Zoonotic Trematodes Research Articles

Relationship between snail population density and infection status of snails and fish with zoonotic trematodes in Vietnamese carp nurseries.

BackgroundFish-borne zoonotic trematodes (FZT) are a food safety and health concern in Vietnam. Humans and other final hosts acquire these parasites from eating raw or under-cooked fish with FZT metacercariae. Fish raised in ponds are exposed to cercariae shed by snail hosts that are common in fish farm ponds. Previous risk assessment on FZT transmission in the Red River Delta of Vietnam identified carp nursery ponds as major sites of transmission. In this study, we analyzed the association between snail population density and heterophyid trematode infection in snails with the rate of FZT transmission to juvenile fish raised in carp nurseries.Methodology/Principal FindingsSnail population density and prevalence of trematode (Heterophyidae) infections were determined in 48 carp nurseries producing Rohu juveniles, (Labeo rohita) in the Red River Delta area. Fish samples were examined at 3, 6 and 9 weeks after the juvenile fish were introduced into the ponds. There was a significant positive correlation between prevalence of FZT metacercariae in juvenile fish and density of infected snails. Thus, the odds of infection in juvenile fish were 4.36 and 11.32 times higher for ponds with medium and high density of snails, respectively, compared to ponds where no infected snails were found. Further, the intensity of fish FZT infections increased with the density of infected snails. Interestingly, however, some ponds with no or few infected snails were collected also had high prevalence and intensity of FZT in juvenile fish. This may be due to immigration of cercariae into the pond from external water sources.Conclusions/SignificanceThe total number and density of potential host snails and density of host snails infected with heterophyid trematodes in the aquaculture pond is a useful predictor for infections in juvenile fish, although infection levels in juvenile fish can occur despite low density or absence infected snails. This suggests that intervention programs to control FZT infection of fish should include not only intra-pond snail control, but also include water sources of allochthonous cercariae, i.e. canals supplying water to ponds as well as snail habitats outside the pond such as rice fields and surrounding ponds.

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

In integrated small-scale aquaculture farming, animal and human excreta maybe used as fish feed and pond fertilizer, thereby enhancing transmission of fish-borne zoonotic trematodes (FZTs) from final hosts, like humans, pigs and chickens, to snails. Areas within a pond could vary in trematode egg-load due to the immediate bordering land, and this might provide implications for control of these trematodes or sampling in field studies measuring FZT prevalence in snails. We therefore estimated the effect of bordering land use on prevalence and FZT burden in snails in different areas within small-scale aquaculture ponds. Nine sampling areas within a pond were assigned in six ponds. For each sampling area, about 120 Melanoides tuberculata snails were collected. Based on land use bordering a sampling area, these were categorized in 5 risk-categories: low-risk (road, rice planted in pond, agriculture, or middle of pond), human access point to pond, livestock sty (pigs or poultry), both human access point and livestock sty, and water connection to canal. In total, 5392 snails were collected. Percentages of snails with parapleurolophocercous cercariae varied between 6% in areas categorized as low-risk and areas with livestock sty only to 15% in areas with both human access point and livestock sty; only this 15% was significantly different from the prevalence in the low-risk category. Percentages of snails with xiphidio cercariae did not differ between risk-categories and varied between 5% and 10%. Mean snail size was 15.2mm, and was significantly associated with both the probability of infection as well as parasite burden. Very small differences in parasite burden were found at different land use areas; the maximum difference was about 11 cercariae. This study demonstrated only small differences between areas surrounding a pond on risk of snails to be infected with fish-borne trematodes within different pond areas. In field studies on FZTs in M. tuberculata snails in ponds, sampling from ponds can therefore be done without considering areas within ponds.

Prevention and Control of Fish-borne Zoonotic Trematodes in Fish Nurseries, Vietnam

Worldwide, >18 million persons were infected with fish-borne zoonotic trematodes in 2002. To evaluate the effectiveness of interventions for reducing prevalence and intensity of fish-borne zoonotic trematode infections in juvenile fish, we compared transmission rates at nurseries in the Red River Delta, northern Vietnam. Rates were significantly lower for nurseries that reduced snail populations and trematode egg contamination in ponds than for nurseries that did not. These interventions can be used in the development of programs for sustained control of zoonotic trematodes in farmed fish.

