Foodborne trematode infections are important in Asia and some parts of Europe, and affect at least 17 million people (1). Opisthorchis felineus affects about 1.5 million in the former Soviet Union (2). Clonorchis sinensis is found in the southern part of Asia, in China, Republic of Korea and Viet Nam, while O. viverrini is confined to Thailand, the Lao People's Democratic Republic and Cambodia (1); in Thailand alone, 7-9 million people are infected by O. viverrini, most of whom live in the north-east region (3). Widespread infection in humans occurs by ingestion of raw or undercooked cyprinoid fish containing metacercariae. After ingestion of the metacercariae, the worms excyst and migrate into the biliary system causing considerable morbidity; they have been implicated in the etiology of cholangiocarcinoma (4, 5). In north-east Thailand, the incidence of cholangiocarcinoma is frequently high in communities with heavy O. viverrini infection (6).
Transmission of O. viverrini from humans to fish via snails is the net result of a complex interplay between hosts and parasites that is invariably regulated by seasonal environmental conditions, especially water temperature and the duration and amount of rainfall (7). Since the investigation by Vichasri et al. (8), there has been no comparable study to confirm the pattern of seasonal variation that they found and determine how representative are their findings for water bodies in north-east Thailand. Moreover, the pattern of seasonal variation in metacercarial intensity could be of considerable importance in planning for parasite control since largescale community-based treatment should be given when transmission is at its lowest in order to substantially reduce the prevalence and intensity of human infection. The present study was conducted to determine the seasonal variation of the density of O. viverrini metacercariae in cyprinoid fish in two reservoirs in communities with low and high O. viverrini infection in humans.
Materials and methods
Cyprinoid fish were caught in two reservoirs in north-east Thailand. These reservoirs were chosen because cyprinoid fish were available all year round in them and because they were located near communities with known levels of O. viverrini infection. The first reservoir is in Ban Non Sang, in Kosumpisai District, Mahasarakham Province, where an intensive national liver fluke control programme has been carried out and is thus currently regarded as a low transmission area (average infection rate, 29.7%; average intensity, 54.2 eggs per g of faeces) (9). The other reservoir is located at Kang La Wa, in Ban Pai District, Khon Kaen Province, where the parasite transmission, rate is high, as is the incidence of cholangiocarcinoma in the surrounding communities (6, 7, 10).
At weekly or fortnightly intervals, samples of cyprinoid fish from these reservoirs were brought to the laboratory. The fish were caught by local fishermen using nets stretched in the water overnight, the trapped fish being collected the following morning. At the laboratory, they were sorted by species and the numbers and weights of each species group recorded. The body lengths and widths of the smallest and the largest fish were noted to provide a range. The metacercarial burden in each group of fish was assessed as described elsewhere (11); briefly, each group of fish was minced in an equal volume of normal saline and the homogenate was digested in 0.05% (w/v) pepsin in 0.01 mol/l hydrochloric acid in a shaken water-bath at 37 [degrees] C for 1 hour. The digest was then filtered through a series of metal sieves of various pore sizes, and the final filtrate was sedimented in conical jars with several changes of saline. The sediment was then examined under a dissecting microscope for metacercariae by experienced technicians. The metacercariae of O. viverrini were differentiated from those of other trematodes and counted. …