Fish Intermediate Hosts Research Articles

Clonorchis sinensis and Opisthorchis viverrini: Development of a mitochondrial-based multiplex PCR for their identification and discrimination

We report a single, one-step PCR approach for detection and discrimination of Clonorchis sinensis and Opisthorchis viverrini in different life-stage forms (adults, metacercariae, and eggs) from fish intermediate hosts and from infected patients. Primers designed for species-specific PCR, amplifying portions of the mitochondrial (mt) genome, were also suitable for a multiplex PCR. The latter was a single, one-step reaction under high stringency conditions, using simultaneously 2 pairs of primers (1 pair for C. sinensis—product size 612 bp, and 1 pair for O. viverrini—product size 1357 bp). Assays using serially diluted templates demonstrated that as little as 0.78 ng of genomic DNA of either species could yield amplicons. Genomic DNA extracted from different life-stage forms including adult worms (of both species), eggs (of O. viverrini), eggs possibly of several trematode species (collected from patients infected with C. sinensis in Vietnam) and mixed metacercariae of common trematodes (collected from fishes in the C. sinensis endemic areas), yielded specific bands of the correct size and their identity was confirmed by sequence analysis. The multiplex PCR approach described here proved to be a species-specific, sensitive and fast tool for accurate diagnosis of clonorchiasis and/or opisthorchiasis, permitting the detection of their metacercariae in infected fishes or adult/eggs from patients in endemic areas.

Alternative life-history and transmission strategies in a parasite: first come, first served?

Alternative transmission strategies are common in many parasitic organisms, often representing discrete phenotypes adopted in response to external cues. The facultative truncation of the normal 3-host life-cycle to a 2-host cycle in many trematodes provides an example: some individuals mature precociously, via progenesis, in their intermediate host and produce eggs without the need to reach a definitive host. The factors that determine how many and which individuals adopt the truncated life-cycle within a parasite population remain unknown. We investigated the occurrence of progenesis in the trematode Stegodexamene anguillae within its fish intermediate host. Location within the host was a key determinant of progenesis. Although the size and egg output of progenetic metacercariae encysted in host gonads did not differ from those of the few progenetic metacercariae in other host tissues, the likelihood of metacercariae becoming progenetic was much higher for those in the gonads than those elsewhere in the host. Progenetic parasites can only evacuate their eggs along with host eggs or sperm, providing a link between the parasite's transmission strategy and its location in the host. Host size and sex, and the presence of other parasite species in the host, did not affect the occurrence of progenesis in S. anguillae. However, the proportion of metacercariae in host gonads and the proportion of progenetic metacercariae both decreased with increasing numbers of S. anguillae per host. These results suggest that progenesis is adopted mostly by the parasites that successfully establish in host gonads. These are generally the first to infect a fish; subsequent arrivals settle in other tissues as the gonads quickly become saturated with parasites. In this system, the site of encystment within the fish host both promotes and constrains the adoption of a facultative, truncated life-cycle by the parasite.

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.

Posthodiplostomum cuticola(Digenea: Diplostomatidae) in intermediate fish hosts: factors contributing to the parasite infection and prey selection by the definitive bird host

Infection parameters of Posthodiplostomum cuticola, a digenean parasite with a complex life-cycle, were investigated in fish (the second intermediate host) from 6 floodplain water bodies over 2 years. A broad range of factors related to abiotic characteristics of localities, density of the first intermediate (planorbid snails) and definitive (wading birds) hosts and fish community structure were tested for their effects on P. cuticola infection in juvenile and adult fish. Characters of the littoral zone and flood duration were found to be important factors for the presence of the first intermediate and definitive hosts. Visitation time of definitive bird hosts was also related to adult fish host density. Localities with P. cuticola infected fish were visited by a higher number of bird species. Infection of P. cuticola in fish and similarities in infection among fish host assemblages were correlated with fish host density and fish species composition. Parasite infection in both adult and juvenile fishes was associated with the slope of the bank and the bottom type, in particular in juvenile fish assemblages with snail host density. We conclude that habitat characteristics, snail host density and fish community structure contribute significantly to P. cuticola infection in fish hosts.

