Transplantation of Schistosoma japonicum daughter sporocysts in Oncomelania hupensis

Schistosoma japonicum daughter sporocysts obtained from infected Oncomelania hupensis hupensis were successfully transplanted to parasite-free O. hupensis hupensis. Survival and infection rates of recipient snails were 80% and 75% respectively. Intramolluscan development of transplanted daughter sporocysts in recipient snails appears to proceed in a similar manner as those reported for transplanted S. mansoni and S. haematobium in their respective snail intermediate hosts. Complete colonization of the digestive gland of recipient snails by sporocysts was observed 80 days after transplantation. Cercarial production during a 10-day observation from recipient snails was characterized by periods of high and low and irregular daily emissions. The average daily cercarial production was 150 per snail. Cercariae produced by recipient snails were infective to mice. Of those cercariae exposed to mice, approximately 30% developed to adult schistosomes. These results have definitive utility in the maintenance of S. japonicum in the laboratory.     

Differential expression of LacdiNAc,fucosylated LacdiNAc,and Lewis x glycan antigens in intramolluscan stages of Schistosoma mansoni

We report the expression of 3 well-characterized adult Schistosoma mansoni glycan antigens among molluscan stages of the parasite. These antigens are LacdiNAc (LDN; GalNAcbeta1-4GlcNAc-R), fucosylated LacdiNAc (LDNF; GalNAc[Fucal-3]beta1-4GlcNAc-R), and Lewis x (Le(x); Gal[Fucalpha1-3]beta1-4GlcNAc-R). The presence of the glycans was determined by both immunoblot and immunohistological methods using monoclonal antibodies that specifically recognize each glycan epitope. Immunoblot analyses reveal that LDN and LDNF epitopes are expressed on many different glycoproteins, including eggs, mother sporocysts, daughter sporocysts, and cercariae, although LDN expression among daughter sporocysts is greatly reduced. LDN and LDNF epitopes are localized on the tegument and in the intrasporocyst cell masses of both in vitro-derived and in vivo-derived mother sporocysts and in the daughter sporocysts derived on day 16 after infection. Unexpectedly, high levels of LDN and LDNF glycans were detected in the infected, but not in the uninfected, snail hemolymph, suggesting that the infecting larvae secrete LDN and LDNF glycoconjugates into the snail hosts. In contrast, the expression of Le(x) antigen among the molluscan stages is highly restricted. Le(x) is present on a few high-molecular weight glycoproteins in eggs and cercariae but is undetectable in mother and daughter sporocysts. Taken together with our earlier studies on vertebrate stages of S. mansoni, these results show that LDN and LDNF glycans are conserved during schistosome development. The study further extends the evidence that Le(x) is a developmentally regulated antigen in schistosomes.     

Mechanisms underlying digenean-snail specificity: role of miracidial attachment and host plasma factors

Digenetic trematodes usually show a high degree of specificity for their molluscan intermediate hosts. A panel of 4 digenean species (Echinostoma paraensei, E. trivolvis, Schistosoma mansoni, and Schistosomatium douthitti) and 5 snail species (Biomphalaria glabrata, Helisoma trivolvis, Lymnaea stagnalis, Stagnicola elodes, and Helix aspersa representing 3 gastropod families) was used to assess the relative contributions of miracidial behavior, host plasma osmolality, and host plasma factors in dictating specificity. Additional experiments were undertaken with a fifth digenean, Echinoparyphium sp. Expected patterns of compatibility were first confirmed; each parasite species produced patent infections in its known snail host, but not in the other snail species. One exception was S. douthitti, which unexpectedly did not infect L. stagnalis. As judged by direct observation and by noting their disappearance after exposure to snails, miracidia were generally less likely to attach to or penetrate incompatible than compatible hosts. However, over half of the miracidia of each parasite species attached to or attempted penetration of both compatible and incompatible hosts, suggesting that under the experimental conditions used, miracidial host location and attachment behaviors were not of overriding importance in dictating observed patterns of specificity. For each digenean species, the percentage of larvae that became immobile, rounded, showed tegumental damage, or died over a 6-hr interval in plasma of the various snails was assessed. In no case was plasma from a compatible host harmful to sporocysts or rediae. In contrast, in 8 of 16 (50%) incompatible combinations, snail plasma had a significant negative effect on sporocyst condition. In 4 of 12 (33%) incompatible combinations, plasma had a significant negative effect on rediae. In 9 of 10 combinations tested, lymnaeid plasma was toxic for the parasites of planorbid snails and in 2 of 4 combinations, planorbid plasma was toxic for the parasites of lymnaeid snails. Toxicity was not attributable to differences in plasma osmolality between snail species. The ability of plasma from incompatible snails to affect viability of both sporocysts and rediae was surprisingly strong, suggesting that humoral factors play a greater role in dictating patterns of digenean-snail specificity than previously appreciated.     

