HBV is a member of the hepadnavirus family. It is a 42-nm enveloped virion,1 with an icosahedral nucleocapsid core containing a partially double-stranded circular DNA genome (Figure 41–1 and Table 41–2).
Hepatitis B virus (HBV). Left: Cross-section of the HBV virion. Right: The 22-nm spheres and filaments composed only of hepatitis B surface antigen. Because there is no viral DNA in the spheres and filaments, they are not infectious. (Reproduced with permission from Ryan K. Sherris Medical Microbiology, 3rd ed. New York, NY: McGraw-Hill Education; 1994.)
The envelope contains a protein called the surface antigen (HBsAg), which is important for laboratory diagnosis and immunization.2
Within the core is a DNA polymerase. The genome contains four genes (four open reading frames) that encode five proteins; namely, the S gene encodes the surface antigen, the C gene encodes the core antigen and the e antigen, the P gene encodes the polymerase, and the X gene encodes the X protein (HBx). HBx is an activator of viral RNA transcription and may be involved in oncogenesis because it can inactivate the p53 tumor suppressor protein (see Chapter 43). The DNA polymerase has both RNA-dependent (reverse transcriptase) and DNA-dependent activity.
Electron microscopy of a patient’s serum reveals three different types of particles: a few 42-nm virions and many 22-nm spheres and long filaments 22 nm wide, which are composed of surface antigen (Figure 41–2). HBV is the only human virus that produces these spheres and filaments in such large numbers in the patient’s blood. The ratio of filaments and small spheres to virions is 1000:1.
Hepatitis B virus—electron micrograph. Long arrow points to a typical virion of hepatitis B virus. Short arrow points to a small sphere (just left of arrowhead) and a long rod (just right of arrowhead), both composed only of hepatitis B surface antigen. (Reproduced with permission from Public Health Image Library, Centers for Disease Control and Prevention.)
In addition to HBsAg, there are two other important antigens both located in the core of the virus: the core antigen (HBcAg) and the e antigen (HBeAg). The core antigen, as the name implies, is located on the nucleocapsid protein that forms the core of the virion, whereas the e antigen is soluble and is released from infected cells into the blood. The e antigen is an important indicator of transmissibility.
For vaccine purposes, HBV has one serotype based on HBsAg. However, for epidemiologic purposes, there are four serologic subtypes of HBsAg based on a group-specific antigen, “a,” and two sets of mutually exclusive epitopes, d or y and w or r. This leads to four serotypes—adw, adr, ayw, and ayr—which are useful in epidemiologic studies because they are concentrated in certain geographic areas.
The specificity of HBV for liver cells is based on two properties: virus-specific receptors located on the hepatocyte cell membrane (facilitate entry) and transcription factors found only in the hepatocyte that enhance viral mRNA synthesis (act post-entry).
Humans are the only natural hosts of HBV. There is no animal reservoir.
Summary of Replicative Cycle
The replicative cycle of HBV is depicted in Figure 41–3. After entry of the virion into the cell and its uncoating, the nucleocapsid moves to the nucleus. In the nucleus, the virion DNA polymerase synthesizes the missing portion of DNA, and a double-stranded circular DNA is formed. This DNA serves as a template for mRNA synthesis by cellular RNA polymerase. After the individual mRNAs are made, a full-length positive-strand RNA is made, which is the template for the minus strand of the progeny DNA. The minus strand then serves as the template for the plus strand of the genome DNA. This RNA-dependent DNA synthesis catalyzed by reverse transcriptase encoded by HBV takes place within the newly assembled virion nucleocapsid core in the cytoplasm. The RNA-dependent DNA synthesis that produces the genome and the DNA-dependent DNA synthesis that fills in the missing portion of DNA soon after infection of the next cell are carried out by the same enzyme (i.e., the HBV genome encodes only one polymerase). Progeny HBV with its HBsAg-containing envelope are released from the cell by budding through the cell membrane.
Replication cycle of hepatitis B virus. Note virus-encoded reverse transcriptase synthesizes the progeny DNA genomes using viral RNA as the template. CCC DNA is covalently closed circular DNA in the nucleus (blue circle).
Hepadnaviruses are the only viruses that produce genome DNA by reverse transcription with viral RNA as the template. (Note that this type of RNA-dependent DNA synthesis is similar to but different from the process in retroviruses, in which the genome RNA is transcribed into a DNA intermediate.)
In chronic HBV infection, a carrier state occurs and progeny HBV continues to be made. In this carrier state, most of the circular HBV DNA is found free in the nucleus as an episome. A small amount of HBV DNA is integrated into host cell DNA. How episomal HBV DNA is maintained in the carrier state for many years is unclear.
