Identification and description of Bucephalus minimus (Digenea: Bucephalidae) life cycle in Portugal: morphological, histopathological, and molecular data

The cercaria of Bucephalus minimus infects the digestive gland and gonads of its first intermediate host, the edible cockle, Cerastoderma edule. Light microscopy (LM) and scanning electron microscopy (SEM) of the cercaria showed a tail formed by a central stem, with 2 long contractile arms presenting distinct morphological surfaces. The encysted metacercaria naturally infected the flathead grey mullet, Mugil cephalus. The cysts found in the heart, liver, and spleen were shown to be identical by the internal transcribed spacer (ITS 1) sequence and morphological features and were associated with encapsulation, recruitment of cell infiltrates, and presence of melanomacrophages and adipose tissue. To establish the life cycle, we compared the ITS1 sequence in an adult from the known definitive host, Dicentrarchus labrax; encysted metacercariae from the liver, heart, and spleen of M. cephalus; and a cercaria from C. edule. With this comparison, we determined that they had a 100% similarity. Therefore, the ITSI sequence data clearly indicate that these 3 parasitic stages belong to the same species, i.e., B. minimus.     

Redescription and life cycle of the monorchiid Postmonorcheides maclovini Szidat, 1950 (Digenea) from the Southwestern Atlantic Ocean: Morphological and molecular data

The adult monorchiid, Postmonorcheides maclovini Szidat, 1950, digenean parasite of the Patagonian blennie Eleginops maclovinus (Cuvier) (Eleginopidae) from Puerto Deseado (47° 45′ S, 65° 55′ W), Argentina, was characterized and its life cycle elucidated. P. maclovinus is the only species of the genus Postmonorcheides, proposed by Szidat (1950) from Tierra del Fuego province (~ 54° S), Argentina. This digenean uses the Patagonian blennie as definitive host, and the intertidal bivalve Lasaea adansoni (Gmelin) (Lasaeidae) as both first and second intermediate hosts (metacercariae encyst inside sporocysts), being the first record of this clam as intermediate host of trematode parasites. The cercaria may, in addition to encysting in the sporocyst, emerge and presumably infect other intermediate hosts. This is the second report of a monorchiid species with metacercariae encysting inside the sporocyst. Adults were found parasitizing the fish stomach, pyloric caeca and intestine with a prevalence of 100%; sporocysts with cercariae and/or metacercariae were found parasitizing the gonad of the bivalve with a prevalence of 2.78%. The cercariae possess a well-developed tail and eye-spots are absent. The ITS1 sequence from the adult digeneans found in the Patagonian blennie, identified as P. maclovini, was found to be identical to the ITS1 sequences obtained both from sporocysts containing cercariae and encysted metacercariae found in L. adansoni.     

范尼道佛吸虫的生活史研究

范尼道佛吸虫的生活史包括毛蚴、胞蚴、尾蚴、囊蚴和成虫5个阶段。它的第一中间宿主为淡水壳菜,第二中间宿主为多种小型鲤形目和鲇形目鱼类,终末宿主是鳜。文中对这种吸虫的胞蚴、尾蚴、囊蚴和成虫的形态特征以及尾蚴的行为特征进行了观察和描述。在用人工方法成功地将尾蚴感染青后,对不同发育阶段的囊蚴在鱼体内的形态变化和器官发育进行了观察。       

Infectivity of Ornithodiplostomum ptychocheilus and Posthodiplostomum minimum (Trematoda: Diplostomidae) cercariae following exposure to cadmium

The effect of cadmium (Cd) exposure on infectivity of cercariae of 2 trematode species (Ornithodiplostomum ptychocheilus and Posthodiplostomum minimum) to their second intermediate fish host was evaluated. Individual fathead minnows (Pimephales promelas) were exposed to cercariae that had been treated with Cd solutions at concentrations of 0, 2, 20, or 200 microg/L for 2 hr. Two weeks later, the numbers of encysted metacercariae in the brain (O. ptychocheilus) and body cavity (P. minimum) of the fish were evaluated. ANOVA analyses indicated a strong negative effect of Cd concentration on cercaria infectivity. The species x Cd concentration interaction was not significant, indicating that the magnitude of Cd-induced reduction in infectivity was similar between O. ptychocheilus and P. minimum. The results show that short-term exposure to Cd, even at low concentrations, interferes with transmission processes that affect the recognition and penetration of cercariae, the migration and survival of metacercariae within the second intermediate host, or both.     