Effect of parasites on host adaptation to abiotic environmental factors: host-parasite relationship of trematode parthenites--mollusc system

This laboratory study examined the survival of extreme environmental conditions by the White Sea periwinkle Littorina saxatilis. Molluscs from localities with high and low prevalence of Microphallus piriformes, which is a representative of the "pygmaeus" group of digeneans (Microphallidae), were compared. These parasites have not a stage of free-living cercariae in their life cycle. Metacercariae mature inside the sporocysts parasitizing the molluscan host. No negative influence of infection was found on resistance of molluscs to prolonged desiccation and extremes of air and water temperature. On the contrary, significant lower mortality of high infected snails was observed in some experiments. Exposure to fresh water was the only treatment that caused more intensive mortality of high infected snails in comparison with low infected one. The results of the experiments were discussed taking into consideration the available data on mechanisms of molluscan resistance and features in the relationships of "pygmaeus" group sporocysts with the organism of molluscan host. It was emphasized that the wide spread opinion regarding the only negative influence of trematode infection on the resistance of infected molluscs should be revised.     

Schistosoma mansoni: relationship between cercarial production levels and snail host susceptibility

Two populations of Biomphalaria glabrata snails differing slightly in their susceptibility to Schistosoma mansoni infection showed dramatic differences in cercarial output per snail. Exposed to five or more miracidia, snails from a group with a 90-100% susceptibility rate (Group A) produced nearly twice the number of cercariae as those from a group with a 70-80% susceptibility rate (Group B). Exposure of individual snails to known numbers of miracidia resulted in higher numbers of primary (mother) sporocysts in Group A snails than in Group B snails. However, monomiracidial exposure of snails from both groups resulted in equivalent numbers of cercariae produced per positive snail, indicating that, once established, all primary sporocysts possess a similar reproductive potential. Morphometric analysis of serially sectioned 9-day-old primary sporocysts supported this conclusion; the size of the primary sporocysts and the size and numbers of secondary (daughter) sporocysts within each primary sporocyst were comparable in snails from both groups. The data indicate cercarial production in this system is regulated prior to, and/or during, early development of the primary sporocyst.     

Search for shared antigens in the schistosome-snail combination Trichobilharzia ocellata-Lymnaea stagnalis

Mother and daughter sporocysts of Tricholbilharzia ocellata, developing in the snail host Lymnaea stagnalis, were searched for substances with antigenic similarities to the snail's haemolymph. Antisera to cell-free snail haemolymph and fractions thereof were used in three different immunocytochemical staining methods, applied on sections of parasitized snails. Snail tissue was consistently stained; cercariae were stained, indicating that the applied methods were successful. Most sections through mother and daughter sporocysts were completely unstained. It is concluded that neither mother nor daughter sporocysts are masked by the antigens studied or substances mimicking these. The relevance of the present observations is discussed.     

Observations on the host-parasite relations between Echinostoma revolutum and lymnaeid snails.