Parasite-induced change in host behaviour and susceptibility to predation in an eye fluke–fish interaction

Trophically transmitted parasites may increase their transmission efficiency by altering the behaviour of infected hosts to increase their susceptibility to predation by target hosts (the next host in the life cycle). The parasite Diplostomum spathaceum (Trematoda) reduces the vision of its fish intermediate hosts: its metacercariae lodge themselves in the eyes of fish and induce cataract formation, which gives them the opportunity to affect fish behaviour. We examined whether D. spathaceum eye flukes change the preference of fish for the surface layers of the water column or their escape behaviour, which could make the fish more vulnerable to predation by bird hosts. We also studied the influence of parasites on the susceptibility of fish to artificial aerial predators that were able to catch fish from the water surface. Infected and control fish did not differ in their preference for the surface layers but infected fish showed less escape behaviour when a black plate was drawn over the water surface. They were also more easily caught by human ‘predators’ dipping a net into the tank. Thus, infected fish should be easier prey for gulls and terns, implying that the ability of D. spathaceum eye flukes to alter fish behaviour may be a parasite strategy evolved to enhance transmission.

Detection of Opisthorchis viverrini in experimentally infected bithynid snails and cyprinoid fishes by a PCR-based method.

A PCR procedure for the detection of Opisthorchis viverrini in experimentally infected bithynid snails and cyprinoid fishes was developed. This procedure was based on primers designed from a pOV-A6 specific probe sequence giving a 330 base-pair product. The detection was accomplished in host tissue homogenates to which a single cercaria or metacercaria was introduced. PCR can detect as little as a single cercaria artificially inoculated in a snail or a single metacercaria artificially inoculated in a fish sample. The method gave a 100% positivity rate for all infected snails or fishes. The method did not yield a 330 base-pair amplified product with other digenean fluke DNAs such as Haplorchis taichui, Centrocestus spp., Echinostoma malayanum, Fasciola gigantica, animal schistosomes, Paragonimus heterotremus or Haplorchoides spp. The assay has great potential for application in epidemiological surveys of both snail and fish intermediate hosts as well as for investigation of foodborne parasites in freshwater fishes.

Diplostomum spathaceum cercariae respond to a unique profile of cues during recognition of their fish host

During its normal life cycle, Diplostomum spathaceum cercariae attach to and invade fish intermediate hosts. They are also known to attach to various other aquatic animals in response to water currents, touch and carbon dioxide. The purpose of this study was to identify the specific stimuli used by D. spathaceum cercariae to recognise the appropriate fish host. We characterised the host cues which stimulate them to remain on the host (enduring contact) and to penetrate the skin. Cercariae were exposed to animal skin tissues and fish skin surface mucus, their extracts and chemical modifications integrated into agar or offered via membrane filters. Enduring contact was stimulated by hydrophilic extracts Mr<3 kDa, which were sensitive to oxidation of carbohydrates. The stimulating cues are probably small molecular carbohydrates, as monosaccharides stimulated enduring contacts, but amino acids, urea, electrolytes and peptides did not. Penetration was stimulated by hydrophilic macromolecules, Mr>30 kDa, and by lipids. The hydrophilic stimuli were protease resistant and precipitable with Alcian blue and they were sensitive to alkaline cleavage, to digestion with lysozyme and neuraminidase as well as to oxidation of sialic acids. They were considered to be glycoproteins with O-glycosidically linked carbohydrate chains and bound sialic acids as signal structures. The lipophilic penetration stimuli were contained exclusively in the fatty acid fractions, and the stimulating characteristics of these fatty acids resembled the stimulating penetrations in other cercarial species. Diplostomum spathaceum cercariae respond to a unique profile of cues in their sequence of host-recognition phases. These cues differ from those used in other fish parasites studied to date and underline the diversity of fish recognition strategies.

