Effectiveness of health education in improving knowledge, attitude and practice related to foodborne zoonotic trematodes in Vietnam, with a particular focus on Clonorchis sinensis

OBJECTIVE

Foodborne zoonotic trematodes (FZT), which infect the liver, lungs and intestines of humans, are an emerging public health concern in tropical countries including Vietnam. In northern Vietnam, Clonorchis sinensis is recognised as the most important species of the FZT. Up to now, small-scale studies conducted in the country have indicated that health education could improve participants' knowledge and practices related to clonorchiasis, however strong evidence is still lacking. We conducted an intervention trial between 2020 and 2021 in four communes in Yen Bai and Thanh Hoa province, aiming to evaluate the impact of an extended educational package on knowledge, attitude and practices related to clonorchiasis, as well as on knowledge on the large liver fluke and minute intestinal flukes.

METHODS

To this end, baseline and post-intervention questionnaires were conducted. Generalised estimating equations models were run to analyse the impact of the intervention on knowledge, attitudes and practices over time in the intervention versus control group. Finally, 172 people in the intervention group and 162 in the control group were included for analysis.

RESULTS

Results demonstrated that the intervention significantly improved clonorchiasis knowledge, attitudes and practices, with higher odds for a correct knowledge response (odds ratio (OR) = 2.80, 95% confidence interval (CI) = 1.84-4.27, p-value < 0.001), increased average mean attitude score (mean = 0.363, 95%CI = 0.182-0.544, p-value < 0.001), and a reduced odds of consuming raw fish (OR = 0.15, 95%CI = 0.06-0.40, p = 0.002) in the intervention group compared to the control group post-intervention versus baseline. Additionally, participants in the intervention group exhibited enhanced basic knowledge of the large liver fluke and minute intestinal flukes post-intervention.

CONCLUSIONS

These findings demonstrate the potential of health education programs in preventing clonorchiasis in endemic areas, emphasising the importance of continued health education as a critical component of integrated control programs for clonorchiasis.

Epidemiology of Trematode Infections: An Update

Digenetic trematodes infecting humans are more than 91 species which belong to 46 genera all over the world. According to their habitat in definitive hosts, they are classified as blood flukes (Schistosoma japonicum. S. mekongi, S. mansoni, S. haematobium, and S. intercalatum), liver flukes (Clonorchis sinensis, Opisthorchis viverrini, O. felineus, Metorchis conjunctus, M. bilis, M. orientalis, Fasciola hepatica, F. gigantica, Dicrocoelium dendriticum, and D. hospes), lung flukes (Paragonimus westermani, P. heterotremus, P. skrjabini, P. miyazakii, P. kellicoti, P. mexicanus, P. africanus, and P. uterobilateralis), throat fluke (Clinostomum complanatum), pancreatic fluke (Eurytrema pancreaticum), and intestinal flukes (Metagonimus yokogawai, M. miyatai, M. takahashii, Heterophyes nocens, H. heterophyes, Haplorchis taichui, H. pumilio, H. yokogawai, Centrocestus formosanus, Echinostoma revolutum, E. ilocanum, Isthmiophora hortensis, Echinochasmus japonicus, E. lilliputanus, Artyfechinostomum malayanum, A. sufrartyfex, A. oraoni, Fasciolopsis buski, Gymnophalloides seoi, Neodiplostomum seoulense, Caprimolgorchis molenkampi, Phaneropsolus bonnei, and Plagiorchis muris). The mode of transmission to humans includes contact with cercariae contaminated in water (schistosomes) and ingestion of raw or improperly cooked fish (liver and throat flukes, heterophyids, and echinostomes), snails (echinostomes and gymnophallids), amphibia, reptiles (neodiplostomes), aquatic vegetables (amphistomes), or insect larvae or adults (plagiorchiids, lecithodendriids, and pancreatic fluke). Praziquantel has been proved to be highly effective against most species of trematode infections except fascioliasis. Epidemiological surveys and detection of human infections are required for better understanding of the geographical distribution and endemicity of each trematode species.

Efficacy and toxicity of praziquantel in helminth-infected barbel (Barbus barbus L.)

This study evaluated efficacy and toxicity of the pyrazinoisoquinoline anthelmintic praziquantel (PZQ) in barbel infected with metacercariae of Diplostomum spathaceum and adult Pomphorhynchus laevis, and assessed antioxidant biomarkers and the lipid peroxidation response in juvenile barbel post-treatment. The estimated 96-hr LC50 of PZQ was 28.6 mg/L. For evaluation of efficacy, barbel naturally infected with D. spathaceum were exposed to a 10 and 20 mg/L PZQ 4-day bath treatment. Both concentrations were 100% effective against D. spathaceum and significantly (p < .01) affected the activity of catalase, superoxide dismutase, glutathione reductase and glutathione-S-transferase as well as levels of reduced glutathione in liver and muscle. The efficacy of orally administered PZQ was assessed in adult barbel naturally infected with P. laevis. Fish were administered 10, 30 and 50 mg/kg of body weight and examined via gut dissection after 6 days. The 50 mg/kg dose significantly decreased the intensity of infection. Praziquantel is a feasible bath treatment for barbel infected with D. spathaceum and has potential for oral treatment of broodfish infected with P. laevis.

Nanoemulsion from essential oil of Pterodon emarginatus (Fabaceae) shows in vitro efficacy against monogeneans of Colossoma macropomum (Pisces: Serrasalmidae)

Outbreaks of diseases pose a major threat to sustainable aquaculture development worldwide. Application of herbal products to combat parasitic diseases provides an alternative approach for sustainable aquaculture. This study investigated the in vitro antiparasitic effects of an oil-in-water nanoemulsion prepared using the essential oil from Pterodon emarginatus, against monogeneans infesting Colossoma macropomum. Gill arches from C. macropomum (47.6 ± 14.5 g and 13.5 ± 1.4 cm) that were naturally parasitized by Anacanthorus spathulatus, Notozothecium janauachensis and Mymarothecium boegeri were immersed in different dispersions of the P. emarginatus nanoemulsions (0, 50, 100, 200, 400 and 600 mg/L). The major compounds presented in the essential oil of P. emarginatus were β-elemene, β-caryophyllene and α-humulene. Characterization of these nanoemulsions showed that they have a small mean droplet size and low polydispersity index, which is concordant with stable systems. In this in vitro trial, the P. emarginatus nanoemulsion concentrations of 100, 200, 400 and 600 mg/L presented 100% helminthic efficacy against monogeneans of the gills of C. macropomum. The highest two concentrations used (400 and 600 mg/L) were seen to immobilize the parasites after only 15 min. Therefore, it would be worthwhile testing these concentrations in therapeutic baths against monogeneans of C. macropomum.

[Investigation of natural foci of Metorchis taiwanensis in Wuhu area]

OBJECTIVE

To explore the existence of natural foci of Metorchis taiwanensis in Wuhu area.

