Abstract
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.
Highlights
Myxozoans are obligate, microscopic, spore forming endoparasites, initially considered as Protozoa and long regarded as an enigmatic clade
Cellular and molecular evidence clearly places the myxozoans as parasitic cnidarians, this obscure group is still not broadly recognized by biologists [1]
Studies on Myxozoans were first published in the late 1800s, a burst of publications has occurred in recent decades with the advent of “omic” technologies (Figure 1)
Summary
Microscopic, spore forming endoparasites, initially considered as Protozoa and long regarded as an enigmatic clade. Myxozoans demonstrate extreme morphological simplification and miniaturization as a result of their parasitic lifestyle [2] This subphylum comprises only two classes, Myxosporea and Malacosporea. Exploitation of the fish immune system is one of the most promising This has led to an increasing interest in deciphering aspects of the host-parasite relationship that relate to invasion, migration and replication of the parasite in the fish, and the host response at each of these stages of infection. Furthering our understanding of the fish immune response against myxozoans will require establishing their life cycles and their route of entry into the fish host. Most known life cycles involve freshwater fish, with only few described from marine species [see S1 in Holzer et al [27]]. One known exception to this twohost life cycle are species of Enteromyxum, which can be transmitted directly between fish hosts [21]
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