Larval Tapeworms (Cestoda: Dilepididae) from the Mummichog Fundulus heteroclitus (Linnaeus, 1766) and Striped Killifish Fundulus majalis (Walbaum, 1792) from South Carolina, U.S.A.

Larval stages (metacestodes) of 2 dilepidid tapeworms (Eucestoda: Cyclophyllidea) are reported from the mesenteries and liver of the mummichog Fundulus heteroclitus and striped killifish Fundulus majalis from South Carolina, U.S.A. Cyclustera ibisae (Schmidt and Bush, 1972) is reported for the first time from a fish intermediate host, whereas metacestodes of Glossocercus caribaensis (Rysavy and Macko, 1973) (syn. Parvitaenia heardi Schmidt and Courtney, 1973) have only recently been found in fish from southeastern Mexico (Yucatan).

Interspecific associations among larval helminths in fish

Various processes can generate associations between the larvae of different helminth species in their fish intermediate or paratenic host. We investigated the pairwise associations among larval helminth species in eight different fish populations, using two different coefficients of associations, in order to determine in what situations they are strongest. All helminth species included use the fish studied as either their second intermediate host or their paratenic host, and are acquired by the fish when it ingests an infected first intermediate host. The intensity of infection correlated positively with fish length for most helminth species. Pairs of species which both exhibited positive correlations with fish length tended to be more strongly associated with one another, although this tendency was not pronounced. Similarity in life cycle had a more important influence on pairwise associations. Among the 62 pairwise associations that could be computed, pairs of helminth species that shared both first intermediate hosts and definitive hosts were the most strongly associated, followed by pairs that shared only one other host, and finally by pairs that did not share other hosts. The results suggest that assemblages of larval helminth parasites in fish are not random collections of locally available species, but rather structured packets of larval parasites that travel together along common transmission routes.

Host manipulation by Ligula intestinalis: accident or adaptation?

Numerous studies have demonstrated that parasites with complex life-cycles can cause phenotypic modifications in their hosts that lead to an increased rate of transmission, and suggest that these modifications are the result of parasitic adaptations to manipulate the host. Little attention is paid, however, to separating the possibility of adaptive host manipulation from incidental (if fortuitous) side-effects of infection. In this study we combine statistical and analytical tools to interpret the impact of the macroparasite Ligula intestinalis L. (Cestoda, Pseudophyllidea) on the behaviour of its intermediate fish host (the roach, Rutilus rutilus L.), using field data on a natural system. Two distinct sets of generalized linear models agree that both the presence and the intensity of infection contribute to a modified behavioural response in the host. This was illustrated by a preference for the lake-edge in infected fish during autumn. Furthermore, the effect of parasites upon their host is heterogeneous with respect to parasite size, with larger parasite individuals having a disproportionate impact. A series of game-theoretic models of adaptive host manipulation illustrate a potential rationale for a size-dependent manipulation strategy in parasites. These findings illustrate the potential complexity and functionality of the impact of L. intestinalis upon its fish host, which together reduce the parsimony of the alternative 'incidental effect' hypothesis.

Observations on the morphology and life-cycle of Procerovum varium (Onji & Nishio, 1916) (Trematoda: Heterophyidae).

The life-cycle of Procerovum varium (Digenea: Haplorchiinae) was studied experimentally and the morphology of stages in the life-cycle has been described and illustrated. Infections with adult flukes were found in the pond heron Ardeola grayii and heavy infections with metacercariae were found, attached to the liver of the fish Oryzias melastigma (Oryziatidae) occurring in a freshwater stream situated in Visakhapatnam, India. The cercariae developing in the snail Thiara tuberculata possessed typical haplorchiine features and were characterised by the presence of numerous cystogenous glands. Early stages of metacercarial development occurred free in the muscles of the fish intermediate host. The larvae reached the liver at 5 days post-infection, encystment commenced 2 days later and 15-day-old metacercariae were found to be infective to chicks, ducks and mice that served as suitable experimental hosts. The adult flukes obtained from natural and experimental infections showed many intraspecific variations, especially in the size and shape of the expulsor which depend on the quantity of sperm it contains. The validity of various species described in the genus and differentiated on the basis of differences in the size of the expulsor has been examined. It is concluded that only three species of the genus, namely P. varium, P. cheni and P. calderoni, are valid. "P. sisonli" of Chen (1949) is confirmed as a synonym of P. varium. P. varium is reported for the first time from India.

