Whirling disease of salmonid fish: life cycle, biology, and disease

Myxobolus cerebralis is the myxozoan parasite responsible for causing whirling disease in salmonid fish. Although the parasite was first described nearly 100 yr ago, it received relatively little attention until the discovery of its 2-host life cycle in the mid 1980s. This was the first, complete, myxozoan life cycle to be described, and it was greeted with some skepticism because it united 2 stages of M. cerebralis that were previously classified in 2 separate taxa. In the last decade, there has been a renewed interest in this parasite because whirling disease has been implicated in the decline of wild trout populations in several western states in the United States. Subsequent research efforts have dramatically increased the understanding of the biology of M. cerebralis and the numerous factors that affect the severity of whirling disease in salmonid hosts. These efforts also have provided a great deal of new information concerning interactions between M. cerebralis and its aquatic oligochaete host Tubifex tubifex. This review examines the current state of M. cerebralis in relation to 3 categories: the life cycle, the salmonid hosts, and the oligochaete host.     

Variability in triactinomyxon production from Tubifex tubifex populations from the same mitochondrial DNA lineage infected with Myxobolus cerebralis, the causative agent of whirling disease in salmonids

Myxobolus cerebralis, the causative agent of whirling disease, infects both salmonid fish and an aquatic oligochaete, Tubifex tubifex. Although M. cerebralis has been detected in river drainages throughout the United States, disease severity among wild fish populations has been highly variable. Tubifex tubifex populations have been genetically characterized using sequences from the 16S mitochondrial DNA (mtDNA) gene, the 18S ribosomal RNA gene, the internal transcribed spacer region 1 (ITS1), and randomly amplified polymorphic DNA (RAPD). Our earlier work indicated that large differences in compatibility between the parasite and populations of T. tubifex may play a substantial role in the distribution of whirling disease and resulting mortality in different watersheds. In the present study, we examined 4 laboratory populations of T. tubifex belonging to 16S mtDNA lineage III and 1 population belonging to 16S mtDNA lineage I for triactinomyxon (TAM) production after infection with M. cerebralis myxospores. All 4 16S mtDNA lineage III populations produced TAMs, but statistically significant differences in TAM production were observed. Most individuals in the 16S mtDNA lineage III-infected populations produced TAMs. The 16S mtDNA lineage I population produced few TAMs. Further genetic characterization of the 16S mtDNA lineage III populations with RAPD markers indicated that populations producing similar levels of TAMs had more genetic similarity.     

Interactions among two strains of Tubifex tubifex (Oligochaeta: Tubificidae) and Myxobolus cerebralis (Myxozoa)

Host-parasite interactions influence host population growth, host evolution and parasite success. We examined the interactions among Myxobolus cerebralis, the parasite that causes salmonid whirling disease, and resistant and susceptible strains of the oligochaete host Tubifex tubifex. Strains of T. tubifex with diverse genotypes often coexist in nature and have variable susceptibilities to M. cerebralis infection. Further, parasite proliferation differs by several orders of magnitude among T. tubifex strains. We examined total biomass produced by individual T. tubifex, including progeny production and adult growth, parasite proliferation and prevalence of infection using 2 strains of T. tubifex at 2 myxospore doses in a response-surface experimental design. Total biomass production per individual oligochaete and progeny biomass produced by an individual adult oligochaete were density-dependent for both resistant and susceptible individuals and the effects did not change with the addition of myxospores. However, both resistant and susceptible adults had highest growth when exposed to M. cerebralis. The presence of resistant oligochaetes in mixed cultures did not reduce the infection prevalence or parasite proliferation in susceptible individuals. In natural aquatic communities, resistant strains of T. tubifex may not reduce the effects of M. cerebralis on the salmonid host, particularly if sufficient numbers of susceptible T. tubifex are present.     

