., water supply, restaurant, day-care center). Asymptomatic shedding and a long (15 to 40 days) incubation period make it difficult to identify the source. Day-care settings are a major source for spread of the virus among classmates and their parents. A further problem is posed by the fact that because the children and personnel in day-care centers may be transient, the number of contacts at risk for HAV infection from a single day-care center can be great.

A relatively high incidence of HAV infection is directly related to poor hygienic conditions and overcrowding. Most people infected with HAV in developing countries are children who have mild illness and then lifelong immune protection against reinfection. In the populations of more highly developed countries, infection occurs later in life. The seropositivity rate of adults ranges from a low of 13% of the adult population in Sweden to highs of 88% in Taiwan and 97% in Yugoslavia, with a 41% to 44% rate in the United States.

Clinical Syndromes

The symptoms caused by HAV are very similar to those caused by HBV and stem from immune-mediated damage to the liver. As already noted, disease in children is generally milder than that in adults and is usually asymptomatic. The symptoms occur abruptly 15 to 50 days after exposure and intensify for 4 to 6 days before the icteric (jaundice) phase (Figure 63-3). Initial symptoms include fever, fatigue, nausea, loss of appetite, and abdominal pain. Dark urine (bilirubinuria), pale stool, and then jaundice may be accompanied by abdominal pain and itch. Jaundice is observed in 70% to 80% of adults but in only 10% of children (<6 years of age). Symptoms generally wane during the jaundice period. Viral shedding in the stool precedes the onset of symptoms by approximately 14 days but stops before the cessation of symptoms. Complete recovery occurs 99% of the time within 2 to 4 weeks of onset.

Figure 63-3 Time course of hepatitis A virus (HAV) infection. IgG, Immunoglobulin G; IgM, immunoglobulin M.

Fulminant hepatitis in HAV infection occurs in one to three persons per 1000 and is associated with an 80% mortality rate. Unlike HBV, immune complex–related symptoms (e.g., arthritis, rash) rarely occur in people with HAV disease.

Laboratory Diagnosis

The diagnosis of HAV infection is generally made on the basis of the time course of the clinical symptoms, the identification of a known infected source, and most reliably, the results yielded by specific serologic tests. The best way to demonstrate an acute HAV infection is by finding anti-HAV IgM, as measured by an enzyme-linked immunosorbent assay (ELISA) or radioimmunoassay. Virus isolation is not performed because efficient tissue culture systems for growing the virus are not available.

Treatment, Prevention, and Control

The spread of HAV is reduced by interrupting the fecal-oral spread of the virus. This is accomplished by avoiding potentially contaminated water or food, especially uncooked shellfish. Proper hand washing, especially in day-care centers, mental hospitals, and other care facilities, is vitally important. Chlorine treatment of drinking water is generally sufficient to kill the virus.

Prophylaxis with immune serum globulin given before or early in the incubation period (i.e., less than 2 weeks after exposure) is 80% to 90% effective in preventing clinical illness.

Killed HAV vaccines have been approved by the U.S. Food and Drug Administration (FDA) and are available for all children and for adults at high risk for infection, especially travelers to endemic regions. The vaccine is administered to infants at 2 years of age and can be administered with the HBV vaccine to adults. A live HAV vaccine is in use in China. There is only one serotype of HAV, and HAV infects only humans, factors that help ensure the success of an immunization program.

Structure

HBV is a small, enveloped DNA virus with several unusual properties (Figure 63-4). Specifically, the genome is a small, circular, partly double-stranded DNA of only 3200 bases. Although a DNA virus, it encodes a reverse transcriptase and replicates through an RNA intermediate.

Figure 63-4 Hepatitis B virus (Dane particle) and hepatitis B surface antigen (HBsAg) particles. The spheric HBsAg consists mainly of the S form of HBsAg, with some M. The filamentous HBsAg has S, M, and L forms. bp, Base pair; DNA, deoxyribonucleic acid; L, gp42; M, gp36; S, gp27.

