Life cycle and structure of the fish digenean Brachyphallus crenatus (Hemiuridae).

Cystophorous cercariae from Retusa obtusa (Montagu) (Gastropoda, Opisthobranchia, Retusidae) develop into adults of Brachyphallus crenatus (Rudolphi, 1802) Odhner, 1905 (Hemiuridae). The free-swimming cercariae were ingested by laboratory-reared Acartia tonsa Dana, and the cercarial body was injected into the hemocoel of the copepod. Two-week-old metacercariae held at 15 C were infective to stickleback Gasterosteus aculeatus Linnaeus. The cercariae of B. crenatus are very similar to the cercariae of Hemiurus luehei Odhner, 1905, and Lecithocladium excisum (Rudolphi, 1819) Lühe, 1901 (Hemiuridae), which develop in closely related opisthobranch snails. Scanning electron microscopy of metacercariae and adults of B. crenatus revealed the annular plications of most of the external surface to be scalelike. The area surrounding the genital pore and the presomatic pit was densely plicated.     

Acanthoparyphium tyosenense (Digenea: Echinostomatidae): experimental confirmation of the cercaria and its complete life history in Korea

Cercaria yamagutii Ito, 1957, was found in the marine mesogastropods Lunatia fortuni and Glossaulax didyma from the tidelands of Simpo located at the estuary of the Mankyoung River, which runs to the western coast of Korea. Metacercariae were found in a marine bivalve Mactra veneriformis after being infected with C. yamagutii experimentally. When a sea gull, Larus crassiostris, was fed with the metacercariae collected from the infected M. veneriformis, adult worms were recovered 10 days later. It was confirmed that the parasites collected from L. crassiostris were Acanthoparyphium tyosenense Yamaguti, 1939. From the results of this life cycle study, it was determined that the first intermediate hosts of A. tyosenense are L. fortuni and G. didyma. The second intermediate and final hosts are M. veneriformis and L. crassiostris, respectively. Mactra veneriformis was experimentally infected with C. yamagutii isolated from L. fortuni and G. didyma by maintaining them in a water tank for 30 min at about 20 C. The cercariae entered M. veneriformis through their incurrent siphons. Five hours after infection, the cercariae tails began to separate from the bodies, and the cercariae formed cysts. Mature cysts were formed 340 hr (14 days) after infection and identified as the metacercariae of A. tyosenense. The prevalence of A. tyosenense metacercariae was 99.5% in naturally infected M. veneriformis. This is the first report of C. yamagutii as the cercaria of A. tyosenense, and the complete life cycle of A. tyosenense was established in Korea.     

A description of the parthenogenetic metacercaria and cercaria of <Emphasis Type="Italic">Cercaria falsicingulae I</Emphasis> larva nov. (Digenea: Gymnophallidae) from the snails <Emphasis Type="Italic">Falsicingula</Emphasis> spp. (Gastropoda), with speculation on an unusual life-cycle

Previously unknown gymnophallid parthenogenetic metacercariae (PM), referred to as Cercaria falsicingulae I larva nov., were found in the extrapallial cavities of the snails Falsicingula mundana (Yokoyama) and F. athera (Bartsch) on coasts of Sakhalin and the Kuril Islands. Unlike all other known PM, rather than producing metacercariae infective to the definitive host, their furcocercariae emerge into the environment. The developing cercariae and metacercariae of C. falsicingulae I are described and compared with other gymnophallid larvae from littoral molluscs in the region. Experimental evidence and analysis of metacercarial group composition in naturally infected molluscs indicate that some cercariae leave their molluscan hosts and penetrate other specimens of Falsicingula in which they develop into new cercariae-producing PM. Metacercariae with developing hermaphroditic reproductive organs were never observed in naturally infected molluscs. A probable life-cycle for C. falsicingulae I is presented in which cercarial/metacercarial production is switched seasonally in order to enable the infection of the definitive hosts by PM. Importantly, the 'cercaria - PM - cercaria' component of the life-cycle appears to be somewhat autonomous.     