The effects of Myxobolus cerebralis myxospore dose on triactinomyxon production and biology of Tubifex tubifex from two geographic regions

The aquatic oligochaete Tubifex tubifex is an obligate host of Myxobolus cerebralis, the causative agent of salmonid whirling disease. Tubifex tubifex can become infected by ingesting myxospores of M. cerebralis that have been released into sediments upon death and decomposition of infected salmonids. Infected worms release triactinomyxons into the water column that then infect salmonids. How the dose of myxospores ingested by T. tubifex influences parasite proliferation and the worm host are not well understood. Using replicated laboratory experiments, we examined how differing doses of myxospores (50, 500, 1,000 per worm) influenced triactinomyxon production and biomass, abundance, and individual weight of 2 geographically distinct populations of T. tubifex. Worm populations produced differing numbers of triactinomyxons, but, within a population, the production did not differ among myxospore doses. At the lowest myxospore dose, 1 worm population produced 45 times more triactinomyxons than myxospores received, whereas the other produced only 6 times more triactinomyxons than myxospores. Moreover, total T. tubifex biomass, abundance, and individual weight were lower among worms receiving myxospores than in myxospore-free controls. Thus, T. tubifex populations differ in ability to support the replication of M. cerebralis, and infection has measurable consequences on fitness of the worm host. These results suggest that variability in whirling disease severity observed in wild salmonid populations may partially be attributed to differences in T. tubifex populations.     

Efficacy of passive sand filtration in reducing exposure of salmonids to the actinospore of Myxobolus cerebralis

The aquatic oligochaete Tubifex tubifex parasitized by Myxobolus cerebralis releases triactinomyxon (TAM) actinospores that can infect some species of salmonids and cause salmonid whirling disease. Silica sand was tested as a filtration medium for removal of TAMs from water containing the parasite. Laboratory tests indicated sand filtration removed > 99.99% of TAMs. In 2 different field tests, groups of 1 mo old rainbow trout Oncorhynchus mykiss were exposed for 2 wk to filtered and unfiltered water from a spring-fed pond enzootic for M. cerebralis. In November 2000, the exposure dose was estimated as between 3 and 5 TAMs fish(-1). During a March 2001 exposure, the estimated dose was between 286 and 404 TAMs fish(-1). Fish were held for 6 mo post exposure (p.e.) in laboratory aquaria for observation and evidence of clinical signs of whirling disease. We used 4 diagnostic techniques to assess the prevalence and severity of infection by M. cerebralis among fish exposed to filtered and unfiltered water. These included polymerase chain reaction (PCR) for genomic DNA of the parasite, histological evaluation for tissue damage, tissue digestion for quantification of cranial myxospores of the parasite, and total non-sampling mortality that occurred over 6 mo p.e. All diagnostic tests verified that the prevalence and severity of infection was significantly reduced among fish in treatment groups exposed to filtered water compared to those exposed to unfiltered water in both the low-dose and high-dose exposures.     

Prevalence of Myxobolus cerebralis infections among genetic lineages of Tubifex tubifex at three locations in the Madison River, Montana

Host biodiversity can impact disease risk and influence the transmission of parasitic disease. Stream sediment-dwelling worms, Tubifex tubifex (Clitellata: Oligochaeta), are the definitive host of the parasite Myxobolus cerebralis (Myxozoa: Myxosporea), which causes whirling disease in salmonid fishes. Genetic diversity of T. tubifex is correlated with host susceptibility to M. cerebralis , and mitochondrial Lineage III is generally shown to be more likely to be infected and produce the triactinomyxon (TAM) spores than other lineages. We determined the mitochondrial lineage, relative abundance, and prevalence of infection of T. tubifex collected at 3 sites in the Madison River, Montana, where previous study had shown variation in whirling disease prevalence and severity in caged trout fry. We also compared visual identification of TAMs released from cultured worms with a molecular genetic assay (diagnostic polymerase chain reaction [PCR]) for parasite detection of both infected and uninfected worms. We estimated that mitochondrial Lineage III was most abundant at the site previously shown to have high fish disease and was also most likely to be infected. The 2 techniques for detecting parasite infection did not always agree, and the likelihood of PCR (+) and spore (-) was not significantly different from PCR (-) and spore (+). Differences in the relative infection prevalence for these 2 lineages may explain the wide range of infection in natural streams.     