The virion, also called the Dane particle, is 42 nm in diameter. The virions are unusually stable for an enveloped virus. They resist treatment with ether, low pH, freezing, and moderate heating. These characteristics assist transmission from one person to another and hamper disinfection.

The HBV virion includes a protein kinase and a polymerase with reverse transcriptase and ribonuclease H activity, as well as a P protein attached to the genome. All of this is surrounded by an icosahedral capsid formed by the hepatitis B core antigen (HBcAg) and an envelope containing three forms of the glycoprotein hepatitis B surface antigen (HBsAg). A hepatitis Be antigen (HBeAg) protein shares most of its protein sequence with HBcAg but is processed differently by the cell, is primarily secreted into serum, does not self-assemble (like a capsid antigen), and expresses different antigenic determinants.

HBsAg-containing particles are released into the serum of infected people and outnumber the actual virions. These particles can be spheric (but smaller than the Dane particle) or filamentous (see Figure 63-4). They are immunogenic and were processed into the first commercial vaccine against HBV.

HBsAg, originally termed the Australia antigen, includes three glycoproteins (L, M, and S) encoded by the same gene and read in the same frame but translated into protein from different AUG (adenine, uracil, guanine) start codons. The S (gp27; 24 to 27 kDa) glycoprotein is completely contained in the M (gp36; 33 to 36 kDa) glycoprotein, which is contained in the L (gp42; 39 to 42 kDa) glycoprotein; all share the same C-terminal amino acid sequences. All three forms of HBsAg are found in the virion. The S glycoprotein is the major component of HBsAg particles; it self-associates into 22-nm spheric particles that are released from the cells. The filamentous particles of HBsAg found in serum contain mostly S but also small amounts of the M and L glycoproteins and other proteins and lipids. The glycoproteins of HBsAg contain the group-specific (termed a) and type-specific determinants of HBV (termed d or y and w or r). Combinations of these antigens (e.g., ady and adw) result in eight subtypes of HBV that are useful epidemiologic markers.

Replication

The replication of HBV is unique for several reasons (see Box 63-1). First, HBV has a distinctly defined tropism for the liver. Its small genome also necessitates economy, as illustrated by the pattern of its transcription and translation. In addition, HBV replicates through an RNA intermediate and produces and releases antigenic decoy particles (HBsAg) (Figure 63-5).

Figure 63-5 Replication of hepatitis B virus (HBV). After entry into the hepatocyte and uncoating of the nucleocapsid core, the partially double-stranded deoxyribonucleic acid (DNA) genome is completed by enzymes in the core and then delivered to the nucleus. Transcription of the genome produces four messenger RNAs (mRNAs), including an mRNA larger than the genome (3500 bases). The mRNA then moves to the cytoplasm and is translated into protein. Core proteins assemble around the 3500-base mRNA, and negative-sense DNA is synthesized by a reverse transcriptase activity in the core. The ribonucleic acid (RNA) is then degraded as a positive-sense (+) DNA is synthesized. The core is enveloped before completion of the positive-sense DNA and then released by exocytosis. HBsAg, hepatitis B surface antigen.

The attachment of HBV to hepatocytes is mediated by the HBsAg glycoproteins. Several liver cell receptors have been suggested, including the transferrin receptor, the asialoglycoprotein receptor, and human liver annexin V. The mechanism of entry is not known, but HBsAg binds to polymerized human serum albumin and other serum proteins, and binding and uptake of these proteins may facilitate virus uptake by the liver.

