Hepatitis B

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Hepatitis B
tenofovir, interferon), liver transplantation[4]
Frequency296 million (2019)[1]
Deaths820,000 resulting from hepatitis B (2019)[1]

Hepatitis B is an

chronic infection.[1]

Many people have no symptoms during an initial infection. For others, symptoms may appear 30 to 180 days after becoming infected and can include a rapid onset of sickness with nausea, vomiting, yellowish skin, fatigue, dark urine, and abdominal pain.[1] Symptoms during acute infection typically last for a few weeks, though some people may feel sick for up to six months.[8] Deaths resulting from acute stage HBV infections are rare.[9] An HBV infection lasting longer than six months is usually considered chronic.[1] The likelihood of developing chronic hepatitis B is higher for those who are infected with HBV at a younger age. About 90% of those infected during or shortly after birth develop chronic hepatitis B,[8] while less than 10% of those infected after the age of five develop chronic cases.[5] Most of those with chronic disease have no symptoms; however, cirrhosis and liver cancer eventually develop[2] in about 25% of those with chronic HBV.[4]

The virus is transmitted by exposure to infectious

tenofovir or interferon may be useful; however, these drugs are expensive.[4] Liver transplantation is sometimes recommended for cases of cirrhosis or hepatocellular carcinoma.[4]

Hepatitis B infection has been preventable by

condoms.[1] In 2016, the WHO set a goal of eliminating viral hepatitis as a threat to global public health by 2030. Achieving this goal would require the development of therapeutic treatments to cure chronic hepatitis B, as well as preventing its transmission and using vaccines to prevent new infections.[15][16][17]

An estimated 296 million people, or 3.8% of the global population, had chronic hepatitis B infections as of 2019. Another 1.5 million developed acute infections that year, and 820,000 deaths occurred as a result of HBV.[1] Cirrhosis and liver cancer are responsible for most HBV-related deaths.[18] The disease is most prevalent in Africa (affecting 7.5% of the continent's population) and in the Western Pacific region (5.9%).[19] Infection rates are 1.5% in Europe and 0.5% in the Americas.[19] According to some estimates, about a third of the world's population has been infected with hepatitis B at one point in their lives.[18] Hepatitis B was originally known as "serum hepatitis".[20]

Signs and symptoms

Acute infection with hepatitis B virus is associated with acute viral

fulminant hepatic failure and may die as a result. The infection may be entirely asymptomatic and may go unrecognized.[21]

Chronic infection with hepatitis B virus may be asymptomatic or may be associated with chronic inflammation of the liver (chronic hepatitis), leading to cirrhosis over a period of several years. This type of infection dramatically increases the incidence of hepatocellular carcinoma (HCC; liver cancer). Across Europe, hepatitis B and C cause approximately 50% of hepatocellular carcinomas.[22][23] Chronic carriers are encouraged to avoid consuming alcohol as it increases their risk for cirrhosis and liver cancer. Hepatitis B virus has been linked to the development of membranous glomerulonephritis (MGN).[24]

Symptoms outside of the liver are present in 1–10% of HBV-infected people and include

hematological disorders, such as essential mixed cryoglobulinemia and aplastic anemia have been described as part of the extrahepatic manifestations of HBV infection, but their association is not as well-defined; therefore, they probably should not be considered etiologically linked to HBV.[28]

Cause

Transmission

Transmission of hepatitis B virus results from exposure to infectious blood or body fluids containing blood. HBV is 50 to 100 times more infectious than human immunodeficiency virus (HIV).[32] HBV can be transmitted through several routes of infection. In vertical transmission, HBV is passed from mother to child (MTCT) during childbirth.[1] Without intervention, a mother who is positive for HBsAg has a 20% risk of passing the infection to her offspring at the time of birth. This risk is as high as 90% if the mother is also positive for HBeAg.[citation needed]

Early life horizontal transmission can occur through bites, lesions, certain sanitary habits, or other contact with secretions or saliva containing HBV.[33][34] Adult horizontal transmission is known to occur through sexual contact,[35] blood transfusions and transfusion with other human blood products,[36] re-use of contaminated needles and syringes.[37] Breastfeeding after proper immunoprophylaxis does not appear to contribute to mother-to-child-transmission (MTCT) of HBV.[38]