METHODS

The ecological environment and daily living habits of residents were investigated; the eggs of M. taiwanensis in the dung of ducks in local region were collected by the sedimentation method; the Parafossarulus striatulus were disposed by the tabletting method and examined with a microscope to isolate the rediae and cercariae; the metacercariae in Pseudorasbora parve were isolated by using the homogenation. The duckling were fed directively by metacercaria of M. taiwanensis or by P. parve infected with metacercaria of M. taiwanensis for artificial infection, and M. taiwanensis were separated from ducks by dissecting the gallbladder and bile ducts.

RESULTS

The survey revealed that there were staggered lake river, rich vegetation, overgrown shrubs, aquatic plants, rich variety of freshwater snails and fish resources in Wuhu area, and it was home to a variety of waterfowl and duck. The average detection rate of eggs of M. tanwanensis in stool of ducks was 33.33% (10/30); that of rediae and cercarriae in P. striatulus was 1.17%(7/600); and that of encysted metacercaria in P. parve was 13.33% (8/60); the number of M. taiwanensis detected in the infected ducklings in 3 groups were 31, 8 and 0, respectively, the natural M. tanwanensis infection rate of ducks was 23.33% (7/30).

CONCLUSION

We have confirmed the existence of natural foci of M. tanwanensis in Wuhu area.

Habitat of in vivo transformation influences the levels of free radical scavengers in Clinostomum complanatum: implications for free radical scavenger based vaccines against trematode infections

Background

Since free radical scavengers of parasite origin like glutathione-S-transferase and superoxide dismutase are being explored as prospective vaccine targets, availability of these molecules within the parasite infecting different hosts as well as different sites of infection is of considerable importance. Using Clinostomum complanatum, as a model helminth parasite, we analysed the effects of habitat of in vivo transformation on free radical scavengers of this trematode parasite.

Methods

Using three different animal models for in vivo transformation and markedly different sites of infection, progenetic metacercaria of C. complanatum were transformed to adult ovigerous worms. Whole worm homogenates were used to estimate the levels of lipid peroxidation, a marker of oxidative stress and free radical scavengers.

Results

Site of in vivo transformation was found to drastically affect the levels of free radical scavengers in this model trematode parasite. It was observed that oxygen availability at the site of infection probably influences levels of free radical scavengers in trematode parasites.

Conclusion

This is the first report showing that habitat of in vivo transformation affects levels of free radical scavengers in trematode parasites. Since free radical scavengers are prospective vaccine targets and parasite infection at ectopic sites is common, we propose that infections at different sites, may respond differently to free radical scavenger based vaccines.

[The prevalence and prevention of alariosis]

High Alaria alata infestation rates were found in the animals of the Kirov Region. The functioning of alariosis foci is maintained by a wide range of primary and intermediate hosts. The purpose of this article was to evaluate the activity of disinfectants against A. alata trematode eggs and their effects on soil microbiocenoses.

Models of macroparasitic infections in domestic ruminants: a conceptual review and critique

A mathematical model is just a means of representing and manipulating something that would not otherwise be accessible. Decision theorists argue that a right decision is one that makes the best use of the available information and using mathematical models of infectious and parasitic disease can help make sure the decision-makers do just that. Seen in this light, models are simply aids to thought--and thus, by definition, good models are useful. This paper deals with the history of mathematical models of parasitic infections of domestic ruminants. It is argued that the early simple forecasting models were very successful, and, although the more complicated models that were constructed to improve the resolution of the forecasts were mostly failures, the experience gained generated a slew of useful, robust models that are still valuable decision-making tools.

Snails and trematode infection after Indian Ocean tsunami in Phang-Nga Province, southern Thailand

The tsunami and non-tsunami affected areas of Takua Pa District, Phang-Nga Province were investigated for fresh- and brackish-water snails that transmit human parasitic diseases during 2006 and 2007. Among 46 snail species found, 17 species of 8 families were freshwater snails, 28 species of another 7 families were brackish-water snails, and 1 species was a land snail. Of these species, 11 freshwater snails, 4 brackish-water snails and 1 land snail were of medical importance. The fresh-water snails were Pomacea canaliculata, Pila angelica, P. gracilis, P. polita, Filopaludina (S.) martensi, F. (F.) s. polygramma, Melanoides tuberculata, Indoplanorbis exuxtus, Radix rubiginosa, Helicorbis umbilicalis, Gyraulus convexiusculus. Four brackish-water snails were Cerithidea cingulata, C. djadjarensis, C. alata, Sermyla riqueti and Achatina fulica was the land snail. I. exutus, M. tuberculata and F. (F.) s. polygramma harbored Xiphidio, Microcercus, Furocercus, Echinostome cercariae, and cercaria without eyespots or tail with hair. Three species of brackish-water snails, Cerithidia cingulata, C. djadjariensis, and C. alata presented with 6 types of trematode cercariae and rediae. Knowledge of medically important snails and their parasitic diseases, and prevention were given to Takua Pa people by poster, pamphlets and broadcasting through community radio.

[Gastrodiscoidosis is a dangerous zoonosis]

The paper gives data on the helminthiasis--gastrodiscoidosis, a zoonotic disease caused by the trematode Gastrodiscoides hominis (Lewis et McConnall, 1876) parasitizing in the animal and human intestine in the endemic foci of both Russia and foreign countries. It also presents information on the morphology of the helminth, the biological cycle of development of the parasite and its habitat and spread, as well as its induced abnormalities and on the method of diagnosing of the disease.

Human waterborne trematode and protozoan infections

Waterborne trematode and protozoan infections inflict considerable morbidity on healthy, i.e., immunocompetent people, and may cause life-threatening diseases among immunocompromised and immunosuppressed populations. These infections are common, easily transmissible, and maintain a worldwide distribution, although waterborne trematode infections remain predominantly confined to the developing countries. Waterborne transmission of trematodes is enhanced by cultural practices of eating raw or inadequately cooked food, socio-economical factors, and wide zoonotic and sylvatic reservoirs of these helminths. Waterborne protozoan infections remain common in both developed and developing countries (although better statistics exist for developed countries), and their transmission is facilitated via contacts with recreational and surface waters, or via consumption of contaminated drinking water. The transmissive stages of human protozoan parasites are small, shed in large numbers in feces of infected people or animals, resistant to environmental stressors while in the environment, and few are (e.g., Cryptosporidium oocysts) able to resist standard disinfection applied to drinking water.

