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Fish protozoan pathogens

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Introduction

Important fish diseases are caused by unicellular eukaryotic organisms (protists, protozoans) from the following taxonomic groups: Euglenozoa, Dinophyceae (flagellated alveolates, dinoflagellates), Apicomplexa (apicomplecans), and Diplomonadida (diplomonads).

Protozoan parasites of fish are especially harmful in aquaria, fisheries, ponds and other closed systems.

Apart from specific pathognomonic clinical signs of a particular disease there is a group of symptoms that can be suggestive of the infection. These include: flashing (rubbing on the bottom of the tank or pond); lethargy; cutaneous lesions including scale loss, ulcerations, and increased mucus production; rapid, opercular movements (increased "gilling"); gasping or piping; weight loss; "yawning"; osmoregulatory disruption, and deaths.

Whitespot disease is caused by Cryptocaryon irritans in saltwater fish and by Ichthyophthirius multifiliis in freshwater fish. Ciliated alveolates Tetrahymena ("guppy disease") and Uronema cause skin lesions, hemorrhages and necrosis in freshwater and saltwater fishes respectively. Coccidia and diplomonads cause gastrointestinal chronic and acute infection. Chronic gill infections are often caused by species in family Trichodinidae. Sedentary Epistylis produces white, fluffy lesions that can easily be mistaken for fungal lesions or columnaris disease. Dinoflagellates Amyloodinium ocellatum, Piscinoodinium are agents of velvet disease in saltwater and freshwater fishes respectively.

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Ciliated alveolates

Cryptocaryon irritans (saltwater white spot)

Taxonomy

cellular organisms - Eukaryota - Alveolata - Ciliophora - unclassified Ciliophora - Cryptocaryon - Cryptocaryon irritans

Cryptocaryon irritans is one of the most important protozoan ciliate pathogens of marine fish, causing the white spot disease and posing a significant problem to marine aquaculture due to its widespread distribution, indiscriminate host specificity, and high level of virulence.

Although C. irritans differs taxonomically from Ichthyophthirius multifiliis, its similarities in biology, host-parasite relationship, and epizootiology give good reason to regard this ciliate as the marine analogue of I. multifiliis. The parasite feeding stage of C. irritans invades the base of the integumentary epithelium and upon completion of its growth schedule, leaves the host and only then begins division. Differences between the two ciliates lie mainly in their cytological details, mode of division (unequal vs. equal), and length of the reproductive process (prolonged and variable [3-28 days] vs. short and synchronous [15-48 h]).

Pathology

Similarities between C. irritans and I. multifiliis in the site and mode of development, and feeding on the fish integument lead to similarities in the pathological process. However, C. irritans demonstrates a predilection to the cornea; therefore, severe eye damage and blindness is more characteristic of C. irritans infestations. The proliferation response of the epithelial tissue, in particular the mucus cells, is also more pronounced in C. irritans infections; thus affected fish produce copious mucus.

Symptoms

Clinical signs include white, raised nodules up 0.5mm on the skin and gills, flashing, increased mucus production, lethargy, respiratory distress, secondary bacterial or fungal infections, and osmoregulatory compromise due to the epithelial and gill damage caused by the parasite. Microscopic evaluation of the gills can show hyperplasia, necrosis, excess mucus, and necrosis. Ich is a large parasite, entirely covered in cilia, moves in a characteristic slow-rolling motion; the nucleus of Cryptocaryon is lobulated with four bead-like segments.

Distribution

C. irritans is found in warm water throughout the world. Its optimal development and reproduction occur at 25-30°C while below 10°C development and reproduction are interrupted. Until recently, all records of epizootic (and enzootic) infestations came from marine aquaria holding tropical marine fishes or native fishes at regulated temperatures above 20°C.

