Echinostoma mekongi: Discovery of Its Metacercarial Stage in Snails, Filopaludina martensi cambodjensis, in Pursat Province, Cambodia.

Echinostoma mekongi was reported as a new species in 2020 based on specimens collected from humans in Kratie and Takeo Province, Cambodia. In the present study, its metacercarial stage has been discovered in Filopaludina martensi cambodjensis snails purchased from a local market nearby the Tonle Sap Lake, Pursat Province, Cambodia. The metacercariae were fed orally to an experimental hamster, and adult flukes were recovered at day 20 post-infection. They were morphologically examined using light and scanning electron microscopes and molecularly analyzed by sequencing of their mitochondrial cox1 and nad1 genes. A total of 115 metacercariae (1-8 per snail) were detected in 60 (60.0%) out of 100 Filopaludina snails examined. The metacercariae were round, 174 µm in average diameter (163-190 µm in range), having a thin cyst wall, a head collar armed with 37 collar spines, and characteristic excretory granules. The adult flukes were elongated, ventrally curved, 7.3 (6.4-8.2)×1.4 (1.1-1.7) mm in size, and equipped with 37 collar spines on the head collar (dorsal spines in 2 alternating rows), being consistent with E. mekongi. In phylogenetic analyses, the adult flukes showed 99.0-100% homology based on cox1 sequences and 98.9-99.7% homology based on nad1 sequences with E. mekongi. The results evidenced that F. martensi cambodjensis snails act as the second intermediate host of E. mekongi, and hamsters can be used as a suitable experimental definitive host. As local people favor to eat undercooked snails, these snails seem to be an important source of human infection with E. mekongi in Cambodia.

Digenean Holostephanus (Trematoda: Digenea: Cyathocotylidae) metacercariae in common carp (Cyprinus carpio Linnaeus, 1758) muscle: zoonotic potential and sensitivity to physico-chemical treatments.

Metacercariae of various species within the genus Holostephanus Szidat, 1936 (Trematoda: Digenea: Cyathocotylidae) occur in muscles of both farmed and wild fish, including common carp (Cyprinus carpio Linnaeus, 1758). The life cycle includes a snail as first intermediate host, fish as second intermediate host and birds or mammals as final hosts. We studied the zoonotic potential and the viability of Holostephanus metacercariae from common carp following exposure to various physical and chemical treatments. Muscle tissue samples of common carp specimens from a fish farm in the north-eastern part of Hungary were examined and metacercariae recovered. The zoonotic potential was evaluated experimentally by using small mammals as models (albino mice, n = 2; and Syrian hamsters, n = 4) infected per os with Holostephanus cysts. Parallelly, Metagonimus metacercariae were used as positive controls. We could not confirm the zoonotic potential of Holostephanus metacercariae as they did not survive in the mammalian intestine whereas Metagonimus metacercariae developed to the adult stage. We assessed the viability of metacercariae isolated from common carp specimens during exposure to different physical treatments (temperatures of -18°C, +20°C, +40°C and +60°C) and chemical agents (5% and 10% acetic acid and 10% sodium chloride (NaCl)). Metacercariae lost viability by freezing at -18°C (2 h), heating at 60°C (20 min), incubation in 5% and 10% acetic acid (5 min) and 10% NaCl (2 h). These methods served as models to investigate the effectiveness of food preparation techniques (such as cold and hot smoking, freezing, salting and pickling) on the survival of metacercariae.

Characteristics of liver fibrosis associated with chronic Opisthorchis felineus infection in Syrian hamsters and humans.

The food-borne liver trematode Opisthorchis felineus causes severe liver damage, including fibrosis. This study shows a comparison of the characteristics between cholangiofibrosis and periductal fibrosis in infected people and in the golden hamster as an experimental model. Comparative evaluation was carried out regarding collagen composition, the number of basic-producing cells, and extracellular-matrix degradation. The results revealed that characteristics of chronic opisthorchiasis due to O. felineus infection in humans and in Syrian hamsters are similar and include well-pronounced development of fibrotic complications in the liver parenchyma. Besides, a difference in fibrogenesis development was demonstrated between chronic O. felineus infection and noninfectious cholecystitis. In this study, we for the first time compared fibrogenesis between humans and model animals against the background of chronic O. felineus infection.

Genetic variant strains of Leishmania (Viannia) braziliensis exhibit distinct biological behaviors.