The biology of gyrodactylid monogeneans: the "Russian-doll killers"

This article reviews the history of gyrodactylid research focussing on the unique anatomy, behaviour, ecology and evolution of the viviparous forms while identifying gaps in our knowledge and directions for future research. We provide the first summary of research on the oviparous gyrodactylids from South American catfish, and highlight the plesiomorphic characters shared by gyrodactylids and other primitive monogeneans. Of these, the most important are the crawling, unciliated larva and the spike sensilla of the cephalic lobes. These characters allow gyrodactylids to transfer between hosts at any stage of the life cycle, without a specific transmission stage. We emphasise the importance of progenesis in shaping the evolution of the viviparous genera and discuss the relative extent of progenesis in the different genera. The validity of the familial classification is discussed and we conclude that the most significant division within the family is between the oviparous and the viviparous genera. The older divisions into Isancistrinae and Polyclithrinae should be allowed to lapse. We discuss approaches to the taxonomy of gyrodactylids, and we emphasise the importance of adequate morphological and molecular data in new descriptions. Host specificity patterns in gyrodactylids are discussed extensively and we note the importance of host shifts, revealed by molecular data, in the evolution of gyrodactylids. To date, the most closely related gyrodactylids have not been found on closely related hosts, demonstrating the importance of host shifts in their evolution. The most closely related species pair is that of G. salaris and G. thymalli, and we provide an account of the patterns of evolution taking place in different mitochondrial clades of this species complex. The host specificity of these clades is reviewed, demonstrating that, although each clade has its preferred host, there is a range of specificity to different salmonids, providing opportunities for complex patterns of survival and interbreeding in Scandinavia. At the same time, we identify trends in systematics and phylogeny relevant to the G. salaris epidemics on Atlantic salmon in Norway, which can be applied more generally to parasite epidemiology and evolution. Although much of gyrodactylid research in the last 30 years has been directed towards salmonid parasites, there is great potential in using other experimental systems, such as the gyrodactylids of poeciliids and sticklebacks. We also highlight the role of glacial lakes and modified river systems during the ice ages in gyrodactylid speciation, and suggest that salmon infecting clades of G. salaris first arose from G. thymalli in such lakes, but failed to spread fully across Scandinavia before further dispersal was ended by rising sea levels. This dispersal has been continued by human activity, leading to the appearance of G. salaris as a pathogen in Norway. We review the history and current status of the epidemic, and current strategies for elimination of the parasite from Norway. Finally, we consider opportunities for further spread of the parasite within and beyond Europe.

Effect of niclosamide on the tegumental surface of Haplorchis taichui using scanning electron microscopy

The effect of niclosamide on the tegument of adult Haplorchis taichui (Trematoda: Heterophyidae) exposed in vitro was observed by scanning electron microscope. Adult worms were incubated in Tyrode's solution containing 0.01, 0.1, 1.0, and 10 microg ml(-1) of niclosamide for 30 min, 1, 6, 12 and 24 h. Control groups were incubated in Tyrode's solution without niclosamide and worms remained active until 24 h. In 0.01 microg ml(-1) of niclosamide, worms showed slightly active movements up to 1 h after incubation, while in 0.1 microg ml(-1) solution a few worms showed only slightly active movements after 30 min. Tegumental changes were determined by scanning electron microscopy. Swelling and blebbing of the tegument were observed on both ventral and dorsal sides. After longer periods, extensive swelling and blebbing of the tegument became more severe and there was a loss of the apical plasma membrane in some regions. Empty spine sockets occurred, and small perforations penetrated the basal lamina, followed by some lesions. Destruction of both surfaces was more pronounced on the posterior compared with the anterior regions.

The pathogenic helminth parasites of eels

Although 63 and 55 species of helminths have been reported from each species of Atlantic eel and from 29 to 19 for each species of Pacific eel only the monogeneans Pseudodactylogyrus bini and P. anguillae and the nematode Anguillicola crassus, originally specific to species of Pacific eels, can be considered serious pathogens. None of the three are normally pathogenic to their preferred natural eel host species in the wild. Pseudodactylogyrus spp. only cause serious local gill damage when present on a host in large numbers under optimal conditions that facilitate transmission. This is the case in eel aquaculture, where infections can be controlled by drugs. Anguillicola crassus is only pathogenic to Anguilla anguilla and A. rostrata when Atlantic eels are introduced to the far east or when the parasites have been introduced to Europe. Here the parasite life cycle differs in that A. crassus can infect a wide range of intermediate hosts, employ paratenic hosts and survive as larvae for months in the swimbladder wall. This makes it an excellent colonizer. Its major pathogenic effects on eels result from haemorrhaging in, and thickening of, the swimbladder wall. It reduces the oxygen concentration in the swimbladder, reducing its ability to function as a hydrostatic organ, and increases the stress response of eels. In shallow lakes at warm temperatures this can result in mass mortalities. It is also feared that the parasite affects the ability of eels to migrate to the Sargasso Sea and so contributes to the decline in eel populations. Control by drug treatment is possible in culture, but not in the wild.

Cream formulations protecting against cercarial dermatitis by Trichobilharzia

Dermatitis caused by penetrating bird schistosome cercariae is an emerging global public health problem. Infections may be prevented by the use of topical formulations that inhibit cercarial skin penetration. We evaluated nine water resistant formulations by exposing treated arms of volunteers to Trichobilharzia szidati cercariae. Six formulations protected from cercarial invasion. However, after immersion of the treated skin in water (2 x 20 min), only two formulations offered full protection: (1) Safe Sea, a cream protecting against jelly fish, (2) niclosamide in water resistant sun protecting cream formulations at concentrations as low as 0.05%. In an in vitro system Safe Sea and a 0.1% niclosamide formulation caused a high damage rate in T. szidati (92% and 99% after 5 min; only niclosamide with lethal effect) but not in Schistosoma mansoni (1% and 72%; both formulations with lethal effect). However, a 1% niclosamide formulation damaged S. mansoni sufficiently (100% after 5 min) and might offer full penetration protection.

Immobilization antibodies of tiger puffer Takifugu rubripes induced by i.p. injection against monogenean Heterobothrium okamotoi oncomiracidia do not prevent the infection

We examined whether infection by the monogenean Heterobothrium okamotoi induces production of specific antibodies against oncomiracidia and their cilia, larvae on the gills, and adults on the branchial cavity wall of tiger puffer Takifugu rubripes. We also investigated whether specific antibody production participates in acquired protection against H. okamotoi. Sera from persistently infected fish immobilized H. okamotoi oncomiracidia 89 days after exposure and antibody levels (measured by enzyme-linked immunosorbent assays) in the sera against oncomiracidia and their cilia increased compared with sera from control (naïve) fish. Antibody levels in these sera against the larvae and adult stages did not increase. The number of H. okamotoi on persistently infected fish was significantly lower than for control fish (P<0.05) when persistently infected fish and control fish were exposed to oncomiracidia in the same tank. Thus tiger puffer produced specific antibodies against oncomiracidia and their cilia, and acquired partial protection against H. okamotoi. Intraperitoneal injection of proteins of sonicated oncomiracidia or their cilia with an adjuvant also produced oncomiracidium agglutination antibodies in sera from tiger puffer; the antibody levels in these sera against oncomiracidia and their cilia increased compared with sera from control fish (injection of BSA with an adjuvant) at 14, 44, and 75 days after the booster immunization. However, in a parasite challenge at 54-58 days after the booster immunization, the infection levels of fish immunized with parasites of sonicated oncomiracidia or their cilia were the same as the control fish. Western blot showed that sera from persistently infected fish and fish immunized with sonicated oncomiracidia or their cilia recognized similar antigenic bands, suggesting that tiger puffer tends to react against these antigens compared with other antigens. These results indicated that specific antibodies against these cilia and oncomiracidia induced by i.p. injection do not prevent H. okamotoi infection.