Diet and Lung Parasites of Swamp Forest Dwarf Crocodiles (Osteolaemus tetraspis osborni) in the Northern Congo Republic

Abstract Wild-caught and market-derived adult specimens of the Dwarf Crocodile Osteolaemus tetraspis osborni, caught in forests bordering the Congo and Oubangui Rivers in the northern Congo Republic, were examined for food residues in the stomach and pentastomid infections in the lungs. Stomach content analysis revealed that Osteolaemus was an opportunistic predator, taking a variety of invertebrates and small vertebrate prey in the dry season. Three species of pentastomids infected the lungs (prevalence 81%), representing three genera (all belonging to the family Sebekidae); every specimen was identified to instar. Two species, Agema silvaepalustris and Alofia parva, were represented by substantial numbers of infective larvae. Because this instar is also thought to occur in fish intermediate hosts, we postulated that some transmission continued in the dry season. Amphibious catfish (Clarias sp.) were common in swamp forest pools and may be the putative intermediate host.

Life cycle and seasonal dynamics of Cucullanus cirratus O.F. Müller, 1777 (Nematoda, Ascaridida, Seuratoidea, Cucullanidae) in Atlantic cod, Gadus morhua L.

Mature specimens of Cucullanus cirratus O.F. Müller, 1777 (Cucullanidae) were obtained from the pyloric caeca and intestine of Atlantic cod, Gadus morhua L., from Danish waters. Eggs embryonate in seawater. Third-stage larvae about 400 µm long, with amphids and dereids, hatch from the egg. Experimental studies indicated that third-stage larvae were infective to calanoid and cyclopoid copepods and sand gobies, Pomatoschistus minutus (Pisces, Gobiidae). Larvae entered the haemocoel of copepods but did not grow. In gobies, the third-stage larvae entered the intestinal mucosa and grew to 800 µm in length within 6 months. They were not encapsulated. Experimental infections of cod (8-30 cm long) showed that free-living third-stage larvae are not infective, whereas &gt;700 µm long third-stage larvae from gobies survived in the cod. Third-stage larvae 700-1200 µm long occur in the stomach mucosa, where they develop and moult to fourth-stage larvae. The fourth-stage larvae then migrate to the pyloric caeca and anterior part of the intestine, where they moult and develop to the mature adult stage. No developmental stage became encapsulated. Naturally infected cod (&gt;20 cm total length) harboured moulting third-stage larvae and &lt;2 mm long fourth-stage larvae 2 months post capture. Naturally infected 4- to 5-month-old codlings (8-10 cm total length) harboured 2-3 mm long fourth-stage larvae only, indicating that they acquired the third-stage larvae as planktivorous fry only a few centimetres long. Cucullanus cirratus may have a life-cycle that involves copepod transport hosts and fish (gobies or cod fry) intermediate hosts. Postcyclic development occurs in gadoids when an infected cod is consumed by another cod (cannibalism). Examinations of 350 (8-78 cm total length) naturally infected cod showed that group 1 and older cod are infected throughout the year, with maximum prevalence of third-stage larvae in spring and summer. The greatest prevalence of gravid worms was observed in autumn.

Otobothrium cysticum (Cestoda: Trypanorhyncha) from the muscle of butterfishes (Stromateidae).

