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High seroprevalence of hepatitis A virus among migrant workers from Myanmar, Cambodia and Laos who are living in Thailand

In Thailand, the socio–economic development that has occurred over the last few decades has brought general improvements in living standards, sanitation and hygiene. In consequence, there has been a gradual decline in the general seroprevalence of infection with hepatitis A virus (HAV), from a hyper-endemic situation to the current meso-endemicity (Poovorawan et al., 1997; Chatchatee et al., 2002; Chatproedprai et al., 2007). There is, however, some concern that areas with large populations of migrants from the neighbouring countries of Myanmar, Cambodia and Laos may still be hyper-endemic for HAV, although little is known about the HAV situation among these migrants or in their countries of origin. In 2004, the Thai Department of Employment estimated that there were .1,200,000 people from Myanmar, Cambodia and Laos working in Thailand (lib.doe.go.th/doeinfor/pagedata/ebookdoc/ 020400006369_1.pdf), and this estimate took no account of unregistered and illegal migrants. The annual influx of migrants is predicted to increase over the next few decades, and it is clear that the health problems of these workers could well have an impact on the non-migrant population. In a cross-sectional study in 2008, the sero–epidemiology of HAV among migrants from Myanmar, Cambodia and Laos working in Thailand was explored. It was hoped that the data collected would help define the sero–epidemiology of HAV within the migrants’ countries of origin and thus be useful in planning preventive strategies in those areas. All legal immigrant workers are registered with the Thai government and are required to have annual health check-ups. The subjects of the present study were legal workers from Myanmar, Cambodia and Laos who were aged between 16 and 60 years and had health insurance at the Bangkok 9 International Hospital, Bangkok. All had immigrated to Thailand within the previous 5 years and worked either in Bangkok or in provinces near Bangkok, such as Samutsakhon and Samutsongkhram. Potential subjects who had chronic illness, were receiving immunosuppressive therapy, and/or had the clinical signs or symptoms associated with HIV/AIDS or any other immunodeficiency-related disease were excluded. The protocol was approved by the ethics committee of Chulalongkorn University’s Faculty of Medicine and by the director of the Bangkok 9 International Hospital. The recruitment of subjects was sequential — as migrant workers presented at the Bangkok 9 International Hospital for their yearly check-ups, a blood sample was collected from each of them (a routine part of such check-ups). The blood (of each eligible subject) that was left after the routine haematology was used in the present study, with each subject given a code number to preserve his or her anonymity. Serum was separated off and stored at 220uC until it could be tested for anti-HAV IgG, in a commercial ELISA (Murex Biotech, Dartford, U.K.), at the Center of Excellence in Clinical Virology in Chulalongkorn University’s Faculty of Medicine. Annals of Tropical Medicine & Parasitology, Vol. 103, No. 4, 361–363 (2009)