Synergism between trematode infection and pesticide exposure: a link to amphibian limb deformities in nature?

The apparently rapid increase in the prevalence of amphibian limb deformities has led to substantial interest from ecologists and public health professionals. Hypotheses proposed to explain the deformities fall into two broad categories: chemical contaminants and trematode infection. Although there are convincing experimental demonstrations that certain factors can lead to some deformities, the causes for recent increases in amphibian malformation remain controversial. Moreover, no experimental studies on amphibian deformities have been conducted in the field, and no studies have attempted to examine the synergistic effects of trematode infection and exposure to chemical contaminants. Here, I present the results of field and laboratory experiments that link increased trematode infection, and increased limb deformities, to pesticide exposure. Field experiments conclusively demonstrated that exposure to trematode infection was required for the development of limb deformities in wood frogs, Rana sylvatica. However, deformities were more common at sites adjacent to agricultural runoff. Laboratory experiments corroborated the association between pesticide exposure and increased infection with pesticide-mediated immunocompetency as the apparent mechanism. Given the conservative contaminant exposure levels used [Environmental Protection Agency (EPA) drinking water standards] and the widespread use of many pesticides, these negative impacts may help to explain pathogen-mediated amphibian declines in many regions.

The biology of gyrodactylid monogeneans: the "Russian-doll killers"

This article reviews the history of gyrodactylid research focussing on the unique anatomy, behaviour, ecology and evolution of the viviparous forms while identifying gaps in our knowledge and directions for future research. We provide the first summary of research on the oviparous gyrodactylids from South American catfish, and highlight the plesiomorphic characters shared by gyrodactylids and other primitive monogeneans. Of these, the most important are the crawling, unciliated larva and the spike sensilla of the cephalic lobes. These characters allow gyrodactylids to transfer between hosts at any stage of the life cycle, without a specific transmission stage. We emphasise the importance of progenesis in shaping the evolution of the viviparous genera and discuss the relative extent of progenesis in the different genera. The validity of the familial classification is discussed and we conclude that the most significant division within the family is between the oviparous and the viviparous genera. The older divisions into Isancistrinae and Polyclithrinae should be allowed to lapse. We discuss approaches to the taxonomy of gyrodactylids, and we emphasise the importance of adequate morphological and molecular data in new descriptions. Host specificity patterns in gyrodactylids are discussed extensively and we note the importance of host shifts, revealed by molecular data, in the evolution of gyrodactylids. To date, the most closely related gyrodactylids have not been found on closely related hosts, demonstrating the importance of host shifts in their evolution. The most closely related species pair is that of G. salaris and G. thymalli, and we provide an account of the patterns of evolution taking place in different mitochondrial clades of this species complex. The host specificity of these clades is reviewed, demonstrating that, although each clade has its preferred host, there is a range of specificity to different salmonids, providing opportunities for complex patterns of survival and interbreeding in Scandinavia. At the same time, we identify trends in systematics and phylogeny relevant to the G. salaris epidemics on Atlantic salmon in Norway, which can be applied more generally to parasite epidemiology and evolution. Although much of gyrodactylid research in the last 30 years has been directed towards salmonid parasites, there is great potential in using other experimental systems, such as the gyrodactylids of poeciliids and sticklebacks. We also highlight the role of glacial lakes and modified river systems during the ice ages in gyrodactylid speciation, and suggest that salmon infecting clades of G. salaris first arose from G. thymalli in such lakes, but failed to spread fully across Scandinavia before further dispersal was ended by rising sea levels. This dispersal has been continued by human activity, leading to the appearance of G. salaris as a pathogen in Norway. We review the history and current status of the epidemic, and current strategies for elimination of the parasite from Norway. Finally, we consider opportunities for further spread of the parasite within and beyond Europe.