Experimental infection with the small intestinal trematode, Haplorchis pumilio, in young dogs

Fishborne zoonotic trematodes (FZT) are highly prevalent in Southeast Asia. Recent studies on the role of domestic animals in the transmission of FZT in Northern Vietnam found that dogs, mainly infected with Haplorchis pumilio, contributed widely to the transmission of FZT. On this background, we conducted an experimental infection with H. pumilio to elucidate population dynamics and host reactions in dogs. Eight household-reared dogs (3–6 months old), were each orally infected with 500 H. pumilio metacercariae obtained by artificial digestion of naturally infected fish. Another eight dogs were included as uninfected controls. Faecal examination for eggs was performed twice weekly using a sieving and sedimentation technique. Body temperature and weight of the dogs were measured as was total white blood cells, blood eosinophils and packed cell volume. Subsets of dogs were examined post-mortem for presence of adult FZT at three different time points post infection by sectioning of the small intestine and caecum into four parts. Patent infections established in all eight infected dogs. The worm establishment ranged from 15 to 121 flukes (3–24%, mean 12%). Faecal egg excretion was measured in all eight infected dogs but no more than two eggs per g faeces (epg) were found at any time. Infections lasted for at least two months as documented by the presence of adult flukes in all three dogs necropsied on day 58 post infection. The predilection site of the flukes was identified as the lower part of jejunum (93% of total worm burden). The results of the haematological tests did not differ between the infected and uninfected group. Further, no clinical symptoms were observed in the infected group and no macroscopic pathological changes could be assigned to the trematode infections, neither did histopathological examination of the intestine reveal any differences between the infected and the control dogs. This study provides the first basic knowledge on the establishment, duration and location of H. pumilio infection in dogs. However, before any control measures can be recommended, knowledge regarding infection dynamics, epidemiology, health impact and control is needed.

Evaluation of the role of rats as reservoir hosts for fishborne zoonotic trematodes in two endemic northern Vietnam fish farms

Fishborne zoonotic trematodes (FZTs) pose a food safety risk for the aquaculture industry in Vietnam. The risk of being infected from eating raw fish applies not only to humans but also to domestic animals which can serve as reservoir hosts in fish farms. The role of rodents, commonly found in fish farms, as reservoir hosts has not been adequately evaluated. To study this question, commensal and rice field rats were collected from fish farms in Nghia Lac and Nghia Phu communes, Nam Dinh province, Vietnam and examined for FZT infection. A total of 250 rats (Rattus norvegicus, Rattus argentiventer, and Bandicota indica) were collected and examined; the rats were trapped in the farm households (commensal) and in rice fields adjoining the fish ponds (field). The total prevalence of all parasites was 52.8 %: nematodes, 42.4 %; trematodes 18.0 %; and cestodes, (3.6 %), including two zoonotic cestode species, Raillietina celebenisis and Hymenolepis diminuta. Although overall prevalence and intensity of parasite infection did not differ significantly among rat species or between the two communes, the intensity of nematode infection was significantly higher in commensal rats (p < 0.05). The only FZTs recovered were the intestinal flukes Echinostoma cinetorchis and Centrocestus formosanus, both at low prevalence. Because the most common FZTs found in fish from these communes are Haplorchis pumilio and Haplorchis taichui, neither of which were found in the rats, we conclude that rats are not significant reservoir hosts for FZT in these Vietnamese fish farms.

Higher attack rate of fish-borne trematodes (Heterophyidae) in common carp fingerlings (Cyprinus carpio) at lower fish weight