Molecular identification of developmental stages in Opecoelidae (Digenea)

Nuclear ribosomal DNA sequences represent a useful tool for distinction of poorly differentiated developmental stages, such as trematode cercariae or metacercariae. Here, the complete internal transcribed spacer region of the ribosomal DNA (ITS 1 + 5.8S + ITS 2) was sequenced for 29 specimens of the digenean family Opecoelidae, including 16 adult specimens and 13 undescribed larval stages (nine cercariae and four metacercariae) occurring in various marine host organisms. Six cercariae and three metacercariae were found to match their corresponding adult form. This work also revealed that cercariae of the same species are able to infect more than one gastropod host species, suggesting that the specificity for the first intermediate host within the Digenea may be lower than previously thought.     

洞庭湖外睾吸虫新种及其生活史

本文报告洞庭湖区鲶鱼肠道寄生的洞庭湖外睾吸虫Exorchis dongtinghuensis sp.nov(新种)及其全程生活史,其第一中间宿主为湖北钉螺Oncomelania hupensis;第二中间宿主为鲤鱼、鲫鱼和金鱼;终宿主为鲶鱼Parasilurus asotus。作者对各期宿主作了人工感染试验和现场自然感染调查。对其发育过程作了观察比较。     

Infection status with trematode metacercariae in pond smelts, Hypomesus olidus

Many Koreans usually eat raw pond smelts, Hypomesus olidus, in the winter. This study was performed to evaluate the infection status with trematode metacercariae in pond smelts from January 1998 through February 1999. Among 1,305 fish collected, 459 were purchased from wholesale dealers in Chinchon-gun, Chungchongbuk-do, and the rest of them were caught with a casting net in Soyangho (Lake), Taehoman (Bay) and Paekkokchosuchi (Pond). Seven species of trematode metacercariae including two unidentified ones were detected from 1,305 pond smelts. The number of detected trematode metacercariae according to the species are as follow: Clonorchis sinensis 8, Holostephanus nipponicus 7, Cyathocotyle orientalis 24, Diplostomum sp. 14, and Metorchis orientalis 7. From the above results, it was confirmed that H. olidus plays a role as the second intermediate host of some kinds of trematode including C. sinensis in Korea. Our report shows possible clonorchiasis caused by eating raw pond smelts.     

Life history and life cycles: production and behavior of trematode cercariae in relation to host exploitation and next-host characteristics

Life history trade-offs affect trematode parasites reproducing inside their 1st intermediate hosts. Within the constraint of the effect on host survival, parasite production of cercariae is subject to a size-numbers trade-off. Within each cercaria, resources must be partitioned between host-seeking and subsequent developmental functions. Three species of microphallid trematodes with the same 1st intermediate host (the gastropod Littorina saxatilis) were investigated. Maritrema arenaria periodically released many small cercariae. Microphallus similis released fewer, 15% larger, cercariae without periodicity. Microphallus similis cercariae were strong swimmers, moving toward the dark and downward in turbulent water, whereas Ma. arenaria cercariae remained suspended. Maritrema arenaria cercariae, although smaller in body and tail size, were produced at an average daily volume nearly twice that of M. similis. These differences are interpreted as transmission adaptations related to mobility and predictability of the 2nd intermediate host. Microphallus similis, with a mobile and less predictable crab host, adopted a 'bethedging' prolonged production of fewer cercariae by less intensive host exploitation, each cercaria having a high allocation to host-seeking behavior. Maritrema arenaria, with predictable sessile barnacle hosts, produced less mobile but potentially longer-lived cercariae in larger numbers. Microphallus piriformes metacercariae remain in the gastropod host. The number of M. piriformes metacercariae increased in larger hosts. The 3 species differed in the number of sporocysts and (meta)cercariae per sporocyst within the gastropod but not in the within-host volume of parasites. Variation in host exploitation and life history appeared adaptive for transmission to the next host.     