Expression of immune response genes in rainbow trout skin induced by Gyrodactylus derjavini infections

By means of semi-quantitative RT-PCR, expression of a number of immune relevant genes was studied in skin of small rainbow trout Oncorhynchus mykiss (Walbaum, 1792) fry during both primary and secondary infections with the ectoparasitic monogenean Gyrodactylus derjavini Mikailov, 1975. The target genes studied included the cyto- and chemokines TNF-alpha1, TNF-alpha2, TGF-beta and IL-8, the inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX-2) genes and finally, two cell markers, the beta-chains of TCR and MHC II, from the adaptive arm of the immune system. In general, constitutive expression of all studied genes was apparent. Significant increases in expression of the TNF-alpha1 isoform could be observed at day 8 p.i. in primary infections and although less marked, the alpha2 isoform of TNF showed a similar trend. With the cytokine TGF-beta, 8-10 times increase in the transcription levels was observed in secondary infections compared to uninfected hosts. However, no parasite related changes in expression patterns could be observed for IL-8. Parasite infections elicited strong iNOS expression by 4 days p.i., but significant differences were not detected before day 8 p.i., when transcript levels were increased 5.5-9.6 times compared to uninfected controls. Augmented expression of COX-2 could also be observed in primary, but not secondary, infections at later stages of infections. No clear parasite related changes in transcript levels of the two cell markers TCRbeta and MHC IIbeta could be observed. Although the cellular source(s) was not determined, most of the examined factors appear to take part in a local signalling network of pivotal importance for the initiation, orchestration, effectuation and modulation of immune responses in rainbow trout against the ectoparasite G. derjavini.

Frequency distributions of parasites in a population of three-spined sticklebacks, Gasterosteus aculeatus L., with particular reference to the negative binomial distribution

All three species of parasites found in sticklebacks from Priddy had overdispersed distributions, similar in shape to host–parasite distributions described by other authors.

The distributions of both Echinorhynchus clavula and Diplostomum gasterostei were fitted closely by the negative binomial. The values of k were similar for both species, being always less than unity; the Diplostomum distributions were more overdispersed and therefore had higher values of p.

The Schistocephalus distributions were mostly described by the log normal, but the high percentage infection and relatively low overdispersion meant that the fit was often not close. There was also some truncation at the upper end.

The relationship between the parameters of the negative binomial (p and k) and the shape and parameters of the original distribution have been investigated, both theoretically and with respect to the distributions of the stickleback parasites.In general p varied as the overdispersion and k as the percentage infection.

The advantages of an overdispersed distribution to the host and parasite populations are discussed and related to the life cycles of the parasites.

I am most grateful to Dr H. D. Crofton for his help and encouragement during this study, and to the staff of the Computer Unit, University of Bristol for their assistance with programming problems. This work was supported by a Science Research Council research studentship and a NATO studentship.

Fasciola hepatica and Paramphistomum daubneyi: changes in prevalences of natural infections in cattle and in Lymnaea truncatula from central France over the past 12 years

A retrospective study was carried out over a 10- to 12-year period to analyse the changes in prevalences of natural fasciolosis and paramphistomosis among cattle and snails in central France, and to determine the causes which had induced these changes. The prevalences of natural fasciolosis in cattle increased from 1990 to 1993 (13.6% to 25.2%) and diminished afterwards up to 1999 (at 12.6%). Those of natural paramphistomosis showed a progressive increase between 1990 and 1999 (from 5.2 to 44.7%). The prevalences of natural infections and the numbers of free rediae counted in the snails (Lymnaea truncatula) infected with F. hepatica did not show any significant variations over time. By contrast, the prevalences of natural paramphistomosis in snails significantly increased from 1989 to 1996 and remained afterwards in the same range of values (3.7-5.3%), while the number of free rediae significantly increased up to 2000 (from a mean of 6.5 to 13.8 rediae per infected snail, respectively). Three hypotheses may explain the increase of paramphistomosis in cattle and snails: a better quality of diagnosis for the detection of P. daubneyi eggs in veterinary analysis laboratories, the use of specific molecules in the treatment of cattle fasciolosis since 1993, and the lack of an effective treatment up to now against cattle paramphistomosis. Since the objective of most farmers in central France is to obtain the highest antiparasitic efficiency with a single treatment of cattle per year, it is reasonable to assume that the prevalence of bovine paramphistomosis will continue to increase in the future.

