Clinical features and molecular characterization of hepatitis A virus outbreak in a child care center in Thailand

Abstract

Background: As a result of declining hepatitis A endemicity in Thailand, an increasing number of children and adolescents have become susceptible to hepatitis A virus (HAV) infection. Objective: The present study was aimed at both investigating the clinical features and determining molecular characterization of HAV during an outbreak, which occurred in a childcare center located in a suburban area of Bangkok between November 2002 and February 2003. Methods: Serum samples obtained from all children in the center were tested for anti-HAV IgG and anti-HAV IgM. Testing for HAV–RNA was performed in sera, saliva and stool samples by the reverse transcription-polymerase chain reaction (RT-PCR) with primers located at the VP1-2A region. To further characterize the HAV genotype serum derived HAV–RNA-positive PCR products were sequenced. Results: Anti-HAV IgG and anti-HAV IgM were detected in 74 and 70 of 112 children in the center, respectively. Among those positive for anti-HAV IgM, 65 cases were asymptomatic, while five children had acute clinical hepatitis. The ratio between symptomatic and asymptomatic children was 1:13. Among the asymptomatic cases, 31 (47.7%) displayed biochemical hepatitis with elevated alanine aminotransferase (ALT) levels. All the isolates from this outbreak were found to be of subgenotype IA, which showed a high level of sequence homology with previous Thai isolates. HAV–RNA could not be detected in saliva, but was found in stool for at least 3 weeks after initial diagnosis of clinical or biochemical hepatitis. Conclusion: Because of the infection’s characteristically asymptomatic spread, hepatitis A poses an increased risk to childcare centers. The presence of a single sub-genotype indicates that this HAV strain has been predominantly circulating in Thailand.

Introduction

Hepatitis A virus (HAV) infection has been a major public health problem in many developing countries worldwide (Cuthbert, 2001). In Thailand during the previous two decades, HAV infection has undergone a remarkable regression from high to intermediate endemic levels. This shifting epidemiology of hepatitis A has been attributed to general improvements in hygiene, living standards and socioeconomic progress (Poovorawan et al., 2002). As a result, the proportion of children and adolescents susceptible to the infection has increased and major outbreaks caused by contaminated water and food have been periodically reported (Poovorawan et al., 2000). For instance in 1992, an outbreak caused by contaminated drinking water occurred among schoolchildren in a province in southern Thailand (Sinlaparatsamee et al., 1995). Between September 2001 and April 2002, two more recent major community outbreaks occurred in another province in southern Thailand (Theamboonlers et al., 2002).

HAV is a 7.5 kb positive-stranded RNA virus belonging to the Picornaviridae family (Yokosuka, 2000). The virion comprises three functional regions namely, P1, P2 and P3. The P1 region encodes the structural proteins VP1, VP2, VP3, and putative VP4, while the P2 and P3 regions encode nonstructural proteins associated with viral replication (Cuthbert, 2001). Since the region spanning VP1-2A has demonstrated substantial sequence heterogeneity, it is suitable for differentiating between HAV strains (Jansen et al., 1990). Based on VP1-2A sequencing, different HAV strains have been classified into seven genotypes designated I–VII, which are distinguishable by 15–25% sequence diversity (Robertson et al., 1992). Genotype I is the most common strain in humans worldwide, with genotype III as the second most prevalent. Genotypes I and III have been further divided into subgenotypes A and B. Sub-genotype IA is the major genotype in America and Asia, whereas subgenotype IB appears to predominate in Europe and the Mediterranean region (Robertson et al., 1992). In Thailand, all isolates obtained from recent outbreaks were found to be of subgenotype IA (Theamboonlers et al., 2002) showing a high level of sequence homology with those previously described (Robertson et al., 1992).

Between November 2002 and February 2003, an outbreak of hepatitis A occurred in a childcare center located in a suburban area of Bangkok. We conducted an investigation to determine the clinical and biochemical features of children infected during this outbreak. We further analyzed the phylogenetic relations between HAV isolates obtained from this outbreak and recent epidemics in order to clarify HAV genotypes that have been circulating among Thai populations. Thus, this study provides valuable information on the clinical aspects of hepatitis A and particularly, the molecular epidemiology of the virus in Thailand.