Fish-borne zoonotic trematodes (FZTs) can cause pathology in humans. Fish weight was reported as important risk factor for transmission from snail to fish. However, in fingerlings, the relation between fish weight and infection is unknown. Aim was quantifying the effect of fish weight on infection probability, attack rate, and metacercariae burden of FZTs in common carps (Cyprinus carpio) between 1 and 20 g. Fish were either used as controls (n = 66) or exposed to 250 parapleurolophocercous cercariae (n = 254). Fish weight was analysed as continuous explanatory variable or classified in four categories with average weights of 0.7 g (n = 116), 4.0 g (n = 58), 8.2 g (n = 57) and 14.2 g (n = 23). The inverse relation between percentage of fish with metacercariae and fish weight is reflected in lower percentages of infected fish at higher weights [%infected = 100/(1 + e([-2.02+0.15 fish weight (g)])); p < 0.01], i.e. 89 %, 85 %, 63 % and 61 %, respectively, in the four groups. Control fish did not get infected. Attack rates were 0.0087, 0.0073, 0.0040 and 0.0033 fish infected per cercariae, respectively; the first two attack rates being significantly higher than the latter two. Mean number of metacercariae per weight group was 5, 5, 2 and 1, respectively, (p < 0.05), with an inverse relation using weight as continuous explanatory variable [p < 0.01; number metacercariae = e(1.76-0.13fish weight(g))]. Concluding, an inverse relation exists between fish weight and probability of infection, attack rate and parasite burden in common carp fingerlings. Reducing transmission to fingerlings might be an effective intervention method to improve food safety, reduce the absolute amount of FZTs in the environment and eventually reduce incidence in humans.

Prevalence and Intensity of Fish-Borne Zoonotic Trematodes in Cultured Freshwater Fish from Rural and Urban Areas of Northern Vietnam

Consumption of raw freshwater fish produced in both rural farm and urban wastewater ponds is a common practice in Vietnam. The present study assessed the risk of fish-borne zoonotic trematode (FZT) infection from fish raised in both these aquaculture systems in northern Vietnam. The diversity, prevalence, and infection intensity of FZT metacercariae in 1,500 freshwater fish collected from 6 sites located in rural and urban areas in northern Vietnam were investigated. The specific diagnosis of species was made by morphologic methods. The overall FZT prevalence in fish from both urban wastewater ponds and rural farm ponds was 11.2%. In wastewater ponds, the overall prevalence was 5.1%, ranging from 2.0% in tilapia to 7.3% in common and grass carp. In fish from farm ponds, the prevalence was 17.3%, and ranged from 6.7% in mud carp to 26.7% in common carp. The mean intensity of FZT infection was also higher in fish from farm ponds than that in fish from wastewater ponds (6.0% and 8.4%, respectively). The FZT species recovered from infected fish included both liver (Clonorchis sinensis) and intestinal flukes (Haplorchis taichui, Haplorchis pumilio, and Centrocestus formosanus). The prevalence of FZT in fish raised in these common farm systems represents a significant public health risk for a population with a strong cultural preference for consuming raw or inadequately prepared fish. These research results should encourage the public health and agriculture sectors to conduct the risk factor research required to develop control programs for FZT.

Characterization of microRNAs in Taenia saginata of zoonotic significance by Solexa deep sequencing and bioinformatics analysis

The beef tapeworm Taenia saginata infects human beings with symptoms ranging from nausea, abdominal discomfort to digestive disturbances and intestinal blockage. In the present study, microRNA (miRNA) expressing profile in adult T. saginata was analyzed using Solexa deep sequencing and bioinformatics analysis. A total of 15.8 million reads was obtained by Solexa sequencing, and 13.3 million clean reads (1.73 million unique sequences) was obtained after removing reads smaller than 18 nt. Ten conserved miRNAs corresponding to 607,382 reads were found when matching the reads against known miRNAs of Schistosoma japonicum in miRBase database. The miR-71 had the most abundant expression in T. saginata, followed by miR-219-5p, but some other common miRNAs such as let-7, miR-40, and miR-103 were not identified in T. saginata. Nucleotide bias analysis found that the known miRNAs showed high bias and the uracil was the dominant nucleotide, particularly at the first and 11th positions which were almost at the beginning and middle of conserved miRNAs. One novel miRNA (Tsa-miR-001) corresponding to ten precursors was identified and confirmed by stem-loop RT-PCR. To our knowledge, this is the first report of miRNA profiles in T. saginata, which will contribute to better understanding of the complex biology of this zoonotic trematode. The reported data of T. saginata miRNAs should provide valuable references for miRNA studies of closed related zoonotic Taenia cestodes such as Taenia solium and Taenia asiatica.

