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Preliminary findings of a high prevalence of Clonorchis sinensis in wild-caught fish in a North Vietnam reservoir (Thac Ba reservoir, Yen Bai Province) prompted a longitudinal epidemiological study of fish infections. Monthly collections of fish from September 2014 to August 2015 were processed for recovery of metacercariae; 1219 fish, representing 22 species, were examined. Seven species were infected with C. sinensis metacercariae. Four species, Toxabramis houdemeri, Hemiculter leucisculus, Cultrichthys erythropterus, and Culter recurvirostris, had high prevalence (31.1 to 76.7 %); metacercarial intensities ranged from 3.9 to 65.7 metacercariae/fish. A seasonal variation of C. sinensis prevalence was observed in T. houdemeri. Variation in intensity of infection occurred in C. erythropterus and H. leucisculus. Intensity and prevalence of C. sinensis in the most highly infected species, T. houdemeri, varied by fish size; prevalence was higher in fish weighing more than 3 g, and intensity was higher in fish weighing more than 5 g. The distribution of metacercariae in the body region of T. houdemeri was significantly higher in the caudal fin (14.7 metacercariae/g), compared to the body and head regions (0.7 and 1.4 metacercariae/g, respectively). Further epidemiological investigations on C. sinensis in this reservoir region should include assessing the relative risk of the different fish species for humans based on the latter's food preferences, and the prevalence of C. sinensis in the community. The snail intermediate host(s) in the reservoir should also be identified along with the ecological factors influencing its exposure to C. sinensis eggs and its subsequent transmission of cercariae to fish. Also needed are investigations on the relative importance of wild and domestic reservoir hosts as sources of egg contamination of the reservoir.

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A total of 21 Pharao flyingfish Cypselurus naresii (Günther) from the Gulf of Tonkin off Vietnam was examined for monogeneans. Ten individuals were parasitised by 72 specimens of two new axinid species of two rare and little known genera, Unnithanaxine Price, 1962 containing only one species, U. parawa (Unnithan, 1957), and Loxuroides Price, 1962 containing two species, L. sasikala (Unnithan, 1957) and L. fungilliformis Zhang, Ding, Liu & Wang, 1999. Unnithanaxine naresii n. sp. and Loxuroides pricei n. sp. are described and differentiated from the related species. Unnithanaxine naresii n. sp. is morphologically similar to U. parawa but is distinguished by the size of the clamps and reproduction organs, the number of spines in the lateral groups of the genital atrium, and in parasitism in a host fish species of a different genus. Loxuroides pricei n. sp. differs from L. fungilliformis in the greater size of the body, the number of clamps, testes, spines on cirrus and genital atrium, and in parasitism in a different host family. Similarly, L. pricei can be separated from L. sasikala in having a shorter distance from the anterior extremity to genital atrium or vaginal region, fewer testes, and a slightly greater number of spines on cirrus and genital atrium.