Prevalence of trematode metacercariae in cyprinoid fish of Ban Pao district, Chiang Mai Province, northern Thailand.

The prevalence of trematode metacercariae in natural cyprinoid fish was carried out in Ban Pao district, Chiang Mai Province, northern Thailand. Fish were collected from both man-made water reservoirs and natural sources during July 1996-May 1997. Nine species of cyprinoid fish harbored the metacercariae of Opisthorchis viverrini and small intestinal flukes of the family Heterophyidae, ie Haplorchis taichui, H. pumilio and Centrocestus spp. Metacercariae of H. taichui were found approximately 384 times more than O. viverrini. Among the fish collected, Puntius leiacanthus contained the highest number of metacercariae of H. taichui (182 metacercariae/fish) whereas P. orphoides contained the highest number of metacercariae of O. viverrini (1.4 metacercariae/fish). Metacercariae of H. taichui found in this area were year-round, with the highest and lowest incidence rates being in winter and rainy season, respectively. The difference between H. taichui and O. viverrini with respect to their prevalences are discussed.

Comparative studies on the morphology of the eggs of Opisthorchis viverrini and lecithodendriid trematodes.

Iodine staining and scanning electron microscopy were used to determine the morphological features distinguishing Opisthorchis viverrini from lecithodendriid eggs in human feces. The embryonated eggs of lecithodendriid trematodes differ from O. viverrini by the presence of an iodophilic body, a large mass at the posterior end of miracidium that stains brown in 0.2% iodine solution and the curved miracidium of Phaneropsolus bonnei. All forms of lecithodendriid eggs can be differentiated from those of O. viverrini by a set of morphological features of the shell surface, the knob, the operculum, the shoulder, the shape and size. On the basis of these differences, it is possible to perform differential egg counts.

Laboratory studies on host-parasite relationship of Bithynia snails and the liver fluke, Opisthorchis viverrini.

The infection rate of Bithynia snails to Opisthorchis viverrini eggs was studied in relation to exposure intensity, age and species of host. It was found that 50 miracidial eggs per snail yielded the highest percentage of living surviving positive snails. Bithynia funiculata and Bithynia siamensis siamensis were highly susceptible to O. viverrini, about four to seven times higher than Bithynia siamensis goniomphalos. Young snails, 1-3 months old, appeared more susceptible than old snails.

Studies on the Transmissibility of Pathogenic-Organisms to Liver by Larvae of Liver Fluke and Hookworm

In order to confirm whether the migrating larvae of parasites could carry pathogenic organisms into liver and cause hepatitis, a series of experiments has been carried out. The summary of the results is as follows: 1. Clonorchis sinensis A few of the excysted larvae of Clonorchis sinensis penetrated into the peritoneal cavity, but they could not penetrate the liver tissues. The artificially introduced Clonorchis sinensis in the tissues were all destroyed within 3-5 days. There was no manifestation of diffuse inflammatory changes due to the inoculation of the parasites, though the sampled micro-organisms, Staphylococcus aureus, were confirmed from the surrounding area. 2. Hookworm The larvae carried pathogenic organisms to liver tissues either by cutaneous or oral infection, but there was no manifestation of hepatitis due to the micro-organisms: In conclusion, it is indicated that liverfluke and hookworm may transmit pathogenic organisms to the liver during their migration.