Echinostoma revolutum from Taiwan was studied in lymneid snails at 29 +/- 0.5 degrees C. Given 3-5 miracidia, 95% of Lymnaea ollula and 40% of Lymnaea swinhoei became positive; the prepatent periods were 18 and 25 days, respectively. The following are based on the observations in Lymnaea ollula: The time required for miracidial penetration was about one hour. The sporocysts developed only in the ventricle of the snail but mother rediae developed in the heart and other organs. Mature daughter rediae were not found in the heart cavity. The sporocysts reached the ventricle within 3 days. Mother rediae were released after 6 days and daughter rediae after 8 days. Given 5 miracidia, 1-3 sporocysts reached the heart and 2-20 mother rediae were found per snail. The number of mature daughter rediae was usually 100-200 although more than a thousand may develop in a snail. The sporocysts and mother rediae attained maximal size 9 days postinfection and started degeneration 13 days postinfection. Daughter rediae were largest at the beginning of cercarial emergence and decreased in size thereafter. Simultaneous production of daughter rediae and cercariae by the mother redia was seen only once in this snail mature cercariae were obtained in 10 days postinfection. The cercariae emerged from a small area of mantle collar near the posterior corner of shell aperture. They were negatively phototactic and positively geotactic. An estimation showed that each snail shed about 350 cercariae a day. The cercariae reached the pericardial cavity of snail in one hour via the renal orifice and metacercariae were seen 4-5 hours after exposure. The infectivity of cercariae at various times after shedding, as expressed by cyst recovery rates, were: 51.6%, O-hr old; 76.1%, 2-hr; 68% 4-hr; 32%, 6-hr; 3%, 8 and 10-hr. Cyst recovery rates were not different within the dosage of 50-500 cercariae per snail. Most metacercariae recovered 1-2 days after cercarial exposure were viable; only 5 among 6,533 cysts were dead.     

Plagiorchis elegans (Digenea: Plagiorchiidae) infections in Stagnicola elodes (Pulmonata: Lymnaeidae): host susceptibility, growth, reproduction, mortality, and cercarial production.

Eggs of Plagiorchis elegans were readily ingested by Stagnicola elodes of all ages, but were more infective to immature than mature snails. Infection enhanced the growth of the host in a dose-dependent manner. The number of cercariae released by immature snails increased with the age of the snail host; mature snails yielded fewer cercariae. Heavily infected snails tended to die prematurely, thereby reducing their total production of cercariae to levels below those of more lightly infected individuals. Even light infections castrated the snail host. Snails that acquired the infection as juveniles never produced eggs. Actively reproducing snails ceased egg laying within days of infection and never recovered. All parasite effects on the growth and reproduction of the snail host first manifested themselves during the early stages of infection, long before the development of daughter sporocysts and cercariae, and are therefore attributable to the effects of mother sporocysts. The study provides insight into how this natural enemy of mosquito larvae may be established in natural snail populations by means of strategically timed introductions of parasite eggs, with a goal of maximizing cercarial production for the biological control of sympatric mosquito larvae.     

Echinostoma paraensei: hemocytes of Biomphalaria glabrata as targets of echinostome mediated interference with host snail resistance to Schistosoma mansoni

Earlier in vivo work by Lie et al. (1977) indicated that the innate resistance of the 10R2 strain of Biomphalaria glabrata to PR1 Schistosoma mansoni could be interfered with if the snails were infected previously with another trematode, Echinostoma paraensei. We have studied this interference phenomenon using in vitro methods in an attempt to understand its mechanistic basis. Hemolymph, derived from 10R2 snails infected with E. paraensei for 14-28 days, killed 25% of S. mansoni sporocysts in vitro, significantly less (P less than 0.001) than the 90% killing rate observed with hemolymph from uninfected, control 10R2 snails. Hemolymph from the infected 10R2 snails and from schistosome susceptible M line snails did not differ significantly (P greater than 0.1) in their relative inability to kill S. mansoni sporocysts in vitro. The defect in sporocyst killing exhibited by echinostome infected 10R2 snails was traced to the cellular, rather than the humoral, component of the hemolymph. Preparations containing uninfected 10R2 snail hemolymph and echinostome daughter rediae exhibited significantly less (P less than 0.001) killing of S. mansoni sporocysts than did controls containing only 10R2 hemolymph and S. mansoni sporocysts. Our results suggest that echinostome larvae release factors that interfere with the ability of B. glabrata hemocytes to kill S. mansoni sporocysts.     