Virology

Main article: Hepatitis B virus

Structure

The structure of hepatitis B virus

Hepatitis B virus (HBV) is a member of the

nucleocapsid core composed of core protein. These virions are 30–42 nm in diameter. The nucleocapsid encloses the viral DNA and a DNA polymerase that has reverse transcriptase activity.[40] The outer envelope contains embedded proteins that are involved in viral binding of, and entry into, susceptible cells. The virus is one of the smallest enveloped animal viruses. The 42 nm virions, which are capable of infecting liver cells known as hepatocytes, are referred to as "Dane particles".[41] In addition to the Dane particles, filamentous and spherical bodies lacking a core can be found in the serum of infected individuals. These particles are not infectious and are composed of the lipid and protein that forms part of the surface of the virion, which is called the surface antigens (HBsAg), and is produced in excess during the life cycle of the virus.[42]

Genome

The genome organisation of HBV. The genes overlap.

The

codon is preceded by an upstream in-frame AUG start codon from which the pre-core protein is produced. HBeAg is produced by proteolytic processing of the pre-core protein. In some rare strains of the virus known as Hepatitis B virus precore mutants, no HBeAg is present.[44]
The DNA
polypeptides of three different sizes called large (the order from surface to the inside: pre-S1, pre-S2, and S ), middle (pre-S2, S), and small (S)[45] are produced.[46] There is a myristyl group, which plays an important role in infection, on the amino-terminal end of the preS1 part of the large (L) protein.[47] In addition to that, N terminus of the L protein have virus attachment and capsid binding sites. Because of that, the N termini of half of the L protein molecules are positioned outside the membrane and the other half positioned inside the membrane.[48]

The function of the protein coded for by gene X is not fully understood but it is associated with the development of liver cancer. It stimulates genes that promote cell growth and inactivates growth regulating molecules.[49]

Pathogenesis

Hepatitis B virus replication

The life cycle of hepatitis B virus is complex. Hepatitis B is one of a few known

mRNAs by host RNA polymerase. The largest mRNA, (which is longer than the viral genome), is used to make the new copies of the genome and to make the capsid core protein and the viral DNA polymerase. These four viral transcripts undergo additional processing and go on to form progeny virions that are released from the cell or returned to the nucleus and re-cycled to produce even more copies.[46][51]
The long mRNA is then transported back to the cytoplasm where the virion P protein (the DNA polymerase) synthesizes DNA via its reverse transcriptase activity.

Serotypes and genotypes

The virus is divided into four major serotypes (adr, adw, ayr, ayw) based on antigenic epitopes presented on its envelope proteins, and into eight major genotypes (A–H). The genotypes have a distinct geographical distribution and are used in tracing the evolution and transmission of the virus. Differences between genotypes affect the disease severity, course and likelihood of complications, and response to treatment and possibly vaccination.[52][53] There are two other genotypes I and J but they are not universally accepted as of 2015.[54] The diversity of genotypes is not shown equally in the world. For example, A, D, and E genotypes have been seen in Africa prevalently while B and C genotypes are observed in Asia as widespread.[55]

Genotypes differ by at least 8% of their sequence and were first reported in 1988 when six were initially described (A–F).[56] Two further types have since been described (G and H).[57] Most genotypes are now divided into subgenotypes with distinct properties.[58]

Mechanisms

Hepatitis B virus primarily interferes with the functions of the liver by replicating in

NTCP.[50] There is evidence that the receptor in the closely related duck hepatitis B virus is carboxypeptidase D.[59][60] The virions bind to the host cell via the preS domain of the viral surface antigen and are subsequently internalized by endocytosis. HBV-preS-specific receptors are expressed primarily on hepatocytes; however, viral DNA and proteins have also been detected in extrahepatic sites, suggesting that cellular receptors for HBV may also exist on extrahepatic cells.[61]

During HBV infection, the host

platelets activated at the site of infection may facilitate the accumulation of CTLs in the liver.[63]

Diagnosis

Hepatitis B viral antigens and antibodies detectable in the blood following acute infection
Hepatitis B viral antigens and antibodies detectable in the blood of a chronically infected person