On penetration into the cell, the partial DNA strand of the genome is completed by being formed into a complete double-stranded DNA circle, and the genome is delivered to the nucleus. Transcription of the genome is controlled by cellular transcription elements found in hepatocytes. The DNA is transcribed from different starting points on the circle but have the same 3′ end. There are three major classes (2100, 2400, and 3500 bases) and two minor classes (900 bases) of overlapping messenger RNAs (mRNAs) (Figure 63-6). The 3500-base mRNA is larger than the genome. It encodes the HBc and HBe antigens, the polymerase, and a protein primer for DNA replication and acts as the template for replication of the genome. The HBe and HBc are related proteins that are translated from different in-phase start codons of closely related mRNA. This causes differences in their processing and structure, with shedding of the HBe and incorporation of HBc into the virion. Similarly, the 2100-base mRNA encodes the small and medium glycoproteins from different in-phase start codons. The 2400-base mRNA, which encodes the large glycoprotein, overlaps the 2100-base mRNA. The 900-base mRNA encodes the X protein, which promotes viral replication as a transactivator of transcription and as a protein kinase.

Figure 63-6 DNA, RNA, mRNA, and proteins of hepatitis B virus. The inner green circles represent the DNA genome with the nucleotide number at the center. DR1 and DR2 are direct repeat sequences of DNA and are important for replication and integration of the genome. The 3500-base transcript (outer black thin-line circle) is larger than the genome and is the template for replication of the genome. Bold arcs represent mRNA for viral proteins. Note that several proteins are translated from the same mRNA but from different AUG codons and that different mRNAs overlap. AAA, 3′ polyA (polyadenylate) at end of mRNA; AUG, adenine, uracil, guanine; C, C mRNA (hepatitis B core antigen [HBcAg]); E, E mRNA (hepatitis Be antigen [HBeAg]); HBsAg, hepatitis B surface antigen; l, large glycoprotein; m, medium glycoprotein; P, polymerase-protein primer for replication; s, small glycoprotein; S, S mRNA (HBsAg); X, X mRNA.

(Modified from Armstrong D, Cohen J: Infectious diseases, St Louis, 1999, Mosby.)

Replication of the genome utilizes the larger-than-genome 3500-base mRNA. This is packaged into the core nucleocapsid that contains the RNA-dependent DNA polymerase (P protein). This polymerase has reverse transcriptase and ribonuclease H activity, but HBV lacks the integrase activity of the retroviruses. The 3500-base RNA acts as a template, and negative-strand DNA is synthesized using a protein primer from the P protein, which remains covalently attached to the 5′ end. After this, the RNA is degraded by the ribonuclease H activity as the positive-strand DNA is synthesized from the negative-sense DNA template. However, this process is interrupted by envelopment of the nucleocapsid at HBsAg-containing intracellular membranes, thereby capturing genomes containing RNA-DNA circles with different lengths of RNA. Continued degradation of the remainder of the RNA in the virion yields a partly double-stranded DNA genome. The virion is then released from the hepatocyte by exocytosis without killing the cell, not by cell lysis.

The entire genome can also be integrated into the host cell chromatin. HBsAg, but not other proteins, can often be detected in the cytoplasm of cells containing integrated HBV DNA. The significance of the integrated DNA in the replication of the virus is not known, but integrated viral DNA has been found in hepatocellular carcinomas.

Pathogenesis and Immunity

HBV can cause acute or chronic, symptomatic or asymptomatic disease. Which of these occurs seems to be determined by the person’s immune response to the infection (Figure 63-7). Detection of both the HBsAg and the HBeAg components of the virion in the blood indicates the existence of an ongoing active infection. HBsAg particles continue to be released into the blood, even after virion release has ended and until the infection is resolved.

Figure 63-7 Major determinants of acute and chronic hepatitis B virus (HBV) infection. HBV infects the liver but does not cause direct cytopathology. Cell-mediated immune lysis of infected cells produces the symptoms and resolves the infection. Insufficient immunity can lead to chronic disease. Chronic HBV disease predisposes a person to more serious outcomes. Purple arrows indicate symptoms; green arrows indicate a possible outcome.

The major source of infectious virus is blood, but HBV can be found in semen, saliva, milk, vaginal and menstrual secretions, and amniotic fluid. The most efficient way to acquire HBV is through injection of the virus into the bloodstream (Figure 63-8). Common but less efficient routes of infection are sexual contact and birth.

