Welcome to the ICOPA 2022 Online Program

Introduction

In the present lecture, data on tapeworms (Cestoda) as pathogens of fishes are reviewed. Tapeworms represent a species-rich (about 5,000 species) group of parasitic flatworms (Neodermata) and parasitise all groups of vertebrates including humans.

This lecture is based on a comprehensive search of the literature and long-term (>30 years) personal experience of the author with fish tapeworms.

About 1,000 species occur as adults in elasmobranchs and nearly 500 species in in ray-finned fish. Tapeworms are common parasites of cultured fish, both as adults and larvae (metacestodes), but only relatively few adult tapeworms are actually pathogenic for their fish hosts. In contrast, larvae (metacestodes) can be harmful for fish, especially plerocercoids migrating throughout their tissue and internal organs.

Current knowledge of life cycles and host-parasite relationships, including immune response of fish infected with tapeworms, is still insufficient to enable adequate control of cestodoses. Treatment of fish infected with adult tapeworms is effective, whereas treatment of metacestodes is problematic. Control measures include interruption of the complex life cycle and prevention of transport of uninspected fish to new region.

Introduction

Understanding the impacts of parasite infection on host species in the wild is important for the proper managements of host’s conservation. A negative correlation between parasite infection and host body condition is generally assumed to be the causal effects of parasites. However, this could also be the consequence of host condition; poor conditioned individuals may be more susceptible to parasite infection. Moreover, infected hosts with poor body condition may suffer further infection. This positive feedback is likely to occur but has rarely been demonstrated in natural populations.

Methods

Here we conducted a mark-recapture survey on a stream salmonid-ectoparasite system: white-spotted charr Salvelinus leucomaenis vs. Salmincola sp., a parasitic copepod infecting the mouth cavity of the host in Hokkaido, Japan. Field surveys were carried out at three seasons (June, July, October) and body condition (residuals from body weight-length relationships), growth rate (mm/day) and parasite number were recorded. We conducted generalized linear mixed models (GLMMs) and structural equation modeling (SEM) to infer the causality among these variables.

Results

We found fish with poor condition were susceptible to further infections and copepod infections significantly reduced host body condition. These results suggest that parasite infection can be both cause and consequence of reduced host condition, and positive feedback is also possible.

Conclusions

This is the first study that demonstrated both causes and consequences of parasite infections and host condition through direct observations in the wild. Positive feedback creates heavily infected host, which could be the main infection sources and thus it can be the key for understanding the infection dynamics.

Introduction

Cod is an important demersal fish of northern Atlantic waters. Two main stocks have been identified in Norwegian waters, the Northeast Arctic cod (NEAC) and the coastal cod (NCC). Parasitic ascaridoids commonly occur in cod in high numbers.

Methods

Fish were sampled from the Barents and Norwegian Seas during 2019–2022. Fish were inspected for ascaridoids by the UV-press method and genotyped with the PanI locus, which discriminates between NEAC and NCC. Ascaridoids (n= 500) were identified by sequencing the mtDNA cox2 gene.

Results

Anisakis simplex (s.s.), Pseudoterranova decipiens (s.s.), P. krabbei, P. bulbosa, Contracaecum osculatum s.s., C. osculatum sp. A, C. osculatum sp. B, Phocascaris cystophorae, Hysterothylacium aduncum, and the seuratoid nematode Cucullanus cirratus were identified, often in mixed infections. Ascaridoids abundance were significantly positively related to fish length. Parasites were mostly located in viscera. Only A. simplex (s. s), P. krabbei and P. decipiens (s.s.) were found in flesh. P. decipiens (s.s.) often occurred in dorsal and caudal muscle, A. simplex (s. s) and P. krabbei mostly in belly flaps and P. bulbosa in liver. Massive infections by C. osculatum sp. B and A. simplex (s.s.) were found in caeca and of adult H. aduncum in the stomach lumen and intestine. Differences in species diversity and infection levels were found between NEAC and NCC. A. simplex (s.s.) showed high infection levels in both stocks. P. krabbei infection was significantly higher in NCC than NEAC, whereas Contracaecum spp. was higher in NEAC.

Conclusions

The high ascaridoid diversity in cod is sustained by the wide diversity of cetacean and pinniped hosts inhabiting the northern NE Atlantic and may be used as ecological markers of fish stocks.

Introduction

M. ellipsoides has been identified morphologically and by molecular methods in European chub (Squalius cephalus) only from plasmodia of the fins until now. The aim of our targeted study was to investigate the site selection and possible histopathological effects of this parasite.

Methods

Tissue samples of 13 European Chub from the river Traun in Austria were examined. They included 11 different organs of each fish. Spore morphology was assessed by microscopy and transmission electron microscopy and compared to published data from M. ellipsiodes. All organ samples were submitted to a nested PCR, based on 18S sequences with subsequent phylogenetic analyses. To differentiate M. ellipsoides unambiguously from other myxozoan species in the tissues, histological methods including ISH (results pending) were used.