The population dynamics of Gyrodactylus bullatarudis (Monogenea) within laboratory populations of the fish host Poecilia reticulata

Experiments were undertaken to investigate the factors which influence the transmission dynamics of Gyrodactylus bullatarudis within populations of laboratory guppies. The parasites possess a number of biological attributes which are almost unique amongst helminth parasites of vertebrates. These include the ability to reproduce viviparously and directly on the surface of the host, the ability to survive death of the host (for a short period of time) and the ability to transfer between hosts (despite the absence of a specialized transmission stage in the parasite's life-cycle). Long-term laboratory experiments demonstrated the inability of the parasite to persist within populations of the host in the absence of the continual introduction of naive susceptible fish. With regular addition of susceptible fish, the parasite population exhibited recurrent epidemic behaviour. The magnitudes of the epidemics and the time interval between them, were dependent on the rate at which fish were added to the populations. The parasite was over-dispersed in its distribution within the experimental fish populations and was a significant cause of host mortality (in a manner related to parasite burden). The experimental results suggest that acquired resistance to infection is an important factor determining epidemic behaviour.

Host-specificity of monogenean (platyhelminth) parasites: a role for anterior adhesive areas?

Monogeneans (flatworms) are among the most host-specific of parasites in general and may be the most host-specific of all fish parasites. Specificity, in terms of a restricted spatial distribution within an environment, is not unique to parasites and is displayed by some fungi, insects, birds, symbionts and pelagic larvae of free-living marine invertebrates. The nature of cues, how "habitats" are recognised and how interactions between partners are mediated and maintained is of interest across these diverse "associations". We review some experiments that demonstrate important factors that contribute to host-specificity at the level of infective stages (larvae of oviparous monogeneans; juveniles of viviparous gyrodactylids) and adult parasites. Recent research on immune responses by fish to monogenean infections is considered. We emphasise the critical importance of host epidermis to the Monogenea. Monogeneans live on host epidermis, they live in its products (e.g. mucus), monopisthocotyleans feed on it, some of its products are "attractants" and it may be an inhospitable surface because of its immunological activity. We focus attention on fish but reference is made to amphibian hosts. We develop the concept for a potential role in host-specificity by the anterior adhesive areas, either the specialised tegument and/or anterior secretions produced by monogeneans for temporary but firm attachment during locomotion on host epithelial surfaces. Initial contact between the anterior adhesive areas of infective stages and host epidermis may serve two important purposes. (1) Appropriate sense organs or receptors on the parasite interact with a specific chemical or chemicals or with surface structures on host epidermis. (2) A specific but instant recognition or reaction occurs between component(s) of host mucus and the adhesive(s) secreted by monogeneans. The chemical composition of fish skin is known to be species-specific and our preliminary analysis of the chemistry of some monogenean adhesives indicates they are novel proteins that display some differences between parasite families and species.

Relationship of tadpole stage to location of echinostome cercariae encystment and the consequences for tadpole survival

The effect of echinostome infections on the survival of Rana pipiens tadpoles was examined in relation to developmental stage of tadpoles. Individual tadpoles of Gosner stages 25, 27, 32-33, and 37-39 were exposed to 1 of 4 levels of cercariae (0, 20, 50, or 100). Only tadpoles at stage 25, the earliest stage infected, died within a 5-day experimental period. This stage-specific mortality rate could be explained, in part, by the stage-specific location of encystment of cercariae, which was documented in a separate experiment. In accordance with kidney development, cercariae predominately encysted in the pronephroi during early stages of tadpole development (stages 25 through 31-32) and only in the mesonephroi and associated ducts at later stages (stages 37 through 46). As the mesonephros develops, renal capacity presumably increases. Thus, tadpoles died only when metacercariae concentrated in the functional portion of the kidney with the most limited renal capacity. As tadpoles aged, they also became less susceptible to infections. On average, 69.5% of cercariae that were exposed to stage 25-26 tadpoles successfully encysted. compared with only 8.4% of cercariae exposed to stage 37-38 tadpoles. Exposures of metamorphic frogs (poststage 46) to cercariae revealed that these individuals can become infected with echinostomes. Collectively, our data highlight the host stage-dependent dynamics of tadpole-echinostome interactions.