A variety of clinical forms of American cutaneous leishmaniasis (ACL) caused by Leishmania braziliensis, as well as differing immune responses of patients, have been reported for an ACL focus in the state of Minas Gerais, Brazil. In addition, two genetic profiles of L. braziliensis have been described, of which one variant profile (hsp70-variant) has been associated with atypical lesions. We investigated the biological behavior of genetic variant strains of L. braziliensis isolated from patients with different clinical manifestations of ACL. Experimental infections were performed with golden hamsters for five L. braziliensis strains in standardized doses of 1 × 106 parasites per inocula. The characteristics of skin lesions, histopathological features, and parasite burden were independently analyzed at 30 and 60 days post-infection. The data revealed distinct patterns in the onset time of visible skin lesions as well as in lesion size and parasite burden among the strains. The extent and density of the inflammatory infiltrate differed among strains, although cellular composition of granulomas appeared similar. Multivariate analysis indicated the occurrence of two clusters: one comprising native strains (cluster 1) and one comprising the reference strain (cluster 2). Within cluster 1, the genetic variants of L. braziliensis did not group with the non-variant strain suggesting that the distinct patterns of biological behavior of these strains could be associated with the known genetic diversity previously described for them.

Survey of intestinal helminths collected from pet rodents in México.

In this survey, intestinal helminths from pet rodents in Mérida, México, were analyzed. A total of 46 mice Mus musculus, 28 hamsters Mesocricetus auratus, 23 rats Rattus norvegicus, and 1 gerbil Meriones unguiculatus were purchased from six pet shops and one black market for wildlife in the city of Mérida. The overall prevalence of helminths in rodents was 61.2% (60/98). Six species of helminths were identified: the zoonotic cestode Rodentolepis nana, and the nematodes Aspiculuris tetraptera, Dentostomella translucida, Syphacia obvelata, Syphacia mesocriceti, and Syphacia muris. Of the 60 infected rodents, 25 (41.7%) harbored 2 or 3 species of helminths. Rodentolepis nana was found in 4.3% of mice and 17.9% of hamsters. This is the first report of infection with S. muris in pet rats. Considering the close physical contact between pet rodents and humans, the presence of R. nana in pets represents a potential risk of transmission, especially to children and immunocompromised individuals.

Secretion of Thioredoxin Peroxidase Protein of Cat Liver Fluke Opisthorchis felineus during Modeling of Experimental Opisthorchiasis.

Mechanisms of thioredoxin peroxidase secretion by Opisthorchis felineus were studied in vivo and in vitro. Specific antibodies were obtained and used for western blotting and immunohistochemical detection in Syrian hamster model of opisthorchiasis. Secreted thioredoxin peroxidase protein was accumulated in the worm incubation medium under conditions of oxidative stress and in bile duct cells of hamsters with chronic opisthorchiasis.

Laboratory maintenance of the bacterial endosymbiont, Neorickettsia sp., through the life cycle of a digenean, Plagiorchis elegans.

The Digenea (Platyhelminthes: Trematoda) are a diverse and complex group of internal metazoan parasites. These parasites can serve as hosts to obligate intracellular bacteria belonging to the genus Neorickettsia (Family: Anaplasmataceae). Neorickettsiae persist within all stages of the fluke life cycle and thus are maintained through vertical transmission. However, the low prevalence of Neorickettsia in nature limits study of their transmission biology at different steps of digenean life cycles. To resolve this dilemma, we have developed for the first time a laboratory model allowing to maintain Neorickettsia sp. through the whole life cycle of a digenean, Plagiorchis elegans. The laboratory life cycle of P. elegans consists of a snail first intermediate host, Lymnaea stagnalis, an aquatic arthropod second intermediate host, Culex pipiens (mosquito larva), and a vertebrate definitive host, Mesocricetus auratus (Syrian hamster). This paper focuses on the development of the laboratory life cycle, as well as outlines its potential uses in studying the transmission biology of Neorickettsia and its evolutionary relationship within its digenean host.

Metagonimus yokogawai (Trematoda: Heterophyidae): From Discovery to Designation of a Neotype.