14C uptake by Schistosoma mansoni from Biomphalaria glabrata exposed to 14C-glucose

Exposure of Biomphalaria glabrata infected with Schistosoma mansoni to ¹⁴C-glucose results in a greater uptake of original total snail label by the parasitized digestive gland-gonad, site of the developing daughter sporocysts and cercariae, than by the digestive gland-gonad of control animals. As a consequence of this greater uptake by the infected digestive gland-gonad, the albumen gland and remainder of the carcass of parasitized snails receive less label than do those areas in normal snails. Emergence of cercariae from the snail and daughter sporocyst mass account for a diversion of 12.6% of original total label from the infected snail itself. This diversion of label from the snail to the parasite may explain carbohydrate depletion in parasitized snails.     

Parthenogenetic generations of Helicometra fasciata Rud., 1819 (Trematoda: Opecoelidae) in the Black Sea molluscs Gibbula adriatica

Morphology of the Helicometra fasciata Rud., 1819 parthenogenetic generation from the Black Sea gastropods Gibbula adriatica (Phil.) was studied for the first time. Data on seasonal dynamics of the hemipopulation of daughter sporocysts are given. Daughter sporocysts of H. fasciata infest 10 +/- 0.2 % of G. adriatica (mainly specimens of larger size and elder age classes). As a rule, local microhemipopulations of daughter sporocysts castrate mollusc hosts. Reproduction of H. fasciata daughter sporocysts is asynchronous: daughter sporocysts born specimens of next sporocyst generation during autumn and winter, and then they begin producing cercaria. In winter development of the cercaria embryo is blocked. Second change of the character of the each sporocyst' posterity is impossible because of the annual life cycle of G. adriatica. Endogenous agglomeration of the H. fasciata daughter sporocysts is extremely little: individuals of next sporocyst generation develop from no more than 2 % of embryonic balls. Energy resources of the mollusc host are used by the H. fasciata daughter sporocysts mainly for producing cercaria; this fact can be interpreted as an adaptation of H. fasciata to using medium-sized, short-living mollusc hosts.     

Description and proposed life cycle of Maritrema novaezealandensis n. sp. (Microphallidae) parasitic in red-billed gulls, Larus novaehollandiae scopulinus, from Otago Harbor, South Island, New Zealand

Maritrema novaezealandensis n. sp. is described from Otago Harbor, South Island, New Zealand, on the basis of adult specimens collected from the Red-billed gull, Larus novaehollandiae scopulinus, and excysted metacercariae obtained from crabs. It belongs to the "eroliae group" and differs from other related species mainly in the shape, size, and patterns of distributions of the spines on the cirrus, the shape of the metraterm, the presence of an unlobed ovary, and the complete ring of the vitelline follicles. Based on morphometric features of metacercariae and adult specimens, the trophic relationships among invertebrate and vertebrate hosts, experimental infections, and previous reports of species of Maritrema with similar transmission patterns, the life cycle of M. novaezealandensis n. sp. is described. A 3-host life cycle is proposed for this parasite. The first intermediate host is the mud snail, Zeacumantus subcarinatus, in which the cercarial stage is produced in sporocysts located within the gonad of the snail. At least 3 crab species (Hemigrapsus crenulatus, Macrophtalmus hirtipes, and Halicarcinus whitei) and several species of amphipods act as second intermediate hosts, with metacercariae encysted in the body cavity of the crustacean host. Finally, the definitive host, the gull, L. n. scopulinus, harbors the adult worms in its intestine.     