Transmission & Epidemiology
The three main modes of transmission are via blood, during sexual intercourse, and perinatally from mother to newborn. The observation that needle-stick injuries can transmit the virus indicates that only very small amounts of blood are necessary. HBV infection is especially prevalent in addicts who use intravenous drugs. Screening of blood for the presence of HBsAg has greatly decreased the number of transfusion-associated cases of hepatitis B.3
However, because blood transfusion is a modern procedure, there must be another natural route of transmission. HBV is found in semen and vaginal fluids, so it is likely that sexual transmission is important. Transmission from mother to child during birth is another important natural route. Transplacental transmission, if it occurs, is rare. There is no evidence that transmission of HBV occurs during breast feeding.
Note that enveloped viruses, such as HBV, are more sensitive to the environment than nonenveloped viruses and hence are more efficiently transmitted by intimate contact (e.g., sexual contact). Nonenveloped viruses, such as HAV, are quite stable and are transmitted well via the environment (e.g., fecal–oral transmission).
Hepatitis B is found worldwide but is particularly prevalent in Asia. Globally, more than 300 million people are chronically infected with HBV, and about 75% of them are Asian. There is a high incidence of hepatocellular carcinoma (hepatoma, HCC) in many Asian countries—a finding that indicates that HBV is a human tumor virus (see Chapter 43). Immunization against HBV has significantly reduced the incidence of hepatoma in children. It appears that the HBV vaccine is the first vaccine to prevent a human cancer.
After entering the blood, the virus infects hepatocytes, and viral antigens are displayed on the surface of the cells. Cytotoxic T cells mediate an immune attack against the viral antigens and inflammation and necrosis occur. Immune attack against viral antigens on infected hepatocytes is mediated by cytotoxic T cells. The pathogenesis of hepatitis B is probably the result of this cell-mediated immune injury, because HBV itself does not cause a cytopathic effect. Antigen–antibody complexes cause some of the early symptoms (e.g., arthralgias, arthritis, and urticaria) and some of the complications in chronic hepatitis (e.g., glomerulonephritis, cryoglobulinemia, and vasculitis).
About 5% of adult patients with HBV infection become chronic carriers. In contrast, 90% of infected newborns become chronic carriers (see later). A chronic carrier is someone who has HBsAg persisting in their blood for 6 months or longer. The chronic carrier state is attributed to a persistent infection of the hepatocytes, which results in the prolonged presence of HBV and HBsAg in the blood. The main determinant of whether a person clears the infection or becomes a chronic carrier is the adequacy of the cytotoxic T-cell response. HBV DNA exists primarily as an episome in the nucleus of persistently infected cells; a small number of copies of HBV DNA are integrated into cell DNA.
A high rate of HCC occurs in chronic carriers. The HBx gene may be an oncogene because the HBx protein inactivates the p53 tumor suppressor protein (see Chapter 43). In addition, HCC may be the result of persistent cellular regeneration that attempts to replace the dead hepatocytes. Alternatively, malignant transformation could be the result of insertional mutagenesis, which could occur when the HBV DNA integrates into the hepatocyte DNA. Integration of the HBV DNA could activate a cellular oncogene, leading to a loss of growth control. Almost all HCC cells have HBV DNA integrated into the cell DNA. Note that although viral DNA is integrated into cell DNA in most HCC cells, integration of viral DNA is not a required step in HBV replication. (See section on viral replication above.)
Chronic carriage is more likely to occur when infection occurs in a newborn than in an adult, probably because a newborn’s immune system is less competent than that of an adult’s. Approximately 90% of infected neonates become chronic carriers. Chronic carriage resulting from neonatal infection is associated with a high risk of HCC.
Some chronic carriers make e antigen (they are said to be e antigen positive) and therefore have a high probability of making infectious virions and being able to transmit the disease. The e antigen is the indicator of transmissibility because it is encoded by the same gene that encodes the core protein indicating that the HBV DNA genome is present. Some chronic carriers do not make e antigen (they are said to be e antigen negative), and therefore have a low probability of making infectious virions and are less likely to transmit the disease.
Lifelong immunity occurs after the natural infection and is mediated by antibody against HBsAg (HBsAb). HBsAb is protective because it binds to surface antigen on the virion and prevents it from interacting with receptors on the hepatocyte. Another way of saying this is that HBsAb neutralizes the infectivity of HBV. Note that antibody against the core antigen (HBcAb) is not protective because the core antigen is inside the virion and the antibody cannot interact with it.