Genetic variation amongClonorchis sinensisisolates from different geographic regions in China revealed by sequence analyses of four mitochondrial genes

The present study examined sequence variation in four mitochondrial (mt) genes, namely cytochrome c oxidase subunits 1 (cox1) and 2 (cox2), and NADH dehydrogenase subunits 1 and 2 (nad1 and nad2) among Clonorchis sinensis isolates from different endemic regions in China, and their phylogenetic relationships with other zoonotic trematodes were reconstructed. A portion of the cox1 and cox2 genes (pcox1 and pcox2), and nad1 and nad2 genes (pnad1 and pnad2) were amplified separately from individual liver flukes by polymerase chain reaction (PCR) and the amplicons were subjected to sequencing from both directions. The intra-specific sequence variations within C. sinensis were 0-1.6% for pcox1, 0-1.4% for pcox2, 0-0.9% for pnad1 and 0-1.0% for pnad2. Phylogenetic analyses based on the combined sequences of pcox1, pcox2, pnad1 and pnad2 revealed that all the C. sinensis isolates grouped together and were closely related to Opisthorchis felineus. These findings revealed the existence of intra-specific variation in mitochondrial DNA (mtDNA) sequences among C. sinensis isolates from different geographic regions, and demonstrated that mtDNA sequences provide reliable genetic markers for phylogenetic studies of zoonotic trematodes.

Freshwater Aquaculture Nurseries and Infection of Fish with Zoonotic Trematodes, Vietnam

Residents of the Red River Delta region of northern Vietnam have a long tradition of eating raw fish. Fish-borne zoonotic trematodes (FZTs) are estimated to infect ≈1 million persons in Vietnam. It remains uncertain at what stages in the aquaculture production cycle fish become infected with FZTs. Newly hatched fish (fry) from 8 hatcheries and juveniles from 27 nurseries were therefore examined for FZT infection. No FZTs were found in fry from hatcheries. In nurseries, FZT prevalence in juveniles was 14.1%, 48.6%, and 57.8% after 1 week, 4 weeks, and when overwintered in ponds, respectively. FZT prevalence was higher in grass carp (p<0.001) than in other carp species. Results show that nurseries are hot spots for FZT infections in fish. Thus, sustainable FZT prevention strategies must address aquaculture management practices, particularly in nurseries, to minimize the risk of distributing infected juveniles to grow-out ponds and, subsequently, to markets for human consumption.

Raw-Fish-Eating Behavior and Fishborne Zoonotic Trematode Infection in People of Northern Vietnam

Raw fish consumption in restaurants, for example, Sashimi style, is popular worldwide. In Vietnam, raw fish dishes are also traditionally prepared and consumed in private households. However, the habits of eating raw or otherwise inadequately cooked fish can be associated with risks of acquiring fishborne zoonotic trematode (FZT) infection. The present study was done in a fish-farming community in Nam Dinh, Vietnam, to obtain information about habits of eating raw fish dishes and risks for human FZT infection. Discussions were held in different groups divided by gender and age on raw-fish-eating behavior. A total of 180 household members were interviewed and their stool samples analyzed to identify risk factors of FZT infection. There was awareness about the risk of liver fluke infections from eating raw fish. However, many older people accepted these risks and continued eating raw fish, as they know effective drug treatment is available. Raw fish dishes are consumed at social gatherings from shared plates and dipping sauces using the same chop sticks. This is likely to pose risks of crosscontamination with FZT metacercariae to different food items as indicated by the finding that 25.8% of household members that stated not to have eaten raw fish were infected. In total, 32.2% fish farm household members were infected with FZT. The odds of FZT infection was 2.3 times higher (p = 0.013) for those eating raw fish than for those who did not eat raw fish. Among the people eating raw fish, those eating raw fish in restaurants had 3.6 times higher odds of FZT infection (p = 0.009) than people eating raw fish at home. A successful program to control FZT must be based on in-depth knowledge on the social and anthropological determinants of people's raw-fish-eating behavior and hygiene practices as well as production of FZT-free fish for human consumption.

Field trial of praziquantel for control of fishborne zoonotic trematodes in reservoir hosts in Vietnam

This field trial was conducted to determine whether 40 or 75mg/kg of praziquantel is suitable for treatment of fishborne zoonotic trematodes (FZT) in naturally infected dogs (n=10) and cats (n=11). Three days after treatment all animals at either dose were negative for small trematode eggs. In two cats and one dog treated with 75mg/kg, however, a few damaged eggs were found 3 days post-treatment; no small trematode eggs were seen in these animals at day 14 post-treatment. In addition, at the 75mg dose, two cats and two dogs experienced vomiting or diminished appetite. Therefore a praziquantel dose of 40mg/kg is suggested for treatment of FZT in dogs and cats.