Review of the Trematode Genus Ribeiroia (Psilostomidae): Ecology, Life History and Pathogenesis with Special Emphasis on the Amphibian Malformation Problem

Trematodes in the genus Ribeiroia have an indirect life cycle involving planorbid snails as first intermediate hosts, fishes or amphibians as second intermediate hosts and birds or mammals as definitive hosts. Although rarely pathogenic in definitive hosts, Ribeiroia infection can cause severe pathology and mortality in snails and amphibians. This group of parasites has gained notoriety for its prominent rol in the recent rash of amphibian deformities in North America. Under some circumstances, these malformations may enhance parasite transmission by rendering infected amphibian hosts more susceptible to definitive host predators. However, increasing reports of malformations in North American amphibian populations emphasize the importance of understanding infection patterns. Here we review important aspects of the biology, ecology, life cycle and pathogenesis of parasites in the genus Ribeiroia and identify priorities for future research. Based on available morphological descriptions and preliminary molecular data, three species of Ribeiroia are recognized: R. ondatrae in the Americas, R. marini in the Caribbean and R. congolensis/C. lileta in Africa. We further evaluate the influence of abiotic and biotic factors in determining the intensity and prevalence of Ribeiroia infection and malformations in amphibians, highlighting the importance of habitat alteration and secondary factors (e.g. aquatic eutrophication, contaminants) in promoting infection. Although not a "new" parasite, Ribeiroia may have increased in range, prevalence, or intensity in recent years, particularly within amphibian hosts. Nevertheless, while much is known about this intriguing group of parasites, there remains much that we do not know. Particular importance for future research is placed on the following areas: evaluating the phylogenetic position of the genus, establishing the molecular mechanism of parasite-induced malformations in amphibians, isolating the drivers of parasite transmission under field conditions and studying the consequences of malformations for parasite and host populations. Investigation of these questions will benefit enormously from a multidisciplinary approach that effectively integrates parasitology, developmental biology, immunology, herpetology and aquatic ecology.

The Hydrobia ulvae-Maritrema subdolum association: influence of temperature, salinity, light, water-pressure and secondary host exudates on cercarial emergence and longevity

The effects of environmental factors and exudates from the amphipod Corophium volutator on the emergence of Maritrema subdolum cercariae (Digenea: Microphallidae) from the snail Hydrobia ulvae were investigated in the laboratory. Increasing the temperature (15 to 25 degrees C) caused an overall 11-fold increase in emergence rate under varying salinities (24 to 36 per thousand). The effect of salinity depended on the experimental temperature. Emergence increased with increasing salinity at higher temperatures, but decreased with increasing salinity at 15 degrees C. Whereas the different levels of salinity had no effect, increasing the temperature significantly reduced the life span of cercariae. In comparison with complete darkness, light caused a two-fold increase in emergence, whereas an increment of the water pressure from 1.0 to 1.3 ATM (corresponding to 0 and 3 m of depth) left the shedding rate unaffected. Unidentified exudates from the second intermediate host, C. volutator, significantly depressed the cercarial emergence rate. The main transmission window of M. subdolum seems to occur during low water in tidal pools where light levels are high and solar radiation rapidly elevates the water temperature, as well as salinity through evaporation. The consequence of such a transmission strategy is discussed in relation to the impact of M. subdolum on the population dynamics of the second intermediate host.

Premature parturition in the California sea lion

Twenty percent of the California sea lion pups born on San Miguel Island die due to premature parturition. Specimens collected from premature-partus animals resulted in recovery of a virus, San Miguel Sea Lion Virus, indistinguishable from Vesicular Exanthema of Swine Virus, and Leptospira pomona from some of the premature cows and pups. The age range of 10 females delivering healthy pups in June was 10-14 years. With one exception, the ages in 10 aborting females was 6-8 years. The p,p'-DDE levels of the premature parturient cows' blubber and liver were 7.6 and 4.8 times greater, respectively, than corresponding tissue concentrations in the full-term animals. Polychlorinated biphenyls residues were 4.4 and 3.8 times greater in aborting animals' blubber and liver than in the same tissues of full-term sea lions. Premature-partus females had tissue imbalances of mercury, selenium, cadmium and bromine. Pathology, parasitology, serum enzyme and hormone results are also presented. These data suggest an interrelationship of disease agents and environmental contaminants as the cause of premature parturition.

