Thai Studies of Aplastic Anemia and Agranulocytosis

Objectives

To document the incidence of the two blood dyscrasias in Thailand, and to identify risk factors. It is thought that the incidence of aplastic anemia is higher than in Western countries, which suggests the possibility of a unique etiology. Factors of particular interest include pesticides, other environmental exposures, and hepatitis viruses, as well as drugs.

Methods

The study is of a population-based case-control design, with methods similar to the IAAAS. All cases of the dyscrasias meeting specified criteria that occur in three regions of Thailand , Bangkok , Khonkaen in the northeast, and Songkla in the south, (total population, 21.4 million) were identified, and interviewed if possible. Up to four hospital controls were matched to each interviewed case according to age, sex, and region. The study ended in Songkla in December 1994; and in Bangkok and Khonkaen in June 2002, with 541 cases of aplastic anemia, 36 cases of agranulocytosis, and 2261 hospital controls enrolled.

The information obtained included demographic details, medical history, current occupation and exposure to chemicals and other occupational factors, and detailed histories of pesticide and medication use in the six months before hospital admission. In addition, for as many cases as possible and at least one control per case, blood samples were obtained and tested for infection with hepatitis A, B, and C. Samples are also stored for the evaluation of future hypotheses.

The study is the largest epidemiologic investigation of aplastic anemia that has been conducted. Publications to date have shown an incidence in Bangkok that is approximately double that in Western countries, a strong inverse association between aplastic anemia and socioeconomic status, a low drug attributability, the relative safety of household pesticides, a strong association with grain farming in rural Thailand and an association with agricultural pesticides, and the relation of the disease to hepatitis viruses. Analyses of the full dataset are now in progress.