The tests, called assays, for detection of hepatitis B virus infection involve serum or blood tests that detect either viral antigens (proteins produced by the virus) or antibodies produced by the host. Interpretation of these assays is complex.[64]

The hepatitis B surface antigen (

IgM antibodies specific to the hepatitis B core antigen (anti-HBc IgM) may be the only serological evidence of disease. Therefore, most hepatitis B diagnostic panels contain HBsAg and total anti-HBc (both IgM and IgG).[65]

Shortly after the appearance of the HBsAg, another antigen called hepatitis B e antigen (HBeAg) will appear. Traditionally, the presence of HBeAg in a host's serum is associated with much higher rates of viral replication and enhanced infectivity; however, variants of the hepatitis B virus do not produce the 'e' antigen, so this rule does not always hold true.[66] During the natural course of an infection, the HBeAg may be cleared, and antibodies to the 'e' antigen (anti-HBe) will arise immediately afterwards. This conversion is usually associated with a dramatic decline in viral replication.

Ground glass hepatocytes as seen in a chronic hepatitis B liver biopsy. H&E stain

If the host is able to clear the infection, eventually the HBsAg will become undetectable and will be followed by IgG antibodies to the hepatitis B surface antigen and core antigen (anti-HBs and anti HBc IgG).[39] The time between the removal of the HBsAg and the appearance of anti-HBs is called the window period. A person negative for HBsAg but positive for anti-HBs either has cleared an infection or has been vaccinated previously.

Individuals who remain HBsAg positive for at least six months are considered to be hepatitis B carriers.

alanine aminotransferase (ALT) levels and inflammation of the liver, if they are in the immune clearance phase of chronic infection. Carriers who have seroconverted to HBeAg negative status, in particular those who acquired the infection as adults, have very little viral multiplication and hence may be at little risk of long-term complications or of transmitting infection to others.[68]
However, it is possible for individuals to enter an "immune escape" with HBeAg-negative hepatitis.

The five phases of chronic hepatitis B infection as defined by European Association for the Study of the Liver

PCR tests have been developed to detect and measure the amount of HBV DNA, called the viral load, in clinical specimens. These tests are used to assess a person's infection status and to monitor treatment.[69] Individuals with high viral loads, characteristically have ground glass hepatocytes on biopsy.

Prevention

Vaccine

Main article: Hepatitis B vaccine

Vaccines for the prevention of hepatitis B have been routinely recommended for babies since 1991 in the United States.[70] The first dose is generally recommended within a day of birth.[71] The hepatitis B vaccine was the first vaccine capable of preventing cancer, specifically liver cancer.[72]

Most vaccines are given in three doses over a course of days. A protective response to the vaccine is defined as an anti-HBs antibody concentration of at least 10 mIU/ml in the recipient's serum. The vaccine is more effective in children and 95 percent of those vaccinated have protective levels of antibody. This drops to around 90% at 40 years of age and to around 75 percent in those over 60 years. The protection afforded by vaccination is long lasting even after antibody levels fall below 10 mIU/ml. For newborns of HBsAg-positive mothers: hepatitis B vaccine alone, hepatitis B immunoglobulin alone, or the combination of vaccine plus hepatitis B immunoglobulin, all prevent hepatitis B occurrence.[73] Furthermore, the combination of vaccine plus hepatitis B immunoglobulin is superior to vaccine alone.[73] This combination prevents HBV transmission around the time of birth in 86% to 99% of cases.[74]

Tenofovir given in the second or third trimester can reduce the risk of mother to child transmission by 77% when combined with hepatitis B immunoglobulin and the hepatitis B vaccine, especially for pregnant women with high hepatitis B virus DNA levels.[75] However, there is no sufficient evidence that the administration of hepatitis B immunoglobulin alone during pregnancy, might reduce transmission rates to the newborn infant.[76] No randomized control trial has been conducted to assess the effects of hepatitis B vaccine during pregnancy for preventing infant infection.[77]

All those with a risk of exposure to body fluids such as blood should be vaccinated, if not already.[70] Testing to verify effective immunization is recommended and further doses of vaccine are given to those who are not sufficiently immunized.[70]