[Ecological bases of the combination of natural foci of Trematoda infections in the floodplain-river ecosystem of the Konda River. Communication 2. Host population-combined foci of Trematoda infections]

In the context of the present-day teaching of parasitocenoses and the proposition that the pathogen's population is the only compulsory and specific component of a natural focus, the author discloses the ecological bases of the combination of natural foci of opisthorchiasis and methorchiasis (M. bilis), methorchiasis (M. bilis) and methorchiasis (M. xanthosomus). These foci are host population-combined. While analyzing the combination of foci, it is expedient to consider them in pairs since this provides a way of identifying only the combination bases that are unique to these foci. The parasitic systems of flukes, the parasitocenoses of co-acting parasitic systems of "twin types", the structure of foci, the species-specific composition of ecosystems, and the ecological relations of the Opisthorchis fluke hosts act as the biotic bases of a combination of foci of Trematoda infections. By coinciding, the multihost hemipopulations of parasites and the susceptibility of host populations predetermine the combination of Opisthorchis fluke foci. The susceptibility of hosts, the multihost pattern of Opisthorchis flukes, the identity of parasitic systems, and the common mechanism of pathogen transmission act as the epizootic bases of a combination of invasion foci. The morphological structure and hydrological regime of a landscape act as the abiotic bases of a combination of foci. The hydrological regime is by its nature a universal mechanism of pathogen transmission. The foci of Opisthorchis flukes at the level of parasitocenosis of metacercarium populations and fish populations in the Konda River ecosystem are combined in the age groups of only carp (Cyprinidae) underyearlings and yearlings. The abiotic, biotic, and epizootic bases of a combination of natural foci of Opisthorchis flukes are, in the aggregate, the ecological bases of a combination of foci.

Antiparasitic effect of calcium and magnesium ion-free buffer treatments against a common monogenean Neobenedenia girellae

This study investigated a new effective method for controlling the capsalid monogenean Neobenedenia girellae. We examined in vitro and in vivo the effect on the percentage survival of N. girellae in buffers containing different metallic ions. Decreased survival was observed in buffer solutions lacking two ions. In particular, the percentage survival of N. girellae was significantly decreased after 10 min exposure to buffer containing neither Ca(2+) nor Mg(2+). Transmission electron microscopic observations showed that treatment with this buffer disrupted intercellular junctions. This significant effect on percentage survival of N. girellae using Ca(2+)/Mg(2+)-free buffer was confirmed in an in vivo assay. Ca(2+)/Mg(2+)-free buffer had no effect on the condition of the host, spotted halibut Verasper variegates (Pleuronectidae). These results suggest that treatment with Ca(2+)/Mg(2+)-free buffer is a new effective control method, which could replace existing control methods.

[Infection rates of mollusks and fishes with the larval stages of Trematoda of the family Opisthorchidae in the Yaroslavl Region]

The paper presents the results of a study of the infection rates of mollusks of the family Bythiniidae (Bithynia tentaculata, Opisthorchophorus troscheli) and fishes with the larval stages of Opisthorchidae in different districts of the Yaroslavl Region. A total of 3,708 mollusks from the water reservoirs of different types (small rivers, lakes, ponds, and canals) were examined in the Nekouzsky, Yaroslavsky, and Pereslavsky Districts of the Yaroslavl Region in 1989 to 1999. Opisthorchis cercarias were found in 7 of the 10 studied water reservoirs. The total rate of infection of mollusks with all types with Trematoda partenitas and cercarias in different water reservoirs averaged 18.5 +/- 3.5% for Bithynia tentaculata and 19.1 +/- 4.1% for Opisthorchophorus troscheli. Trematoda metacercarias of the family Opisthorchidae were revealed in roaches, ides, verkhovkas, silver breams, and chubs in four different water reservoirs (small rivers and a lake) of the region. Pseudamphistomum truncates, Opisthorchis felineus, and Metorchis xanthosomus were present among the metacercarias found. It is concluded that there may be sporadic cases of Opisthorchis infection in wild and domestic animals and humans in the Yaroslavl Region.

Radioresistance of Anisakis simplex Third-Stage Larvae and the Possible Role of Superoxide Dismutase

The radioresistance of Anisakis simplex third-stage larvae and the possible role of sublethal radiation on superoxide dismutase (SOD) were investigated. Larvae were isolated from the viscera of the sea eel Anago anago; irradiated with 10, 100, 200, 500, or 1,000 Gy; and then given orally to rats. Worms were recovered at 16 hr postinoculation. Most larvae were found to have invaded the gastric wall, omentum, and abdominal cavity, suggesting that their viability and infectivity were not controlled by irradiation with the doses used. To determine the relationship between SOD activities in parasites and their radiosensitivities, the larvae of A. simplex and the metacercariae of Neodiplostomum seoulense (a radiosensitive control) were irradiated with 0, 30, 100, or 500 Gy, and parasite SOD levels were measured. In nonirradiated A. simplex larvae, the average SOD level was 38.9 U/mg, and this increased to 51.3 U/mg at 500 Gy. However, at all radiation doses applied, SOD activities of N. seoulense metacercariae were significantly (P < 0.05) lower than those of A. simplex larvae. Our results demonstrate that A. simplex third-stage larvae are radioresistant, and suggest that SOD plays a role in this radioresistance.

[To the detection of the trematode mesocercariae Alaria alata in the blood of domestic dogs and cats]

Blood samples taken from 2128 domestic dogs and 543 cats in Moscow and the Moscow Region were studied by the concentration method using distilled water. The studies revealed the infestation with the mesocercariae Alaria alata in 1.9% of the dogs and in 1.I1% of the cats. A method was first developed for the life-time diagnosis of alariasis in definitive and reserve hosts.

Epidemiological studies of amphistome infections in cattle in the highveld and lowveld communal grazing areas of Zimbabwe

