hepatitis B virus

hepatitis B virus
Name	Hepatitis B virus
Taxon	Orthohepadnavirus
Family	Hepadnaviridae
Order	Blubervirales
Genus	Orthohepadnavirus
Species	Hepatitis B virus

hepatitis B virus. It is a partially double-stranded DNA virus belonging to the family Hepadnaviridae and is a major global cause of liver disease. The virus primarily infects hepatocytes, leading to conditions ranging from acute hepatitis to chronic infection, cirrhosis, and hepatocellular carcinoma. Its discovery is credited to Baruch Blumberg, who identified the Australia antigen, later known as the HBsAg, for which he received the Nobel Prize in Physiology or Medicine.

Virology and structure

The virion, known as the Dane particle, consists of an outer lipid envelope embedded with HBsAg and an inner nucleocapsid core containing the viral genome and HBcAg. The genome is a relaxed-circular, partially double-stranded DNA that is replicated via an RNA intermediate through the action of a viral reverse transcriptase. Key viral proteins include the surface antigen, core antigen, e antigen, and the polymerase, which has reverse transcriptase activity. The virus exhibits a high degree of genetic diversity, classified into ten genotypes (A-J) with distinct geographical distributions, influencing disease progression and treatment response.

Transmission and epidemiology

Transmission occurs through percutaneous or mucosal exposure to infected blood or body fluids, including during childbirth, through sexual contact, sharing of needles, and unsafe medical procedures. The World Health Organization estimates that nearly 300 million people live with chronic infection, with the highest prevalence in the Western Pacific Region and Africa. High-risk groups include infants born to infected mothers, people who inject drugs, and individuals with multiple sexual partners. The implementation of widespread blood donation screening and vaccination programs has significantly reduced incidence in many areas, such as North America and Western Europe.

Pathogenesis and clinical features

Following entry, the virus replicates in hepatocytes, with the host immune response determining the clinical outcome. Acute infection may be asymptomatic or present with jaundice, fatigue, and abdominal pain, while fulminant hepatic failure is rare. Chronic infection, defined by persistence of HBsAg for more than six months, develops in a significant proportion of adults and most perinatally infected infants. Chronic infection can lead to progressive liver inflammation, fibrosis, cirrhosis, and dramatically increases the risk for hepatocellular carcinoma, a primary liver cancer. The e antigen status is a key marker of viral replication and infectivity.

Diagnosis and serology

Diagnosis relies on serological tests to detect viral antigens and specific antibodies. The presence of HBsAg indicates current infection, while anti-HBs signifies immunity from vaccination or resolved infection. Anti-HBc appears during acute infection and persists for life. The e antigen and anti-HBe help determine infectivity and treatment response. Quantitative HBV DNA tests by polymerase chain reaction measure viral load, which is crucial for assessing replication activity and guiding therapy. Additional assessments include liver function tests, alpha-fetoprotein screening for cancer, and imaging studies like ultrasound.

Prevention and vaccination

The cornerstone of prevention is vaccination with the recombinant HBsAg vaccine, which is highly effective and part of routine immunization schedules globally, as recommended by the World Health Organization. The first dose is typically administered at birth, especially to infants of HBsAg-positive mothers, who should also receive hepatitis B immune globulin. Other preventive measures include screening of blood donation, safe injection practices, use of condoms, and harm reduction programs for people who inject drugs. Post-exposure prophylaxis with vaccine and immune globulin is effective for needlestick injuries or sexual exposure.

Treatment and management

Treatment aims to suppress viral replication, reduce liver inflammation, and prevent complications like cirrhosis and hepatocellular carcinoma. First-line antiviral agents include nucleoside analogues like entecavir and tenofovir, which inhibit the viral polymerase. Pegylated interferon-alpha is an immunomodulatory option for select patients. Management decisions are based on viral load, alanine transaminase levels, liver disease stage, and guidelines from bodies like the American Association for the Study of Liver Diseases. For end-stage liver disease, liver transplantation is a definitive treatment, often accompanied by antiviral therapy to prevent reinfection.

Category:Viral diseases Category:Hepadnaviridae