Epidemiology and Geographical Distribution

Hepatitis A occurs endemically in all parts of the world, with frequent reports of minor and major outbreaks. The exact incidence is difficult to estimate because of the high proportion of subclinical infections and infections without jaundice, differences in surveillance, and differing patterns of disease. The degree of underreporting is very high.

HAV enters the body by ingestion. The virus then spreads, probably by the bloodstream, to the liver, a target organ, where it replicates in the hepatocytes. Large numbers of virus particles are detectable in faeces during the incubation period (Figure 6.1), beginning as early as 10-14 days after exposure and continuing, in general, until peak elevation of serum aminotransferases. Virus is also detected in faeces early in the acute phase of illness, but relatively infrequently after the onset of clinical jaundice. IgG antibody to HAV that persists is also detectable late in the incubation period, coinciding approximately with the onset of biochemical evidence of liver damage. The virus does not persist and chronic excretion of HAV does not occur. There is no evidence of progression to chronic liver disease.

The mode of transmission of HAV is by the faecal-oral route, most commonly by person-to-person contact in developed countries, and infection occurs readily under conditions of poor sanitation and hygiene and overcrowding. Common source outbreaks are most frequently initiated by faecal contamination of water and food, but waterborne transmission is not a major factor in maintaining this infection in industrialised communities. On the other hand, many foodborne outbreaks have been reported. This can be attributed to the shedding of large quantities of virus in the faeces during the incubation period of the illness in infected food handlers; the source of the outbreak can often be traced to uncooked food or food that has been handled after cooking. Oysters, clams and other shellfish from contaminated water pose a high risk of infection unless heated or steamed thoroughly. There is a similar risk with uncooked vegetables and crops in countries where raw sewage is used as a fertiliser. Although hepatitis A remains endemic and common in the developed countries, the infection occurs mainly in small clusters, often with only a few identified cases.

Hepatitis A is recognised as an important travel-related infection in travellers from low-prevalence areas to endemic countries. As a generalisation, low-prevalence areas include western Europe, the USA and Canada, Australia, New Zealand and Japan. The infection is much more prevalent in other areas of the world, and people travelling to developing countries, including many holiday destinations, are at risk of infection, and are at particularly high risk of infection in rural areas.

Two other sectors of the population are at an increased risk of infection with HAV: those engaging in oral-anal sexual practices and male homosexuality, and injecting drug users. The latter group is at risk because of a combination of poor personal hygiene, faecal contamination of injection equipment which is often shared, the use of water drawn from toilet pans to dissolve drugs, and possible contamination of illicit drugs that are transported in the intestine after swallowing or are carried in the rectum.

Hepatitis A is rarely transmitted by blood transfusion, although transmission by inadequately inactivated and treated blood coagulation products has been reported, as have cases in patients with cancer treated with lym-phokine-activated killer cells and interleukin 2 prepared with tissue culture medium supplemented with pooled human serum.

The incubation period of hepatitis A is 3-5 weeks, with a mean of 28 days. Subclinical and anicteric cases are common and, although the disease has in general a low mortality rate, patients may be incapacitated for many weeks. There is no evidence of progression to chronic liver damage.

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