During the period between January 1999 and December 2000, the distribution and seasonal patterns of amphistome infections in cattle in the highveld and lowveld communal grazing areas of Zimbabwe were determined through monthly coprological examination. Cattle faecal samples were collected from 12 and nine dipping sites in the highveld and lowveld communal grazing areas, respectively. Patterns of distribution and seasonal fluctuations of intermediate host-snail populations and the climatic factors influencing the distribution were also determined by sampling at monthly intervals for a period of 24 months (November 1998 to October 2000) in six dams and six streams in the highveld and in nine dams in the lowveld communal grazing areas. Each site was sampled for relative snail density and the vegetation cover and type, physical and chemical properties of water, and mean monthly rainfall and temperature were recorded. Aquatic vegetation and grass samples 0-1 m from the edges of the snail habitats were collected monthly to determine the presence or absence of amphistome metacercariae. Snails collected at the same time were individually checked for the emergence of larval stages of amphistomes. A total of 16,264 (calves 5418, weaners 5461 and adults 5385) faecal samples were collected during the entire period of the study and 4790 (29.5%) of the samples were positive for amphistome eggs. For both regions the number of animals positive for amphistome eggs differed significantly between the 2 years, with the second year having a significantly higher prevalence (P < 0.01) than the first year. Significantly higher prevalences were found in the highveld compared to the lowveld (P < 0.001), for adult cattle than calves (P < 0.01), and in the wet over the dry season (P < 0.01). Faecal egg output peaked from October to March in both years of the study. Bulinus tropicus, Bulinus forskalii and Biomphalaria pfeifferi were recorded from the study sites. The main intermediate host for amphistomes was B. tropicus with a prevalence of infection of 8.5%. However, amphistome cercariae were also recorded in Biom. pfeifferi and B. forskalii. Amphistome cercariae were recorded from both the highveld and lowveld areas with peak prevalence during the post-rainy season (March to May). Metacercariae were found on herbage from the fringes of the snail habitats between February and August, with most of the metacercariae concentrated on herbage 0-1 m from the edges of the habitats. Based on the epidemiological findings a control programme was devised. From this study, large burdens of immature flukes could be expected in cattle during the dry months. Since adult cattle would be resistant to the pathogenic effects of the migrating immature amphistomes the target for control would be young animals being exposed to the infection for the first time. Therefore, the first anthelmintic treatment can be administered in calves in mid June when maximum migration of immature amphistomes starting 3-4 weeks after infection in the early dry season would be expected. A second treatment could be given in late July or early August to remove potentially dangerous burdens of immature flukes acquired later in the dry season. Where resources permit, another strategy would be to treat against the mature flukes in March or April in order to reduce the number of eggs deposited on pastures and the opportunity for infection of the intermediate host snails. To reduce cercarial shedding by the intermediate host snails molluscicides can also be applied during the peak transmission periods (April/May and August/September).

Epidemiology and control of trematode infections in cattle in Zimbabwe: a review

In this paper the main epidemiological aspects of the major domestic ruminant trematode infections in Zimbabwe are reviewed and discussed with regard to the available options for control. Seasonal occurrence of amphistomes, Fasciola gigantica and Schistosoma mattheei are considered both in the definitive and intermediate hosts. The regional distribution of the trematodes is reviewed in relation to the distribution patterns of their snail intermediate hosts. Based on the epidemiological features of the trematodes, practical control measures are suggested.

In vitro effects of aqueous extract from Artocarpus takoocha Roxb on tegumental surface of Haplorchis taichui

The tegumental surface of the adult intestinal fluke, Haplorchis taichui, after in vitro incubation in Tyrode's solution containing 25, 50, 75 and 100% of aqueous extract of Artocarpus takoocha for 1, 6, 12 and 24 hours, was monitored by Scanning Electron Microscopy (SEM). The effect on worm motility was studied in contemporizing period. The adult worms stopped their motility immediately after being incubated with 25, 50, 75 and 100% of aqueous extract of A. takoocha. Observed under a light microscope, most worms showed bubbling from the oral sucker, ventral sucker and excretory pore. After exposure to the solution, SEM observation showed large blebs protruding from the oral sucker, ventral sucker and excretory pore. The surface damage was composed of numerous small blebs, followed later by the disruption of tegument. The severity of damage increased due to the increasing of aqueous extract of A. takoocha and incubation periods.

Defeating diplostomoid dangers in USA catfish aquaculture

Diplostomoid digenean metacercariae have caused widescale mortalities of channel catfish, Ictalurus punctatus (Rafinesque), at aquaculture farms in Louisiana, Mississippi and Arkansas, USA. Originally, based on a tentative diagnosis, the industry considered the primary harmful agent to be an introduced species from Europe, Bolbophorus confusus (Krause, 1914), frequently reported from the American white pelican, Pelecanus erythrorhynchos Gmelin. Our group has now shown, using ITS 1-2 plus three more-conservative gene fragments, that two sympatric species of Bolbophorus exist in the American white pelican. One, B. damnificus Overstreet et Curran, 2002, infects the musculature of catfish, and the other, probably not B. confusus, does not infect catfish. However, at least four other pathogenic diplostomoids and a clinostomoid infect the catfish, and they use at least four different snail hosts, including the planorbids Planorbella trivolvis (Say) and Gyraulus parvus (Say), the physid Physella gyrina (Say) and a lymnaeid. Two metacercariae, B. damnificus and Bursacetabulus pelecanus Dronen, Tehrany et Wardle, 1999, infect the catfish and mature in the pelican; two others, Austrodiplostomum compactum (Lutz, 1928) and Hysteromorpha cf. triloba (Rudolphi, 1819), mature in cormorants; one, Diplostomum sp., matures in seagulls and at least one, Clinostomum marginatum (Rudolphi, 1819), matures in herons, egrets and other wading birds. Consequently, management of catfish ponds relative to digenean infections requires considerable biological information on the fish, bird, and snail hosts as well as the parasites.

Parasite resistance and avoidance behaviour in preventing eye fluke infections in fish

This paper examines the efficiency of acquired resistance in protecting the fish host, rainbow trout (Oncorhynchus mykiss), against the trematode parasite Diplostomum spathaceum, and the hypothesis that fish recognize areas where infective stages are aggregated and show avoidance behaviour. We found that when fish with a low level of infection were held in restricted cages in natural conditions they became infected and developed cataracts as a result of this infection. This suggests that acquired resistance is insufficient in protecting fish against the parasite or the deleterious effects of infection in conditions where fish could not avoid the parasite. Behavioural experiments in the laboratory showed that fish reacted to the parasite cercariae by avoiding the infection source, which decreased the rate of parasite establishment. We conclude that by using a combination of behavioural avoidance and physiological resistance, fish could defend against the parasite more effectively.

Qualitative risk assessment of routes of transmission of the exotic fish parasite Gyrodactylus salaris between river catchments in England and Wales

Gyrodactylus salaris is a freshwater, monogenean ecto-parasite of Atlantic-salmon. Infection of its natural host, the Baltic strain of Atlantic-salmon, is inapparent. G. salaris also can infect rainbow-trout (Oncorhynchus mykiss) permanently, and cause infection of < or =50 days in several other species. It is only on Atlantic stocks of Atlantic-salmon (Salmo salar) that the parasite multiplies unchecked by an immune response, causes death in juveniles and dramatic reductions in wild populations. In Norway, the parasite has been introduced into 45 rivers, resulting in reductions in Atlantic-salmon stocks of up to 98%. It is probably the most-important exotic fish-disease threat to the UK. We used risk analysis to assess the most-important routes of spread for G. salaris between rivers in England and Wales. The movement of live rainbow-trout was identified as the most-important route of transmission; this route is likely to lead rapidly to the wide geographic spread of the parasite. The movement of other species of fish (especially from sites holding rainbow-trout) is also an important risk. Other routes of spread (including mechanical transmission on farm equipment and vehicles, angling equipment, canoes, etc.) might allow limited local spread (mainly to neighbouring rivers).