Cryptocaryon in PubMed

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Ichthyophthirius multifiliis (freshwater white spot)

Taxonomy

cellular organisms - Eukaryota - Alveolata - Ciliophora - Intramacronucleata - Oligohymenophorea - Hymenostomatida - Ophryoglenina - Ichthyophthirius - Ichthyophthirius multifiliis

Ichthyophthirius multifiliis, freshwater white spot fish disease taxonomy, facts, pathogenicity, bibliography at MetaPathogen

 

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Family Trichodinidae (saltwater and freshwater)

Taxonomy

cellular organisms - Eukaryota - Alveolata - Ciliophora - Intramacronucleata - Oligohymenophorea - Peritrichia - Trichodinidae Trichodina

Brief facts

Members of the family Trichodinidae are best known as ciliated ectoparasites of fishes. About 300 species of trichodinids have been described from fishes, mostly from freshwater environments.

Among the trichodines, Trichodina heterodentata is the most widespread species in warm water systems. A large group of ubiquitous trichodines (T. acuta, T. kupermani, T. mutabilis, T. nigra, T. nobilis, T. reticulata, Trichodinella epizootica, and Tripartiella bulbosa), are preferencially associated with cultured goldfish and common, grass, silver, and bighead carp, and other cyprinids, as well as some other fish groups.

These parasites are often associated with high levels of organic debris in the water, poor nutrition, overcrowding, and poor water quality.

Pathology and symptoms

Most of these parasites has a predilection for skin and gill epithelium, but some will parasitize the urinary bladder or oviduct. Clinical signs of heavy infestations include flashing, increased mucus production giving a cloudy appearance to the skin, cutaneous hemorrhages, frayed fins and tail, lethargy, and chronic low level mortalities; with severe branchial infestations, respiratory signs may be present. Secondary bacterial and fungal infections may occur because of extensive tissue damage. Observation of the circular, ciliated parasite with a prominent internal denticular ring on a wet-mount examination of gills or skin is diagnostic. The parasite has been described as a flying saucer or "scrubbing bubble". The motion of this parasite has been described as rotating, scooting, erratic, whirling, and hyperactive.

Denticle morphology of trichodinids
Denticle morphology of different trichodinids, arrows mark the blade of the denticle and the yellow lines indicate the Y-axis. A–B T. reticulata (Tang and Zhao, 2010); C T. modesta (present work); D T. heterodentata (Gong et al. 2006); E Trichodina nobilis (Gong et al. 2006); F Trichodina paraheterodentata (Tang and Zhao 2012); G Trichodina sinonovaculae (Xu et al. 1999); H T. meretricis (Xu et al. 1999); I T. ruditapicis (Xu et al. 2000); J Trichodinella sp. (Gong et al. 2006); K Trichodinella epizootica (present work). (Scale bar 20 μm).
Phylogenetic analyses of Trichodinids (Ciliophora, Oligohymenophora) inferred from 18S rRNA gene sequence data. Tang FH et al. Curr Microbiol. 2013 Mar;66(3):306-13.

Trichodinidae in PubMed

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Chilodonella (freshwater)

Taxonomy

Ciliophora - Intramacronucleata - Phyllopharyngea - Cyrtophorida Chilodonella

Brief facts

Chilodonella sp. is a ciliated parasite shaped like a heart or an onion, with a flattened appearance. Cilia are located longitudinally on the parasite and may be seen as visible striations. Chilodonella can survive a wide variety of temperature ranges, is found worldwide, and can survive in brackish water. These parasites are highly pathogenic and severe tissue damage can occur before any gross pathology is visible. Chilodonella feed on epithelial cells of the integument causing abrasions. Clinical signs include respiratory distress (gasping, piping, opercular flaring, increased gilling), clamped fins, a ragged appearance to the skin, excess mucus production, secondary cutaneous ulcers, pathologic gill changes including hyperplasia and fusion of the lamellae, depression, and mortalities. Diagnosis is based on observation of the parasite on wet mount examinations of skin and gills. Chilodonella moves in a gliding motion or circular motion on wet-mount preparation.