A new cercaria and metacercaria of Acanthoparyphium (Echinostomatidae) found in an intertidal snail Zeacumantus subcarinatus (Batillaridae) from New Zealand

A new 23-collar-spined cercaria and metacercaria are described from intertidal molluscs of the coast of New Zealand. The new cercaria found emerging from the mud snails Zeacumantus subcarinatus (Sowerby) (Prosobranchia: Batillariidae) is characterized mainly by the number and arrangement of the cephalic glands, the size of the suckers, and the size and number of the collar spines. The cercaria encysts in the cockle Austrovenus stutchburyi (Wood) (Bivalvia: Veneridae) which lives in sympatry with the first intermediate host throughout New Zealand. Laboratory infections of cockles by cercariae from naturally infected snails resulted in metacercariae identical to those found in naturally infected cockles. The main features of the cercaria and metacercaria are the presence of a reniform collar with 23 spines, two pairs of small cephalic glands at the oral sucker level, another two pairs of much longer ones posterior to the pharynx, and the excretory vesicle Y-shaped with the main collecting canals extending to the anterior level of the ventral sucker. Each of the main collecting canals had 10-11 pairs of bilateral diverticula between the anterior edge of the ventral sucker and anterior body end. A brief discussion of its possible life cycle and ecology is also provided.     

The life cycle of Brachylaima ruminae n. sp. (Trematoda: Brachylaimidae), a parasite of rodents

The life cycle of Brachylaima ruminae n.sp. (Trematoda: Brachylaimidae), a duodenal parasite of rodents on the Mediterranean island of Formentera (Spain) is elucidated. The new species follows a terrestrial triheteroxenous life cycle. Eggs passed in the faeces of the definitive host must be ingested by a specific first intermediate host, the land snail Rumina decollata. Branched cercariogenous sporocysts develop in the digestive gland. Microcercous cercariae come out through the terminal birth pores of the branches. Cercariae shed by the snail are terrestrial, crawling on humid substratum. They contact the second intermediate host, another land snail, principally the species R. decollata and less frequently slugs and Helicids. Cercariae enter via the excretory pore and kidney duct to their specific final location, the kidney. Unencysted metacercariae develop in the kidney (also, less frequently, in the pedal glands) to the mature, infective stage. Infective metacercariae infest the definitive host when ingested together with the snail.     

Molecular Description of Macroorchis spinulosus (Digenea: Nanophyetidae) Based on ITS1 Sequences

We performed a molecular genetic study on the sequences of 18S ribosomal RNA (ITS1 region) gene in 4-day-old adult worms of Macroorchis spinulosus recovered in mice experimentally infected with metacercariae from crayfish in Jeollanam-do Province, Korea. The metacercariae were round, 180 μm in average diameter, encysted with 2 layers of thick walls, but the stylet on the oral sucker was not clearly seen. The adult flukes were oval shape, and 760-820 μm long and 320-450 μm wide, with anterolateral location of 2 large testes. The phylogenetic tree based on ITS1 sequences of 6 M. spinulosus samples showed their distinguished position from other trematode species in GenBank. The most closely resembled group was Paragonimus spp. which also take crayfish or crabs as the second intermediate host. The present study is the first molecular characterization of M. spinulosus and provided a basis for further phylogenetic studies to compare with other trematode fauna in Korea.     

Gymnophallid digenean Parvatrema duboisi uses Manila clam as the first and second intermediate host

To identify the metacercariae of a gymnophallid trematode in the Manila clam Ruditapes philippinarum from the Ariake Sea, experimental infection and molecular analysis were conducted. Based on the morphology of adult worms obtained from experimentally infected mice, the parasite was identified as Parvatrema duboisi. Comparison of internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) sequences for metacercariae and sporocysts from Manila clams and adult worms collected from wild wigeon Anas penelope showed minor differences ranging from 0 to 0.8%. These data strongly suggest that in the Ariake Sea, the parasite has a lifecycle using the Manila clam as the first and second intermediate hosts and wigeon as the definitive host.     

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.     

Experimental life history of Echinostoma hortense

The complete life cycle of Echinostoma hortense has been maintained in the laboratory, using Lymnaea pervia snails and Rana nigromaculata tadpoles as the first and second intermediate hosts. ICR mice was used as the definitive host. Within the egg of E. hortense, the miracidium was fully matured in 13 days of incubation at 29-30 degrees C. The miracidium was 93.8 x 53.6 microns in average size, covered with numerous cilia of 7-11 microns length. The epidermal plates were arranged in 6-8-4-2 formula. The first generation rediae (1.19 x 0.27 mm in average size) were observed in 14 days after miracidial challenge to the snails, and the second generation rediae (1.40 x 0.26 mm in average size) in 30 days. The average size of the cercaria was 295.5 x 145.0 microns. Their head crown was poorly developed, and collar spines were not yet observed. After a cercarial challenge to the tadpoles, all of the tadpoles became infected and the average worm recovery rate was 88.5%. The majority of the metacercariae (75.5%) were recovered from the muscle of the tadpole's posterior body and the rest (24.3%) from their gills. The metacercariae from the tadpoles were elliptical, and 167.7 x 129.9 microns in average size. The recovery rate of adults from the mice was different by the age of the metacercariae grown in the tadpoles. The metacercariae younger than 5 hrs could not infect mice whereas those older than 6 hrs could infect mice. The recovery rate became higher as the metacercaria matured, with the peak recovery rate of 90.0% at the metacercarial age of 9 days. Thereafter the recovery rate decreased to 55.0% at the age of 50 days. As shown by the above results, the whole life cycle of E. hortense has been completed in the laboratory. At least 55-58 days were required to maintain one egg-to-egg cycle of E. hortense.     

