Distribution of trematodes in snails in ponds at integrated small-scale aquaculture farms

Environmental influence on abundance and infection patterns of snail intermediate hosts of liver and intestinal flukes in North and Central Vietnam

Liver and intestinal flukes (LIF) are important groups of foodborne zoonotic trematodes (FZTs) in Southeast Asia, including Vietnam. Their complex life cycles require specific freshwater snail species as the obligatory first intermediate hosts. In 2019, we conducted a longitudinal study in Yen Bai and Thanh Hoa provinces in North and Central Vietnam, respectively, to investigate the diversity of LIF and their infection prevalence in relation to snail host abundance and environmental factors. Using a combination of morphological and molecular identification techniques, we identified 10 LIF species infecting 11 snail host species. We observed significant seasonal variation in the mean abundance of several snail host species, with the majority of snails collected during the spring. We also detected seasonal changes in LIF species composition, with the highest species richness reported in the spring. Clonorchis sinensis and Fasciola gigantica, two medically important human liver flukes in Asia, were found only in the spring in Yen Bai. Our study revealed that not all snail host species have the same probability of becoming infected, and we recorded seasonal variations in the prevalence of LIF infection in different snail species in relation to water parameters.

The prevalence of cercarial infection and development of a duplex PCR for detection of the cercarial stage of Haplorchis taichui and H. pumilio in first intermediate hosts from Chai Nat province, Thailand

This study aimed to investigate the prevalence of cercarial infections in freshwater snails collected from the Chai Nat province of Central Thailand. Moreover, we aimed to develop a duplex PCR technique, using the ITS2 and candidate MT-ND1 gene, to determine the dissemination of H. taichui and H. pumilio, respectively. Six types of cercariae were discovered with an overall prevalence of 4.71% (59/1,252). The parapleurolophocercous cercariae were demonstrated to be the dominant type, infecting only Melanoides tuberculata snails. The duplex PCR was optimized for specific amplification of ITS2 for H. taichui (115 bp) and MT-ND1 for H. pumilio (335 bp). Both specific primers confirmed the specificity of the duplex PCR reaction, with no cross-reactivity with other heterophyids or related species. In addition, this duplex PCR could be used to detect co-infection at a concentration of 3.0 ng/µL. For the molecular identification, 9 of 22 parapleurolophocercous cercaria specimens in Chai Nat province generated the specific DNA fragment of H. pumilio. These results proved that the MT-ND1 gene is a species-specific method for heterophyid detection and provides a rapid method for identification based on larval and adult stages of H. taichui and H. pumilio in their intermediate and/or definitive hosts in the infected area.

The rapid detection method by polymerase chain reaction for minute intestinal trematodes: Haplorchis taichui in intermediate snail hosts based on 18s ribosomal DNA

The minute intestinal trematode, Haplorchis taichui, is an important parasite species that can infect humans and other mammals. This study investigated the outbreak of H. taichui in thiarid snails in the lower part of the Chao Phraya Basin, Thailand by employing morphological and molecular-based methods. In development of a specific primer of H. taichui, the PCR reaction was conducted with no cross-reaction to their hosts and other related trematode species. The highest level of sensitivity that could be amplified was 0.50 ng/μl and this was detected with only one egg in the sample. In terms of the epidemic results, the parapleurolophocercous cercaria infected only two species of thiarid snails (Melanoides tuberculata and Tarebia granifera) with an overall prevalence of 3.80% (23/605). The process of molecular identification revealed positive results indicating that eleven from twenty-three of parapleurolophocercous cercariae specimens in the lower part of the Chao Phraya Basin were H. taichui. In conclusion, this study has developed a rapid detection method, which can discriminate H. taichui from other parapleurolophocercous cercaria in intermediate snail hosts with a high level of sensitivity. Moreover, the high proportion of H. taichui in parapleurolophocercous cercaria (47.83%) indicated that H. taichui was the dominant species of this cercarial type and could infect cyprinoid fish in the lower part of the Chao Phraya Basin leading to public health problems in this area. Thus, a specific primer could be useful in the detection and surveillance of H. taichui outbreaks in their hosts. Recognition of this has resulted in the creation of important prevention programs in these infected areas in the further study.

Transmission of Calicophoron daubneyi and Fasciola hepatica in Galicia (Spain): Temporal follow-up in the intermediate and definitive hosts

Background

Paramphistomosis caused by Calicophoron daubneyi and fasciolosis caused by Fasciola hepatica are common parasitic diseases of livestock animals. Transmission of the diseases depends on the presence of intermediate hosts, i.e. freshwater gastropods such as lymnaeids. We carried out a 2-year-long study of the dynamics of the snail population acting as the intermediate host for these parasites, considering the population structure in terms of size/age and infection status. In addition, we determined the kinetics of trematode egg excretion in grazing cows. Generalized Additive Models (GAMs) were used to analyze the associations between different response variables and snail size, sampling month and weather-related variables.

Results

Of the molluscan species examined, Galba truncatula, Radix peregra, Anisus (Anisus) leucostoma and Pisidium casertanum (n = 2802), only G. truncatula was infected with C. daubneyi or F. hepatica, at prevalence rates of 8.2% and 4.4% respectively. The probability of infection with C. daubneyi or F. hepatica was linearly related to snail size, although in different ways (negative for C. daubneyi and positive for F. hepatica). The total snail population increased in winter, when specimens of all size classes were found. Infected snails were more abundant during spring-autumn. Mature cercariae of both parasites were found in most seasons. In the statistical models, the sampling month accounted for a high percentage (71.9–78.2%) of the observed variability in snail abundance. The inclusion of climatic variables in the models moderately increased the percentage of deviance explained (77.7–91.9%). Excretion of C. daubneyi eggs in cow faeces was always higher than that of F. hepatica eggs.

Conclusions

Particular care should be taken to prevent pastures and the surrounding environment being contaminated with parasite eggs during winter-spring, when the number of snails susceptible to miracidial infections is maximal. This is therefore the optimal time for treating grazing animals. Nevertheless, control of trematodosis based only on chemotherapy is difficult in an area such as the study area, where environmental factors favour the regular appearance of snail populations harbouring mature cercariae.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1892-8) contains supplementary material, which is available to authorized users.