Eye fluke-induced cataract formation in fish: quantitative analysis using an ophthalmological microscope

We examined the parasite-induced cataract formation in rainbow trout (Oncorhynchus mykiss) using slit-lamp microscopy to determine the relationship between cataract intensity and number of Diplostomum spathaceum parasites that were established in the lens. Cataract intensity increased significantly with parasite burden, but was also affected by the pattern of exposure to the parasite cercariae. The slit-lamp methodology proved useful in scoring the cataracts since it provides a 3-dimensional view into the lens and gives an actual picture of the location and intensity of the cataracts, which allows detailed investigations of mechanisms underlying cataract formation in various fish species. Potential applications of the method in fish farming and parasitological studies are discussed.

Selection by parasites in spate conditions in wild Trinidadian guppies (Poecilia reticulata)

Gyrodactylids are ubiquitous fish parasites and yet, with the notable exception of Gyrodactylus salaris, few studies have reported the effect of these parasites on host survival in natural populations. Here, we assess the impact of the parasite load of gyrodactylids (G. turnbulli and G. bullatarudis) on the survival and migration of guppies (Poecilia reticulata) in their natural habitat of the Aripo River in Trinidad. The recapture rate of males declined by 19% with every additional parasite, a remarkably high figure given that the parasite load in this study ranged from zero to 20 worms. In addition, with an increased number of parasites, males were more prone to be recovered downstream. In contrast, no effect of parasitism was observed in females. The mean parasite load sharply declined after a series of flushing events during heavy seasonal downpours. The parasite load varied significantly between fish depending on their location in the river, and the size of the fish explained variation in parasite load between individuals. The present study indicates that tropical gyrodactylid parasites can play an important role in the ecology of natural fish populations, causing intense bouts of natural selection in guppies during heavy rains in the wet season.

Gyrodactylus derjavini infection elicits IL-1beta expression in rainbow trout skin

Specific gene expression of the pro-inflammatory cytokine IL-1beta and the type II IL-1beta receptor (IL-1RII, 'decoy receptor') was studied in skin of rainbow trout Oncorhynchus mykiss (Walbaum, 1792) fry during infection with the ectoparasitic monogenean Gyrodactylus derjavini Mikailov, 1975. Cytokine expression was studied in both primary and secondary infections using a semi-quantitative RT-PCR technique. Generally, low levels of specific IL-1beta1, IL-1beta2 and IL-1RII gene transcription were found in uninfected hosts. In contrast, a clear induction of both IL-1beta isoforms could be observed during the initial phases of primary G. derjavini infections. A less obvious induction of IL-1beta expression was seen in secondary infections imposed just after recovery from the priming infection. However, challenge infections given one-month post full recovery of the primary infections did not elicit transcription levels above those seen in skin samples from uninfected hosts. Despite this, hosts carrying a secondary infection elicited a stronger response toward G. derjavini thus expelling parasites quicker than their nai;ve counterparts. When induced, indications of a more tight regulation of the IL-1beta1 isoform compared to IL-1beta2 could be observed. Increased expression of IL-1RII was seen later during the course of a primary infection and could be involved in the observed down regulation of IL-1, especially the beta1 isoform. The current study represents the first example of cytokine expression in fish induced by an ectoparasitic infection and indicates the importance of localised mucosal immune reactions in responses of fish towards gyrodactylids.

Molecular and morphological differentiation between species of the Plagiorchis vespertilionis group (Digenea, Plagiorchiidae) occurring in European bats, with a re-description of P. vespertilionis (Müller, 1780)

The taxonomic history of the species of Plagiorchis Lühe, 1899 occurring in European bats has been very confused because of high morphological similarity between different forms/species and the inadequate initial description of P. vespertilionis (Müller, 1780). As morphological data alone have not provided enough convincing arguments to solve the problem, the sequences from the nuclear rDNA ITS region (ITS1, 5.8S and ITS2) of three species of the P. vespertilionis group (P. vespertilionis, P. muelleri Tkach & Sharpilo, 1990 and P. koreanus Ogata, 1938) occurring in European bats were used to test the validity of these species and evaluate some of the morphological characters used for the species differentiation within this group. P. elegans from birds was used as the outgroup in the analysis. All three ingroup species were clearly distinguishable using ITS sequences. Among them, P. koreanus occupied a basal position, while P. vespertilionis and P. muelleri appeared as a cluster of two closely related, derived species. ITS sequences of the specimens obtained from different hosts and/or geographical areas did not exhibit any intraspecific variability. Morphological study of the material in collections revealed characters which enable the species of Plagiorchis from bats in Europe to be distinguished. Taking into account that the type-material of P. vespertilionis. described during the 18th Century, has been lost, for nomenclatural stability, a neotype is established and described. An amended description of the type-material of P. muelleri and keys for the determination of Plagiorchis spp. from European bats are presented.