The effect of various metals on Gyrodactylus salaris (Platyhelminthes, Monogenea) infections in Atlantic salmon (Salmo salar)

Atlantic salmon (Salmo salar) parr (age 0+), infected by the ectoparasite Gyrodactylus salaris, were exposed to aqueous aluminium (Al), copper (Cu), zinc (Zn), iron (Fe) and manganese (Mn), at 4 different concentrations. There was a negative correlation between G. salaris infections and metal concentrations in both Zn- and Al-exposed salmon. In the Zn-experiment, all 4 concentrations tested caused a decrease in the G. salaris infections, while in the Al-experiment the G. salaris infection did not decline at the lowest concentration. The number of G. salaris increased continuously during the experiments in all control groups, and in all groups exposed to Cu, Fe and Mn. At the highest concentration, however, copper seemed to impair the growth of G. salaris infection. The results show that aqueous Al and Zn are environmental factors of importance controlling the distribution and abundance of the pathogen G. salaris. Other pollutants might also have an influence on the occurrence of G. salaris. Finally, the results demonstrate that aqueous Al and Zn have a stronger effect on the parasite than on the salmonid host, suggesting that both metals may be used as a pesticide to control ectoparasites such as G. salaris.

The food-borne trematode zoonoses of Vietnam

During the past couple of decades, numerous surveys for the occurrence and distribution of food-borne trematodes in Vietnam have been carried out. However, the majority of the data obtained have not been published in international journals, and therefore, the seriousness of these trematode parasitic zoonoses in the country has not been fully appreciated. To correct this, over 40 Vietnamese language reports and local publications were translated and organized as a status review for an international audience. The results show that such serious trematode zoonoses as clonorchiasis, opisthorchiasis, paragonimiasis and fascioliasis are common in many regions of Vietnam, and, in the case of fascioliasis and paragonimiasis, are increasing. Data on the species of intermediate hosts involved in the transmission of these zoonoses, and the effect of host sex and age on infection frequencies are presented, along with findings on food preference/behavior investigations. Finally, the authors present recommendations for further research to provide a more comprehensive picture of the status of these zoonoses, and to obtain the risk assessment information needed to design prevention and control programs.

Immunization of rainbow trout Oncorhynchus mykiss against Discocotyle sagiffata (Monogenea)

Rainbow trout Oncorhynchus mykiss were injected intraperitoneally with 2 different Discocotyle sagittata extracts dissolved in PBS and subsequently exposed to controlled infection. Immunization resulted in significantly reduced (p < 0.0001) worm intensities in > 50% of vaccinated fish (response arbitrarily defined as parasite burdens < mean control intensity - 1 SD), irrespective of the immunization regime (different parasite extracts, dosing and application schedules) employed. The protective effect of worm extract applied in Freund's complete adjuvant (FCA) did not differ significantly from extract given in PBS. Vaccination with embryonated parasite eggs extract and with FCA alone did not result in partial immunity, suggesting the observed protective effect is specific. Immunized fish had significantly higher specific antibody titres at the time of dissection (as determined by ELISA) than both naive and control fish. Overall, a significant negative correlation was found between antibody titres and worm burdens, suggesting immunoglobulins are implicated in mediating partial immunity. Western blot tests indicated the 2 different worm extracts used to immunize fish share antigens, but each one primarily induced recognition of a distinct band (30 and 38 kDa). Immunization seems to promote a shift between 2 equilibria, rather than progressively increasing protection. This would explain why boosting did not increase immunity, and why 2 different extracts primarily inducing recognition of 2 distinct antigens provide similar degrees of protection. Although several other non-specific and cellular factors are likely to be involved in controlling parasite numbers, it cannot be excluded that antibodies could be involved in mediating the observed partial immunity.

Gymnophalloides seoi: a new human intestinal trematode

A new minute intestinal fluke, recovered from a Korean woman suffering from acute pancreatitis and gastrointestinal difficulties, was described as Gymnophalloides seoi. The coastal village where the patient resided was found to be a highly endemic area for this fluke, and 24 other endemic areas have been found. This fluke infection is now recognized as an emerging parasitic disease. Its second intermediate host is the oyster, and migrating birds are a natural definitive host. This new trematode might exist in other countries.

Intestinal heterophyidiasis: an emerging food-borne parasitic zoonosis in southern Philippines

Heterophyidiasis is an infection of the small bowel by minute intestinal flukes of the genus Heterophyes or related members of the family Heterophyidae. To provide a better understanding of this rarely reported condition, this study attempted to determine the infection rate, intensity of infection as well as the clinical spectrum of heterophyid infection. A stool survey was conducted in barangay San Isidro, Monkayo, Compostela Valley. Thirty-six percent of patients with history of bowel disturbance (abdominal discomfort/pain and/or diarrhea) in the past 4 weeks were found to have heterophyidiasis. All age groups were infected, with the youngest patient being 1 year 7 months of age, while the oldest patient was 73 years of age. Prevalence was highest in the 15 to 30 years old group at 55.3%. The most common clinical manifestations of heterophyidiasis were signs and symptoms of acid peptic or peptic ulcer disease. Early diagnosis and treatment are important to ensure prompt resolution of heterophyid infection, hence, decreased morbidity and decreased chances for complications like heart or brain involvement. Proficiency of laboratory staff should be enhanced to ensure accurate diagnosis which will then make appropriate treatment possible.

Detection of bird schistosomes in lakes by PCR and filter-hybridization

Many lakes around the world are contaminated with bird schistosome cercariae, which penetrate into human skin, causing an itching dermatitis called "swimmers' itch." Bathers could be forewarned from exposure to the larvae and ecological examinations could be performed, when a sensitive method to detect the parasites in aquatic systems, where lots of organisms hinder microscopic examinations, would be available. For this purpose we cloned, sequenced, and analyzed a 396 bp tandem repeated DNA sequence from Trichobilharzia ocellata (ToSau3A), and employed it for developing molecular detection assays. It hybridized with less than 100 pg DNA from different Trichobilharzia species (T. ocellata, Trichobilharzia franki, and Trichobilharzia regenti), but not with 10 ng DNA from other related or sympatric trematodes. A PCR assay, amplifying this sequence with the same specificity, detected 100 fg T. ocellata DNA, 1 cercaria in 0.5 g plankton, and 2 cercariae in 0.5 g host snail tissues.

Motility of minute intestinal fluke, Haplorchinae spp, metacercariae in fish dishes prepared by different uncooked methods

CONTEXT

Fish-borne trematode is a worldwide problem, with the number of people infected by liver flukes alone estimated at 21 million. In addition to the major liver flukes, Opisthorchis viverrini and Clonorchis sinensis, several types of intestinal flukes also use cyprinoid fish as the intermediate host. Traditional ingestion of undercooked and uncooked fish preparations is a major factor in pathogen acquisition.

OBJECTIVE

To examine the motility of Haplorchinae spp metacercariae in matched minnows using a number of preparation methods that mimic traditional practices.