Effect of temperature on emergence, survival and infectivity of cercariae of the marine trematode Renicola roscovita (Digenea: Renicolidae)

Marine bivalves harbour a diversity of trematode parasites affecting population and community dynamics of their hosts. Although ecologically and economically important, factors influencing transmission between first (snail) and second (bivalve) intermediate hosts have rarely been studied in marine systems. In laboratory experiments, the effect of temperature (10, 15, 20, 25 degrees C) was investigated on (1) emergence from snails, (2) survival outside hosts and (3) infectivity in second intermediate hosts of cercariae of the trematode Renicola roscovita (Digenea: Renicolidae), a major parasite in North Sea bivalves. Emergence of cercariae peaked at 20 degrees C (2609 +/- 478 cercariae snail(-1) 120 h(-1)) and was considerably lower at 10 degrees C (80 +/- 79), 15 degrees C (747 +/- 384) and 25 degrees C (1141 +/- 334). Survival time decreased with increasing temperature, resulting in 50% mortality of the cercariae after 32.8 +/- 0.6 h (10 degrees C), 26.8 +/- 0.8 h (15 degrees C), 20.2 +/- 0.5 h (20 degrees C) and 16.6 +/- 0.3 h (25 degrees C ). Infectivity of R. roscovita cercariae in cockles Cerastoderma edule increased with increasing temperature and was highest at 25 degrees C (42.6 +/- 3.9%). However, mesocosm experiments with infected snails and cockle hosts in small aquaria, integrating cercarial emergence, survival and infectivity, showed highest infection of cockles at 20 degrees C (415 +/- 115 metacercariae host(-1)), indicating 20 degrees C to be the optimum temperature for transmission of this species. A field experiment showed metacercariae of R. roscovita to appear in C. edule with rising water temperature in April; highest infection rates were in August, when the water temperature reached 20 degrees C. Since another trematode species (Himasthla elongata; Digenea: Echinostomatidae) occurring at the experimental site showed a similar temporal pattern, trematode transmission to second intermediate bivalve hosts may peak during especially warm (> or = 20 degrees C) summers in the variable climate regime of the North Sea.     

Experimental Life History and Biological Characteristics of Fasciola gigantica (Digenea: Fasciolidae)

This study was conducted to investigate the life history, morphology, and maturation of larval stages and adult worms of Fasciola gigantica in experimental mice. Lymnaea auricularia rubiginosa was used as the intermediate host, and Oryza sativa was used for encystment of the metacercariae, while Mus musculus was used as the definitive host for maturation study. Fresh eggs from the gall bladder of water buffaloes fully developed into embryonated ones and hatched out at days 11-12 after incubation at about 29ºC. Free-swimming miracidia rapidly penetrated into the snail host, and gradually developed into the next larval stages; sporocyst, redia, and daughter redia with cercariae. Fully-developed cercariae were separated from the redia and shed from the snails on day 39 post-infection (PI). Free-swimming cercariae were immediately allowed to adhere to rice plants, and capsules were constructed to protect metacercariae on rice plants. Juvenile worms were detected in intestines of mice at days 3 and 6 PI, but they were found in the bile duct from day 9 PI. Juvenile and adult flukes were recovered from 16 mice experimentally infected with metacercariae, with the average recovery rate of 35.8%. Sexually mature adult flukes were recovered from day 42 PI. It could be confirmed that experimentally encysted metacercariae could infect and develop to maturity in the experimental host. The present study reports for the first time the complete life history of F. gigantica by an experimental study in Thailand. The obtained information can be used as a guide for prevention, elimination, and treatment of F. gigantica at environment and in other hosts.     

The life cycle of Echinochasmus bagulai (Trematoda: Echinostomatidae)

The life history of Echinochasmus bagulai, an echinostomatid trematode in birds, is reported and stages in the life cycle are described. Natural infections with cercariae, which are of the gymnocephalous type, were found in the thiarid snail Thiara tuberculata. Metacercarial cysts were found in the gills of Aplocheilus panchax, Oryzias melastigma, Gambusia affinis and Channa punctata. Adults were obtained in the small intestine of Ardeola grayi. In laboratory experiements development of cercariae into infective metacercariae took 10 days in the gills of A. panchax. Mature flukes were recovered in 10 days in the small intestine of experimentally infected 1-day-old leghorn chicks. The eggs collected from the faeces of these infected chicks were incubated at 39 degrees C. The free-swimming miracidia were found on the fifth day of incubation.     