Epizootiology of Myxobolus cerebralis, the causative agent of salmonid whirling disease in the Rock Creek drainage of west-central Montana

Whirling disease, caused by the myxozoan parasite Myxobolus cerebralis, remains a health threat to salmonid fish in the western United States. Although various aspects of this host-parasite system have been studied, investigations examining the overall epizootiology of whirling disease in an ecosystem are lacking. Therefore, in June 1998, studies were initiated in the Rock Creek watershed of west-central Montana and continued through 2003 to assess the intensity of infection in trout using sentinel cages stationed throughout the drainage. Additional studies determined the percentage of the annelid worm, Tubifex tubifex, releasing M. cerebralis at various localities in Rock Creek and whether there was a seasonal or daily periodicity in the release of the triactinomyxon stage of the parasite from T. tubifex. Lastly, habitat and water quality parameters, and the effects of habitat restoration on transmission of M. cerebralis, were assessed. Overall, the intensity of M. cerebralis infections in sentinel trout increased significantly throughout the drainage between June of 1998 and 2003, with the biggest jump occurring between 1998 and 1999. In addition, the range of M. cerebralis expanded considerably over the period of study. There was no strict correlation between habitat condition and the occurrence of the parasite; fish became heavily infected in optimal and marginal habitats. However, fish exposed at a locality that had the lowest habitat ranking consistently had the highest intensity of infection. The parasite has apparently caused a dramatic decline in rainbow trout densities, but the brown trout population numbers have increased, and the overall fish density remains high. Although a major habitat restoration project did not seem to have an effect on decreasing disease intensity, this was not surprising because the restored area was located just downstream from a "hotspot" of infected T. tubifex.     

Prevalence and susceptibility of infection to Myxobolus cerebralis,and genetic differences among populations of Tubifex tubifex

The prevalence of infection and susceptibility of the aquatic oligochaete Tubifex tubifex to Myxobolus cerebralis, was examined in 2 studies on the upper Colorado River, Colorado, USA, where whirling disease occurs in wild trout populations. In the first study, the prevalence of infection ranged from 0.4 to 1.5%, as determined by counting the number of T. tubifex releasing triactinomyxons of M. cerebralis directly following their collection from the field. The susceptibility of those T. tubifex not releasing triactinomyxons was assessed by the number of these oligochaetes releasing triactinomyxons 3 mo following experimental exposures to spores of M. cerebralis. The prevalence of infection following experimental exposures of these T. tubifex ranged from 4.2 to 14.1%. In a second study, all T. tubifex collected at 2 different times directly from the 2 field sites in Colorado were exposed to spores of M. cerebralis. Individual oligochaetes representing those groups of T. tubifex releasing and those groups not releasing triactinomyxons at 3 mo were screened with molecular genetic markers. T. tubifex populations found at the 2 study sites consisted of 4 genetically distinct lineages that varied with respect to their susceptibility to experimental exposure to M. cerebralis. Lineages I and III contained the most oligochaetes susceptible to M. cerebralis and were the most prominent lineages at Windy Gap Reservoir, a site of high infectivity for wild rainbow trout on the upper Colorado River. In contrast, at the Breeze Bridge site which is below Windy Gap Reservoir and where M. cerebralis infections are less severe in wild trout, oligochaetes in lineages V and VI that are resistant to M. cerebralis were more prominent. These results suggest that certain habitats, such as Windy Gap Reservoir, are conducive to large and more homogenous populations of susceptible T. tubifex lineages that may serve as point sources of infection for M. cerebralis. Although not a direct objective of this study, there was no evidence of M. cerebralis infections among any oligochaetes other than those that would be classified as T. tubifex by standard morphological characteristics.     

Tubifex tubifex from Alaska and their susceptibility to Myxobolus cerebralis

Although widespread throughout the continental United States, Myxobolus cerebralis, the myxozoan parasite that causes whirling disease in salmonids, has not been reported from the state of Alaska. As part of a risk assessment for the introduction and establishment of M. cerebralis into Alaska, the distribution of the invertebrate host Tubifex tubifex was surveyed, and its genetic composition and susceptibility to the parasite were determined. Many oligochaetes, but no T. tubifex, were collected from southeastern Alaska; however, 4 lineages of T. tubifex (I, III, IV, and VI) were identified from south-central Alaska. Lineage IV had not been previously described in North America, and its susceptibility to M. cerebralis was unknown. When lineage IV T. tubifex and 3 mixed lineage (I, III, IV, and VI) groups were exposed to M. cerebralis, only lineage III became infected under our experimental conditions. Infection occurred in this lineage even when it made up just 3% of the population. Implications of the presence of nonsusceptible lineages of T. tubifex on Alaskan salmonids would be significant in areas where these lineages dominate T. tubifex populations.     