Glucocorticosteroid Hormone Treatment of Larval Treefrogs Increases Infection by Alaria Sp. Trematode Cercariae

In many amphibian species, an apparent increase has occurred in the prevalence of limb deformities caused by parasitic trematodes. We are interested in the role of environmental stressors in increasing these infections in amphibians. One mechanism by which environmental stressors could act to increase disease prevalence is to increase circulating levels of glucocorticosteroid hormones, which are released in response to stressors and can be immunosuppressive. In the present study, we treated gray treefroZg tadpoles (Hyla versicolor) with exogenous corticosterone, which is the main glucocorticosteroid "stress" hormone in amphibians. We then exposed treated tadpoles to Alaria sp. cercariae and scored the number of mesocercariae that successfully infected the tadpoles. In addition, we assayed one function of the immune response by counting the number of circulating eosinophilic granulocytes, which are thought to be important in immune responses to macroparasites. Tadpoles treated with exogenous corticosterone developed higher parasite loads than control tadpoles did, and they had lower numbers of circulating eosinophilic granulocytes. These results provide evidence of glucocorticosteroid-mediated immunosuppression in tadpoles that may help to explain apparent increases in the numbers of trematode-induced deformities in amphibian populations during recent decades.

Co-existence of nine gill ectoparasites (Dactylogyrus: monogenea) parasitising the roach (Rutilus rutilus l.): history and present ecology

Co-existence among potentially competing species can be favoured by niche specialisation and/or by reducing the overall intensity of competition via aggregated utilisation of fragmented resources. We investigated the respective roles of niche specialisation and aggregation in the case of nine congeneric monogenean parasites on the gills of Roach (Rutilus rutilus L.) belonging to the genus Dactylogyrus. The position of each individual parasite of the nine Dactylogyrus species was recorded. Niche breadth and niche overlap of parasite species were estimated. Comparative methods, which take into account phylogenetic information of the analysed species, were used. We reconstructed a phylogeny of the nine Dactylogyrus species based on morphological characters. We used the 'aggregation model of co-existence' in the model to test if species co-existence is facilitated when intraspecific aggregation exceeds interspecific aggregation. We observed a lack of negative correlation in abundance between pairs of parasites, and a negative correlation between niche size and parasite aggregation, for both intraspecific and interspecific aggregation. Our comparative analysis showed that parasite abundance is positively correlated with niche breadth. Then parasite abundance, and not interactions between Dactylogyrus species, seems to be the most important factor determining niche size This result gives some support to niche segregation by specialisation. Niche size was negatively correlated with both intraspecific and interspecific aggregation. No relationship was found between an increase of interspecific aggregation with an increase of niche overlapping, which suggests that competition may play little role. A lack of competition could be also confirmed by the lack of negative correlation in abundance between species pairs. A parsimony analysis of the evolution of gill distribution indicates a change in one parameter of the niche (arch, segment and/or area) at each branching event.

Development and application of an eDNA method to detect and quantify a pathogenic parasite in aquatic ecosystems

Approaches based on organismal DNA found in the environment (eDNA) have become increasingly utilized for ecological studies and biodiversity inventories as an alternative to traditional field survey methods. Such DNA-based techniques have largely been used to establish the presence of free-living organisms, but have much potential for detecting and quantifying infectious agents in the environment, which is necessary to evaluate disease risk. We developed an eDNA method to examine the distribution and abundance of the trematode Ribeiroia ondatrae, a pathogenic parasite known to cause malformations in North American amphibians. In addition to comparing this eDNA approach to classical host necropsy, we examined the detectability of R. ondatrae in water samples subject to different degradation conditions (time and temperature). Our test exhibited high specificity and sensitivity to R. ondatrae, capable of detecting as little as 14 fg (femtograms) of this parasite's DNA (1/2500th of a single infectious stage) from field water samples. Compared to our results from amphibian host necropsy, quantitative PCR was -90% concordant with respect to R. ondatrae detection from 15 field sites and was also a significant predictor of host infection abundance. DNA was still detectable in lab samples after 21 days at 25°C, indicating that our method is robust to field conditions. By comparing the advantages and disadvantages of eDNA vs. traditional survey methods for determining pathogen presence and abundance in the field, we found that the lower cost and effort associated with eDNA approaches provide many advantages. The development of alternative tools is critical for disease ecology, as wildlife management and conservation efforts require reliable establishment and monitoring of pathogens.