Infectivity of a microsporidium of mosquitoes (Nosema algerae) to larval stages of Schistosoma mansoni in Biomphalaria glabrata

Lai P. F. and Canning E. U. 1980. Infectivity of a microsporidium of mosquitoes (Nosema algerae) to larval stages of Schistosoma mansoni in Biomphalaria glabrata. International Journal for Parasitology10: 293–301. Nosema algerae derived from a closed colony of Anopheles stephensi was fed to Biomphalaria glabrata infected with Schistosoma mansoni. Mother and daughter sporocysts became hyperinfected but the snail tissues remained free of the microsporidia except for rare small aggregates of spores. These lay close to the sites occupied by mother or daughter sporocysts and were probably liberated from them. Irrespective of dose, fewer snails contained infected sporocysts when spores were given at 7 days post-miracidial infection than when given at 14 days. These periods corresponded respectively to stages when mother sporocysts only or daughter sporocysts as well were present in the snails. Infection of the sporocysts began in the tegumental cells, spread to the brood chamber and ultimately to the cercariae themselves. Heavily infected sporocysts contained fewer developing embryos. Doses of 10⁶ and 10⁷ spores/snail caused significant depression of cercaria output when given at 14 days but not at 7 days.     

The invasion of Biomphalaria pfeifferi by Schistosoma mansoni miracidia and the development of daughter sporocysts

Laboratory experiments have been carried out to determine the susceptibility of Gezira Biomphalaria pfeifferi snails to S. mansoni miracidia and the relationship between miracidia and daughter sporocyst production at the 10–17 day development stage. The relationship between snail numbers, miracidia numbers and water volume has also been studied. Two non susceptible snails, Bulinus truncatus and Cleopatra bulimoides, both of which occur naturally in Gezira canals, were tested to see if they act as decoys for S. mansoni miracidia.The results showed that the B. pfeifferi are 100% susceptible to S. mansoni invasion, at least to the daughter sporocyst development stage. The more miracidia that penetrated the more daughter sporocysts were produced, however individual variation and overlap were great. When one miracidium was released to find one snail it succeeded in low water volumes (5 m, 50 ml), but failed in 5 litres. When 100 miracidia were released mortality of snails was high suggesting superinfection particularly when only one or five snails were available. Among survivors daughter sporocyst counts were very high. Cleopatra and Bulinus snails do have a decoy effect when present in large numbers. In their presence the number of infected snails was marginally reduced and the number of daughter sporocysts greatly reduced. However, if superinfection is reduced by decoy effect, it is conceivable that Biomphalaria may be protected by decoy snails in circumstances where miracidia counts are high.     

Phylogenetic analysis of Alloglossidium (Digenea: Macroderoididae) and related genera: life-cycle evolution and taxonomic revision

A phylogenetic analysis was performed on 13 species of digenetic trematodes in the Macroderoididae, including 10 species of Alloglossidium, 2 species of Alloglossoides, and Hirudicolotrema richardsoni. The evolution of the unusual life-cycle patterns in the group was assessed in light of the proposed phylogeny. The results support previous hypotheses that taxa with a 3-host life cycle involving catfish as definitive hosts are basal to taxa with a 2-host life cycle involving invertebrates such as crustaceans and leeches as definitive hosts. Our results also strongly suggest that species maturing in leeches evolved from an ancestor that matured in crustaceans. Our phylogeny places Alloglossoides and Hirudicolotrema within Alloglossidium, showing Alloglossidium to be paraphyletic. To achieve a natural classification, Alloglossoides and Hirudicolotrema are synonymized with Alloglossidium, and a revised generic diagnosis for Alloglossidium is given.     