Chilodonella in PubMed

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Tetrahymena (freshwater, guppy disease)

Taxonomy

cellular organisms - Eukaryota - Alveolata - Ciliophora - Intramacronucleata - Oligohymenophorea - Hymenostomatida - Tetrahymenina - Tetrahymenidae Tetrahymena

Brief facts

Tetrahymena spp. are ciliated parasites that cause external (skin and gill) lesions and internal, systemic infections. Fish with systemic infections may show nonspecific signs such as anorexia and lethargy. Death can occur rapidly once infection is established. Tetrahymena, also known as "guppy disease" or "guppy killer", is found most often in guppies, other livebearers, tetras, and cichlids. This organism can also be found colonizing organic debris in the water. Poor water quality, bacterial infections, and other stressors may predispose fish to Tetrahymena infections. Muscle swelling and periocular lesions can be seen with Tetrahymena infections. Keratitis can also occur due to the close connection of the skin and cornea. Deep or systemic infections carry a poor prognosis. Diagnosis is made with wet-mount examination or histopathology of skin and gill tissue.

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Uronema (saltwater)

Taxonomy

cellular organisms - Eukaryota - Alveolata - Ciliophora - Intramacronucleata - Oligohymenophorea - Scuticociliatia - Philasterida - Uronematidae Uronema

Brief facts

Uronema species infect a wide variety of marine species and over a wide temperature range (8–28°C). External signs include small, white patches on the skin, skin hemorrhages, sloughing, and necrosis, and gill aneurysms. Fish with systemic infections may show nonspecific signs such as anorexia and lethargy. Death can occur rapidly once infection is established. Diagnosis is made with wet-mount examination or histopathology of skin and gill tissue.

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Epistylis (freshwater)

Taxonomy

cellular organisms - Eukaryota - Alveolata - Ciliophora - Intramacronucleata - Oligohymenophorea - Peritrichia - Epistylidae Epistylis

Brief facts

Epistylis spp. are sedentary (sessile) ciliates. Epistylis produces white, fluffy lesions on fins and tail margins, opercular margins, and oral cavity. These lesions can easily be mistaken for fungal lesions or columnaris disease owing to their similar appearance.

Epistylis plicatilis.jpg
Lobban CS, Schefter M. Freshwater biodiversity of Guam. 1. Introduction, with new records of ciliates and a heliozoan. Micronesica. 2008;40(1-2):273-293.

Epistylis in PubMed

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Flagellated alveolates

Amyloodinium ocellatum (saltwater velvet disease)

Taxonomy

cellular organisms - Eukaryota - Alveolata - Dinophyceae - Blastodiniales - Oodiniaceae Amyloodinium - Amyloodinium ocellatum

Brief facts

This species of dinoflagellates can be found in marine tropical fish. A. ocellatum can parasitize both elasmobranchs (sharks, rays and skates) and teleosts (ray-finned fishes). The gills and skin are the preferential site of infestation and heavy infestation can result in edematous changes, hyperplasia, inflammation, hemorrhage, osmoregulatory compromise, and necrosis of the gill filaments. Mortalities, in as little as 12 hours, result from hypoxia, secondary bacterial infections, and osmoregulatory compromise. In addition to respiratory distress, another clinical sign that may be seen is a dusty, gold appearance to the skin, hence the names "velvet disease", "gold dust disease", and "rust disease". Diagnosis is made by wet-mount cytology or histopathology of the skin and gills. In addition to the introduction of infected fish to a system, fomites and the introduction of infected water (including in form of aerosol) may play a role in transmission.

Pathology

A. ocellatum is attached to and feeds on the host epithelial cell by means of rhizoids that penetrate the host cells, subsequently leading to degeneration and collapse of the host cells. Damage of the affected cells leads to focal erosion of the epithelium. Proliferation of the epithelium causes obliteration of the gill lamellae, while the inner strata of the epithelium become spongious and, in some cases, undergoes complete lysis.