Metagonimus yokogawai (Katsurada, 1912) Katsurada, 1912 (Trematoda: Heterophyidae) is parasitic in the small intestine of mammals including man and birds in Far Eastern Russia, Korea, Japan, China, and Taiwan. In the present study, the metacercariae and adults of M. yokogawai were redescribed to designate a neotype of this fluke together with reviews of previous studies including study histories from the first discovery to now. We particularly, attempted to review the study histories and morphological descriptions of M. yokogawai for the species validity, and compared with the morphological characteristics and life cycles of the closely related species, Metagonimus takahashii and Metagonimus miyatai. Finally, we proposed a differential key for the 8 known Metagonimus species distributed in East Asia. Metacercariae were obtained from the body muscles of sweetfish (Plecoglossus altivelis) collected in the Asahi River at Takebe-cho, Kita-ku, Okayama City, Okayama Prefecture, Japan. Adults were recovered from the small intestine of Syrian golden hamsters, to which the metacercariae had been fed 14 days before. A neotype was selected out of the present adult specimens. The Asahi River at Takebo-cho became the type locality of M. yokogawai. In conclusion, the present review shows that M. yokogawai, M. takahashii, and M. miyatai are valid and discriminated by means of morphological characteristics.

Transcriptional profiling of the spleen in progressive visceral leishmaniasis reveals mixed expression of type 1 and type 2 cytokine-responsive genes.

BACKGROUND: The Syrian golden hamster (Mesocricetus aureus) has been used as a model to study infections caused by a number of human pathogens. Studies of immunopathogenesis in hamster infection models are challenging because of the limited availability of reagents needed to define cellular and molecular determinants. RESULTS: We sequenced a hamster cDNA library and developed a first-generation custom cDNA microarray that included 5131 unique cDNAs enriched for immune response genes. We used this microarray to interrogate the hamster spleen response to Leishmania donovani, an intracellular protozoan that causes visceral leishmaniasis. The hamster model of visceral leishmaniasis is of particular interest because it recapitulates clinical and immunopathological features of human disease, including cachexia, massive splenomegaly, pancytopenia, immunosuppression, and ultimately death. In the microarray a differentially expressed transcript was identified as having at least a 2-fold change in expression between uninfected and infected groups and a False Discovery Rate of <5%. Following a relatively silent early phase of infection (at 7 and 14 days post-infection only 8 and 24 genes, respectively, were differentially expressed), there was dramatic upregulation of inflammatory and immune-related genes in the spleen (708 differentially expressed genes were evident at 28 days post-infection). The differentially expressed transcripts included genes involved in inflammation, immunity, and immune cell trafficking. Of particular interest there was concomitant upregulation of the IFN-γ and interleukin (IL)-4 signaling pathways, with increased expression of a battery of IFN-γ- and IL-4-responsive genes. The latter included genes characteristic of alternatively activated macrophages. CONCLUSIONS: Transcriptional profiling was accomplished in the Syrian golden hamster, for which a fully annotated genome is not available. In the hamster model of visceral leishmaniasis, a robust and functional IFN-γ response did not restrain parasite load and progression of disease. This supports the accumulating evidence that macrophages are ineffectively activated to kill the parasite. The concomitant expression of IL-4/IL-13 and their downstream target genes, some of which were characteristic of alternative macrophage activation, are likely to contribute to this. Further dissection of mechanisms that lead to polarization of macrophages toward a permissive state is needed to fully understand the pathogenesis of visceral leishmaniasis.

Light and electron microscopy observations of embryogenesis and egg development in the human liver fluke, Opisthorchis viverrini (Platyhelminthes, Digenea).

Eggs of most species digenean flukes hatch in the external environment to liberate larvae that seek and penetrate a snail intermediate host. Those of the human liver flukes, Opisthorchis viverrini, hatch within the gastrointestinal canal of their snail hosts. While adult parasites are primarily responsible for the pathology in cases of human opisthorchiasis, their eggs also contribute by inducing granulomata and in serving as nidi for gallstone formation. In view of the peculiar biology of O. viverrini eggs and their contribution to pathology, we investigated embryogenesis in this species by light and transmission electron microscopy. Egg development was traced from earliest stages of coalescence in the ootype until full embryonation in the distal region of the uterus. Fully mature eggs were generally impermeable to resin and could not be examined by conventional electron microscopy methods. However, the use of high-pressure freezing and freeze-substitution fixation of previously fixed eggs enabled the internal structure of mature eggs, particularly the subshell envelopes, to be elucidated. Fertilization occurs in the ootype, and the large zygote is seen therein with a single spermatozoon wrapped around its plasma membrane. As the zygote begins to divide, the spent vitellocytes are pushed to the periphery of the eggs, where they progressively degrade. The early eggshell is formed in the ootype by coalescing eggshell precursor material released by approximately six vitelline cells. The early eggs have a thinner eggshell and are larger than, but lack the characteristic shape of, mature eggs. Characteristic shell ornamentation, the "muskmelon" appearance of eggs, appears after eggshell polymerization in the ootype. Pores are not present in the shell of O. viverrini eggs. The inner and outer envelopes are poorly formed in this species, with the outer envelope evident beneath the eggshell at the opercular pole of the mature egg. The miracidium has a conical anterior end that lacks the distinctive lamellar appearance of the terebratorium of other digeneans, such as the schistosomes. The miracidium is richly glandular, containing an apical gland in the anterior end, large cephalic gland, and posterior secretory glands. Each gland contains a secretory product with different structure. The paucity of vitelline cells associating with eggs, the reduced size of eggs, and reduced complexity of the extraembryonic envelopes are interpreted as adaptations to the peculiar hatching biology of the miracidia.