Farm-Level Risk Factors for Fish-Borne Zoonotic Trematode Infection in Integrated Small-Scale Fish Farms in Northern Vietnam

BackgroundNorthern Vietnam is an endemic region for fish-borne zoonotic trematodes (FZT), including liver and intestinal flukes. Humans acquire the FZT infection by eating raw or inadequately cooked fish. The production of FZT-free fish in aquaculture is a key component in establishing a sustainable program to prevent and control the FZT transmission to humans. Interventions in aquaculture should be based on knowledge of the main risk factors associated with FZT transmission.Methodology/Principal FindingsA longitudinal study was carried out from June 2006 to May 2007 in Nam Dinh province, Red River Delta to investigate the development and risk factors of FZT infections in freshwater cultured fish. A total of 3820 fish were sampled six times at two-month intervals from 96 fish farms. Logistic analysis with repeated measurements was used to evaluate potential risk factors based on information collected through questionnaire interviews with 61 fish farm owners. The results showed that the FZT infections significantly increased from first sampling in June to July 2006 (65%) to sixth sampling in April to May, 2007 (76%). The liver fluke, Clonorchis sinensis and different zoonotic intestinal flukes including Haplochis pumilio, H. taichui, H. yokogawai, Centrocestus formosanus and Procerovum varium were found in sampled fish. Duration of fish cultured (sampling times), mebendazole drug self-medication of household members, presence of snails in the pond, and feeding fish with green vegetation collected outside fish farms all had a significant effect on the development of FZT prevalence in the fish.Conclusions/SignificanceThe FZT prevalence in fish increased by 11 percentage points during a one-year culture period and the risk factors for the development of infection were identified. Results also highlight that the young fish are already highly infected when stocked into the grow-out systems. This knowledge should be incorporated into control programs of FZT transmission in integrated small-scale aquaculture nursery and grow-out systems in Vietnam.

Combined mitochondrial 16S and 12S rDNA sequences: an effective genetic marker for inter-species phylogenetic analysis of zoonotic trematodes

The present study studied the genetic variation among Schistosoma japonicum isolates from different endemic regions in mainland China and examined the phylogenetic relationships of zoonotic trematodes using the combined mitochondrial 16S and 12S ribosomal DNA sequences. The fragments of 16S and 12S rDNA were amplified from 22 S. japonicum isolates, and sequenced, and the relevant sequences of other nine trematode species belonging to six genera in four families were downloaded from GenBank, and their phylogenetic relationships were re-constructed by unweighted pair-group method with arithmetic averages analyses using the combined 16S and 12S rDNA sequences, with Trichinella spiralis as outgroup. The results showed that the partial sequences of mitochondrial 16S and 12S rDNA of S. japonicum were 757 and 797 bp, respectively, and they were quite conserved among the S. japonicum isolates. Phylogenetic analysis revealed that the combined 16S and 12S rDNA sequences were not able to distinguish S. japonicum isolates in mountainous areas from those in lake/marshland areas in mainland China. However, the combined sequences could distinguish different species of zoonotic trematodes. Therefore, the combined mitochondrial 16S and 12S rDNA sequences provide an effective molecular marker for the inter-species phylogenetic analysis and differential identification of zoonotic trematodes.

Distribution of freshwater snails in family-based VAC ponds and associated waterbodies with special reference to intermediate hosts of fish-borne zoonotic trematodes in Nam Dinh Province, Vietnam

