LIVER FLUKES AND CANCER IN SOUTH-EAST ASIA*

STITAYA SIRISINHA

ABSTRACT: Different lines of evidence currently available suggest that microorganisms and parasites may play a role in the development of a diverse group of human cancers. The invol'lement and the mechanisms of oncogenic viruses in the development of cancers in man and animals are well documented. Many of these cancers including nasopharyngeal, cervical and hepatocellular carcinomas and cholangiocarcinomas are highly prevalent in Southeast Asian countries. The parasites known to play an etiological role in tumor development in man include liver flukes (Opisthorchis viverrini, O. felineus and Clonorchis sinensis) and blood flukes (Schistosoma haematobium).^1-4 S. mansoni, S. japonicum, whipworms and hookworms have also been suggested to be associated with colon and rectal carcinomas and even lymphomas but causal evidence for this is lacking. The incidence of liver cancer in Southeast Asian countries is among the highest in the world, with a total of more than 30,000 new cases per year.^5-7 In the endemic northeast Thailand, a crude annual incidence of CCA for both sexes is 54/100,000 with an: age-standarized incidence of 87/100,000, comparing with 2/100,000 in Western countries.⁶ It has been estimated that in Thailand and Leas alone, 8 million people are at risk of developing CCA.⁶ On the average, the age of the patients is 50-60 years with a male to female ratio of 4: 1. In some parts of Thailand, the incidence among the male population is estimated to be over 135/100,000, with an age-standardized incidence of as high as 334/ 100,000 (comparing with only 104/100,000 for the female). Of the two major kinds of primary liver cancer, that of the biliary system, Le., cholangiocarcinoma (CCA) occurs at higher frequency in areas where liver fluke infections are endemic.^3,5-7 For example, in Korea a hepatocellular carcinoma (HCC) to cholangiocarcinoma ratio (HCC:CCA) of 4:1 was found in the Pusan area where C. sinensis infection occurs, while a ratio of 10:1 was found in Seoul where C. sinensis does not occur.⁴ Similarly, a ratio of 3-4: 1 was reported in the endemic northeastern part of Thailand where O. viverrini infection may be as high as 90% in some villages. This compares with an average ratio of 5:1 to 10:1 throughout the country.⁵ It should be noted that in countries where liver fluke infections do not occur, CCA is rarely found and the HCC: CCA ratios as high as 56:1 have been reported.

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Department of Microbiology, Faculty of Science, Mahidol University and Laboratory of Immunology, Chulabhorn Research Institute, Bangkok, Thailand.

Received December 8, 1993