Results

Myxobolus ellipsoides was detected in each of the organs except for blood and in each of the examined fish in at least two of the sampled organs. Histology showed mostly no severe effect of the infestations on the organs. Plasmodia were evident in tissues of kidney, liver, heart and intestine. Spores were found in melano-macrophage centers, granulomas, connective tissues, capillaries, in the pericardium and between myofibrils and in the mucosal as well as submucosal layers of the intestine.

Myxobolus ellipsoides specific PCR resulted in the identification of 11 subtypes, of which the most prevalent subtype was detected in all organs (excl. blood). In skin samples, this genotype was the only one detected.Three fish had genotypes which were not detected in any other fish.

Conclusions

Our findings present new data regarding tissue tropism, molecular genetic variability and spore morphology of M. ellipsoides in European Chub.

Introduction

Kudoa thyrsites infects the skeletal muscle of Atlantic mackerel. Heavy infections are associated with post-mortem myoliquefaction of the fish flesh which reduces the quality of the fillets. The sites of the parasite in mackerel are poorly known. We examined the distribution of K. thyrsites in various organs and tissues of mackerel, and investigate the relationship between parasite density and distribution in the musculature, and the extent of visible flesh myoliquefaction.

Methods

30 fish were examined for the presence and density of K. thyrsites in eight sections of the somatic muscle, heart and visceral organs by qPCR, whereas 6 myoliquefacted fish were histologically analysed for the distribution of parasite stages in the musculature. The degree of myoliquefaction was scored in all fish.

Results

qPCR analyses showed that K. thyrsites is unevenly distributed in the somatic musculature of mackerel, with highest density in the anterior belly flaps. A correlation was observed between the degree of myoliquefaction and parasite density in the muscle. Histology indicated an association between the dispersion of free myxospores and the level of muscle myoliquefaction, and the possibility that the myxospores may be dissolved in the most deteriorated areas. The parasite was also detected in heart and visceral organs with qPCR, although at low level.

Conclusions

For the first time, we describe the tissue distribution of K. thyrsites in NEA mackerel. The parasite shows a clear tropism towards the somatic musculature, where the distribution is uneven. The heart, where myocardial cells have been found infected in other host species, showed low K. thyrsites density. The low parasite density in the visceral organs could be caused by positive blood, which is likely in K. thyrsites.

Introduction

Background and aims: Parasites play important roles in ecosystems; even so, when the parasites are invasive, they may need to be eradicated from the host population. However, the eradication of one parasite species may have collateral impacts on other native parasite species. In this study, we assess the long-term recovery of native parasite communities following the eradication of the monogenean Gyrodactylus salaris from a lake system.

Methods

Methods: We examined the metazoan parasite community of Arctic charr Salvelinus alpinus, among three lakes, 6 to 9 years after an ecosystem-wide piscicide (rotenone) treatment which eliminated all fish and their parasites from the lake. We contrast short- and long-term recovery of the parasite community following the re-establishment of the charr population post-treatment.

Results

Results: Of the 14 parasite taxa present pre-treatment, 71% had re-established within nine years post-treatment, with most taxa attaining similar or greater abundance to pre-treatment levels. The parasite species that re-established first were those with complex life cycles and those with Snails or bivalves as first intermediate hosts. However, the most recently re-established taxa showed lower prevalence.

Conclusions

Conclusion: Our results highlight how native parasite re-establishment shows taxon-specific trajectories following an ecosystem-wide eradication treatment event, with depleted taxon richness still observed after nine years.

Introduction

The Lepocreadiidae, trematodes of marine fishes, infect a wide range of 2^nd intermediate hosts - annelids, cnidarians, ctenophores, echinoderms and molluscs. Here we report those found in “jellyfish” (cnidarians and ctenophores) from off eastern Australia.

Methods

Jellyfish were sampled from off the east coast of Australia between Brisbane and Hobart from 2019, including on two cruises of the RV Investigator. Animals were collected with trawled bongo nets and hand nets. Metacercariae were preserved in 96% ethanol and studied by ITS2 rDNA and cox1 mtDNA sequence data. Sequences were compared with those from adult lepocreadiids from fishes of eastern Australia and those available on GenBank.

Results

384 individuals of nine cnidarian species and four ctenophore species were examined. 133 ITS2 metacercarial sequences generated relate to six species of Cephalolepidapedon, Opechona, Opechonoides and Prodistomum. Abundance ranged from a single specimen of P. keyam to 63 for C. warehou. All six species are known as adults (variously) from the fishes Monodactylus argenteus, Scomber australasicus, two Seriolella species and multiple species of Pomacentridae. Two lepocreadiid species infected only ctenophores, two only cnidarians, and two infected both groups.

Conclusions

The fish definitive hosts are plausible predators of jellyfish, although for the Pomacentridae the predation is perhaps largely opportunistic. The data therefore suggest that jellyfish are important in the transmission of a small but distinct cohort of lepocreadiids. Exploitation of the available jellyfish fauna is uneven, with some species showing distinct specificity and others not. The evidence suggests that comparable lepocreadiid transmission occurs in many parts of the world.