DESIGN

Several freshly killed freshwater fish, Cyclocheilichthys armatus, were purchased from the local market and sent to the authors' laboratory to be examined under a stereoscopic microscope for active Haplorchinae spp metacercariae. A total of 10 fish were infected with many active metacercariae and used in the preparation of uncooked fish dishes. Five different raw fish dishes that mimic traditional meals (2 fishes/dish) were prepared using the following methods: 1) left to dry at room temperature; 2) frozen at -20 Celsius; 3) refrigerated at 4 Celsius; 4) marinated in saline (5% sodium chloride solution); and 5) marinated in 5% acetic acid solution to mimic traditional vinegar. The motility of the metacercariae in each of these dishes was examined under a stereoscopic microscope. Motility was estimated as the level of activity or degeneration of the metacercariae at the start of the experiment and then every 30 minutes until all the parasites appeared degenerate (and therefore, presumably not viable).

RESULTS

Degeneration of the parasites was slowed by cooling: degeneration of all metacercariae took approximately 5 hours in the refrigerated or frozen fish, compared with 3 hours in all other dishes left at room temperature.

CONCLUSION

The study substantiates the epidemiologic data demonstrating transmission of these types of pathogens despite freezing, pickling, and salting. Although the tradition of eating uncooked fish is deeply rooted in Thailand, constant efforts need to be made to discourage this practice.

The contribution of molecular biology to the development of vaccines against nematode and trematode parasites of domestic ruminants

Rapid developments in molecular biology have had an enormous impact on the prospects for the development of vaccines to control the major nematode and trematode infestations of livestock. Vaccine candidates are purified using conventional protein chemistry techniques but the limitations imposed by the scarcity of parasite material provide an insurmountable barrier for commercial vaccine production by this means. The ability to purify mRNA from different parasite life-cycle stages and to prepare cDNA expression libraries from it has proven central to the identification of immunogenic parasite proteins. Potentially, protective parasite antigens can now be produced in recombinant form in a variety of vectors and this represents a key breakthrough on the road to commercial vaccine production. The contribution of molecular biology to this process is discussed using several examples, particularly in vaccine development against the pathogenic abomasal nematode of sheep and goats, Haemonchus contortus, and the liver fluke of sheep and cattle, Fasciola hepatica. The difficulties of producing recombinant proteins in the correct form, with appropriate post-translational modification and conformation, are discussed as well as emerging means of antigen delivery including DNA vaccination. The opportunities offered by genome and expressed sequence tag analyses programmes for antigen targeting are discussed in association with developing microarray and proteomics technologies which offer the prospect of large scale, rapid antigen screening and identification.

[Role of gastropods in epidemiology of human parasitic diseases]

Pulmonate and prosobranch snails, being necessary hosts for parthenogenetic generations of digenetic trematodes, participate in transmission of all trematodoses important from medical point of view. Role of particular gastropods in epidemiology of these diseases is discussed in details. Invasion of land snails and slugs is mainly passive by eggs containing developed miracidia, while enter of these larvae into snails inhabiting fresh-water environments is usually active. Generations in the snail host between miracidia and cercariae vary considerable, depending upon the fluke species. Generally, the cercaria is produced by the sporocyst or the redia. Cercariae usually actively penetrate out of the infected snail and enter water. Leaving out of account cases of encystation of cercariae in external environment (liver flukes of the family Fasciolidae) and active penetration into the final host (blood flukes of the family Schistosomatidae), encystation of these larvae takes place in the second intermediate host. Cercariae of medically important flukes may develop into metacercariae in tissues of fish (liver flukes - Opisthorchis felineus, Clonorchis sinensis, intestinal flukes--Heterophyes heterophyes, Metagonimus yokogawai, Troglotrema salmincola), as well as in crustaceans (pulmonary flukes of the genus Paragonimus), insects (the bipathogenic liver fluke Dicrocoelium dendriticum and other species of the family Plagiorchiidae incidentally found in man), and gastropods (flukes of the family Echinostomatidae, among them perhaps the best known is the Oriental species--Echinostoma ilocanum). In prevention and control of human trematodoses, especially in endemic foci, elimination of snail hosts is a great importance. Using molluscides may be objectionable from the stadpoint of environmental modification through their toxicity to other organisms. Biological control of snail hosts is more attractive. It includes introduction and management of predators, parasites, or pathogens, and intramolluscan competition. Moreover, certain slugs and terrestrial snails participate in transmission of nematode larval stages, including species known as pathogenic for man - the strongylid nematodes of the genus Angiostrongylus). Such infections may be prevented by abstanding from eating raw or inadequately cooked molluscs in endemic areas.

Fasciolopsiasis: is it a controllable food-borne disease?

Fasciolopsiasis, endemic to the Orient and Southeast Asia, is a snail-transmitted, intestinal, food-borne parasitic zoonosis caused by a trematode, Fasciolopsis buski, which also infects farm pigs. Fasciolopsiasis remains a public health problem despite changes in eating habits, alterations in social and agricultural practices, health education, industrialization, and environmental alterations. The disease occurs focally and is most prevalent in school-age children. In foci of parasite transmission, the prevalence of infection in children ranges from 57% in mainland China to 25% in Taiwan and from 50% in Bangladesh and 60% in India to 10% in Thailand. Control programs implemented for food-borne zoonoses are not fully successful for fasciolopsiasis because of century-old traditions of eating raw aquatic plants and using untreated water. Fasciolopsiasis is aggravated by social and economic factors such as poverty, malnutrition, an explosively growing free-food market, a lack of sufficient food inspection and sanitation, other helminthiases, and declining economic conditions.

Recovery of avian schistosome cercariae from water using penetration stimulant matrix with an unsaturated fatty acid

Avian schistosome cercariae that emerge from aquatic snails can penetrate human skin causing cercarial dermatitis resulting in serious skin disease in sensitized and immunocompromised people. A trap developed for Schistosoma mansoni cercariae was tested for recovery of avian schistosome cercariae. A matrix with an unsaturated fatty acid, linoleic acid stimulates attachment and penetration of Trichobilharzia spp. cercariae, and the immobilized larvae can be subsequently visualized. The number of trapped cercariae exceeded by 3 to 7 times the number of larvae expected on the surface of the trap, based on their random distribution in the water. Recognition, attachment, and penetration of Trichobilharzia spp. cercariae led to injection of more secretory products into the stimulant matrix than by Schistosoma mansoni cercariae. This method can assist in the identification of waters infected with avian schistosome cercariae so that human exposure to these parasitic larvae can be minimized.

