Ecological Niche Model based on Maximum Entropy for mapping distribution of Bithynia siamensis goniomphalos, first intermediate host snail of Opisthorchis viverrini in Thailand.

The snail Bithynia siamensis goniomphalos acts as first intermediate host of the liver fluke, Opisthorchis viverrini, which causes opisthorchiasis in humans. In this study, we used a geographic information system (GIS), remote sensing (RS) and software using the maximum entropy (MaxEnt) algorithm to predict the distribution of B. s. goniomphalos in Thailand on the basis of environmental and climatic factors. The MaxEnt model for B. s. goniomphalos was excellent, with average test AUC values of 0.89. The predicted distribution of B. s. goniomphalos was affected by altitude, land cover, normalized difference vegetation index (NDVI), precipitation in the driest month (BIO 14), land surface temperature (LST) and soil pH. The areas suitable for B. s. goniomphalos were mostly in Northeast Thailand and some northern parts of the country. The presence of B. s. goniomphalos decreases with increasing altitude and increasing NDVI value. Bithynia s. goniomphalos is most likely to occur in paddy fields and cropland. Opisthorchiasis prevalence was directly proportional to the likelihood of snail occurrence as predicted by the model. This is the first time this ecological niche model has been used to predict Bithynia snail distribution and hence to provide a basis for future work of opisthorchiasis prevention in opisthorchiasis-endemic countries.

Effects of albendazole, artesunate, praziquantel and miltefosine, on Opisthorchis viverrini cercariae and mature metacercariae.

OBJECTIVE: To explore larvicidal effects of anthelmintic drugs on Opisthorchis viverrini (O. viverrini) for alternative approach to interrupting its cycle for developing a field-based control program. METHODS: The larvicidal activities of albendazole (Al), artesunate (Ar), praziquantel (Pzq) and miltefosine (Mf) on O. viverrini cercariae and mature metacercariae were investigated. Lethal concentrations (LC50 and LC95) of these drugs were determined. Mature metacercariae previously exposed to various concentrations of the drugs were administered to hamsters. Worms were harvested 30 d post infection and worm recovery rates calculated. Al, Ar, Pzq and Mf produced morphological degeneration and induced shedding tails of cercariae after 24 h exposure. RESULTS: The LC50 and LC95 of Al, Ar, Pzq and Mf on cercariae were 0.720 and 1.139, 0.350 and 0.861, 0.017 and 0.693, and 0.530 and 1.134 ppm, respectively. LC50 and LC95 of Ar on mature metacercariae were 303.643 and 446.237 ppm and of Mf were 289.711 and 631.781 ppm, respectively but no lethal effect in Pzq- and Al-treated groups (up to 1 ppt). No worms were found in hamsters administered Pzq-treated metacercariae. The adult worms from Al-treated metacercariae were significantly bigger in size compared to the control group (P < 0.05). Fecundity and body width were greater in adults from Mf-treated metacercariae compared to the control group (P < 0.05). CONCLUSIONS: The larvicidal effects of these drugs were high efficacy to O. viverrini cercariae but lesser efficacy to metacercariae. It should be further studied with the eventual aim of developing a field-based control program.

Experimental and modelling investigations of Opisthorchis viverrini miracidia transmission over time and across temperatures: implications for control.

Transmissibility is a significant factor in parasite fitness. The rate and magnitude of parasite transmission affect prevalence and infection intensity in individual hosts and are influenced by environmental factors. In this context, the objectives of this study were: (i) to experimentally assess Opisthorchis viverrini miracidia survival and infectivity over time and across temperatures; and (ii) to combine these experimental results with environmental data to build a key component of a transmission model, identifying seasonal windows of transmission risk in hyper-endemic northeastern Thailand. Five replicates of 50 O. viverrini eggs were randomly distributed and maintained under four temperature conditions (25°C, 30°C, 35°C, 40°C). Microscopic observations were performed on all experimental units over a period of 3months to record miracidia motility and mortality trends. Six infection trials were also conducted to assess infectivity of miracidia over time and across temperatures, using observations of egg hatching success and infection rates. Upon completion of experiments, data were integrated into a transmission model to create a transmission risk index and to simulate seasonal transmission risk. Miracidia survival rate and motility decreased steadily with 50% mortality observed after 2weeks. Hatching and infection success also decreased significantly after 3weeks. Temperatures over 30°C were associated with increased mortality and decreased infectivity. When incorporating local environmental parameters into our model, we observed low transmission risk during the dry season and increasing transmission risk at the onset of the rainy season, culminating with the highest risk in September. We believe that our results provide the first estimates of O. viverrini miracidia survival and transmission potential under variable temperature conditions and suggest that high temperature treatment (>40°C) of fecal waste could be an efficient control strategy.