Changes in human ecology and behavior have had discernable effects on the epidemiology of the hepatitis viruses in different ways and to different degrees. Following is a brief summary of each virus and how it has interacted with its host. In some cases the view is but a glimpse because the existence of three of the five viruses has been recognized for less than 20 years.

HEPATITIS A VIRUS

History

HAV may be traced back to epidemics of ''campaign jaundice" that afflicted the armies of the Middle Ages and that has continued to be a serious problem up to and including the Korean and Vietnamese conflicts (see ref. 1). The first civilian epidemics of hepatitis were recorded in Europe in the 17th and 18th centuries. The differentiation of hepatitis A, then called infectious hepatitis, from hepatitis B, then called serum hepatitis, came principally from studies in volunteers in Europe and the United States from the 1940s through the 1960s. It was not until the 1970s that HAV was transmitted to laboratory animals (marmoset monkeys and chimpanzees), and the virus was isolated in cell culture in 1979 (2). It remains the only one of the five hepatitis viruses that has been unequivocally isolated and serially propagated in cell culture. Effective inactivated whole virus vaccines have been developed. The first was licensed in Europe in 1991 and two inactivated hepatitis A vaccines should be licensed in the United States within the next 1 or 2 years.

Epidemiology

HAV is commonly transmitted by the fecal–oral route, either by person-to-person spread (the most common) or in common-source epidemics caused by contamination of food or water. Viremia occurs during the incubation period and the early acute phase of hepatitis A, and transmission by transfusion or recently by contaminated commercial factor VIII (3) has been reported, but such blood-borne transmissions are rare.

Virology

HAV is classified in the family Picornaviridae, genus Hepatovirus. Based on genomic sequence heterogeneity, seven genotypes of HAV have been identified, but one serotype comprises all of these (4). The



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