Larval trematodes (Digenea) of the great pond snail, Lymnaea stagnalis (L.), (Gastropoda, Pulmonata) in Central Europe: a survey of species and key to their identification

A survey of cercariae and metacercariae (Trematoda, Digenea) from the great pond snail (Lymnaea stagnalis) in Central Europe (Austria, Czech Republic, South-East Germany, Poland and Slovak Republic) is presented, based on a study of 3,628 snails examined from 1998 to 2005. A total of 953 (26.3%) L. stagnalis were infected with 24 trematode species comprising 19 species of cercariae and 11 species of metacercariae (six species occurred both as cercarie and metacercarie) of eight families. The dominant cercariae were those of Opisthioglyphe ranae (159 hosts infected), Plagiorchis elegans (141) (both family Plagiorchiidae) and Echinoparyphium aconiatum (153) (Echinostomatidae); 14 double infections were found. The most frequent metacercariae were those of Neoglyphe locellus (71) (Omphalometridae), E. aconiatum (66), Echinostoma sp. (59) and Moliniella anceps (48) (Echinostomatidae). In the previous studies carried out in Central Europe, a very similar spectrum of nine trematode families of 22 cercariae determined to species level and 43 types of cercariae reported under generic or provisional names, which can be in many cases conspecific with the previous taxa, were found. A simple key to identification of cercariae and metacercariae, together with their illustrations, is provided.     

Infection by Paragonimus miyazakii cercariae of their crab hosts, Geothelphusa dehaani, by percutaneous penetration

Identification of the transmission routes of the trematode parasite Paragonimus miyazakii into different intermediate hosts would help to explain the natural distribution of the parasite. The behavior of P. miyazakii cercariae released from snails into water and in the presence of a living host or a whole crab leg was observed by stereoscopic or light microscopy at various times after exposure started. On encountering a crab leg or cheliped, the cercariae became entangled with the host via mucoid strands arising from the cercariae. Within 3 hr, most cercariae were attached to the host; cuticular penetration took between 5 and 6 hr, after which cercariae were found in the cavity of the leg. Crabs examined 102-149 days after exposure to the cercariae contained fully developed metacercariae. The metacercariae were fed to 2 rats, and the rats were killed 83 or 111 days later. Some of the metacercariae had reached maturity in the rats. That the cercariae were not ingested by the crabs but penetrated the crabs percutaneously (through hard as well as soft tissue) means that transmission can occur even in areas in which crabs and the host snails do not coexist, as they would if the usual route were oral (when the crabs ate infected snails).     

Cipangopaludina chinensis malleata (Gastropoda: Viviparidae): a new second molluscan intermediate host of a human intestinal fluke Echinostoma cinetorchis (Trematoda: Echinostomatidae) in Korea

Two species of the viviparid snails have been reported in Korea, e.g., Cipangopaludina chinensis malleata and Cipangopaludina japonica. Cipangopaludina chinensis malleata collected at 3 of 12 localities were found to be infected with metacercariae of Echinostoma cinetorchis, one of the snail-borne human intestinal trematodes in Korea. Metacercariae from these snails were fed to rats (S/D strain), and adult worms of E. cinetorchis, characterized by 37-38 collar spines on the head crown, were recovered from the ileocecal regions. However, no C. japonica collected from 2 localities harbored the metacercariae. In experiments with laboratory-bred viviparid snails, all viviparids were not susceptible to miracidia of E. cinetorchis. To confirm the identity of second intermediate hosts of E. cinetorchis experimentally, 2 species of viviparid snails were exposed to the cercariae from Segmentina hemisphaerula that had been infected with miracidia of E. cinetorchis. Both species of snails were susceptible to cercariae of E. cinetorchis. This is the first report of Cipangopaludina spp. serving as the second intermediate host of E. cinetorchis and as a potential source of human infection.     