Persistent infection of Myxobolus cerebralis, the causative agent of salmonid whirling disease, in Tubifex tubifex

The objective of this study was to quantify and determine the periodicity in the release of the triactinomyxon (TAM) stage of Myxobolus cerebralis, the causative agent of salmonid whirling disease, by its aquatic oligochaete host Tubifex tubifex. For this, 24 individual T. tubifex (infected as a group at 15 C) were examined daily for the release of M. cerebralis TAMs, and the number of waterborne TAMs released by each worm was quantified. The duration of the infection in these worms was also monitored using a polymerase chain reaction (PCR) diagnostic test. TAMs were first released 74 days postexposure (PE) and continued to be released until 132 days PE. During this period, each worm released on average, 1.5 x 10(3) waterborne TAMs 12 times; however, no pattern or periodicity was noted. The results of the PCR diagnostic tests conducted at 5, 7, 9, and 15 mo PE were positive, and the persistent infection was confirmed at 606 days PE (approximately 20 mo) when the remaining worms began releasing TAMs again. Similar results were observed in naturally infected T. tubifex, indicating that these worms remain infected for the duration of their natural lifespan and are capable of shedding viable TAMs, in temporally separate periods. These findings open the possibility of a seasonal periodicity in TAM release by T. tubifex.     

The parasite that causes whirling disease, Myxobolus cerebralis, is genetically variable within and across spatial scales

Understanding the genetic structure of parasite populations on the natural landscape can reveal important aspects of disease ecology and epidemiology and can indicate parasite dispersal across the landscape. Myxobolus cerebralis (Myxozoa: Myxosporea), the causative agent of whirling disease in the definitive host Tubifex tubifex, is native to Eurasia and has spread to more than 25 states in the USA. The small amounts of data available to date suggest that M. cerebralis has little genetic variability. We examined the genetic variability of parasites infecting the definitive host T. tubifex in the Madison River, MT, and also from other parts of North America and Europe. We cloned and sequenced 18S ribosomal DNA and the internal transcribed spacer-1 (ITS-1) gene. Five oligochaetes were examined for 18S and five for ITS-1, only one individual was examined for both genes. We found two different 18S rRNA haplotypes of M. cerebralis from five worms and both intra- and interworm genetic variation for ITS-1, which showed 16 different haplotypes from among 20 clones. Comparison of our sequences with those from other studies revealed M. cerebralis from MT was similar to the parasite collected from Alaska, Oregon, California, and Virginia in the USA and from Munich, Germany, based on 18S, whereas parasite sequences from West Virginia were very different. Combined with the high haplotype diversity of ITS-1 and uniqueness of ITS-1 haplotypes, our results show that M. cerebralis is more variable than previously thought and raises the possibility of multiple introductions of the parasite into North America.     

Effects of water flow on the infection dynamics of Myxobolus cerebralis

Myxobolus cerebralis, the myxozoan parasite responsible for whirling disease in salmonid fishes, has a complex life-cycle involving an invertebrate host and 2 spore stages. Water flow rate is an environmental variable thought to affect the establishment and propagation of M. cerebralis; however, experimental data that separates flow effects from those of other variables are scarce. To compare how this parameter affected parasite infection dynamics and the invertebrate and vertebrate hosts, dead, infected fish were introduced into a naive habitat with susceptible hosts under 2 experimental flow regimes: slow (0 x 02 cm/s) and fast (2 x 0 cm/s). Throughout the 1-year study, uninfected fry were held in both systems, the outflows were screened weekly for spores and the annelid populations were monitored. We found clear differences in prevalence of infection in the worms, prevalence and severity of infection in the fish, and host survival. Both flows provided environments in which M. cerebralis could complete its life-cycle; however, both the parasite and its invertebrate host proliferated to a greater extent in the slow flow environment over the 1-year study period. This finding is of significance for aquatic systems where the flow rate can be manipulated, and should be incorporated into risk analysis assessments.     

Alternation of Actinosporean and Myxosporean Phases in the Life Cycle of Zschokkella nova (Myxozoa)

ABSTRACT. Experimental evidence has been gathered to show that the life cycle of the myxozoan gallbladder parasite Zschokkella nova Klokacewa, 1914, which infects the fish Carassius carassius , has a complex life cycle with alternation of two hosts (fish and Oligochaeta) and two developmental phases (myxosporean and actinosporean). The gut epithelium of the oligochaete, Tubifex tubifex , exposed experimentally to Z. nova , obtained from C. carassius , became infected with organisms resembling Actinosporea. The spore structure and cube-like network of the interconnected spores is reminiscent of Siedleckiella silesica Janiszewska, 1952, although the spores are very different in size and number of sporoplasm nuclei. The life cycle of Z. nova resembles that of the whirling disease agent Myxosoma cerebralis described by Wolf and Markiw, which also alternates between fish and oligochaete hosts.     