On the basis of the tentacular armature, surface ultrastructure, and morphological measurements of plerocerci obtained from the musculature of butterfishes (Stromateidae), we corroborate an earlier proposal that Otobothrium crenacolle, a commonly reported trypanorhynch cestode from the northwestern Atlantic coast, is a junior synonym of O. cysticum. This action exemplifies at least an Atlantic Ocean and Indian Ocean distribution for O. cysticum. The infection in commercially important butterfishes shows that an otobothriid trypanorhynch may heavily infect fish flesh and influence the market value of some fish species yet also be restricted to the body cavity of other fish intermediate hosts. Infections of O. cysticum in the flesh of Peprilus burti (Gulf butterfish) and P. alepidotus (harvestfish) in the Gulf of Mexico has varied annually since 1970, with samples ranging in prevalence between 20% and 100% and in mean intensity between 1 and 3,500 or more plerocerci per fish. Comparative infections in P. burti from the Gulf of Mexico and P. triacanthus (butterfish) from the Atlantic Ocean demonstrate a present geographic difference in infections. The prevalence and mean intensity in 4 collections of butterfishes ranged from 9% to 98% of the fish and from 1 to 678 plerocerci in a subsample of tissue, respectively, with prevalent and heavy infections being observed in the Gulf of Mexico fish and relatively few individuals being infected with few worms in the Atlantic fish. A slight host response in the butterfishes involving some fatty infiltration and inflammatory infiltration was associated with the metacestode. In some larger fish, encapsulations were yellow, and in a few cases, worms had degenerated. This finding and an increase in intensity with fish weight suggest a continual accumulation of the worms in association with little host resistance.

A description of Agema, a new monotypic pentastomid genus from the lungs of the African dwarf and slender-snouted crocodiles

The lungs of wild-caught and market-derived crocodiles, including 15 dwarf crocodiles Osteolaemus tetraspis Cope and one slender-snouted crocodile Crocodylus cataphractus Cuvier were dissected for pentastomids. Of the three species recovered, one, found in both hosts, accounted for 66% of the specimens (N = 236) and most of these (N = 150) came from a single host. On the basis of these specimens a new genus, Agema, is described. The type-series consisted of nine adult males and five adult females, together with a complete sequence of instars beginning with the ‘double-hooked’ infective nymph which gains entry to crocodiles when these consume infected fish intermediate hosts. The evidence suggests that three instars separate the adult female from this infective instar, whereas only two are required in the case of males. Adult males are claviform and females are banana-shaped; in both sexes the tiny trapezoid head is strongly united with the abdomen. Males possess 52–58 abdominal annuli, females 58–63, and this number does not change with increasing age. The terminal part of the abdomen of females is abruptly tapered to a bluntly rounded point. Adult hook morphology is identical in both sexes; hooks are smooth with a gently curved barb and they become progressively more chitinised with each ecdysis. The buccal cadre is bridged anteriorly by chitin which appears to be crescentic when viewed from the ventral aspect and another crescent is present towards the pharynx – such a structure most closely resembles that of certain members of the genus Sebekia. The closest counterpart of the Agema male copulatory spicule is also found within this genus, but the two genera have very different hook morphologies. Infective Agema nymphs (= instar I) have large, strongly recurved hooks overlain by a spike which is typical of all members of the family Sebekidae. However, all but one of the succeeding instars have smooth hooks and, uniquely, instar III has a patch of minute spines at the base of the hook blade. The fulcrum of both instars II and III is provided with an anterior extension in the form of a delicate cowl into which hooks can probably retract. In adult males and pre-adult females (instar IV) and adult females (instar V), this is absent. Small differences in hook size between the anterior and posterior hooks, mostly accounted for by the blade, and apparent in all instars except instar I, are most pronounced in the final instar of both sexes. Smooth, gently curved hooks, and a singular permutation of characteristics (notably mouth morphology, body shape and the form of the copulatory spicule), which are important in the generic diagnosis of three other genera within the family Sebekiidae (Alofia, Sebekia and Selfia), define this new genus.