Figure 63-8 Spread of hepatitis B virus (HBV) in the body. Initial infection with HBV occurs through injection, heterosexual and homosexual sex, and birth. The virus then spreads to the liver, replicates, induces a viremia, and is transmitted in various body secretions in addition to blood to start the cycle again. Symptoms are caused by cell-mediated immunity (CMI) and immune complexes between antibody and hepatitis B surface antigen (HBsAg). IV, Intravenous.

The virus starts to replicate in the liver within 3 days of its acquisition, but as already noted, symptoms may not be observed for 45 days or longer, depending on the infectious dose, the route of infection, and the person. The virus replicates in hepatocytes with minimal cytopathic effect. Infection proceeds for a relatively long time without causing liver damage (i.e., elevation of liver enzyme levels) or symptoms. During this time, copies of the HBV genome integrate into the hepatocyte chromatin and remain latent. Intracellular buildup of filamentous forms of HBsAg can produce the ground-glass hepatocyte cytopathology characteristic of HBV infection.

Cell-mediated immunity and inflammation are responsible for causing the symptoms and effecting resolution of the HBV infection by eliminating the infected hepatocyte. Epitopes from the HBc antigen are prominent T-cell antigens. An insufficient T-cell response to the infection generally results in the occurrence of mild symptoms, an inability to resolve the infection, and the development of chronic hepatitis (see Figure 63-7). Chronic infection also exhausts CD8 T cells preventing them from killing infected cells. Antibody (as generated by vaccination) can protect against initial infection by preventing delivery of the virus to the liver. Later in the infection, the large amount of HBsAg in serum binds to and blocks the action of neutralizing antibody, which limits the antibody’s capacity to resolve an infection. Immune complexes formed between HBsAg and anti-HBs contribute to the development of hypersensitivity reactions (type III), leading to problems such as vasculitis, arthralgia, rash, and renal damage.

Antibody to HBc is present in serum but is nonprotective. The HBeAg protein, like HBsAg, is released into serum, and, during its production, anti-HBeAg is bound to the antigen and undetectable.

Infants and young children have an immature cell-mediated immune response and are less able to resolve the infection, but they suffer less tissue damage and milder symptoms. As many as 90% of infants infected perinatally become chronic carriers. Viral replication persists in these people for long periods.

During the acute phase of infection, the liver parenchyma shows degenerative changes consisting of cellular swelling and necrosis, especially in hepatocytes surrounding the central vein of a hepatic lobule. The inflammatory cell infiltrate is mainly composed of lymphocytes. Resolution of the infection allows the parenchyma to regenerate. Fulminant infections, activation of chronic infections, or co-infection with the delta agent can lead to permanent liver damage and cirrhosis.

Epidemiology

In the United States, more than 12 million people have been infected with HBV (1 out of 20), with 5000 deaths per year. In the world, one out of three people have been infected with HBV, with approximately a million deaths per year. More than 350 million people worldwide have chronic HBV infection. In developing nations, as many as 15% of the population may be infected during birth or childhood. High rates of seropositivity are observed in Italy, Greece, Africa, and Southeast Asia (Figure 63-9). In some areas of the world (southern Africa and southeastern Asia), the seroconversion rate is as high as 50%. PHC, a long-term sequela of the infection, is also endemic in these regions.

Figure 63-9 Worldwide prevalence of hepatitis B carriers and primary hepatocellular carcinoma (PHC). HBsAg, Hepatitis B surface antigen.

(Courtesy Centers for Disease Control and Prevention, Atlanta.)

The many asymptomatic chronic carriers with virus in blood and other body secretions foster the spread of the virus. In the United States, 0.1% to 0.5% of the general population are chronic carriers, but this is very low in comparison with many areas of the world. Carrier status may be lifelong.