Factors influencing the distribution of Myxobolus cerebralis,the causative agent of whirling disease,in the Cache la Poudre River,Colorado

Oligochaetes, triactinomyxons (TAMs), and age-0 trout were sampled in the upper Cache la Poudre River, Colorado, to determine the distribution of Myxobolus cerebralis during 1997 and 1998. Densities of the intermediate host, the oligochaete Tubifex tubifex, were 3.5 orders of magnitude higher in the M. cerebralis-infected Poudre Rearing Unit (PRU) trout rearing ponds than at any of the river sampling reaches. Oligochaetes, including T. tubifex, were rare in the river (1 oligochaete m(-2)), except in a few stream side alcoves and eddies (50 oligochaete m(-2)). Species composition of oligochaetes in the river reaches was more diverse than in the PRU. Tubifex tubifex constituted 50% or less of the oligochaete community in the river and 98% in the PRU. Infection rates of T tubifex were 1% in the area above the PRU, 2% in the PRU, and 6% below the PRU. An increased M. cerebralis intensity of infection in age-0 trout below the PRU could not be attributed entirely to the high numbers of TAMs in its effluent (3.7 TAMs l(-1)). Low densities of TAMs ranging from 0 to 0.2 TAMs l(-1) were found in the river reaches, yet nearly all of the age-0 trout were infected soon after emergence. This suggests that very few TAMs, as measured by filtration, need be present in the water column to bring about infection in the majority of trout present. This also indicates that the parasite can persist and potentially cause reduced juvenile trout recruitment in cold, oligotrophic, sediment poor, high-gradient streams.     

The effect of cohabitation of Tubifex tubifex (Oligochaeta: Tubificidae) populations on infections to Myxobolus cerebralis (Myxozoa: Myxobolidae)

The competitive interactions between susceptible and resistant Tubifex tubifex (Oligochaeta: Tubificidae) exposed to Myxobolus cerebralis (Myxozoa: Myxobolidae) infections were investigated in two laboratory trials. Competition was assessed by the total parasite production over the course of the trials in mixed and pure cultures of M. cerebralis exposed worms, and by the genetic analyses of worms from the control and experimental groups at the beginning and end of the experiments. Mixed cultures of resistant and susceptible worms showed a 70% reduction in production of parasites released when compared with pure cultures of susceptible worms. In studies with laboratory and field-collected oligochaetes the mixed cultures at the end of the cohabitation experiments were dominated by resistant Tubifex from lineage V (HB strain) this strain of Tubifex has a competitive advantage over worms from other lineages. The results of this study suggest that certain species of Tubifex may be dead-end hosts to M. cerebralis by absorbing or inactivating the parasite and may also show greater survival compared to susceptible oligochaetes in certain whirling disease enzootic habitats.     

Molecular phylogeny of tubificid oligochaetes with special emphasis on Tubifex tubifex (Tubificidae)

Tubifex tubifex is a cosmopolitan freshwater oligochaete whose presence has been studied as a health indicator of the aquatic environment and as a host for several myxozoan parasites of fish. Unfortunately, current morphological criteria used to distinguish Tubifex spp. (Tubificidae) are inadequate. We therefore developed mitochondrial 16S ribosomal DNA markers to examine phylogenetic relationships among aquatic oligochaetes and to distinguish species of Tubifex that might serve as hosts for a particular myxozoan parasite, Myxobolus cerebralis. Our phylogenetic analyses of oligochaetes based on a 378-bp segment yielded one most parsimonious tree with three major groups that corresponded to the families Lumbricidae, Sparganophilidae, and Tubificidae. T. tubifex and T. ignotus formed a monophyletic assemblage, and a sister relationship between the genera Tubifex and Limnodrilus was strongly supported. A second analysis of the relationship within the genus Tubifex identified six genetically distinct lineages of T. tubifex from North America and Europe that were separated by genetic distances comparable to those found for "well-defined" species of Limnodrilus. Therefore, the existence of several morphologically indistinguishable, thus cryptic, species of Tubifex in North America and Europe is suggested.     