Morphogenesis ofContracaecum rudolphii(Nematoda: Ascaridoidea), a parasite of fish-eating birds, in its copepod precursor and fish intermediate hosts

Eggs of Controcaecum rudolphii obtained from female worms in double-crested cormorants (Phalacrocorax auritus ) in Nova Scotia, Canada, hatched in 9-17 days in sea water at 15-20 °C. The newly-emerged, free-living (and presumably second-stage) larva is described in detail, as are larvae from experimentally-infected copepods (Tigriopus sp.), amphipods (Gammarus sp.), and fish (Lebistes reticulatus, Fundulus heteroclitus) . Copepods are considered precursor hosts and morphogenesis of the parasite in them primarily involved body size, the ventricular appendix, and the excretory system. Infection of amphipods and fish was much more successful when invading larvae were from copepods than when they were free living; amphipods served as paratenic hosts and fish as intermediates. In fish, second-stage larvae 20 or fewer days postinfection were within the intestinal wall. At 44 or more days postinfection, larvae were considered third stage; all were in the abdominal cavity, many within a closely adhering « sleeve » of material resembling cuticle. The longest larvae (3,1-3.9 mm) in fish were associated with the oldest infection (152 d), suggesting that larvae continue to grow for a considerable period of time; growth was also found to be asynchronous, however.

A reassessment of the pentastomid genusLeiperia Sambon, 1922, with a description of a new species from both the Indopacific crocodileCrocodylus porosus and Johnston's crocodileC. johnsoni in Australia

The two pentastomid species currently included in the genusLeiperia Sambon, 1922 (family Sebekidae Fain, 1961) are re-assessed. Measurements of chitinous structures, notably the hooks, fulcra and oral cadre of three adult females of the otherwise well-characterised speciesL. cincinnalis Sambon, 1922, from the bronchi of the Nile crocodileCrocodylus niloticus, exhibit a degree of variation that is usual in pentastomid morphometrics. Similar measurements of six nymphs from the aorta and heart of two other crocodiles, although equally variable, do not differentiate these from infective nymphs removed from cysts within the body cavity of a South African fish (Pelamatachromis robustus) intermediate host. Adult females of another species,L. australiensis n. sp., from the bronchi of the Indopacific crocodileCrocodylus porosus from Northern Australia are described. Females (N=3) are smaller and more slender than their African counterparts, and their chitinous structures, although equally variable, are correspondingly smaller. A mature male, from the lung of Johnston's crocodileC. johnsoni, is also allocated to this species because the ranges and diet of the two crocodilian hosts overlap. Also, this zone of syntopy is sufficient to maintain populations of two other species belonging to the family Sebekidae (Sebekia spp.) in both hosts. This male is the first of the genusLeiperia to be fully described, and its copulatory spicules are quite distinctive. Three other immature specimens ofL. australiensis from the lung, or near to the lung, of the definitive host are also described. In the light of these findings the literature is re-appraised, and it is evident that at least two moreLeiperia spp. exist in South America, and that presently these are known only as nymphs. The existing South American species,L. gracilis Heymons & Vitzthum, 1935 (=Pentastoma gracile Diesing, 1835) was established only on the basis of immature or nymphal forms, almost certainly encompassing several genera. Despite claims to the contrary in the literature, this ‘species’ should be regarded as aspecies inquirenda. Aspects of the life-cycle ofLeiperia spp., from the infective stage in fish and the subsequent instars, are also discussed.