Comparison of percutaneous vs oral infection of hamsters with the hookworm Ancylostoma ceylanicum: Parasite development, pathology and primary immune response.

BACKGROUND: Hundreds of millions of people in poor countries continue to suffer from disease caused by bloodfeeding hookworms. While mice and rats are not reliably permissive hosts for any human hookworm species, adult Golden Syrian hamsters are fully permissive for the human and animal pathogen Ancylostoma ceylanicum. Similar to humans, hamsters may be infected with A. ceylanicum third-stage larvae orally or percutaneously. Oral infection typically leads to consistent worm yields in hamsters but may not accurately reflect the clinical and immunological manifestations of human infection resulting from skin penetration. METHODOLOGY/PRINCIPAL FINDINGS: In this study we compared host responses following percutaneous infection to those utilizing an established oral infection protocol. Infected hamsters exhibited a dose-dependent pathology, with 1000 percutaneous larvae (L3) causing anemia and adult worm recovery comparable to that of 50 orally administered L3. A delayed arrival and maturity of worms in the intestine was observed, as was variation in measured cellular immune responses. A long-term study found that the decline in blood hemoglobin was more gradual and did not reach levels as low, with the nadir of disease coming later in percutaneously infected hamsters. Both groups exhibited moderate growth delay, an effect that was more persistent in the percutaneously infected group. Fecal egg output also peaked later and at lower levels in the percutaneously infected animals. In contrast to orally infected hamsters, antibody titers to larval antigens continued to increase throughout the course of the experiment in the percutaneous group. CONCLUSIONS/SIGNIFICANCE: These results demonstrate that the route of infection with A. ceylanicum impacts disease pathogenesis, as well as humoral and cellular immune responses in an experimental setting. These data further validate the utility of the Golden Syrian hamster as a model of both oral and percutaneous infection with human hookworms.

Development of the reproductive system of Echinostoma paraensei in Mesocricetus auratus analyzed by light and confocal scanning laser microscopy.

This study was performed to gain insight into the maturation of the reproductive system of Echinostoma paraensei worms grown in an early infection of Mesocricetus auratus. Hamsters were infected with 100 metacercariae and necropsied on days 3, 5, 7, 10 and 14 post infection (dpi). Recovered flukes stained with hydrochloric carmine were preserved as whole mounts and analyzed by light and confocal scanning laser microscopy. The average worm recovery was 43.7 per host. Images of the male and female reproductive systems were taken. The ovary and anterior and posterior testis were evidenced on day 3, while the ootype and cirrus sac were present on day 5. Confocal imaging showed primordium testis and ovary as a cluster of primordial cells from day 3 onward. The testes, ovary, cirrus sac and uterus organs were already present during the first week of life. The two testes were seen as individual structures on 7 dpi while the cirrus sac and vitelline glands were in development. The ovary was connected to the uterus while the ootype was adjacent to it. Both testes were larger than the ovary, showing cells at different stages of development, but with few bundles of functional spermatozoa in 10 day-old worms. On day 14, eggs and spermatozoa were seen in the uterus and seminal vesicle, respectively, while oocytes appeared in the ootype as fertilized eggs. We conclude that the reproductive system of E. paraensei was functional on 14 dpi in the hamsters.

The mucosal response of hamsters to a low-intensity superimposed secondary infection with the hookworm Ancylostoma ceylanicum.