Susceptibility of two strains of rainbow trout (one with suspected resistance to whirling disease) to Myxobolus cerebralis infection

The susceptibility of 2 strains of rainbow trout Oncorhynchus mykiss, 1 from North America (TL) and 1 from Germany (GR), to Myxobolus cerebralis (the cause of salmonid whirling disease) was assessed following exposure to the infectious stages (triactinomyxons). Two laboratory experiments were conducted with age-matched rainbow trout of each strain. At the beginning of the study, the 2 trout strains were aged ca. 570 degree-days in Expt 1, and ca. 999 degree-days in Expt 2. In both experiments, replicate groups of each trout strain were exposed to 10, 100, 1000 or 10000 triactinomyxons (TAMs) fish(-1) for 2 h. The fish were then held in aquaria receiving 15 degrees C well-water. Severity of infection was evaluated 5 mo after exposure by presence of clinical signs (whirling and/or black tail), prevalence of infection, severity of microscopic lesions, and spore counts. Clinical signs of whirling disease were evident only in the younger fish exposed in Expt 1: These occurred first among TL rainbow trout at the highest dose at 6 to 7 wk post exposure and then 2 wk later in fish at the 1000 TAMs dose. Black tail was also observed among GR rainbow trout at the 10000 TAMs dose only, but in fewer fish. The prevalence of infection, spore numbers, and severity of microscopic lesions due to M. cerebralis among GR rainbow trout were less at all doses compared to TL rainbow trout. Risk of infection analyses showed that TL rainbow trout were more prone to infection at the lower doses than GR trout. Mean spore counts were consistently (10- to 100-fold) less in GR than TL trout at doses of 1000 TAMs or lower. Microscopic lesions increased with increasing dose in both strains of rainbow trout. The mechanisms underlying the greater resistance of the GR strain to M. cerebralis infections are unknown, but are under investigation as part of a long-term project to determine the basis for resistance and susceptibility of salmonid fishes to whirling disease.     

The severity of whirling disease among wild trout corresponds to the differences in genetic composition of Tubifex tubifex populations in central Colorado

We analyzed the geographic distribution of Tubifex tubifex from various river drainages in central Colorado by genetic screening with specific mitochondrial 16S ribosomal DNA (mt 16S rDNA) markers. Four distinct mt 16S rDNA lineages are evident. The sites varied with respect to land- and water-use practices. All sites represented habitats presumed capable of supporting oligochaetes. At the locations where whirling disease has had the greatest impact on resident rainbow trout, T. tubifex, representing lineages I and III (genotypes known to be susceptible to Mxyobolus cerebralis), were most commonly found. In contrast, at sites less affected by whirling disease, T. tubifex of lineages V and VI that are more resistant to M. cerebralis infections were more abundant. The predominance of resistant T. tubifex worms (lineages V and VI) at low-impact sites supports the conclusion that when these genotypes are in greater abundance, the potential for more severe effects of whirling disease on wild rainbow trout populations may be diminished.     

The biology of Myxosoma cerebralis: the causative organism of whirling disease of salmonids

The literature describing the biology and control of Myxosoma cerebralis (whirling disease) is reviewed. New data on the world distribution of the parasite are presented. It is concluded that the presence of M. cerebralis is not an important limiting factor in salmonid fanning per se but only limits methods of production.     

Present knowledge on the life cycle,taxonomy, pathology,and therapy of some Myxosporea spp. important for freshwater fish

In this review some important myxosporean species of fish are depicted and described, with special reference to Myxobolus cerebralis (causing whirling disease of salmonids), Sphaerospora renicola (causing Swimbladder inflammation of common carp), Hoferellus carassii (causing kidney enlargement disease of goldfish and proliferative kidney disease [PKD] of salmonids). Detailed information on the life cycles of M. cerebralis, M. cotti, and M. pavlovskii is presented proving that these myxosporeans develop in an intermediate host, the oligochaete worm, Tubifex tubifex to actinosporeans which represent the only infective stage for susceptible fish. These findings imply that the present taxonomy for the phylum Myxozoa has to be revised. Trends for treatment of myxosporean associated diseases are summarized with special emphasis on the antibiotic Fumagillin DCH.