In 10- to 22-year follow-up studies there were no cases of hepatitis B among those with a normal immune system who were vaccinated. Only rare chronic infections have been documented.[78] Vaccination is particularly recommended for high risk groups including: health workers, people with chronic kidney failure, and men who have sex with men.[79][80][81]

Both types of the hepatitis B vaccine, the plasma-derived vaccine (PDV) and recombinant vaccine (RV) are of similar effectiveness in preventing the infection in both healthcare workers and chronic kidney failure groups.[79][80] With one difference noticed among health worker group, that the RV intramuscular route is significantly more effective compared with RV intradermal route of administration.[79]

Other

In assisted reproductive technology, sperm washing is not necessary for males with hepatitis B to prevent transmission, unless the female partner has not been effectively vaccinated.[82] In females with hepatitis B, the risk of transmission from mother to child with IVF is no different from the risk in spontaneous conception.[82]

Those at high risk of infection should be tested as there is effective treatment for those who have the disease.[83] Groups that screening is recommended for include those who have not been vaccinated and one of the following: people from areas of the world where hepatitis B occurs in more than 2%, those with HIV, intravenous drug users, men who have sex with men, and those who live with someone with hepatitis B.[83] Screening during pregnancy is recommended in the United States.[84]

Treatment

Acute hepatitis B infection does not usually require treatment and most adults clear the infection spontaneously.

alanine aminotransferase, a marker of liver damage, and HBV DNA levels are candidates for therapy.[87] Treatment lasts from six months to a year, depending on medication and genotype.[88] Treatment duration when medication is taken by mouth, however, is more variable and usually longer than one year.[89]

Although none of the available medications can clear the infection, they can stop the virus from replicating, thus minimizing liver damage. As of 2018, there are eight medications licensed for the treatment of hepatitis B infection in the United States. These include antiviral medications lamivudine, adefovir, tenofovir disoproxil, tenofovir alafenamide, telbivudine, and entecavir, and the two immune system modulators interferon alpha-2a and PEGylated interferon alpha-2a. In 2015, the World Health Organization recommended tenofovir or entecavir as first-line agents.[90] Those with current cirrhosis are in most need of treatment.[90]

The use of interferon, which requires injections daily or thrice weekly, has been supplanted by long-acting PEGylated interferon, which is injected only once weekly.[91] However, some individuals are much more likely to respond than others, and this might be because of the genotype of the infecting virus or the person's heredity. The treatment reduces viral replication in the liver, thereby reducing the viral load (the amount of virus particles as measured in the blood).[92] Response to treatment differs between the genotypes. Interferon treatment may produce an e antigen seroconversion rate of 37% in genotype A but only a 6% seroconversion in type D. Genotype B has similar seroconversion rates to type A while type C seroconverts only in 15% of cases. Sustained e antigen loss after treatment is ~45% in types A and B but only 25–30% in types C and D.[93]

It seems unlikely that the disease will be eliminated by 2030, the goal set in 2016 by WHO. However, progress is being made in developing therapeutic treatments. In 2010, the Hepatitis B Foundation reported that 3 preclinical and 11 clinical-stage drugs were under development, based on largely similar mechanisms. In 2020, they reported that there were 17 preclinical- and 32 clinical-stage drugs under development, using diverse mechanisms.[15]

Prognosis

Estimate of disability-adjusted life year for hepatitis B per 100,000 inhabitants as of 2004

Hepatitis B virus infection may be either acute (self-limiting) or chronic (long-standing). Persons with self-limiting infection clear the infection spontaneously within weeks to months.

Children are less likely than adults to clear the infection. More than 95% of people who become infected as adults or older children will stage a full recovery and develop protective immunity to the virus. However, this drops to 30% for younger children, and only 5% of newborns that acquire the infection from their mother at birth will clear the infection.[94] This population has a 40% lifetime risk of death from cirrhosis or hepatocellular carcinoma.[91] Of those infected between the age of one to six, 70% will clear the infection.[95]

Hepatitis D (HDV) can occur only with a concomitant hepatitis B infection, because HDV uses the HBV surface antigen to form a capsid.[96] Co-infection with hepatitis D increases the risk of liver cirrhosis and liver cancer.[97] Polyarteritis nodosa is more common in people with hepatitis B infection.