An experiment was conducted to assess the mucosal response to low-dose superimposed challenge with Ancylostoma ceylanicum. Hamsters were assigned to five treatment groups (1-5 respectively): naïve controls; primary immunizing infection controls; challenge controls; immunized, anthelmintic-treated, challenged group; immunized, superimposed challenge group. Group 4 hamsters were resistant to challenge, whereas most of the challenge inoculum larvae established in Group 5. Villus height and crypt depth measurements were initially markedly divergent between these two groups but over time post-challenge (pc) values for both parameters drew nearer and by day 31 pc they were indistinguishable. The greatest change was experienced by Group 4 which showed increasing inflammation and gut pathology during the challenge infection. Mitotic activity in crypts and mast cell counts in the mucosa were highest in Group 5 on day 10 pc, but there was little to distinguish between Groups 4 and 5 by day 31 pc. Goblet cell, eosinophil and Paneth cell counts were very similar throughout in both groups but, in the case of Paneth cells, they were consistent with a possible role in protective immunity to challenge. Some adult worms survived throughout the period of intense inflammation, emphasizing their tremendous resilience and resistance to mucosal host protective responses.

[Continuous blood coagulation in chronic opisthorchiasis].

Animal experiments have shown that chronic Opisthorchis invasion results in accelerated continuous blood coagulation, by inducing hypercoagulation and platelet activation. The state of hemostasis depends on the degree of infection and undergoes the largest changes during maximum invasion. Irrespective of invasion rates, chronic opisthorchiasis is accompanied by hypofibrinogenemia.

The effect of tribendimidine and its metabolites against Necator americanus in golden hamsters and Nippostrongylus braziliensis in rats.

The antinematode effect of tribendimidine (TBD) and its metabolites has been studied. A total of 107 hamsters were each infected with 250 Necator americanus third stage infective larvae (NaL3) for 25 days. In the first test, 75 hamsters were divided equally into 15 groups for determination of ED(50) and ED(90.) Among them, five groups were treated orally with TBD or its metabolite, p-(1-dimethylamino ethylimino)aniline (aminoamidine, deacylated amidantel, BAY d 9216, dADT), at single doses of 1, 2, 4, 8, and 16 mg/kg. The remaining five groups were administered with acetylated dADT (AdADT) at single oral doses of 8, 12, 18, 24, and 30 mg/kg. In the second test, 20 hamsters were equally divided into four groups. Two groups were treated intramuscularly with TBD and dADT at a single dose of 16 mg/kg, while in the remaining two groups, single intramuscular dose of AdADT 15 or 30 mg/kg was administered. In the third test, two groups of six hamsters were treated orally with terephthalaldehyde (TPAL) and terephthalic acid (TPAC) at a single dose of 1,000 mg/kg. Other 85 rats, each infected with 300 Nippostrongylus braziliensis third stage infective larvae (NbL3), were used in three tests. For determination of ED(50) and ED(90) in the first test, five groups of five rats were treated orally with TBD or dADT at single doses of 3.0, 4.2, 5.9, 8.2, and 11.5 mg/kg or 2.0, 2.9, 4.2, 6.1, and 8.8 mg/kg, respectively. In the second test, three groups of eight to nine rats were treated orally with TBD at a single 8.4-mg/kg dose (ED(90)) and AdADT 100 or 200 mg/kg, respectively. In the third test, two groups of four rats were treated orally with TPAL and TPAC at a single dose of 1,000 mg/kg. Twenty-four to 48 h post-treatment, all the feces of each hamster and rat were collected for recovery of worms expelled from the feces. Following this period, all of the animals were sacrificed, and the adult hookworm or N. braziliensis from small intestine and large intestine were recovered and counted for calculation of worm burden reduction. The results showed that the ED(50) and ED(90) for TBD, dADT, and AdADT determined in treatment of N. americanus-infected hamsters were 1.849 and 13.598, 3.922 and 54.354, as well as 20.966 and 51.633 mg/kg, respectively. In intramuscular administration of TBD and dADT at single dose of 16 mg/kg or AdADT 30 mg/kg, similar worm burden reductions of 71.4-76.3% were observed. Two other metabolites, i.e., TPAL and TPAC, exhibited no effect against N. americanus. The ED(50) and ED(90) for TBD and dADT determined in treatment of rats infected with N. braziliensis were 3.234 and 8.435, as well as 2.345 and 5.104 mg/kg. Oral administration of AdADT at a higher single dose of 100 or 200 mg/kg resulted in worm burden reductions of 11.9-46.3%, which was significantly lower than 84.5% of worm burden reduction obtained from rats treated with TBD 8.4 mg/kg. The results indicate that in oral administration, TBD exhibits slightly better effect against N. americanus in hamsters than dADT, but AdADT possesses less effect; TBD, dADT, and AdADT show promising effect in intramuscular treatment of N. americanus-infected hamsters; the effect of oral dADT against N. braziliensis in rats is somewhat better than TBD, while AdADT endorses poor effect; and TPAL and TPAC are ineffective metabolites of TBD against both species of nematodes.