Brachylaima phaedusae n. sp. (Trematoda: Brachylaimidae) from door snails in Japan

The metacercarial infections of door snails (Gastropoda: Clausiliidae) with unknown species of the genus Brachylaima (Trematoda: Brachylaimidae) have recently been reported in eastern Honshu and Kyushu, Japan. A large scale snail survey was carried out to clarify their taxonomic status. From the period of 2015 to 2020, a total of 1239 land snails (768 door snails and 471 others) were collected from 32 localities in Honshu, Shikoku, and Kyushu. The resulting trematode isolates were identified as Brachylaima sp. by mitochondrial DNA barcoding. The sporocysts were found only a few from Megalophaedusa sublunellata (Clausiliidae), Tauphaedusa subaculus (Clausiliidae), and Aegista trochula (Camaenidae), while the metacercariae were frequently detected from 14 species of Clausiliidae and 2 species of other families. Although Brachylaima sp. showed a broad range of intermediate hosts, door snails seem to be very important to drive the life cycle. The gravid adults of Brachylaima sp. was experimentally raised from metacercariae using immunosuppressed mice. Morphological, phylogenetical, and ecological considerations prompted us to propose Brachylaima phaedusae n. sp. for this unknown species. The definitive hosts of the new species are completely unknown. The wide geographic distribution and high genetic diversity of the new species suggest a possibility that the definitive host is ground-foraging birds, which prefer door snails.     

The effect of size of M line Biomphalaria glabrata on the course of development of Echinostoma paraensei

M line Biomphalaria glabrata snails of 4-, 6-, 8-, 10-, 12-, or 20-mm shell diameter were individually exposed to 10 miracidia each of Echinostoma paraensei. Snails 10 mm in size or larger were found to be significantly less likely to harbor intraventricular sporocysts than snails in smaller size categories. The percentage of snails with intraventricular sporocysts that also developed hemocyte encapsulation responses generally increased with snail size, whereas the number of snails that ultimately became heavily parasitized with large numbers of daughter rediae decreased significantly with snail size. However, at least some snails in each size category developed such disseminated infections. Comparative histological study of 6- and 12-mm snails revealed that parasites readily penetrated both groups of snails, but were more likely to be encapsulated and destroyed in larger snails. Encapsulation reactions were noted from 1 to 15 days postexposure (dpe) in 12-mm snails, indicating that unlike other commonly studied models of trematode-gastropod interactions, snail resistance is not always manifested during the first few days following exposure. Upon infection with E. paraensei, both 6- and 12-mm snails showed significant increases in the number of circulating hemocytes/mm3 of hemolymph. In 6-mm snails, such increases occurred concurrently with successful parasite development. Hemocyte counts in 6-mm snails were significantly elevated from 4 to 15 dpe whereas in 12-mm snails they were significantly elevated from 2 to 30 dpe. A significant degree of resistance to E. paraensei develops as B. glabrata grows and attains sexual maturity. A mechanistic understanding of this phenomenon awaits further investigation.     

A comparative study of mechanisms underlying digenean-snail specificity: in vitro interactions between hemocytes and digenean larvae

