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Here the local people have a long tradition of eating raw freshwater fish. The objectives of the study were to identify FZT species diversity and to estimate the prevalence and infection densities of FZT in fish species from irrigation canals wild-caught and from family ponds cultured species and SRS. The study was conducted in two communes in Nghia Hung district, Nam Dinh province. Freshwater fish production in the province consists of fish cultured, SRS in ponds, and wild-caught fish from irrigation canals.

In the Nghia Lac and Nghia Phu communes aquaculture is practiced mainly in household ponds stocked with multiple fish species and raised in a continuous production cycle. The major fish species cultured are grass carp Ctenopharyngodon idellus , silver carp Hypophthalmichthys molitrix , common carp Cyprinus carpio , rohu Labeo rohita , and mirgal Cirrhinus mrigala.

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In the studied communes, most of households locate the toilets outside the main house, and some households had it near the pond. Free-roaming domesticated animals seen in this area were dogs, cats, chicken, and ducks. The lists of households in Nghia Lac and Nghia Phu communes were obtained and used for the selection of households for the study. Households with no ponds and households that participated in a previous FZT prevalence study in humans were excluded Dung et al.

A total of 75 households were randomly selected; if the household had more than one pond, one pond was randomly selected. Samples of fish in the ponds were collected using a cast net, which was thrown in each of the four corners and in the middle of each pond. All fish caught from the five throws were mixed and 10 fish were randomly selected for examination.

However, at some ponds less than 10 fish were obtained. In total, fish were collected from aquaculture ponds belonging to 16 fish species of which 8 were SRS. Wild-caught fish were sampled from the main irrigation canals of two communes. Due to difficulties using casting net in canals, lift nets were sometimes used. In each canal, 10—12 fish were sampled from 30 different locations. A total of wild-caught fish representing 19 fish species were collected and examined.

Solutions of 0. When the supernatant was clear, the sediment was examined under a stereo microscope. In some cases to assist their morphological identification, the encysted metacercariae were excysted by pressing on the cover slip or by placing them in trypsin digestion fluid 0.

Approaches to Research on the Systematics of Fish-Borne Trematodes

Identification of the metacercariae was according to the morphological features described in detail by Yamaguti , Velasquez , Pearson and Ow-Yang , Scholtz et al. To validate morphological metacercariae of each FZT species found in fish, in some instances, mice were inoculated with the metacercariae for adult worm recovery to facilitate identification following the published procedures Kay et al. The study was designed as a cross-sectional study.

Data collected in the field and from laboratory analysis were recorded in an Excel spreadsheet Microsoft Office Excel, and then imported into the Statistical Analysis System SAS, version 9. There was a broad range of fish species caught from canals and ponds. Therefore, fish species with less than 50 individuals were excluded from the analysis of differences in FZT prevalence between fish from different origins. When comparing the prevalence of FZT in wild-caught fish in canals and SRS fish in ponds, all common carp and mrigal caught in canals were excluded because these two species are exclusively cultured species and were assumed to be escapes from aquaculture ponds.

Logistic analysis was used to evaluate prevalence differences of FZT between origin canal and pond , location of commune Nghia Lac and Nghia Phu , and noncultured fish species wild-caught species and SRS. Study unit was the individual fish collected from one pond in each household in each commune.

Therefore, a multilevel data structure was used with household randomly sampled within each of the two commune and fish were randomly sampled within each household i.

Household nested within a commune was included as a random effect when using logistic analysis to evaluate differences of FZT prevalence between cultured species and SRS within ponds. Odds ratios were calculated only for significant factors. An analysis of variance of metacercariae density was performed to evaluate differences of FZT density between fish from canals and ponds, as the metacercariae density is a continuous variable.

Due to a skewed distribution of metacercariae density, a rank transformation of densities was applied. Fish from canals harbored a total of five species of zoonotic intestinal trematodes Heterophyidae , namely, Haplorchis pumilio , Haplorchis taichui , Haplorchis yokogawai , Centrocestus formosanus , and Procerovum varium , whereas fish from ponds were infected with four species, namely, H. The liver fluke, Cl. All fish species sampled in the study were infected by FZT at different prevalence. The prevalence of FZT metacercariae in different fish species from ponds cultured species and SRS and canals wild-caught species fish is shown in Table 2.

The prevalence of FZT in fish stratified by commune and different groups of fish ranged from Table 1. FZT, fish-borne zoonotic trematodes. Table 2. Table 3.


SRS, self-recruiting species. There was no significant difference in FZT prevalence between wild-caught fish from canals and ponds from both communes. Table 4. Table 5. The diversity of FZT species recovered from cultured fish species, SRS and wild-caught fish, in this study is similar to the diversity of adult intestinal flukes recovered recently from humans and domestic animals in the same study area of Northern Vietnam Dung et al.

Based on the results obtained in present and other studies Thien et al. The wide distribution of these intestinal fluke species is probably related to their broad host ranges, which include many mammals and fish-eating birds, a condition that increases opportunities for successful life cycle completions. The ubiquity of the snail vector Melanoides tuberculata in Vietnam may also be an important factor favoring the distribution of these intestinal flukes.

Comprehensive studies on the snail, fish, and reservoir host ranges for this trematode in Southeast Asia are, however, as yet not comprehensive enough to permit any conclusions on this issue. This suggests that humans are likely to acquire these FZT species from wild-caught or cultured fish imported from other areas. An evaluation of the influence of the human eating behaviors, including fish species preferences and their sources, may reveal risk factors that could explain these differences in FZT prevalence.

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The prevalence of FZT in fish from ponds For some of fish species, the sample size was very small. Therefore, the prevalence was estimated with high uncertainty for these species. Species with small sample size were excluded in statistic analysis. Although this may be related to regional differences such as fish farming practices, snail and reservoir host distributions, among others, our findings demonstrate for the first time that FZT-infected wild-caught fish from irrigation canals in Vietnam represent a significant food safety risk. The high FZT prevalence in cultured and wild-caught carps, especially silver carp, grass carp, mrigal, and rohu, should increase awareness of public health agencies and of consumers, because these species are frequently used for preparing raw fish dishes.

Therefore, people should not eat raw fish dishes unless the fish were first treated with some procedure that kills metacercariae WHO Further, the wide range and high prevalence of FZT-infected wild-caught and SRS fish species are an often overlooked problem because the fish are often used uncooked in animal feed preparations, presenting a risk to potential reservoir hosts.


In our study, the density of FZT in wild-caught fish from canals was 10 times higher than that in fish from ponds. The higher FZT prevalence and density in wild-caught fish species compared with similar species SRS from ponds may reflect the longer exposure time to cercariae of wild-caught fish. Wild-caught fish in the canals are not captured regularly and consequently probably have much longer life spans than SRS because SRS fish are normally harvested after several months and are seldom subject to exposure periods comparable to wild-caught fish.

Further studies on the effect of different risk factors for differences in FZT density in canal and pond fish are necessary for confirmation.

Approaches to Research on the Systematics of Fish-Borne Trematodes - 1st Edition

The high prevalence of FZT found in cultured and wild-caught species often used to prepare raw fish dishes represents a food safety risk and calls for better aquaculture management practices to control FZT, whereas control of FZT in wild-caught fish will be difficult. The authors appreciate farmers who participated in this study. Special thanks to Nguyen Viet Khue for his arrangement with fish sampling and Vo Van Binh for help in identifying the species of the collected fish.

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