The use of statistical classifiers for the discrimination of species of the genus Gyrodactylus (Monogenea) parasitizing salmonids

This study applies flexible statistical methods to morphometric measurements obtained via light and scanning electron microscopy (SEM) to discriminate closely related species of Gyrodactylus parasitic on salmonids. For the first analysis, morphometric measurements taken from the opisthaptoral hooks and bars of 5 species of gyrodactylid were derived from images obtained by SEM and used to assess the prediction performance of 4 statistical methods (nearest neighbours; feed-forward neural network; projection pursuit regression and linear discriminant analysis). The performance of 2 methods, nearest neighbours and a feed-forward neural network provided perfect discrimination of G. salaris from 4 other species of Gyrodactylus when using measurements taken from only a single structure, the marginal hook. Data derived from images using light microscopy taken from the full complement of opisthaptoral hooks and bars were also tested and nearest neighbours and linear discriminant analysis gave perfect discrimination of G. salaris from G. derjavini Mikailov, 1975 and G. truttae Gläser, 1974. The nearest neighbours method had the least misclassifications and was therefore assessed further for the analysis of individual hooks. Five morphometric parameters from the marginal hook subset (total length, shaft length, sickle length, sickle proximal width and sickle distal width) gave near perfect discrimination of G. salaris. For perfect discrimination therefore, larger numbers of parameters are required at the light level than at the SEM level.

Immunization of cultured juvenile rockfish Sebastes schlegeli against Microcotyle sebastis (Monogenea)

To determine whether immunization with Microcotyle sebastis antigen could induce protection against the parasite's establishment, naive juvenile rockfish were immunized by injection or immersion with whole worm antigen of M. sebastis. The infestation intensities of immunized groups following a challenge (2 wk after boosting) with 5000 M. sebastis eyed-eggs were significantly lower than those of control groups, when determined 7 wk postinfection. The fish in the groups boosted with M. sebastis antigen showed stronger protection than unboosted groups. The control group injected with FCA only showed a significantly smaller number of worms than the control group, which was immersed in PBS containing seawater. The results strongly suggest that both specific and nonspecific immune factors participate in the protection of rockfish against M. sebastis establishment.

Fish- and shellfish--borne trematode infections in Canada

Food-borne trematode infections are endemic in various parts of the world, particularly Southeast Asia. Despite the high prevalence, morbidity and total costs of these infections, they remain poorly recognized by public health authorities and consumers. Factors such as poor sanitation and traditional methods of food preparation hasten the spread of food-borne trematode infections in endemic regions and must be carefully examined in order to develop effective control strategies. There is also a growing risk to consumers in non-endemic countries as a result of international trade. A considerable quantity of freshwater fish and shellfish is imported into Canada from endemic countries in Southeast Asia. Some of these products are imported fresh or processed in such a way that the infective metacercariae may not be destroyed. Further, current inspection procedures in Canada may not detect the presence of all parasites in imported fish products. Therefore, there may be a risk of infection if the fish or shellfish is consumed raw or lightly cooked. Many of the cases of infection in Canada involve recent immigrants from endemic regions who have become infected either before arriving or through the consumption of traditional or ethnic dishes prepared from imported products. International travel and the increasing availability and interest in ethnic foods also contribute to the risk of infection in all Canadians. In addition to these imported trematodes, a number of species are found in freshwater fishes and shellfish in North America and have also caused illness in humans. Although the prevalence of infection remains relatively low in Canada, the need for an increased general awareness of food-borne trematode infections and their causes is indicated.

The prevalence of cercariae from Stagnicola emarginata (Lymnaeidae) over 50 years in northern Michigan

Stagnicola emarginata were collected from 3 northern Michigan lakes and examined for larval trematode infections. The structure of the trematode community was then compared with 2 previous studies conducted on Douglas Lake in order to determine what changes had taken place over a period of more than 50 yr and to examine the possibility of using trematodes as bioindicators of environmental quality. Species richness was reduced by half (from 16 to 8 species) from the first study conducted in 1936, along with a decrease in the overall prevalence from 61% to 13%. No new species had colonized the lake, and the same 8 trematodes were also the only species found in 2 other lakes. The decline in prevalence was most severe in trematodes that used gulls as definitive hosts, whereas species that used ducks as hosts increased slightly. The decrease in species richness and prevalence of infection over time may reflect increasing human impacts on lakes. However, because of patchy host distributions, the use of larval trematodes as bioindicators of environmental change requires further investigation.

Vaccination against worm parasites of animals

The 1990s have seen the culmination of decades of painstaking research with the registration and launch of Tickgard (Hoechst), a recombinant vaccine against Boophilus microplus, and the provisional registration of a Taenia ovis vaccine. Research continues to hold promise for immunological control of Echinococcus, Fasciola, Haemonchus, Trichostrongylus and Ostertagia. Blood-sucking parasites (e.g. ticks and H. contortus) are susceptible to control by vaccines containing 'novel' or 'concealed' antigens where serum antibodies in blood meals attack targets in the gut. Antibodies also provide protection in taeniid models, whereas the protective response to be sought in Fasciola remains unclear. More problematic are formulations and delivery strategies to induce expulsion of gastrointestinal nematodes, using vaccines containing recombinant 'conventional' antigens. The use of computer models to simulate vaccine efficacy in worm control and challenges to the concept of 'hypo-responsiveness' of young lambs will encourage cautious optimism and lively debate as to the prospects for integrated worm control using parasite vaccines. This review covers the aspirations, current success and problems faced by researchers in the parasite arena.

New geographical approaches to control of some parasitic zoonoses

The advent of new technology for geographical representation and spatial analysis of databases from different sectors offers a new approach to planning and managing the control of tropical diseases. This article reviews the geographical and intersectoral aspects of the epidemiology and control of African trypanosomiasis, cutaneous and visceral leishmaniasis, Chagas disease, schistosomiasis, and foodborne trematode infections. The focal nature of their transmission, increasing recognition of the importance of animal reservoirs, and the need to understand environmental factors influencing their distribution are common to all these diseases. Geographical information systems (GIS) open a completely new perspective for intersectoral collaboration in adapting new technology to promote control of these diseases.

Vaccination against animal parasites

A decade of molecular parasitology is beginning to bear fruit, with the appearance of several new, highly effective, practical vaccines against parasitic diseases. Recombinant antigen vaccines have been developed against cestode, nematode, trematode, protozoan and arthropod parasites. Greatest progress has been made with veterinary vaccines, where the ability to test numerous vaccine formulations in challenge trials has allowed more rapid identification of host-protective antigens than is possible with many medically important parasites. Several quite different approaches to vaccine development have been successful. The traditional approach using live, attenuated parasites continues to provide effective vaccines against several protozoan and nematode parasites. Recombinant DNA technology, monoclonal antibody technology, protein chemistry and immunochemistry have played critical roles in the outstanding success which has been achieved over the last 5 years in the development of defined-antigen vaccines. Two approaches have been successful in research towards defined antigen vaccines against parasites: (1) the 'natural antigen' approach where immune responses are stimulated to parasite molecules which are normally antigenic, and possibly host-protective, in infected hosts; (2) the 'naive antigen' approach where parasite molecules which are not antigenic, or of very low antigenicity, in infected hosts are used to raise immune responses capable of killing the parasite. This review examines the successful approaches taken towards the development of effective anti-parasite vaccines and the vaccines which have been produced to date.