Fish-borne zoonotic trematodes, such as Clonorchis sinensis, heterophyids and others, constitute a public health concern in parts of northern Vietnam and infections with these trematodes are often thought to be linked to fish culture. One common fish culture system is the integrated fish-livestock (VAC) ponds where individual households have 1 or more ponds. Fish fry, mainly of various carp species, produced in hatcheries, not necessarily local, are introduced into nursery ponds and after approximately 6 weeks, juvenile fishes are transferred to household ponds, referred to as grow-out ponds. Grow-out ponds are usually fertilized with organic debris, including animal excreta, to stimulate algal growth and subsequently fish growth. This paper describes the distribution of freshwater snails and occurrence of trematode infections in these in VAC ponds and associated habitats as part of a major study on risk factors of FZT infections in cultured fish in two communes, Nghia Lac and Nghia Phu, Nghia Hung District, Nam Dinh Province. The area is under intense rice cultivation with an extensive canal network supplying fields and also household VAC ponds. A total of 16 snail species was found and four were widely distributed i.e. Angulyagra polyzonata, Melanoides tuberculata, Bithynia fuchsiana and Pomacea insularum. Snail diversity and counts were higher in nursery ponds than in grow-out ponds. Species of the families Thiaridae and Viviparidae were more abundant than other species in VAC ponds while species of the Bithyniidae, Stenothyridae and Planorbidae dominated in rice fields and small canals. Trematode infections were found in eight snail species and among these M. tuberculata had the highest overall prevalence of infection (13.28%). No trematode infections were found in species of the Viviparidae and Ampullaridae except for metacercariae. Parapleurolophocercous and pleurolophocercous cercariae constituted the most common type of cercariae recovered, contributing 40.6% of all infections followed by echinostome cercariae (35.0%) and xiphidiocercariae (17.3%). Bithynia fuschiana and M. tuberculata had the most diverse trematode fauna. C. sinensis was not recorded in this study. The VAC pond system in this area, is very important for transmission of minute intestinal trematodes while they play little role in transmission of C. sinensis as its intermediate hosts, bithynid snails, rarely occur in these ponds. From a public health perspective this is positive as the effects of infections with intestinal trematodes are considered mild. On the other hand it is possible that even such subtle effects could have importance in public health as transmission is very intense in the area. And this in combination with the aquaculture importance, reduced marketability of fishes with high metacercariae loads, warrants that control efforts against these trematodes are initiated to reduce transmission in this production system.

Fish-Borne Zoonotic Trematodes in Cultured and Wild-Caught Freshwater Fish from the Red River Delta, Vietnam

Nam Dinh province in Red River Delta, Vietnam, is an endemic area for the human liver fluke Clonorchis sinensis, but little is known about its occurrence in the fish intermediate host in this region. A cross-sectional study was carried out to identify fish-borne zoonotic trematodes (FZT) species diversity and to estimate the prevalence and infection densities of FZT in three major types of fish: cultured species and self-recruiting species from family ponds and wild-caught species from irrigation canals in Nam Dinh province. A total of 714 wild-caught fish from canals and 829 fish from family ponds were examined for FZT infection. Only a single fish from a pond was infected with Cl. sinensis. The intestinal fluke Haplorchis pumilio was very common and found in more than 50% of fish irrespective of origin. Four other intestinal FZT species were found at low prevalence (<4.0%) except Procerovum varium, which was found in 14.4% of wild-caught fish. There was no significant difference (p > 0.05) in FZT prevalence in cultured fish (64.3%) compared with wild-caught fish (68.9%), nor between cultured fish species (65.1%) and self-recruiting species obtained from ponds (58.1%). The prevalence of FZT in silver carp (Hypophthalmichthys molitrix) (82.7%), grass carp (Ctenopharyngodon idellus) (76.9%), and mrigal (Cirrhinus mrigala) (63.8%) was significantly higher than that in rohu (Labeo rohita) (49.1%) (p < 0.001). The density of FZT metacercariae in fish from canals (0.56 metacercariae/g) was significantly higher (p < 0.001) than that in fish from ponds (0.03 metacercariae/g). The frequent occurrence of intestinal flukes in all fish types indicates that wild-caught and cultured fish are equally at risk of infection with FZT, particularly in species commonly used in Nam Dinh to prepare raw fish dishes. These results underscore the need for greater awareness of the risk from raw fish dishes among public health agencies and consumers.

Infection status of zoonotic trematode metacercariae in Sutchi catfish ( Pangasianodon hypophthalmus) in Vietnam: Associations with season, management and host age

The occurrence of four species of potentially zoonotic trematode metacercariae in Sutchi catfish Pangasianodon hypophthalmus from four provinces in the Mekong Delta of the Southern Vietnam is reported. A total of 1127 fish were collected from pond cultures comprising farm house systems (FHS) (279) and from farm household systems (FHHS) (848) and examined by classical and molecular methods. The trematode metacercariae Haplorchis pumilio, H. taichui, Centrocestus formosanus and Procerovum sp. were detected and the infections were analysed in relation to season, farm management type, host size and infection site in the host tissue. Generally, prevalences in the fish were higher during the rainy season from April to October when compared to the dry season. The infection rates and densities were lower in fishes from FHHS than from FHS. Fish at an age of 61 to 90 days post-hatch showed the highest parasite loads reflecting the presence of a continuous infection risk in the ponds following stocking. The parasites were found in the body musculature, head, fins and especially at the base of fins. The importance of Sutchi catfish farming is increasing both at the local and international markets and management practices in farms and industrial processing may reduce the infection levels and thereby improve the food hygienic standard of the products.

