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A study of antibody prevalence for hepatitis A virus (HAV) and hepatitis E virus (HEV) was carried out in southwestern Vietnam in an area adjacent to a known focus of epidemic HEV transmission. The purpose of this investigation was first to provide a prevalence measure of hepatitis infections, and second to determine the outbreak potential of HEV as a function of the susceptible population. Blood specimens collected from 646 persons in randomly selected village hamlets were examined by an ELISA for anti-HEV IgG and anti-HAV IgG. The prevalences of anti-HEV IgG and anti-HAV IgG were 9% and 97%, respectively. There was a significant increase (P < 0.01) in age-specific anti-HEV IgG. A notable increase in anti-HAV IgG prevalence (P < 0.0001) occurred between child populations 0-4 (64%) and 5-9 (95%) years of age. No evidence of familial clustering of anti-HEV IgG-positive individuals was detected, and household crowding was not associated with the spread of HEV. Boiling of water was found to be of protective value against HEV transmission. A relatively low prevalence of anti-HEV indicates considerable HEV outbreak potential, against a background of 1) poor, water-related hygiene/sanitation, 2) dependence on a (likely human/animal waste)-contaminated Mekong riverine system, and 3) periodic river flooding.

A hepatitis outbreak affecting primarily adults occurred in southwestern Vietnam, along the Hau river bordering Cambodia, in June and July 1994. One month after the outbreak, sera and epidemiologic information were collected from 150 subjects: 50 patient cases, 50 matched, healthy community controls, and 50 geographic controls living 50 km upriver. The prevalence of immunoglobulin G (IgG) to hepatitis E virus (HEV) was significantly (P < 0.001) higher (76%) among cases than among the matched (38%) and geographic (38%) control populations. Immunoglobulin M to HEV was detected by enzyme-linked immunosorbent assay and Western blot in 16% of sera collected from patients one month after the outbreak. Hepatitis E virus RNA was detected with the polymerase chain reaction in 6% of sera from patients; RNA was not detected in either control group. These results indicate that HEV was the etiologic agent responsible for the outbreak. Children were under-represented among clinical cases. River water served as the principal source for drinking and bathing among most (96%) of the case and control study populations. Boiling of drinking water was negatively associated (P < 0.05) with IgG anti-HEV seropositivity. Unusually heavy rainfall likely contributed to conditions that favored the outbreak. This is the first recognized outbreak of epidemic HEV transmission in Indo-China.