Recent advances in our knowledge of the Myxozoa

In the last few years two factors have helped to significantly advance our understanding of the Myxozoa. First, the phenomenal increase in fin fish aq uaculture in the 1990s has lead to the increased importance of these parasi tes; in rum this has lead to intensified research efforts, which have incre ased knowledge of the development, diagnosis, and pathogenesis of myxozoans . The hallmark discovery in the 1980s that the life cycle of Myxobolus cere bralis requires development of an actinosporean stage in the Oligochaete. T ubifex 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. Tetra capsula bryosalmonae, from bryozoans. The 18S rDNA gene of many species has been sequenced, and here we add 22 new sequences to the data set. Phylogen etic analyses using all these sequences indicate that: 1) the Myxozoa are c losely related to Cnidaria (also supported by morphological data), 2) marin e 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 myxo zoan evolution, perhaps reflected by their having bryozoan. rather than ann elid hosts; 5) the morphology of actinosporeans offers little information f or determining their myxosporean counterparts (assuming that they exist), a nd 6) the marine actinosporeans from Australia appear to form a clade withi n the platysporinid myxosporeans. Ribosomal DNA sequences have also enabled development of diagnostic tests for myxozoans. PCR and in situ hybridisati on tests based on rDNA sequences have been developed for Myxobolus cerebral is. Ceratomyxa shasta. Kudoa spp,, and Tetracapsula bryosalmonae (PKX). Lec tin-based and antibody tests have also been developed for certain myxozoans , such as PKX and C. shasta. We also review important diseases caused by my xozoans. which are emerging or re-emerging. Epizootics of whirling disease in wild rainbow trout (Oncorhynchus mykiss) have recently been reported thr oughout the Rocky Mountain states of the USA. With a dramatic increase in a quaculture of fishes using marine netpens, several marine myxozoans have be en recognized or elevated in status as pathological agents. Kudoa thyrsites infections have caused severe post-harvest myoliquefaction in pen-reared A tlantic salmon (Salmo salar), and Ceratomyxa spp., Sphaerospora spp., and M yxidium leei cause disease in pen-reared sea bass (Dicentrarchus labrax) an d sea bream species (family Sparidae) in Mediterranean countries.