Behavioral responses of Poecilia vivipara (Osteichthyies: Cyprinodontiformes) to experimental infections of Acanthocollaritrema umbilicatum (Digenea: Cryptogonimidae)

The swimming behavior of Poecilia vivipara was evaluated using an image analysis system comparing laboratory-reared uninfected fish before and after experimental infection with different intensities of cercariae of the trematode Acanthocollaritrema umbilicatum. Two experiments were performed, each with 30 fish which were individually exposed to 30 and 50 cercariae, respectively, shed from experimentally infected molluscs, Heleobia australis. Before and after (17–27 days) infection, the behavior of each fish was monitored in terms of Distance travelled, Ambulatory time, Stereotypic time, Resting time and Average speed. At the end of the experiments, the fish were dissected to count the number of metacercariae recovered. In the experiment with 30 cercariae, fish with 2–10 metacercariae did not exhibit any significant differences in their swimming activity, but those with 11–22 metacercariae had a significantly enhanced Stereotypic time and a reduced Time Resting. In the experiment with 50 cercariae, fish with 5–22 metacercariae had an enhanced Distance travelled and a reduced Average speed; highly significant differences occurred with regard to all behavioral parameters when considering the subgroup 23–36 metacercariae: Distance travelled, Stereotypic time, Resting time, Ambulatory time and Average speed. The swimming behavior of P. vivipara changed influenced by an intensity-dependence on metacercariae of A. umbilicatum, supporting the prediction that parasites are able to alter the behavior of their hosts.     

In vivo and in vitro encystment of Echinochasmus liliputanus cercariae and biological activity of the metacercariae

In vivo and in vitro encystment of the cercariae of Echinochasmus liliputanus and biological activity of the metacercariae were studied. In vivo encystment of cercariae occurred in the gills of goldfish, the second intermediate host. However, the cercariae also encysted in vitro in Locke solution (0.6x to 1.2x strength), 0.7-1.2% NaCI, artificial gastric juice, and human gastric juice. Locke or NaCI solutions were shown to be appropriate for in vitro encystment to occur within 24 hr; however, full-strength Locke solution was shown to be optimal. The 1-day-old metacercariae formed in vivo and treated in 0.1% sodium deoxycholate excystation medium at 37 C for 1 hr showed 88.5% excystation. The metacercariae formed in vitro, however, showed 88.6% and 85.0% excystation for normal and abnormal ones, respectively. Abnormal cysts at room temperature usually die within 10 days. About 70% of the normal cysts, both in vivo and in vitro, can still excyst after being stored in Locke 0.5x solution at 4 C for 3 mo. Cysts formed in vivo and in vitro were equally infective. The encystment of the cercariae in vitro could be inhibited when the cercariae were treated with 1 micromol silver nitrate. Because silver nitrate binds to the papillae, especially to the ciliated papillae, on the cercaria surface, it is suggested that papillary chemoreceptors may be involved in encystment of the cercariae. The finding of E. liliputanus cercariae encysting in vitro, especially in human gastric juice, might be helpful in elucidating mechanisms of the definitive hosts that are directly infected by the cercariae.     

Metacercarial dispersion and intracellular parasitism in a strigeid trematode

Combes C. and Nassi H. 1977. Metacercarial dispersion and intracellular parasitism in a strigeid trematode. International Journal for Parasitology7: 501–503. The life cycle of Apatemon graciliformis Szidat, 1928 (Trematoda, Strigeidae), a parasite of Biorrphalaria glabrata in Guadeloupe, involves a novel mode of transmission, experimentally demonstrated, between the second intermediate host and the definitive host. The furcocercariae penetrate gravid females of the ovoviviparous fish, Poecilia reticulata, and develop into metacercariae in vitelline vesicles of the embryos where they encyst a short time before parturition. The young guppies are born infected with 1–3 metacercariae. It is considered that young infected fish are more prone to predation by the definitive host, thereby increasing the probability of the cycle being completed. Domestic ducks have been experimentally infected with these metacercariae. If cercariae penetrate non-gravid P. reticulata, they enter the oocytes; this represents a phase of intracellular parasitism.     

