Experimental infection of Atlantic salmon (Salmo salar) with marine Eubothrium sp. (Cestoda: Pseudophyllidea): observations on the life cycle, aspects of development and growth of the parasite

Microbiome "Inception": an Intestinal Cestode Shapes a Hierarchy of Microbial Communities Nested within the Host

The concept of a holobiont, a host organism and its associated microbial communities, encapsulates the vital role the microbiome plays in the normal functioning of its host. Parasitic infections can disrupt this relationship, leading to dysbiosis. However, it is increasingly recognized that multicellular parasites are themselves holobionts. Intestinal parasites share space with the host gut microbiome, creating a system of nested microbiomes within the primary host. However, how the parasite, as a holobiont, interacts with the host holobiont remains unclear, as do the consequences of these interactions for host health. Here, we used 16S amplicon and shotgun metagenomics sequencing to characterize the microbiome of the intestinal cestode Eubothrium and its effect on the gut microbiome of its primary host, Atlantic salmon. Our results indicate that cestode infection is associated with salmon gut dysbiosis by acting as a selective force benefiting putative pathogens and potentially introducing novel bacterial species to the host. Our results suggest that parasitic cestodes may themselves be holobionts nested within the microbial community of their holobiont host, emphasizing the importance of also considering microbes associated with parasites when studying intestinal parasitic infections.

Praziquantel use in aquaculture - Current status and emerging issues

Parasitic diseases are major constraints in fish mariculture. The anthelmintic praziquantel (PZQ) can effectively treat a range of flatworm parasites in a variety of fish species and has potential for broader application than its current use in the global aquaculture industry. In this review we report on PZQ's current use in the aquaculture industry and discuss its efficacy against various flatworm parasites of fish. Routes of PZQ administration are evaluated, along with issues related to palatability, pharmacokinetics and toxicity in fish, while PZQ's effects on non-target species, environmental impacts, and the development of drug-resistance are discussed.

Endoparasitic helminths in Baltic salmon Salmo salar: ecological implications

Parasites in fish are ecological indicators, as they reflect the host's migration routes, feeding behavior and immune status. We performed a parasitological investigation of sea-running Baltic salmon to study the use of parasites as indicators for this fish stock. The host-a strain of Atlantic salmon Salmo salar-has been isolated for several millennia in the semi-enclosed brackish Baltic Sea, with limited migration to and from the North Sea. Twenty-four salmon (total body weight: 4.2-14.2 kg; total body length: 80-105 cm) were caught by spoon bait in the southern Baltic Sea during feeding migrations, necropsied shortly afterwards and internal organs subjected to parasitological investigation focusing on endoparasitic helminths. The pyloric region was heavily parasitized by the cestode Eubothrium crassum (100% prevalence; intensity: 97-273 parasites per infected fish), reflecting a diet of smaller pelagic fishes. The stomach contained the hemiurid digeneans Brachyphallus crenatus (95.8% prevalence; intensity: 8-151) and Hemiurus luehei (58.3% prevalence; intensity: 2-13), indicating a diet of clupeids. Schistocephalus solidus (25% prevalence; intensity: 1-2), liberated from ingested sticklebacks, the acanthocephalan Echinorhynchus truttae (54% prevalence; intensity: 1-13) and the adult nematode Hysterothylacium aduncum (29% prevalence; intensity: 1-13) were found in the intestine. The liver was parasitized by third-stage nematode larvae of Contracaecum osculatum (45.8% prevalence; intensity: 1-4), but these were growth-stunted and encapsulated. The parasite fauna differs markedly from salmon in North Atlantic waters, and the lack of purely marine parasite species indicates that the Baltic salmon has remained in the Baltic Sea during its life history.