The virus is spread by sexual, parenteral, and perinatal routes. Transmission occurs through contaminated blood and blood components by transfusion, needle sharing, acupuncture, ear piercing, or tattooing, and through very close personal contact involving the exchange of semen, saliva, and vaginal secretions (e.g., sex, childbirth) (see Figure 63-8). Medical personnel are at risk in accidents involving needlesticks or sharp instruments. People at particular risk are listed in Box 63-4. Sexual promiscuity and drug abuse are major risk factors for HBV infection. HBV can be transmitted to babies through contact with the mother’s blood at birth and in the mother’s milk. Babies born to chronic HBV-positive mothers are at highest risk for infection. Serologic screening of donor units in blood banks has greatly reduced the risk of acquisition of the virus from contaminated blood or blood products. Safer sex habits adopted to prevent human immunodeficiency virus (HIV) transmission and the administration of the HBV vaccine have also been responsible for decreasing the transmission of HBV.

One of the major concerns about HBV is its association with PHC. This type of carcinoma probably accounts for 250,000 to 1 million deaths per year worldwide; in the United States, approximately 5000 deaths per year are attributed to PHC.

Clinical Syndromes

Acute Infection

As already noted, the clinical presentation of HBV in children is less severe than that in adults, and infection may even be asymptomatic. Clinically apparent illness occurs in as many as 25% of those infected with HBV (Figures 63-10 to 63-12).

Figure 63-10 Symptoms of typical acute viral hepatitis B infection are correlated with the four clinical periods of this disease. RUQ, Right upper quadrant.

(Modified from Hoofnagle JH: Type A and type B hepatitis, Lab Med 14:705–716, 1983.)

Figure 63-11 Clinical outcomes of acute hepatitis B infection. HBsAg, Hepatitis B surface antigen.

(Modified from White DO, Fenner F: Medical virology, ed 3, New York, 1986, Academic.)

Figure 63-12 A, The serologic events associated with the typical course of acute hepatitis B disease. B, Development of the chronic hepatitis B virus carrier state. Routine serodiagnosis is difficult during the hepatitis B surface antigen (HBsAg) window, when HBs and anti-HBs are undetectable. Anti-HBc, Antibody to hepatitis B core antigen [HBcAg]; Anti-HBe, antibody to hepatitis Be antigen [HBeAg]; Anti-HBs, antibody to HBsAg.

(Modified from Hoofnagle JH: Serologic markers of hepatitis B virus infection, Annu Rev Med 32:1–11, 1981.)

HBV infection is characterized by a long incubation period and an insidious onset. Symptoms during the prodromal period may include fever, malaise, and anorexia, followed by nausea, vomiting, abdominal discomfort, and chills. The classic icteric symptoms of liver damage (e.g., jaundice, dark urine, pale stools) follow soon thereafter. Recovery is indicated by a decline in the fever and renewed appetite.

Fulminant hepatitis occurs in approximately 1% of icteric patients and may be fatal. It is marked by more severe symptoms and indications of severe liver damage, such as ascites and bleeding.

HBV infection can promote hypersensitivity reactions that are caused by immune complexes of HBsAg and antibody. These may produce rash, polyarthritis, fever, acute necrotizing vasculitis, and glomerulonephritis.

Laboratory Diagnosis

The initial diagnosis of hepatitis can be made on the basis of the clinical symptoms and the presence of liver enzymes in the blood (see Figure 63-12). However, the serology of HBV infection describes the course and the nature of the disease (Table 63-2). Acute and chronic HBV infections can be distinguished by the presence of HBsAg and HBeAg in the serum and the pattern of antibodies to the individual HBV antigens.

Table 63-2 Interpretation of Serologic Markers of Hepatitis B Virus Infection

HBsAg and HBeAg are secreted into the blood during viral replication. The detection of HBeAg is the best correlate to the presence of infectious virus. A chronic infection can be distinguished by the continued finding of HBeAg, HBsAg, or both, and a lack of detectable antibody to these antigens. Antibody to HBsAg indicates resolution of infection or vaccination.

Antibody to HBcAg indicates current or prior infection by HBV and IgM anti-HBc is the best way to diagnose a recent acute infection, especially during the period when neither HBsAg nor anti-HBs can be detected (the window). Detection of antibodies to HBeAg and HBsAg is obscured during infection because the antibody is complexed with antigen in the serum.