Sealworm (Pseudoterranova decipiens) infection in common seals (Phoca vitulina) and potential intermediate fish hosts from the outer Oslofjord

Infection with Pseudoterranova decipiens in 45 common seals (Phoca vitulina) from the outer Oslofjord was investigated. Seals were cellected before and during the phocine distemper virus epizootic in 1988 when the seal population in the area was reduced from about 350 to 100. In 1989, cod (Gadus morhua) and other fish species were sampled for comparison with earlier investigations on prevalence and abundance of P. decipiens infection in fishes from the outer Oslefjord. Seventy-two per cent of cod were infected with P. decipiens larvae in shallow waters between the seal skerries; the corresponding abundance was 2.2. At other fishing sites at increasing distances from the seal colony, abundance dropped to 0.05 – 0.20. Otoliths recovered from stomachs and scats indicated that gadids (Micromesistius poutassu, Trisopterus esmarkii and G. morhua) made up more than 80% of the seal diet. Bullrout (Myxocephalus scorpius) was also heavily infected with P. decipiens (prevalence 93% and abundance 8.4), but was not found in the seal diet. However, uninfected cod that enter shallow water from the surronading deep waters became infected when they feed on bullrout. Recruitment to mature stock of P. decipiens occurs when highly infected cod are eaten by seals. The number of reproducing P. decipiens is very low in comunon seals. Only 2 seals (i.e. less than 5% of the sample) were simultaneonaly infected with mature werms of both sexes. The dramatic reduction of the seal stock in the outer Oslofjord by the epizootic did not seem to affect the abundance of P. decipiens in its intermediate hosts.

The life cycle of Contracaecum osculatum (Rudolphi, 1802) sensu stricto (Nematoda, Ascaridoidea, Anisakidae) in view of experimental infections.

Hatched, ensheathed third-stage larvae of Contracaecum osculatum, 300-320 microns long, were shown to be infective to copepods, to nauplius larvae of Balanus and to small specimens of fishes (sticklebacks, O-group eelpout). Other fishes such as gobies and small flatfishes became infected by ingesting infected crustaceans. Cod were infected by being given infected small fishes. In the crustacean paratenic hosts, little growth of the larvae occurred, if any. In the liver sinusoids of sticklebacks and gobies the length of most of the unencapsulated third-stage larvae had not even doubled within 6 months of infection. The fate of larvae (max. 2 mm long) given to cod via infected intermediate fish hosts was apparently decided by the size of the larvae only. Small larvae became encapsulated and eventually died in the liver and wall of the gastrointestinal tract. Larger larvae migrated to the liver parenchyma, where some grew to a length of as much as 10 mm. The growth of the larvae in sticklebacks was shown not to be affected by an increase in temperature (infected fish being transferred from 8 degrees to 14 degrees and 20 degrees C), by the intensity of infection and, partly, by the age of infection (e.g. some 2-week-old and 6-month-old larvae were of identical size). In the liver and mesentery of plaice the third-stage larvae developed via copepod paratenic hosts to infectivity (i.e. to more than 4 mm in length), showing that the life cycle may be completed with an optional paratenic invertebrate host and only one intermediate fish host.(ABSTRACT TRUNCATED AT 250 WORDS)

Population dynamics of the metacercarial stage of the bucephalid trematode, Labratrema minimus (Stossich, 1887) from Salses-Leucate lagoon (France) during the cercarial shedding period

The population dynamics of Labratrema minimus (Stossich, 1887) metacercariae (Trematoda, Bucephalidae) were studied in its second intermediate fish host Atherina boyeri from Salses-Leucate lagoon (Northwest Mediterranean Sea, South of France), during the cercarial shedding period. The infection parameters (prevalence and mean abundance of infection) showed the high susceptibility of silversides (juveniles and adults) to infection. The observed variation in the mean abundance of infection with respect to time and host size was explained by (i) the susceptibility of A. boyeri to multiple successive infections (increase in abundance of infection with host length and raising of parasite burden until August) and (ii) the loss, probably by more rapid death, of heavily infected silversides from each host size group (decrease in abundance of infection in September within each host size group, stability of the parasite burden within the older class of hosts over the whole period, premature decrease in the condition factor in August). Finally, it was deduced that the infection pattern observed over the May-September period was a dynamic process controlled by: availability of infective cercariae, influx of newborn silversides free of metacercariae until September, high susceptibility of juvenile and adult A. boyeri to infection, metacercarial loss and probably by parasite-induced host mortality.