Schistosoma mansoni infection affects the proteome and lipidome of circulating extracellular vesicles in the host.

Eggs, schistosomula and adult Schistosoma worms are known to release extracellular vesicles (EV) during in vitro incubations and these EVs are postulated to affect the host responses. So far only those EVs released during in vitro incubations of schistosomes have been studied and it is unknown whether in blood of infected hosts the schistosomal EVs can be detected amidst all the circulating EVs of the host itself. In this study we analyzed the protein as well as the phospholipid composition of EVs circulating in blood plasma of S. mansoni infected hamsters and compared those with the EVs circulating in blood of non-infected hamsters. Although neither proteins nor lipids specific for schistosomes could be detected in the circulating EVs of the infected hamsters, the infection with schistosomes had a marked effect on the circulating EVs of the host, as the protein as well as the lipid composition of EVs circulating in infected hamsters were different from the EVs of uninfected hamsters. The observed changes in the EV lipid and protein content suggest that more EVs are released by the diseased liver, the affected erythrocytes and activated immune cells.

Systems metabolic effects of a necator americanus infection in Syrian hamster.

Hookworms (Ancylostoma duodenale and Necator americanus) are blood-feeding intestinal nematodes that infect approximately 700 million people worldwide. To further our understanding of the systems metabolic response of the mammalian host to hookworm infection, we employed a metabolic profiling strategy involving the combination of (1)H NMR spectroscopic analysis of urine and serum and multivariate data analysis techniques to investigate the biochemical consequences of a N. americanus infection in the hamster. The infection was characterized by altered energy metabolism, consistent with hookworm-induced anemia. Additionally, disturbance of gut microbiotal activity was associated with a N. americanus infection, manifested in the alterations of microbial-mammalian cometabolites, including phenylacetylglycine, p-cresol glucuronide, 4-hydroxy-3-methyl-phenylpropionic acid, hippurate, 4-hydroxyphenylactate, and dimethylamine. The correlation between worm burden and metabolite concentrations also reflected a changed energy metabolism and gut microbial state. Furthermore, elevated levels of urinary 2-aminoadipate was a characteristic feature of the infection, which may be associated with the documented neurological consequences of hookworm infection.

The life history of Pygidiopsis macrostomum Travassos, 1928 (Digenea: Heterophyidae).

The life history of the trematode Pygidiopsis macrostomum Travassos, 1928 is described for the first time. Rediae and cercariae were obtained from naturally infected snails Heleobia australis (d Orbigny), a new first intermediate host. Metacercariae were found encysted in the mesenteries of three naturally infected guppies, Phalloptychus januarius (Hensel), Jenynsia multidentata (Jenyns) (new host records) and Poecilia vivipara Bloch and Schneider. Experimental infections were successfully completed in the intermediate hosts H. australis and Poe. vivipara reared in the laboratory and hamsters Mesocricetus auratus Waterhouse were utilised as a definitive host.

Dose-response relationships and tegumental surface alterations in Opisthorchis viverrini following treatment with mefloquine in vivo and in vitro.

The treatment and control of opisthorchiasis relies on a single drug, praziquantel; hence, there is a need to develop novel opisthorchicidal drugs. We investigated the in vitro and in vivo activity of the antimalarial mefloquine against Opisthorchis viverrini. Hamsters infected with O. viverrini for 2 weeks (juvenile infections) and 4 weeks (adult infections) were treated orally with single 200-400-mg/kg oral mefloquine. Worm burden reductions were assessed against untreated control hamsters. Worms were incubated in the presence of 10 and 100 microg/ml mefloquine. Scanning electron microscopy was used to examine adult O. viverrini after recovery from hamsters and following in vitro incubation. A single oral dose of 300-mg/kg mefloquine resulted in worm burden reductions of 88.5% (juvenile infection) and 96.0% (adult infections), respectively. Incubation with 10 and 100 microg/ml mefloquine resulted in rapid death of O. viverrini. Extensive tegumental disruption such as blebbing, sloughing, and furrowing was seen on worms incubated in vitro and on flukes recovered 48 h posttreatment. In conclusion, we have documented promising opisthorchicidal activities in hamsters and in vitro with the tegument being an important drug target. Proof-of-concept studies with mefloquine could be considered in opisthorchiasis patients.