The amount of virus in blood can be determined by quantitative genome assays using polymerase chain reaction (PCR) and related techniques. Knowing the virus load can help in following the course of chronic HBV infection and antiviral drug efficacy.

Treatment, Prevention, and Control

Hepatitis B immune globulin may be administered within a week of exposure and to newborn infants of HBsAg-positive mothers to prevent and ameliorate disease. Chronic HBV infection can be treated with drugs targeted at the polymerase—for example, lamivudine (2′3′dideoxy-3′-thiacytidine), which is also an HIV reverse transcriptase inhibitor—or the nucleoside analogues, adefovir dipivoxil and famciclovir. These FDA-approved treatments are taken for 1 year. Unfortunately, antiviral drug resistance can develop. Pegylated interferon-α can also be effective and is taken for at least 4 months.

Transmission of HBV in blood or blood products has been greatly reduced by screening donated blood for the presence of HBsAg and anti-HBc. Additional efforts to prevent transmission of HBV consist of avoiding sex with a carrier of HBV and avoiding the lifestyles that facilitate spread of the virus. Household contacts and sexual partners of HBV carriers are at increased risk, as are patients undergoing hemodialysis, recipients of pooled plasma products, health care workers exposed to blood, and babies born to HBV-carrier mothers.

Vaccination is recommended for infants, children, and especially people in high-risk groups (see Box 63-4). For newborns of HBsAg-positive mothers and people accidentally exposed either percutaneously or permucosally to blood or secretions from an HBsAg-positive person, vaccination is useful even after exposure. Immunization of mothers should decrease the incidence of transmission to babies and older children, also reducing the number of chronic HBV carriers. Prevention of chronic HBV will reduce the incidence of PHC.

The HBV vaccines form virus-like particles. The initial HBV vaccine was derived from the 22-nm HBsAg particles in human plasma obtained from chronically infected people. The current vaccine was genetically engineered by the insertion of a plasmid containing the S gene for HBsAg into a yeast, Saccharomyces cerevisiae. The protein self-assembles into particles, which enhances its immunogenicity.

The vaccine must be given in a series of three injections, with the second and third given 1 and 6 months after the first. The single serotype and limited host range (humans) help ensure the success of an immunization program.

Universal blood and body fluid precautions are used to limit exposure to HBV. It is assumed that all patients are infected. Gloves are required for handling blood and body fluids; wearing protective clothing and eye protection may also be necessary. Special care should be taken with needles and sharp instruments. HBV-contaminated materials can be disinfected with 10% bleach solutions, but unlike most enveloped viruses, HBV is not readily inactivated by detergents.

Structure and Replication

HCV is the only member of the Hepacivirus genus of the Flaviviridae family. It is 30 to 60 nm in diameter, has a positive-sense RNA genome, and is enveloped. The genome of HCV (9100 nucleotides) encodes 10 proteins, including two glycoproteins (E1, E2). The viral RNA-dependent RNA polymerase is error prone and generates mutations in the glycoprotein and other genes. This generates antigenic variability. Such variability makes the development of a vaccine very difficult. There are six major groups of variants (clades), which differ in their worldwide distribution.

HCV infects only humans and chimpanzees. HCV binds to CD81 (tetraspanin) surface receptors, which is expressed on hepatocytes and B lymphocytes, and can also coat itself with low-density lipoprotein or very-low-density lipoprotein and then use the lipoprotein receptor to facilitate uptake into hepatocytes. The virus replicates like other flaviviruses. The virion assembles at and buds into the endoplasmic reticulum, becoming cell associated. HCV proteins inhibit apoptosis and interferon-α action by binding to the tumor necrosis factor receptor and to protein kinase R. These actions prevent the death of the host cell and promote persistent infection.

