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Fish myxosporean parasites

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Introduction

The myxozoans are a speciose and economically important group of microscopic metazoan parasites. Whereas they are best known for the diseases they cause in commercially important fish hosts, these species represent only a fraction of the some 1,350 described species assigned to about 52 genera of Myxozoa. They also been reported rarely in platyhelminths, reptiles, and amphibians, and recently a myxozoan-like parasite was found in the brain of a mole Talpa europrea.

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 led to the increased importance of these parasites; in turn this has led 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 phylogenetic analyses using all these sequences indicate that the Myxozoa are closely related to Cnidaria (also supported by morphological data). This phylum Cnidaria also contains anemones, corals and jellyfishes.

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Tetracapsuloides bryosalmonae, Proliferative kidney disease (PKD)

Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Cnidaria - Myxozoa - Myxosporea - Malacosporea - Malacovalvulida - Saccosporidae - Tetracapsuloides - Tetracapsuloides bryosalmonae

Brief facts

The myxozoan Tetracapsuloides bryosalmonae is the causative agent of Proliferative Kidney Disease (PKD) targeting primarily the kidney of infected fish where it causes a chronic lymphoid immunopathology. This disease with serious consequences for cultured fish raises presently problems not only in farmed but also in feral trout populations in Europe and North America. The causative agent of PKD, named 'PKX', was postulated to be a developmental stage of Tetracapsuloides bryosalmonae.

PKX-cells, which represent multinuclear developmental stages and form in various organs outside the final site of spore production, can be detected especially in the kidney and spleen. The disease is induced by an intense defense reaction of the host elicited by the parasite. Diagnosis of the disease is largely based on demonstration of PKX cells in histological sections of tissues. Control of this disease remains unsatisfactory, but antibiotics (fumagillin) may prevent PKD when given prophylactically and malachite green bath was found to be successful in suppressing PKD.

Tetracapsuloides bryosalmonae in PubMed

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Myxobolus cerebralis, whirling disease

Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Cnidaria - Myxozoa - Myxosporea - Bivalvulida - Platysporina - Myxobolidae - Myxobolus - Myxobolus cerebralis

Brief facts

M. cerebralis is a multicellular or metazoan parasite of salmonid fish. It represents one of the most pathogenic of the over 1,300 members of the phylum Myxozoa known to parasitize fish causing so-called whirling disease.

M. cerebralis possesses a life cycle well adapted to the natural environments where salmonid fish are found. Whirling disease was first described in Europe in 1898 among farmed rainbow trout but recent occurrences have been devastating to wild trout in North America. The disease is considered a major threat to survival of wild rainbow trout in intermountain west of the United States. Difficulties in containing the spread and potentially eliminating the pathogen are tied to features of a complex life cycle involving two hosts, the salmonid fish and an aquatic oligochaete.

Disease

The name whirling disease comes from the characteristic tail chasing behavior of fish infected with the parasite. Other characteristic signs of the disease include a blackened tail and spinal and cranial deformities. These signs of disease reflect the direct and indirect effects of the developmental stages of the parasite that have reached the principal host target tissue, cartilage. The development and severity of whirling disease in rainbow trout generally depends on fish age and the dose of parasites when exposed.

Control

Disinfection of water containing triactinomyxons spores with ultraviolet light has been demonstrated as a potential method for treatments of hatchery water supplies. Active and prophylactic treatments with drugs and chemicals have also been attempted with whirling disease, with fumagillin shown as potentially the most effective.

Myxobolus cerebralis in PubMed

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Kudoa spp., milky flesh

Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Cnidaria - Myxozoa - Myxosporea - Multivalvulida - Kudoidae - Kudoa

Brief facts

The genus Kudoa is comprised of myxosporean parasites with four valves, each of which contains a polar capsule. All known species of Kudoa infect marine and estuarine fishes. Species within this genus are typically histozoic (living in the tissues of a host) parasites of marine teleosts (ray-finned fishes). This genus is of concern to both aquaculture and commercial fisheries because several of its species either produce unsightly macroscopic cysts in the musculature or are associated with post-mortem myoliquefaction, and thus reduce the market value of the infected fish products.

Infection

Infections by Kudoa spp. as well as with some other multivalvulids are associated with post-mortem myoliquefactive autolysis (enzymatic degradation) commonly referred as "soft flesh". In living fish, even with some localized pathological changes, there are usually no observable effect on physiology, behavior, or life span of the fish host.

Control

At present, there are no allowable chemotherapeutic treatments for myxosporean infections in fish to be sold as food.

Kudoa in PubMed

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Ceratomyxa shasta, gastrointestinal disease

Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Cnidaria - Myxozoa - Myxosporea - Bivalvulida - Variisporina - Ceratomyxidae - Ceratonova - Ceratonova shasta

Brief facts

Ceratomyxa shasta is a microscopic myxosporean protozoan parasite that afflicts salmonid fish of the Pacific Northwest. Intermediate host is a freshwater polychaete.

Transmission

Infectious actinospores are released into freshwater system after fish death and infect other fish.

Signs

Clinical indications of infection include lethargy, loss of body mass, darkening, ascites, exopthalmia, kidney pustules. Internally parasite affects entire digestive tract, liver, gall bladder, spleen, gonads, kidney, heart, gills, and muscle.

Prevention

Treatment of incoming hatchery water supplies using a combination of ultraviolet irradiation, chlorination, and sand filtration or by ozonation.

Ceratomyxa shasta in PubMed

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References