Biology and ecology

The life cycle of fish dinoflagellid parasites is comprised of a parasitic feeding stage (trophont) when the parasite is attached to integumentary epithelial cells and an encysted dividing stage (tomont), which is when the parasite is detached from the host. Division yields motile infective stage (dinospore), which can attach to a new host. If the parasite is accidentally detached from the host integument before completing its growth schedule, it may still proceed in its development. The number of divisions undergone before the formation of dinospores is correlated to the trophont size when detached. Trophonts, having attained their full growth potential (up to 100 pm) on the host integument, undergo up to seven or eight divisions after detachment, yielding 132-264 dinospores. Abrupt detachment of trophonts, as when part of the infested fish dies or are treated with a chemical agent (such as formalin) which causes detachment but does not inhibit division, sharply accelerates infestation levels. Effective reproduction of tomonts is limited to a temperature range of 18-30 °C and a salinity range of 12-50 ppt. Below 18 °C the slow rate of growth and division of the parasite is insufficient to generate serious infestation. Tomonts stop dividing below 16 °C, and gradually die below 10 °C. Temperature and salinity effects act synergistically.

Amyloodinium in PubMed

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Piscinoodinium (freshwater velvet disease)

Taxonomy

cellular organisms - Eukaryota - Alveolata - Dinophyceae - unclassified Dinophyceae - Piscinoodinium

Brief facts

This species of dinoflagellates can be found in freshwater tropical fish and is very similar morphologically and symptomatically to its saltwater counterpart Amyloodinium ocellatum. Only the free-living dinospore is susceptible to treatment. A ocellatum can parasitize both elasmobranchs and teleosts. The gills and skin are the preferential site of infestation and heavy infestation can result in edematous changes, hyperplasia, inflammation, hemorrhage, osmoregulatory compromise, and necrosis of the gill filaments. Mortalities, in as little as 12 hours, result from hypoxia, secondary bacterial infections, and osmoregulatory compromise. In addition to respiratory distress, another clinical sign that may be seen is a dusty, gold appearance to the skin, hence the names "velvet disease", "gold dust disease", and "rust disease". Diagnosis is made by wet-mount cytology or histopathology of the skin and gills. In addition to the introduction of infected fish to a system, fomites and the introduction of infected water (including in form of aerosol) may play a role in transmission.

Piscinoodinium in PubMed

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Kinetolastids

Ichthyobodo sp. (freshwater)

Taxonomy

cellular organisms - Eukaryota - Euglenozoa - Kinetoplastida - Ichthyobodonidae - Ichthyobodo

Brief facts

Ichthyobodo, previously known as Costia, is a very small, flagellated parasite (about the size of a red blood cell) of freshwater fish found in a wide variety of species with a global distribution. The parasite can survive a wide temperature range (2–30 °C) and has been occasionally found on marine fish. The life cycle is direct and transmission occurs from fish to fish. Mortalities are higher in fry, young fish, and stressed, debilitated adult fish. Clinical signs include severe respiratory distress, lethargy, depression, flashing, anorexia, epithelial irritation, heavy mucus production, and deaths. Deaths may occur before any clinical signs. The organism's movement has been described as a "flickering" candle, or erratic spiraling.

Ichthyobodo in PubMed

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Cryptobia sp. (freshwater and saltwater)

Taxonomy

cellular organisms - Eukaryota - Euglenozoa - Kinetoplastida - Bodonidae - Cryptobia

Brief facts

Cryptobia is a genus that encompasses a wide variety of small (10–13 mm long and 2 mm wide) flagellated fish parasites. There are species that infect the gills and skin, the gastrointestinal tract, and the blood.

Clinical signs

Common clinical signs include anorexia, weight loss, abnormal appearing stools, darkening of the skin, lateral recumbency, lethargy, decreased fecundity, and hanging and moving slowly in the water column. Morbidity and mortality rate may be as high as 70% to 90%, especially in younger fish. Fish more than 2 months old suffer high morbidity but low mortality (2%–5%). These older fish show an increased opercular rate and mucus production for up to 1 week before recovering.

