Recent Advances in Our Knowledge of the Myxozoa

Abstract

In the last few years two factors have helped to significantly advance our understanding of the Myxozoa. First, the phenomenal increase in fin fish aquaculture in the 1990s has lead to the increased importance of these parasites; in turn this has lead to intensified research efforts, which have increased knowledge of the development, diagnosis. and pathogenesis of myxozoans. The hallmark discovery in the 1980s that the life cycle of Myxobolus cerebralis requires development of an actinosporean stage in the oligochaete. Tubifex tubifex, led to the elucidation of the life cycles of several other myxozoans. Also, the life cycle and taxonomy of the enigmatic PKX myxozoan has been resolved: it is the alternate stage of the unusual myxozoan, Tetracapsula bryosalmonae, from bryozoans. The 18S rDNA gene of many species has been sequenced, and here we add 22 new sequences to the data set. Phylogenetic analyses using all these sequences indicate that: 1) the Myxozoa are closely related to Cnidaria (also supported by morphological data); 2) marine taxa at the genus level branch separately from genera that usually infect freshwater fishes; 3) taxa cluster more by development and tissue location than by spore morphology; 4) the tetracapsulids branched off early in myxozoan evolution, perhaps reflected by their having bryozoan, rather than annelid hosts; 5) the morphology of actinosporeans offers little information for determining their myxosporean counterparts (assuming that they exist); and 6) the marine actinosporeans from Australia appear to form a clade within the platysporinid myxosporeans. Ribosomal DNA sequences have also enabled development of diagnostic tests for myxozoans. PCR and in situ hybridisation tests based on rDNA sequences have been developed for Myxobolus cerebralis, Ceratomyxa shasta, Kudoa spp., and Tetracapsula bryosalmonae (PKX). Lectin-based and antibody tests have also been developed for certain myxozoans, such as PKX and C. shasta. We also review important diseases caused by myxozoans, which are emerging or re-emerging. Epizootics of whirling disease in wild rainbow trout (Oncorhynchus mykiss) have recently been reported throughout the Rocky Mountain states of the USA. With a dramatic increase in aquaculture of fishes using marine netpens, several marine myxozoans have been recognized or elevated in status as pathological agents. Kudoa thyrsites infections have caused severe post-harvest myoliquefaction in pen-reared Atlantic salmon (Salmo salar), and Ceratomyxa spp., Sphaerospora spp., and Myxidium leei cause disease in pen-reared sea bass (Dicentrarchus labrax) and sea bream species (family Sparidae) in Mediterranean countries.

Fish immune response to Myxozoan parasites

“…However, nowadays, the presence of specific antibodies has been unambiguously reported in fish infected by the myxosporeans Myxobolus cerebralis (Hedrick et al, 1998), Myxobolus artus (Furuta et al, 1993), Tetracapsuloides bryosalmonae (Saulnier & Kinkelin, 1996), Ceratomyxa shasta (Bartholomew, 2001) and Enteromyxum scophthalmi (Sitjà-Bobadilla et al, 2004). The speed of antibody production is relatively low in comparison with available data for other fish immunized or vaccinated with different parasites.…”

“…The commonest picture observed in histopathological studies is the encapsulation of the parasitic stages by connective, fibrotic and epitheliod tissue layers, which is aimed to isolate the parasite and to prevent its dis-T he Myxozoan phylum gathers more than 2,180 species, being most of them fish parasites, though their presence has also been documented in waterfowl, amphibians, reptiles and even mammals (Lom & Dyková, 2006). Myxozoa are long-known organisms, which still today pose important questions about their origin, phylogenetic relationship and life cycle (Kent et al, 2001;Fiala, 2006). They are responsible for important economic losses among fisheries and aquaculture industries, and hence the high interest in studying the immune response of fish against them.…”

Light and electron microscopic studies on turbot Psetta maxima infected with Enteromyxum scophthalmi: histopathology of turbot enteromyxosis

“…In the last few years, due to the phenomenal increase in fish aquaculture, concerns regarding myxosporean infections have increased significantly (Diamant et al 1994, Kent et al 2001, Lom & Dyková 2006, Palenzuela 2006, Feist 2008. Although enormous advances have been made in the last few decades in aspects of myxozoan phylogeny and taxonomy (Fiala 2006, Bartosová et al 2009), the whole life cycle of many species remains unknown.…”

A new species of Henneguya, a gill parasite of Astyanax altiparanae (Pisces: Characidae) from Brazil, with comments on histopathology and seasonality

“…With the growth of aquiculture in several regions of the world since 1990 (Kent et al 2001), the study of fish pathologies has become increasingly important. Among agents responsible for diseases in fish are the myxosporeans, which cause serious damage to economically important fish species.…”