Pathogenesis

The ability of HCV to remain cell associated and prevent host cell death promotes persistent infection but results in liver disease later in life. Cell-mediated immune responses are responsible for both the resolution of infection and tissue damage. As for HBV, chronic infection can exhaust CD8 cytotoxic T cells to prevent resolution of infection. The extent of lymphocytic infiltration, inflammation, portal and periportal fibrosis, and lobular necrosis in liver biopsies can be used to grade the severity of disease. It has been suggested that the cytokines of inflammation and continual liver repair and induction of cell growth occurring during chronic HCV infection are predisposing factors in the development of PHC. Antibody to HCV is not protective.

Epidemiology

HCV is transmitted primarily in infected blood and sexually. Intravenous drug abusers, tattoo recipients, transfusion and organ recipients, and hemophiliacs receiving factors VIII or IX are at highest risk for infection (Box 63-5). Almost all (>90%) HIV-infected people who are or were intravenous drug users are infected with HCV. HCV is especially prevalent in southern Italy, Spain, central Europe, Japan, and parts of the Middle East (e.g., almost 20% of Egyptian blood donors are HCV positive). The high incidence of chronic asymptomatic infections promotes the spread of the virus in the population. Screening procedures have led to a reduction in the levels of transmission by blood transfusion and organ donation, but transmission by other routes is prevalent.

Clinical Syndromes (Clinical Case 63-1)

HCV causes three types of disease (Figure 63-13): (1) acute hepatitis with resolution of the infection and recovery in 15% of cases, (2) chronic persistent infection with possible progression to disease much later in life for 70% of infected persons, and (3) severe rapid progression to cirrhosis in 15% of patients. A viremia can be detected within 1 to 3 weeks of a transfusion of HCV-contaminated blood. The viremia lasts 4 to 6 months in people with an acute infection and longer than 10 years in those with a persistent infection. In its acute form, HCV infection is similar to acute HAV and HBV infection, but the inflammatory response is less intense, and the symptoms are usually milder. More commonly (>70% of cases), the initial disease is asymptomatic but establishes chronic persistent disease. The predominant symptom is chronic fatigue. The chronic, persistent disease often progresses to chronic active hepatitis within 10 to 15 years and to cirrhosis (20% of chronic cases) and liver failure (20% of cirrhotic cases) after 20 years. HCV-induced liver damage may be exacerbated by alcohol, certain medications, and other hepatitis viruses to promote cirrhosis. HCV promotes the development of hepatocellular carcinoma after 30 years in up to 5% of chronically infected patients.

Figure 63-13 Outcomes of hepatitis C virus infection.

Laboratory Diagnosis

The diagnosis and detection of HCV infection are based on ELISA recognition of anti-HCV antibody or detection of the RNA genome. Seroconversion occurs within 7 to 31 weeks of infection. ELISA is used for screening the blood supply from normal donors. As for HIV, results can be confirmed by Western immunoblot procedures. Antibody is not always detectable in viremic people, in immunocompromised patients, or in those receiving hemodialysis. Genome detection and quantitation by RT-PCR, branched-chain DNA, and related techniques is the gold standard for confirming a diagnosis of HCV and for following the success of antiviral drug therapy. Genetic assays are less strain specific and can detect HCV RNA in seronegative people.

Treatment, Prevention, and Control

Recombinant interferon-α or pegylated interferon (treated with polyethylene glycol to enhance its biologic lifetime), alone or with ribavirin, were the only known treatments for HCV. Treatment with pegylated interferon and ribavirin for extended periods may yield up to 50% recovery rates. This therapy can now be supplemented with either of two protease inhibitors, boceprevir or telaprevir. As for HIV, the addition of a protease inhibitor to the previous antiviral protocol is expected to make a significant difference in therapeutic efficacy.

The precautions for transmission of HCV are similar to that of HBV and other blood-borne pathogens. The blood supply and organ donors are screened for HCV. Persons with HCV should not share any personal care items and syringe needles that may get contaminated with blood and should practice safe sex. Alcohol drinking should be limited because it exacerbates the liver damage caused by HCV.