Diagnosis

Gastrointestinal cryptobiosis is most commonly detected by granulomas in squash preparation wet mounts of the stomach. An acid-fast stain should be performed to rule out mycobacteriosis (another common cause of granulomas in ornamental fish). Motile trophozoites are not commonly seen on wet mounts. When present, flagellated trophozoites are elongate (acute infection) to oval or teardrop shape (chronic infection) with a characteristic slow, undulating movement. The organism has two flagella. Species identification requires electron microscopy.

Treatment

There is no effective treatment. Because of the granulomatous gastritis caused by the parasites, it may be difficult to impossible to completely eradicate it in infected fish. Some farmers have reported decreased mortalities with sulfa drugs (e.g., sulfadimethoxine), but this may just help with secondary infections or co-infections. Some reduction of the prevalence of the pathogen in experimentally infected fish was achieved with treatments of dimetridazole and 2-amino-5-nitrothiazole.

Cryptobia in PubMed

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Diplomonads

Hexamita spp.

Taxonomy

cellular organisms - Eukaryota - Fornicata - Diplomonadida - Hexamitidae - Hexamita

Brief facts

Hexamita sp. are found primarily in the gastrointestinal tract of freshwater fish. These flagellates are most commonly found to cause clinical disease in freshwater angelfish, cichlids, and anabantids. Clinical signs of gastrointestinal infections include: severe weight loss, anorexia, lethargy, abdominal distension, mucoid enteritis, mucoid or pale feces, exophthalmos, darkening of the skin, buoyancy disorders, redness at the vent, and deaths. Concurrent infections with other parasites, poor water quality and other stressors such as overcrowding are not uncommon and increase morbidity. Hexamita's trophozoites are small (12.5–20 μm in length), flagellated, actively motile, with an ellipsoid to pear shape. Trophozoites are often localized in the anterior intestinal lumen and therefore may not be present on fecal examination. They have six anterior and two posterior flagella. Species identification and differentiation between Hexamita and Spironucleus requires electron microscopy.

Hexamita in PubMed

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Spironucleus spp.

Taxonomy

cellular organisms - Eukaryota - Fornicata - Diplomonadida - Hexamitidae - Spironucleus

Brief facts

Spironucleus has been isolated from lesions characteristic of "hole-in-the-head-disease" in discus (Symphysodon spp.) and angelfish (Pterophyllum sp) but the role it plays in this multifactorial syndrome is unknown. Hexamita sp. are found primarily in the gastrointestinal tract of freshwater fish. These flagellates are most commonly found to cause clinical disease in freshwater angelfish, cichlids, and anabantids. Clinical signs of gastrointestinal infections include: severe weight loss, anorexia, lethargy, abdominal distension, mucoid enteritis, mucoid or pale feces, exophthalmos, darkening of the skin, buoyancy disorders, redness at the vent, and deaths. Concurrent infections with other parasites, poor water quality and other stressors such as overcrowding are not uncommon and increase morbidity. Spironucleus's trophozoites are small (12.5–20 mm in length), flagellated, actively motile, with an ellipsoid to pear shape. Trophozoites are often localized in the anterior intestinal lumen and therefore may not be present on fecal examination. They have six anterior and two posterior flagella. Species identification and differentiation between Hexamita and Spironucleus require electron microscopy.

Spironucleus in PubMed

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Apicomplexans

Coccidia

Taxonomy

cellular organisms - Eukaryota - Alveolata - Apicomplexa - Conoidasida - Coccidia

Brief facts

Coccidia are group of intracellular parasites that can infect a variety of species of fish. Most species show a predilection for the gastrointestinal system causing emaciation, chronic enteritis, anorexia, mucoid stool, and deaths. Goldfish are the most common pet species presented with infections in a pet fish-practice setting. Immune suppression due to poor water quality and other environmental disorders may predispose the fish to infection. Other organs that may be affected include the reproductive organs, swim bladder, liver, spleen, and kidney. Diagnosis of intestinal infection can be made by wet-mount examination of a fresh fecal sample or cloacal wash.

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References

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