Diagnosis and treatment of multi-species fish mortality attributed to Enteromyxum leei while in quarantine at a US aquarium

To React or Not to React: The Dilemma of Fish Immune Systems Facing Myxozoan Infections

Myxozoans are microscopic, metazoan, obligate parasites, belonging to the phylum Cnidaria. In contrast to the free-living lifestyle of most members of this taxon, myxozoans have complex life cycles alternating between vertebrate and invertebrate hosts. Vertebrate hosts are primarily fish, although they are also reported from amphibians, reptiles, trematodes, mollusks, birds and mammals. Invertebrate hosts include annelids and bryozoans. Most myxozoans are not overtly pathogenic to fish hosts, but some are responsible for severe economic losses in fisheries and aquaculture. In both scenarios, the interaction between the parasite and the host immune system is key to explain such different outcomes of this relationship. Innate immune responses contribute to the resistance of certain fish strains and species, and the absence or low levels of some innate and regulatory factors explain the high pathogenicity of some infections. In many cases, immune evasion explains the absence of a host response and allows the parasite to proliferate covertly during the first stages of the infection. In some infections, the lack of an appropriate regulatory response results in an excessive inflammatory response, causing immunopathological consequences that are worse than inflicted by the parasite itself. This review will update the available information about the immune responses against Myxozoa, with special focus on T and B lymphocyte and immunoglobulin responses, how these immune effectors are modulated by different biotic and abiotic factors, and on the mechanisms of immune evasion targeting specific immune effectors. The current and future design of control strategies for myxozoan diseases is based on understanding this myxozoan-fish interaction, and immune-based strategies such as improvement of innate and specific factors through diets and additives, host genetic selection, passive immunization and vaccination, are starting to be considered.

Effect of a functional feed additive on mitigation of experimentally induced gilthead sea bream Sparus aurata enteromyxosis

In gilthead sea bream Sparus aurata, infection by Enteromyxum leei produces a cachectic syndrome with anorexia, weight loss, severe epaxial muscle atrophy and, eventually, death. Currently, there are neither vaccines nor effective prescription medicines to control this infection. Nutraceutical approaches are raising interest in the aquaculture industry, responding to the lack of therapeutic tools for the management of insidious chronic losses due to parasites. In this study, the effect of a commercially available health-promoting feed additive (SANACORE® GM) at 2 different doses was tested in comparison with a basal diet without the additive during a laboratory-controlled challenge with E. leei. Group performance and biometrical values were monitored, and an in-depth parasitological diagnosis, quantification of parasite loads and histopathological examination were carried out at the end of the trial. Supplemented diets mitigated the anorexia and growth arrestment observed in challenged fish fed the basal diet. This mitigation was maximum in the highest dose group, whose growth performance was not different from that of unchallenged controls. Treated groups also presented lower prevalence of infection and a lower parasite load, although the differences in the mean intensity of infection were not statistically significant. Although the decrease in parasite levels was similar with both doses of additive tested, the pathogeny of the infection was mostly suppressed with the higher dose, while only mitigated with the lower dose. The mechanisms involved in the effects obtained remain to be investigated, but the results point to a modulation of the immunopathological response to the infection.