Cirrhosis

A number of different tests are available to determine the degree of cirrhosis present.

Transient elastography (FibroScan) is the test of choice, but it is expensive.[90] Aspartate aminotransferase to platelet ratio index may be used when cost is an issue.[90]

Reactivation

Hepatitis B virus DNA remains in the body after infection, and in some people, including those that do not have detectable HBsAg, the disease recurs.

Immunosuppressive drugs favor increased HBV replication while inhibiting cytotoxic T cell function in the liver.[104] The risk of reactivation varies depending on the serological profile; those with detectable HBsAg in their blood are at the greatest risk, but those with only antibodies to the core antigen are also at risk. The presence of antibodies to the surface antigen, which are considered to be a marker of immunity, does not preclude reactivation.[103] Treatment with prophylactic antiviral drugs can prevent the serious morbidity associated with HBV disease reactivation.[103]

Epidemiology

Hepatitis B incidence rate in 2017.[105]

At least 296 million people, or 3.8% of the world's population, had chronic HBV infection as of 2019. Another 1.5 million cases of acute HBV infection also occurred that year.[1] Regional prevalences across the globe range from around 7.5% in Africa to 0.5% in the Americas.[19]

The primary method of HBV transmission and the prevalence of chronic HBV infection in specific regions often correspond with one another. In populations where HBV infection rates are 8% or higher, which are classified as high prevalence, vertical transmission (usually occurring during birth) is most common, though rates of early childhood transmission can also be significant among these populations.[106] In 2021, 19 African countries had infection rates ranging between 8-19%, placing them in the high prevalence category.[107]

In moderate prevalence areas where 2–7% of the population is chronically infected, the disease is predominantly spread horizontally, often among children, or vertically.[108] China's HBV infection rate is at the higher end of the moderate prevalence classification with an infection rate of 6.89% as of 2019.[109] HBV prevalence in India is also moderate, with studies placing India's infection rate between 2-4%.[110]

Countries with low HBV prevalence include Australia (0.9%),[111] those in the WHO European Region (which average 1.5%),[19] and most countries in North and South America (which average 0.28%).[112][113] In the United States, an estimated 0.26% of the population was living with HBV infection as of 2018.[114]

History

Findings of HBV DNA in ancient human remains have shown that HBV has infected humans for at least ten millennia, both in Eurasia and in the Americas.[115][116][117] This disproved the belief that hepatitis B originated in the New World and spread to Europe around 16th century.[117] Hepatitis B virus subgenotype C4 is exclusively present in Australian aborigines, suggesting an ancient origin as much as 50,000 years old.[118][119][120] However, analyses of ancient HBV genomes suggested that the most recent common ancestor of all known human HBV strains was dated to between 20,000 and 12,000 years ago,[115] pointing to a more recent origin for all HBV genotypes. The evolution of HBV in humans was shown to reflect known events of human history such as the first peopling of the Americas during the late Pleistocene and the Neolithic transition in Europe.[115] Ancient DNA studies have also showed that some ancient hepatitis viral strains still infect humans, while other strains became extinct.[115][116]

The earliest record of an epidemic caused by hepatitis B virus was made by Lurman in 1885.

hypodermic needles that were used, and, more importantly, reused, for administering Salvarsan for the treatment of syphilis
.

The largest outbreak of Hepatitis B ever recorded was the infection of up to 330,000 American soldiers during World War II. The outbreak has been blamed on a yellow fever vaccine made with contaminated human blood serum, and after receiving the vaccinations about 50,000 soldiers developed jaundice.[122]

The virus was not discovered until 1966 when

electron microscopy.[125] In 1971, the FDA issued its first-ever blood supply screening order to blood banks.[126] By the early 1980s the genome of the virus had been sequenced,[127] and the first vaccines were being tested.[128]

Society and culture

World Hepatitis Day, observed 28 July, aims to raise global awareness of hepatitis B and hepatitis C and encourage prevention, diagnosis, and treatment. It has been led by the World Hepatitis Alliance since 2007 and in May 2010, it received global endorsement from the World Health Organization.[129]

See also

References

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External links

Wikimedia Commons has media related to Hepatitis B.

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