Structure and Replication

The HDV RNA genome is very small (approximately 1700 nucleotides), and unlike other viruses, the single-stranded RNA is circular and forms a rod shape as a result of its extensive base pairing. The virion is approximately the same size as the HBV virion (35 to 37 nm in diameter). The genome is surrounded by the delta antigen core, which in turn is surrounded by an HBsAg-containing envelope. The delta antigen exists as a small (24 kDa) or large (27 kDa) form; the small form is predominant.

The delta agent binds to and is internalized by hepatocytes in the same manner as HBV because it has HBsAg in its envelope. The transcription and replication processes of the HDV genome are unusual. The host cell’s RNA polymerase II makes an RNA copy to replicate the genome. The genome then forms an RNA structure called a ribozyme, which cleaves the RNA circle to produce an mRNA for the small delta antigen. The gene for the delta antigen is mutated by a cellular enzyme (double-stranded RNA-activated adenosine deaminase) during infection, thereby allowing production of the large delta antigen. Production of this antigen limits replication of the virus but also promotes association of the genome with HBsAg to form a virion, and the virus is then released from the cell.

Pathogenesis

Similar to HBV, the delta agent is spread in blood, semen, and vaginal secretions. However, it can replicate and cause disease only in people with active HBV infections. Because the two agents are transmitted by the same routes, a person can be co-infected with HBV and the delta agent. A person with chronic HBV can also be superinfected with the delta agent. More rapid, severe progression occurs in HBV carriers superinfected with HDV than in people co-infected with HBV and the delta agent because, during co-infection, HBV must first establish its infection before HDV can replicate (Figure 63-15), whereas superinfection of an HBV-infected person allows the delta agent to replicate immediately.

Figure 63-15 Consequences of delta virus infection. Delta virus (δ) requires the presence of hepatitis B virus (HBV) infection. Superinfection of a person already infected with HBV (carrier) causes more rapid, severe progression than co-infection (shorter arrow).

Replication of the delta agent results in cytotoxicity and liver damage. Persistent delta agent infection is often established in HBV carriers. Although antibodies are elicited against the delta agent, protection probably stems from the immune response to HBsAg because it is the external antigen and viral attachment protein for HDV. Unlike HBV disease, damage to the liver occurs as a result of the direct cytopathic effect of the delta agent combined with the underlying immunopathology of the HBV disease.

Epidemiology

The delta agent infects children and adults with underlying HBV infection (see Box 63-5), and people who are persistently infected with both HBV and HDV are a source for the virus. The agent has a worldwide distribution, infecting approximately 5% of the 3 × 10⁸ HBV carriers and is endemic in southern Italy, the Amazon Basin, parts of Africa, and the Middle East. Epidemics of HDV infection occur in North America and Western Europe, usually in illicit drug users. HDV is spread by the same routes as HBV, and the same groups are at risk for infection, with parenteral drug abusers, hemophiliacs, and others receiving blood products at highest risk. Screening of the blood supply has reduced the risk for recipients of blood products.

Clinical Syndromes (Box 63-6)

The delta agent increases the severity of HBV infections. Fulminant hepatitis is more likely to develop in people infected with the delta agent than in those infected with the other hepatitis viruses. This very severe form of hepatitis causes altered brain function (hepatic encephalopathy), extensive jaundice, and massive hepatic necrosis, which is fatal in 80% of cases. Chronic infection with the delta agent can occur in people with chronic HBV.

Laboratory Diagnosis

The presence of the agent can be noted by detecting the RNA genome, the delta antigen, or anti-HDV antibodies. ELISA and radioimmunoassay procedures are available for detection. The delta antigen can be detected in the blood during the acute phase of disease in a detergent-treated serum sample. RT-PCR techniques can be used to detect the virion genome in blood.

Treatment, Prevention, and Control

There is no known specific treatment for HDV hepatitis. Because the delta agent depends on HBV for replication and is spread by the same routes, prevention of infection with HBV prevents HDV infection. Immunization with HBV vaccine protects against subsequent delta virus infection. If a person has already acquired HBV, delta agent infection may be prevented by halting illicit intravenous drug use and avoiding HDV-contaminated blood products.