Many HBV infections are asymptomatic and are detected only by the presence of antibody to HBsAg. The mean incubation period for hepatitis B is 10 to 12 weeks, which is much longer than that of hepatitis A (3–4 weeks). The clinical appearance of acute hepatitis B is similar to that of hepatitis A. However, with hepatitis B, symptoms tend to be more severe, and life-threatening hepatitis can occur. Most chronic carriers are asymptomatic, but some have chronic active hepatitis, which can lead to cirrhosis and death.
In addition to liver-related findings, extrahepatic manifestations of HBV infection occur. In acute infection, serum sickness-like symptoms, such as fever, rash, and arthralgias, can occur. In chronic HBV infection, neuropathies, glomerulonephritis, and polyarteritis nodosa (a vasculitis of small and medium-sized arteries) may occur. Autoantibodies, such as cryoglobulins and rheumatoid factor, may be detected.
Chronic hepatitis B can be managed but it cannot be cured. HBV infection persists for the lifetime of the individual, not only in those who have chronic hepatitis, but also in those who are asymptomatic and have antibody to HB surface antigen. The clinical importance of this is that patients who are immunosuppressed later in life can manifest a reactivation of disease, including active hepatitis and liver failure. Patients at high risk for this reactivation include those who are HB surface antigen positive and who are treated with anti-CD20 drugs, such as rituximab< or have a hematopoietic stem cell transplant. Patients in this high-risk category should be treated prophylactically with either tenofovir or entecavir to prevent this severe complication (See Treatment section later).
Patients coinfected with both HBV and human immunodeficiency virus (HIV) may have increased hepatic damage if HIV is treated prior to treating HBV. This occurs because the “immune reconstitution” that results when HIV is treated successfully leads to increased damage to the hepatocytes by the restored, competent cytotoxic T cells. For this reason, it is suggested that HBV be treated prior to treating HIV.
The two most important serologic tests for the diagnosis of early hepatitis B are the tests for HBsAg and for IgM antibody to the core antigen. Both appear in the serum early in the disease. The hallmark test for acute hepatitis B is IgM antibody to core antigen. The test for HBsAg is also positive but that occurs in chronic hepatitis B also so is not a specific marker for acute infection.
HBsAg appears during the incubation period and is detectable in most patients during the prodrome and acute disease (Figure 41–4). It falls to undetectable levels during convalescence in most cases; its prolonged presence (at least 6 months) indicates the carrier state and the risk of chronic hepatitis and hepatic carcinoma.
As described in Table 41–4, HBsAb is not detectable in the chronic carrier state. Note that HBsAb is, in fact, being made but is not detectable in the laboratory tests because it is bound to the large amount of HBsAg present in the blood. HBsAb is also being made during the acute disease but is similarly undetectable because it is bound in antigen–antibody complexes.
A: Important diagnostic tests during various stages of hepatitis B. B: Serologic findings in a patient with acute hepatitis B. C: Duration of increased liver enzyme activity and of symptoms in a patient with acute hepatitis B. D: Serologic findings in a patient with chronic hepatitis B. anti-HBc, hepatitis B core antibody; anti-HBe, hepatitis B e antibody; anti-HBs, hepatitis B surface antibody; HBeAg, hepatitis B e antigen; HBsAg, hepatitis B surface antigen; HBV, hepatitis B virus. (Adapted with permission from Lennette EH. Manual of Clinical Microbiology. 4th ed. Washington, DC: ASM Press; 1985.)
Table 41–4Serologic Test Results in Four Stages of HBV Infection ||Download (.pdf) Table 41–4 Serologic Test Results in Four Stages of HBV Infection
|Test ||Acute Disease ||Window Phase ||Complete Recovery ||Chronic Carrier State1 |
|HBsAg ||Positive ||Negative ||Negative ||Positive |
|HBsAb ||Negative ||Negative ||Positive ||Negative2 |
|HBcAb ||Positive3 ||Positive ||Positive ||Positive |
Note that there is a period of several weeks when HBsAg has disappeared but HBsAb is not yet detectable. This is the window phase. At this time, the HBcAb is always positive and can be used to make the diagnosis. HBcAb is present in those with acute infection and chronic infection, as well as in those who have recovered from acute infection. Therefore, it cannot be used to distinguish between acute and chronic infection. The IgM form of HBcAb is present during acute infection and disappears approximately 6 months after infection. The test for HBcAg is not readily available. Table 41–4 describes the serologic test results that characterize the four important stages of HBV infection.