A panel of 4 digenetic trematode species (Echinostoma paraensei, E. trivolvis, Schistosoma mansoni, and Schistosomatium douthitti) and 5 snail species (Biomphalaria glabrata, Helisoma trivolvis, Lymnaea stagnalis, Stagnicola elodes, and Helix aspersa) was examined to determine if known patterns of host specificity could be explained by the tendency of digenean larvae to be bound by snail hemocytes, or by the ability of larvae to influence the spreading behavior of hemocytes. In short-term (1 hr) in vitro adherence assays, there was no overall pattern to suggest that sporocysts were more likely to be bound by hemocytes from incompatible than compatible snails. Compared with the other parasites, sporocysts of E. paraensei were less likely to be bound by hemocytes from any of the snail species tested. All rediae examined, including those of another species Echinoparyphium sp., were also remarkably refractory to binding by hemocytes from any of the snails. Of all the larvae examined, only sporocysts and young daughter rediae of E. paraensei caused hemocytes to round up in their presence. This was true for hemocytes from the compatible species B. glabrata and the incompatible lymnaeid species S. elodes and L. stagnalis. The patterns of host specificity shown by this particular panel of parasites and snails were not predicted by either the extent of hemocyte adherence to digenean larvae or by the ability of larvae to affect hemocyte spreading behavior. The results of this study suggest that a role for hemocytes, although likely, may require different assays, possibly of a more prolonged nature, for its detection. Also, different parasite species (notably E. paraensei) and intramolluscan stages have distinctive interactions with host hemocytes, suggesting that the determinants of specificity vary with the host-parasite combination, and with the parasite life cycle stage.     

Schistosoma mansoni: Cytotoxicity of hemocytes from susceptible snail hosts for sporocysts in plasma from resistant Biomphalaria glabrata

Sporocysts of Schistosoma mansoni (PR1 strain) survive and grow in Biomphalaria glabrata PR albino strain snails, whereas they are encapsulated and die in B. glabrata 10R2 strain snails. These processes also occur in an in vitro system in which the only living cells are those of sporocysts and snail hemolymph. Hemocytes of the susceptible snail are normally not effective in damaging sporocysts. However, when the encounter occurred in the presence of cell-free plasma from resistant snails, previously impotent hemocytes severely damaged sporocysts in 24 hr. The cytotoxic capacity of resistant strain hemocytes was not altered by plasma from susceptible snails. Furthermore, it was retained even when plasma was replaced by culture medium free of snail components. The nature of the plasma factor(s) which facilitated damage by otherwise impotent hemocytes is discussed, and evidence is evaluated for the hypothesis that snail resistance is dependent upon the specificity of cytophilic factors present both in the plasma and on the hemocyte plasma membranes.     

Life cycle of Brachylaima mascomai n. sp. (Trematoda: Brachylaimidae), a parasite of rats in the Llobregat delta (Spain)

The terrestrial triheteroxenous life cycle of Brachylaima mascomai n. sp. (Trematoda: Brachylaimidae) is elucidated. Operculated, assymetric, embryonated eggs (25.4 x 12.7 microm) are passed with feces of the natural (Rattus norvegicus and R. rattus) and experimental (albino and wild mice, albino rats, Apodemus sylvaticus, Mus spretus [Muridae] and the golden gerbil) definitive hosts and ingested by the helicid gastropod Pseudotachea splendida, the only natural and experimental first intermediate host. Microcaudate cercariae harbored in branched sporocysts in the digestive gland emerge from this snail and contact P. splendida, Otala punctata, Theba pisana, and Helix (C.) aspersa snails developing into unencysted infective metacercariae in the kidney. Definitive hosts are infected by ingestion of infected snails; the adult parasites inhabit the small intestine. Chaetotaxic cercarial pattern specific at acetabular (S(II) 8-10 papillae) and cephalic (C(III) 13-15 papillae, H 16 papillae) levels. Three types of cercarial papillae are observed by scanning electron microscopy (SEM) and their arrangement is correlated with chaetotaxy for the first time in trematodes: argentophilic papillae with fingerlike process (cephalic, body, and acetabular levels), argentophilic papillae with opening (2 papillae in the M body level), and nonargentophilic dome-shaped papillae (alternated with argentophilic S(II) papillae on the ventral sucker). SEM detected interlacing network of ridges covering the metacercarial body. Adults with multidigitate tegumentary spines were observed by SEM. Subequal suckers; the acetabulum located in the posterior part of anterior fifth of body. Vitellaria extend from between middle level and anterior margin of anterior testis to between middle level and posterior margin of acetabulum. Uterus almost reaches the intestinal bifurcation.