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abdominal pain, and jaundice. Adults with HAV illness may be sick enough to require hospitalization. The disease has no known sequelae and is rarely fatal. Apart from the transient viremia that occurs during early HAV infection, there has been no identification of viremic carriers (Mosley, 1975). Socioeconomic status and general hygienic standards are the major risk factors for acquiring disease in most areas of the world. Specific risk factors for hepatitis A infection in developed areas include involvement in day care, homosexuality, personal contact with infected individuals, and foreign travel to endemic areas.

PATHOGEN DESCRIPTION

The HAV is an enterovirus about 28 nm in diameter (McCollum, 1982). It contains a single-stranded RNA and four polypeptides. Comparative studies of HAV from different geographic areas have been limited, but it appears that only one serotype exists. This simplifies potential vaccine development.

Fecal excretion of the virus begins about 25 days after oral infection with HAV. Peak infectivity probably occurs before the onset of symptoms in the fourth week after exposure.

HOST IMMUNE RESPONSE

The host immune response to HAV infection involves both IgM and IgG (McCollum, 1982). Anti-HAV IgM appears as virus excretion begins to subside. Shortly thereafter, IgG levels begin to rise. IgG persists while IgM levels fall over the next 3 to 6 months. Cell-mediated immunity to HAV infection has not been reported.

DISTRIBUTION OF DISEASE

Geographic Distribution

Hepatitis A virus infection and illness occur worldwide. As noted above, the severity of symptoms is related to the age at infection, which is dependent on the socioeconomic development of the region or country. Hence, the proportion of cases with moderate or severe symptoms is likely to be greater in the developed and more advanced developing countries, but the number of cases may be greater in the poorer countries. The rates of disease in different countries are discussed further below.

Disease Burden Estimates

Estimates of the numbers of cases of clinical hepatitis A illness worldwide are based on limited information and are therefore uncertain. Seroprevalence data show that hepatitis A is an infection of early



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