Poultry as reservoir hosts for fishborne zoonotic trematodes in Vietnamese fish farms

Fishborne zoonotic trematodes (FZT) are widespread in Vietnam and Southeast Asia. It is now recognized that the risk of being infected from eating raw fish dishes applies not only to humans, but also to domestic animals (e.g., cats, dogs, and pigs) and fish-eating birds. The role of ducks and chicken, commonly raised on fish farms, as reservoir hosts, however, has not been adequately investigated. To study this question, chickens and ducks from integrated poultry-fish farms in Nghia Lac and Nghia Phu communes, Nam Dinh province, Vietnam were surveyed for FZT infections. A total of 50 ducks and 50 chickens from each commune were examined. Results revealed that 12% of chickens and 30% of ducks were infected with various species of trematodes, including two zoonotic species, Centrocestus formosanus and Echinostoma cinetorchis. Both occurred in chickens whereas only E. cinetorchis was found in ducks. Prevalence of these zoonotic species was 12% and 7% in ducks and chickens, respectively. Among other trematodes, Hypoderaeum conoideum, also a zoonotic fluke, was the most prevalent (20–30%). The feeding of snails and fish remains to poultry, either intentionally or by discharge of waste from the slaughter of ducks and chickens into the ponds, was identified as risk factors for trematode infection. The FZT species and low prevalence found in poultry in these communes indicate their role as reservoir hosts is minor.

Fishborne zoonotic trematodes in raw fish dishes served in restaurants in Nam Dinh Province and Hanoi, Vietnam.

Fishborne zoonotic trematode (FZT) parasites are a serious food safety problem in Asian countries because of their tradition of eating raw fish. In northern Vietnam, the prevalence of FZT metacercariae (MC) in wild-caught and cultured fish is quite high. The objective of this study was to assess the risk for acquiring these parasites from raw fish dishes prepared in restaurants. Samples of fish meat (body muscle) and remainder parts (head, gills, fins, skin, and muscle tissue from the tail) were obtained from 39 restaurants located in Nam Dinh province, an area endemic for FZTs, and from 74 restaurants in the capital city of Hanoi. Six species of zoonotic intestinal trematodes were recovered from a total of nine freshwater and brackish water fish species. Overall, the prevalence of MC in meat samples was 6.1% and in remainder samples, 34.9%; freshwater fish were more often infected (39.4%) than brackish water species (16.0%). Dishes from Nam Dinh restaurants were more often infected (11.8%) than those from Hanoi (3.1%). The dominant trematode species, Haplorchis pumilio, exhibited a strong predilection for encystment in remainder fish parts (36.5%) compared with meat (3.9%). The mean density of MC in meat samples overall was low (1.0 to 1.5 MC per 50 g); however, the potential for parasites to accumulate over years of eating raw fish could result in significant clinical disease, as occurs with liver flukes. Also, the high densities of MC in the remainder parts (2.3 to 72.9 MC per 50 g) pose a threat for cross-contamination during dish preparation.

Survey for Fishborne Zoonotic Metacercariae in Farmed Grouper in Vietnam

Because of the common and growing practice of consuming raw grouper in Vietnam, the potential for transmission of zoonotic parasites is unclear. An investigation of the prevalence of zoonotic parasites in cage-reared grouper (Epinephelus coioides and Epinephelus bleekeri) from marine waters of Cat Ba Island, Hai Phong city, Northern Vietnam, revealed the presence of two zoonotic trematode metacercariae species, Procerovum varium and Heterophysopsis continua. The identity of the metacercariae was confirmed from adult flukes recovered from mice experimentally inoculated with metacercariae. The prevalence of zoonotic parasites in 40 E. coioides was 10.0% and 1.2% in 172 E. bleekeri. Since little is known of their ecology and epidemiology, further investigation of these zoonotic parasites is needed to develop prevention guidelines.