Some aspects of the biology of the cercaria and metacercaria of Stephanostomum baccatum (Nicoll, 1907) Manter, 1934 (Digenea: Acanthocolpidae)

Experimental proof is presented that Cercaria neptunae Lebour is the cercaria of Stephanostomum baccatum and that the gastropods Buccinum undatum (L.) and Neptunea antiqua (L.) both act as its first intermediate hosts in the north-east Atlantic. Heavy experimental infections were associated with mortalities of 0+plaice Pleuronectes platessa (L.). The probable effects on plaice of infections at intensities below the lethal level are discussed. Under natural conditions almost all S. baccatum infection of plaice occurs in the first year of life, but newly emerged cercariae were shown experimentally to be capable of encysting in the musculature of plaice up to three years old. The thicker epidermis of older plaice may account for their resistance to infection. Common dabs were more easily infected than plaice. The ability of S. baccatum cercariae to infect plaice and common dabs is inversely related to the length of time between emergence and contact with the host. The differences in skin structure between opposite sides of the plaice body may account for the asymmetrical distribution of metacercariae in the musculature. The potential use of S. baccatum as a biological tag is discussed.     

Asexual reproduction in Pygospio elegans claparède (Annelida, Polychaeta) in relation to parasitism by Lepocreadium setiferoides (Miller and Northup) (Platyhelminthes, Trematoda)

Life-history theory predicts that parasitized hosts should alter their investment in reproduction in ways that maximize host reproductive success. I examined the timing of asexual reproduction (fragmentation and regeneration) in the polychaete annelid Pygospio elegans experimentally exposed to cercariae of the trematode Lepocreadium setiferoides. Consistent with adaptive host response, polychaetes that became infected by metacercariae of trematodes fragmented sooner than unexposed controls. Parasites were not directly associated with fission in that exposed polychaetes that did not become infected also fragmented earlier than controls. For specimens of P. elegans that were not exposed to trematodes, new fragments that contained original heads were larger than those that contained original tails, whereas original head and tail fragments did not differ in size for infected polychaetes. In infected specimens, metacercariae were equally represented in original head and tail fragments and were more likely to be found in whichever fragment was larger. Despite early reproduction, parasitism was still costly because populations of P. elegans exposed to parasites were smaller than controls when measured 8 weeks later and because exposure to cercariae reduced survivorship of newly divided polychaetes. Taken together, my results suggest that early fragmentation is a host response to minimize costs associated with parasitism.     

Life cycle of Stephanoprora uruguayense (Digenea: Echinostomatidae) in Argentina

The life cycle of Stephanoprora uruguayense Holcman et Olagüe, 1989, was experimentally resolved. In an artificial pond in the Zoological Garden in Buenos Aires City, Argentina, Heleobia parchappei (Hydrobiidae) was found to be releasing large-tailed cercariae with a prepharyngeal body, but lacking collar spines and corpuscles in the excretory system. Metacercariae, which encysted on the gills of naturally and experimentally infected Cnesterodon decemmaculatus (Poecilidae), developed collar spines and corpuscles in the excretory system in 7 days. Sexually mature adults were recovered from chicks and immature adults from mice fed metacercariae from C. decemmaculatus. Eggs shed in chick feces developed to miracidia within 10 days; sporocysts were found on the gills of snails. Stephanoprora uruguayense and S. denticulata from Europe are similar in adult morphology, but can be distinguished by morphological and behavioral features of larvae. Likewise, although S. denticulata and S. paradenticulata from Venezuela are similar to S. uruguayense in adult morphology, they differ considerably in larval morphology and intermediate hosts.     