HBeAg arises during the incubation period and is present during the prodrome and early acute disease and in certain chronic carriers. Its presence in chronic carriers indicates a high likelihood of transmissibility, and, conversely, the absence of HBeAg indicates a low likelihood of transmission. In addition, the finding of HBeAb indicates a lower likelihood, but transmission can still occur. DNA polymerase activity is detectable during the incubation period and early in the disease, but the assay is not available in most clinical laboratories.
The detection of viral DNA (viral load) in the serum is a strong evidence that infectious virions are present. Reduction of the viral load in patients with chronic hepatitis B is used to monitor the success of drug therapy.
No antiviral therapy is typically used in acute hepatitis B.
For chronic hepatitis B, entecavir (Baraclude) or tenofovir (Viread, Vemlidy) are the drugs of choice. They are nucleoside analogues that inhibit the reverse transcriptase of HBV. Interferon in the form of peginterferon alfa-2a (Pegasys) may also be used. Other nucleoside analogues such as lamivudine (Epivir-HBV), adefovir (Hepsera), and telbivudine (Tyzeka) are used less frequently. A combination of tenofovir and emtricitabine (Emtriva) is also used.
These drugs reduce hepatic inflammation, reduce the risk of cirrhosis and hepatocellular carcinoma, and lower the viral load of HBV in patients with chronic active hepatitis. Note however, that neither interferon nor the nucleoside analogues cure the HBV infection. In most patients, when the drug is stopped, HBV replication resumes.
Patients coinfected with HBV and HIV should be prescribed highly active antiretroviral therapy (HAART) with caution because recovery of cell-mediated immunity can result in an exacerbation of hepatitis (immune reconstitution syndrome, IRIS). Consideration should be given to treat the HBV infection prior to starting HAART.
The two main modes of prevention involve the use of either the vaccine or hyperimmune globulin or both.
The subunit vaccine (e.g., Recombivax, Engerix-B, or Heplisav-B) contains HBsAg produced in yeasts by recombinant DNA techniques. The vaccine is highly effective in preventing hepatitis B and has few side effects. The seroconversion rate is approximately 95% in healthy adults. Recombivax and Engerix-B are given in a three-dose regimen whereas Heplisav-B is given as a two-dose regimen.
It is indicated for people who are frequently exposed to blood or blood products, such as certain healthcare personnel (e.g., medical students, surgeons, and dentists), patients receiving multiple transfusions or dialysis, patients with frequent sexually transmitted disease, and abusers of illicit intravenous drugs. Travelers who plan a long stay in areas of endemic infection, such as many countries in Asia and Africa, should receive the vaccine. The U.S. Public Health Service recommends that all newborns and adolescents receive the vaccine.
At present, booster doses after the initial two or three-dose regimen are not recommended. However, if antibody titers have declined in immunized patients who are at high risk, such as dialysis patients, then a booster dose should be considered.
Widespread immunization with the HBV vaccine has significantly reduced the incidence of HCC in children. A vaccine called Twinrix that contains both HBsAg and inactivated HAV provides protection against both hepatitis B and hepatitis A.
Hepatitis B immune globulin (HBIG) contains a high titer of HBsAb. It is used to provide immediate, passive protection to individuals known to be exposed to HBsAg-positive blood (e.g., after an accidental needle-stick injury).
Precise recommendations for use of the vaccine and HBIG are beyond the scope of this book. However, the recommendation regarding one common concern of medical students, the needle-stick injury from a patient with HBsAg-positive blood, is that both the vaccine and HBIG be given (at separate sites). This is true even if the patient’s blood is HBeAb positive.
Both the vaccine and HBIG should also be given to a newborn whose mother is HBsAg positive. This regimen is very effective in reducing the infection rate of newborns whose mothers are chronic carriers. The regimen of vaccine plus HBIG in those with needle-stick injuries and in neonates is a good example of passive–active immunization, in which both immediate protection and long-term protection are provided.
The effectiveness of cesarean section to reduce HBV infection of neonates is uncertain. It is currently not recommended. Breast feeding of immunized neonates by mothers who are chronic carriers entails little risk of infection of the neonate.
All blood for transfusion should be screened for HBsAg. No one with a history of hepatitis (of any type) should donate blood, because non-A, non-B viruses may be present. Screening of high-risk populations to detect chronic carriers using serologic testing should be done because identification and treatment of carriers will reduce transmission.
Note that preexposure prophylaxis using Truvada (tenofovir plus emtricitabine) to prevent HIV infection also prevents HBV infection. These two drugs inhibit the reverse transcriptase of both HBV and HIV.