Does a facultative precocious life cycle predispose the marine trematode Proctoeces cf. lintoni to inbreeding and genetic differentiation among host species?

Intraspecific variability in parasite life cycle complexity (number of hosts and species of hosts in the life cycle) may have an impact how parasite genetic variation is partitioned among individual parasites, host individuals or host species within a given area. Among digenean trematodes, a three-host life cycle is common. However, a few species are precocious and may reach sexual maturity in what is typically regarded as the second intermediate host. The objective of this study was to determine whether a precocious life cycle predisposes digeneans to possible inbreeding or genetic subdivision among host species. As a study system, we used the digenean Proctoeces cf. lintoni whose metacercariae precociously mature (facultative) without a cyst wall in the gonads of multiple sympatric species of keyhole limpets (Fissurella spp.), typically regarded as the second intermediate hosts. Genotyped parasites were collected from four species of limpets and the clingfish Sicyases sanguineus, the third and final host where sexual maturity occurs. We found very high microsatellite diversity, Hardy–Weinberg equilibrium over all genotyped individuals, and little to no genetic structuring among parasites collected from the different host species. The fact that metacercariae do not encyst in the keyhole limpets, coupled with the high mixing potential of an aquatic environment, likely promote panmixia in local populations of P. cf. lintoni.     

Developmental and Phylogenetic Characteristics of Stellantchasmus falcatus (Trematoda: Heterophyidae) from Thailand

This study aimed to investigate the infection status, worm development, and phylogenetic characteristics of the intestinal trematode, Stellantchasmus falcatus. The metacercariae of S. falcatus were detected only in the half-beak (Dermogenus pusillus) out of the 4 fish species examined. Their prevalence was 90.0%, and the intensity of infection was 919 metacercariae on average. Worms were recovered from 33 (97.1%) of 34 chicks that were experimentally infected with 200 S. falcatus metacercariae each, and the average recovery rate was 43.0%. The body size and inner organs of S. falcatus quickly increased in the experimental chicks over days 1-2 post-infection (PI). In addition, ITS2 sequence data of this parasite were analyzed to examine the phylogenetic relationships with other trematodes using the UPGMA method. The results indicated that the ITS2 sequence data recorded from trematodes in the family Heterophyidae appeared to be monophyletic. This study concluded that D. pusillus serves as a compatible second intermediate host of S. falcatus in Thailand and that S. falcatus can develop rapidly in the experimental chicks. Data collected from this study can help to close the gap in knowledge regarding the epidemiology, biology, and phylogenetic characteristics of S. falcatus in Thailand.     

Austropeplea ollula (Pulmonata: Lymnaeidae): a new molluscan intermediate host of a human intestinal fluke, Echinostoma cinetorchis (Trematoda: Echinostomatidae) in Korea

Three freshwater snail species of the family Lymnaeidae have been reported from Korea, Radix auricularia coreana, Austropeplea ollula and Fossaria truncatula. Out of 3 lymnaeid snail species, A. ollula was naturally infected with the Echinostoma cinetorchis cercariae (infection rate = 0.7%). In the experiments with the laboratory-bred snails, F. truncatula as well as A. ollula was also susceptible to the E. cinetorchis miracidia with infection rates of 25% and 40%, respectively. All of three lymnaeid snail species exposed to the E. cinetorchis cercariae were infected with the E. cinetorchis metacercariae. It is evident that A. ollula acts as the first molluscan intermediate host of E. cinetorchis in Korea, and F. truncatula may be a possible candidate for the first intermediate host of this intestinal fluke. Also, three lymnaeid snail species targeted were experimentally infected with E. cinetorchis metacercariae.