The role of light and gravity in the experimental transmission of Echinostoma caproni (Digenea: Echinostomatidae) cercariae to the second intermediate host, Biomphalaria glabrata (Gastropoda: Pulmonata)

Trematode cercariae inhabit predictable environments and respond to trigger cues with genetically fixed releaser responses when foraging for the upstream host. The effect of light and gravity on the transmission of Echinostoma caproni cercariae to Biomphalaria glabrata was investigated experimentally. Transmission chambers were constructed of clear polyvinyl chloride pipe. Snails were constrained within the chamber to prevent movement, while permitting the cercariae to swim freely. A trial consisted of 2 infected B. glabrata shedding E. caproni cercariae placed at the center of the chamber, with 5 uninfected B. glabrata placed 10 cm on either side (or above and below) of the shedding snails as sentinels. There was no significant difference in the prevalence of infection sentinel snails in either experiment (light vs. dark or top vs. bottom); however, mean intensity was significantly higher in sentinel snails in the dark portion of the chamber (42.5 vs. 10.4; P = 0.001) and the top of the transmission chamber (66.1 vs. 38.0; P = 0.0003). There was a high correlation between the number of metacercariae collected from sentinel snails and the total number of infective units (metacercariae + unsuccessful cercariae): r = 0.992 (light vs. dark) and r = 0.957 (top vs. bottom), respectively, at cercariae densities estimated from 22 to 3,304/L. The results suggest that cercariae of E. caproni exhibit negative photo- and geotaxis in searching for a second intermediate host. Stereotypical releaser responses to environmental trigger cues (light and gravity) allow E. caproni cercariae to exploit flexible strategies for completing the life cycle consistent with the broad range second intermediate and definitive hosts used by E. caproni cercariae and adults, respectively.     

Encystment and metacercariae development of Echinostoma cinetorchis cercariae in an in vitro culture system

For some species of 37-collar-spined Echinostoma, their cercariae successfully encyst and develop to metacercariae in vitro. In our study, we cultured Echinostoma cinetorchis cercariae in 12 different media to study the formation of metacercariae. Locke's solution, medium 199, and RPMI 1640 were used as media for culture. RPMI 1640 produced the highest encystment and normal metacercariae development. The osmolality of the media was related to their ability to encyst and develop. The 0.5 X media induced higher encystment and normal metacercaria formation than the 1x media. The addition of fetal bovine serum to RPMI 1640 increased the level of encystment and normal metacercariae development. In the mixture of 0.5 x RPMI 1640 and 10% fetal bovine serum, encystment was highest, at 96.0%, and the development ratio of normal metacercariae was also the highest, at 91.5%, after 48 hr of cultivation. In a viability test, 7 day- and 14 day-cultured metacercariae were successfully matured to adult worms in experimentally infected rats. These results showed that E. cinetorchis cercariae could be cultured to viable metacercariae in an in vitro culture system and that 0.5 x RPMI 1640 plus 10% fetal bovine serum was the most useful medium for cultivation. This culture system can be adapted for additional studies on the E. cinetorchis life cycle, especially to supply large numbers of metacercariae for other studies on this echinostome.     

The distribution and incidence of larval trematodes in the freshwater fauna of the Wentloog level, South Wales

The Wentloog level, in South Wales, is a low-lying coastal plain dissected by a network of drainage channels. The latter contain a rich aquatic invertebrate fauna which harbour a wide range of larval digenetic trematodes. Few of the host species examined were not infected with larval trematodes. Miracidia together with their associated sporocysts, rediae and cercariae were more host specific than metacercariae but many of the miracidia which developed in pulmonate molluscs could do so in more than one genus. Metacercarial host specificity is less rigid, and usually species are only restricted to members of one phylum although several were found which could develop in two or more phyla. Most larval stages showed some host preference when they occurred in several host species and this was attributed to ecological or physiological factors or to mechanical barriers. Seasonal variation of infection rate occurs in all larval stages: infection with developing cercariae showed peaks during June and August, but metacercarial infection rates varied according to the host. In molluscs there may be two peaks of infection with metacercariae (as in the cercarial stages), but for many arthropods infections show a gradual increase through the year. The factors mainly responsible for these seasonal changes are considered to be the life-cycle of the intermediate hosts and the environmental temperature, the latter acting on the rate of development of the trematodes. Rainfall, and patterns of egg production of adult trematodes in the definitive hosts are also important.