While hepatitis B (HBV) infection is endemic in the UK, it is more common in developing countries, where children often acquire infection from their mothers during birth or through close contact in early infancy. The UK is a low prevalence area, with a carriage rate of 0.1-0.5%, although rates may vary between individual communities.
Once inside the host, HBV is transported in the blood to the liver, where it infects liver cells. The incubation period of acute HBV infection is usually about 75 days, but can range from 45 to 200 days. The virus spreads in the liver and causes a spectrum of disease, ranging from acute hepatitis to chronic liver disease and liver tumours. A small proportion of patients with acute infection suffer liver failure, although most recover from the infection. Asymptomatic infection (infection without symptoms) and illness without jaundice does occur, particularly in children and the immunocompromised (those with an impaired immune system). The likelihood of a patient developing chronic infection is inversely related to age at the time of infection. Chronic infection occurs in at least 90% of infected neonates, 25% of children aged 1-5 years and 5% or less of adults. Chronically infected individuals are often referred to as HBV carriers.
HBV is an unusual virus as large quantities of viral proteins are produced, resulting in the production of a range of different particles, some of which are infectious and some of which are not. Viral proteins are secreted into the blood and their presence can be useful markers of infection. In individuals chronically infected with HBV, the persistence in the circulation of viral proteins indicates continuing high-potential infectivity for sexual partners and for babies born to carrier mothers. These chronically infected individuals, who may be totally without symptoms, also present a major risk to non-immune healthcare workers and others accidentally exposed to their blood and body fluids by, for instance, a needle-stick injury. In addition, the continued presence of viral proteins is associated with progressive liver damage (chronic active hepatitis and cirrhosis) and increased risk of primary liver cancer. Much of the damage to the liver in chronic cases is believed to be as a result of immune responses to the infection.
The severity of illness is clearly influenced by host immune responses to the virus. Antibody responses to HBV are induced by infection, and the specificity and type of these antibodies relative to levels of viral proteins is often indicative of the seriousness and nature of disease. The relationships between viral proteins, antibody generation and the progression of this disease are complex.
In Western Europe, North America and other developed countries, infection occurs sporadically by sexual contact and blood transfer, particularly by the sharing of needles and syringes in drug misuse. In England and Wales, injecting drugs is the most frequently reported route of infection. Transmission to babies from infected mothers has been largely attributed to exposure during or after delivery, with trans-placental infection (the passage of infection through the placenta) being apparently rare. Transmission from mother to baby can be prevented by the administration of HBV immunoglobulin (HBIG) and immunising the newborn in cases where the mother is infected with the virus. However, provision of HBIG to babies born to HBV carrier mothers is currently limited to the babies of carriers whose laboratory markers indicate there is a particularly high risk of mother-to-baby transmission of infection, and those whose mothers have had acute HBV infection during pregnancy (See - HBIG for babies born to hepatitis B - infected mothers).
Workplace exposure in the healthcare setting usually occurs as a result of needle-stick injury, injury with other contaminated sharp instruments, or as a result of contamination of the mucous membranes (eyes, nose and mouth). Workplace acquisition of HBV has been significantly reduced due to the availability of an effective vaccine.
The UK falls into the lowest category of prevalence for HBV, as determined by the World Health Organisation. The prevalence rate is believed to be between 0.1% and 0.5% of the UK population. HBV infections are usually acquired in adulthood, principally resulting from sexual activity or injecting drug use. Reports of acute HBV infection have fallen sharply, which is thought to be mainly due to a decline in cases of intravenous drug misuse and possibly changes in behaviour in response to the risks from the HIV/AIDS epidemic. The effect of risk reduction programmes, which include the use of needle exchange centres and the immunisation of injecting drug users (IDU), is also likely to have contributed to this decline. However, data from the Health Protection Agency indicates that the frequency of chronic infection is increasing in the UK because of migration from high prevalence areas of the world.