Viruses by Crawford Dorothy H.;

Author:Crawford, Dorothy H.;
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2011-01-14T16:00:00+00:00

Hepatitis viruses

Hepatitis, meaning inflammation of the liver, can be caused by a variety of viruses as well as toxic chemicals such as alcohol and the drug paracetamol. The liver is a huge organ with plenty of spare capacity, so mild inflammation often passes unnoticed. The main indication of more severe damage is the yellow discolouration of the skin known as jaundice, often most noticeable in the whites of the eyes.

Several viruses, including Epstein–Barr and herpes simplex viruses, can cause hepatitis as part of a generalized infection, but for others the liver is their main site of replication, causing them to be lumped together as ‘the hepatitis viruses’ although they belong to quite different virus families. To date, five human hepatitis viruses have been discovered and named A, B, C, D, and E. With the exception of HDV, all these viruses either infect silently or produce clinical hepatitis varying in severity from mild and self-limiting to fulminant – that is, acute liver failure which is generally fatal unless a liver transplant can be performed as an emergency procedure.

Hepatitis A and E viruses spread by the faecal–oral route causing epidemics of ‘infectious jaundice’, and where standards of hygiene are low most children are infected at an early age. Although the illness may be prolonged, recovery is the rule, and the viruses do not persist thereafter. In contrast, hepatitis B and C viruses may persist after primary infection, and this can lead to chronic hepatitis, cirrhosis, and liver cancer. Hepatitis D virus (HDV), also known as delta virus, is unique among human viruses in being defective and requiring the assistance of HBV for its transmission. Specifically, HDV particles consist of an RNA genome surrounded by their own protein but enveloped in HBV surface antigen that acts as its receptor for getting in and out of liver cells. So this virus can only replicate in cells already infected with HBV and manufacturing HBV surface antigen. HDV may be transmitted along with HBV or may infect an HBV carrier, and in both cases it tends to worsen the infection by increasing the liver damage and accelerating the onset of chronic liver disease.

Hepatitis C virus is mainly spread by blood contamination. Once routine testing of donor blood excluded most HBV-infected units in the 1970s, HCV became the commonest cause of viral hepatitis following blood transfusion. But after its discovery in 1989, when blood and blood products were screened for HCV, the commonest route of transmission became needle sharing by intravenous drug users. Around 10% of carrier mothers pass the virus to their newborn offspring, but household and sexual contacts are not thought to be at increased risk.

HCV presently infects around 170 million people. Infection occurs worldwide but shows marked geographical variation, with 1-2% of the population infected in the USA, northern Europe, and Australia, and rates of up to 5% in southern and central Europe, Japan, and parts of the Middle East (Figure 14). The highest levels of around 20% are recorded in Egypt, where a

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