Parasite resistance and avoidance behaviour in preventing eye fluke infections in fish

This paper examines the efficiency of acquired resistance in protecting the fish host, rainbow trout (Oncorhynchus mykiss), against the trematode parasite Diplostomum spathaceum, and the hypothesis that fish recognize areas where infective stages are aggregated and show avoidance behaviour. We found that when fish with a low level of infection were held in restricted cages in natural conditions they became infected and developed cataracts as a result of this infection. This suggests that acquired resistance is insufficient in protecting fish against the parasite or the deleterious effects of infection in conditions where fish could not avoid the parasite. Behavioural experiments in the laboratory showed that fish reacted to the parasite cercariae by avoiding the infection source, which decreased the rate of parasite establishment. We conclude that by using a combination of behavioural avoidance and physiological resistance, fish could defend against the parasite more effectively.

Trichobilharzia regenti, a pathogen of the avian and mammalian central nervous systems

The development of nasal avian schistosomes of the genus Trichobilharzia in their final host is poorly known. Therefore, an experimental infection of ducklings (Anas platyrhynchos f. dom.) by T. regenti was performed. The infection resulted in leg paralysis and orientation/balance disorders of birds. The examination of the duck's spinal cord and brain confirmed the presence of developing parasites in pre-patent as well as patent periods. The absence of the worms in other tissues strongly supports our hypothesis that the parasite migrates through the central nervous system (CNS) to its final location in bird nasal mucosa. The injury level is probably dependent on number of parasites as well as yet unknown host factors. The affinity to the CNS seems to be high; also by exposure of experimental animals to low cercarial doses the growing worms in the CNS were found. In addition to the generally accepted view that bird schistosomes may cause cercarial dermatitis of mammals (including man), there is evidence of a partial development of T. regenti in mouse CNS; in certain cases leg paralysis was also recorded. Therefore, the pathogenesis spectrum caused by bird schistosomes in birds/mammals needs to be reconsidered.

Pollution toxicity to the transmission of larval digeneans through their molluscan hosts

The increased occurrence of pollutants in ecosystems is a continuing area of concern. It is known that numerous diseases of wild aquatic animals can occur with decreased or increased prevalences in areas associated with high or chronic levels of pollution. This may have serious implications for environmental health. There has consequently been an increasing number of laboratory and field studies on disease transmission under polluted conditions, especially focusing on digeneans of medical or economic importance. The effect of pollutants to the transmission of larval digeneans (miracidia, cercariae, metacercariae) and snail-digenean interactions is therefore considered. An overview and interpretation of the published literature on laboratory and field studies is provided. It is apparent from these studies that the influence of pollutants on digenean transmission is highly complex with much of the observed effects in the laboratory often masked by a complexity of other factors in the field. Future studies would benefit from a standardisation of experimental procedures, increasing the number of combined laboratory and field studies, and increasing the complexity of the experiments undertaken.

Trichobilharzia regenti n. sp. (Schistosomatidae, Bilharziellinae), a new nasal schistosome from Europe

Members of the genus Trichobilharzia are parasitic in visceral or nasal body parts of their avian hosts. The examination of water snails in South Bohemia revealed a schistosome infection in Radix peregra snails. The experimental infection of ducklings (Anas platyrhynchos, Cairina moschata) confirmed that a new Trichobilharzia species--T. regenti n. sp.--was found. The adults, eggs and larvae (miracidia and cercariae) of the species were morphologically characterized. The adult occupy the nasal blood vessels where they lay the eggs; the miracidia hatch from the eggs directly in the tissue and leave the host during drinking/feeding of the infected birds.