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Fish fungal pathogens

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Introduction

Compared with other animals, there are relatively few fungi that affect fish. Fungal infections in fish are most frequently attributed to members of the family Saprolegniaceae in the taxon Oomycetes or the water molds. Oomycetes were re-classified and taxonomically separated from true fungi. They don't have chitin in the cell wall, diploid, and their spores are motile (zoospores).

Fungal classification is based on life cycle, the morphology of hyphae, and the reproductive units, as well as the type of spores produced. Fungi are incapable of synthesizing their own nutrients. They are heterotrophs, i.e., they require organic matter for their growth and reproduction. They can be categorized as saprophytes, which use dead organic matter or as parasites that infect living organisms for their sustenance. Most fungi, however, are facultative rather than obligate parasites/saprophytes.

Many of the fungi that affect fish are considered opportunists, attacking the fish when they are stressed or immunocompromised because of unfavorable environmental conditions, or secondary to bacterial or viral infections, or when they have lost their mucus protection because of trauma or excessive handling. There are, however, primary pathogens even in the family Saprolegniaceae that cause disease without predisposing factors.

Some of fish fungal pathogens can cause infection and disease in humans.

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Ascomycetes (yeasts)

Genus Exophalia (black yeasts)

Taxonomy

cellular organisms - Eukaryota - Opisthokonta - Fungi - Dikarya - Ascomycota - saccharomyceta - Pezizomycotina leotiomyceta - Eurotiomycetes - Chaetothyriomycetidae - Chaetothyriales - Herpotrichiellaceae - Exophiala

Brief facts

Exophiala spp are ubiquitous melanized yeast found in both terrestrial and aquatic environments. The genus was first described in cutthroat trout (Salmo clarkia) in 1966. In humans, they have been associated with numerous and varied clinical signs from superficial infections, subcutaneous infections, keratitis, pneumonia, and brain abscesses, to disseminated disease. These infections are more often seen in immunocompromised individuals.

Clinical signs

Clinical signs of infection with Exophiala spp. vary based on the species affected and species of Exophiala. In weedy and leafy sea dragons, clinical signs included weakness, loss of appetite, lethargy, increased respiratory effort and rate, abnormal buoyancy, listing, piping at the surface of the water, or death. The duration of clinical signs was 1 week to 6 months, with a mean of 8 weeks before death.

Diagnosis

A tentative diagnosis of Exophiala can be made during postmortem examination. The fungal plaques are a distinct olive to black-brown color. Lesions in sea dragons are most consistently found in the skeletal muscle, skin, and kidney, as well as in other numerous locations. Colonies of Exophiala on fungal culture will appear as a velvety, olive or black-brown mold.

Treatment

There is no curative treatment of Exophiala in fish.

Exophiala aquamarina, CBS 119918. Conidial apparatus and conidia.
Exophiala aquamarina, CBS 119918. Conidial apparatus and conidia. — Scale bar = 10 μm.
Phylogenetic analyses of Trichodinids (Ciliophora, Oligohymenophora) inferred from 18S rRNA gene sequence data. de Hoog GS et al. Persoonia. 2011 Dec;27:46-72.

Exophiala in PubMed

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Oomycetes (water molds)

The water molds are the most common of all fungal pathogens of freshwater and brackish water fish. Saprolegnia, Achlya, and Aphanomyces are the genera typically isolated. The water molds differ from true fungi in several important ways. At one life stage, unicellular, biflagellated zoospores with two different types of flagella are produced. Nuclei within the fungal hyphae are diploid. Their cell wall is comprised of cellulose and glycans, and not chitin.

Order Saprolegniales

Taxonomy

cellular organisms - Eukaryota - Stramenopiles - Oomycetes - - Saprolegniales - Saprolegniaceae - Saprolegnia

Brief facts

Saprolegniasis is a disease of freshwater and brackish water species caused by Saprolegnia, although several other related species of Oomycetes such as Achlya, all ubiquitous in soil and freshwater, can present clinically in a similar way. Saprolegniasis in fish eggs is a significant problem in the culture of many freshwater and brackish water species. Water molds normally do not penetrate or infect normal, healthy fertilized eggs. Infertile eggs are very susceptible to fungal infection, however, and fungi that begin their growth on infertile eggs may rapidly spread and smother adjacent fertile eggs, preventing adequate oxygenation and gas exchange.

Clinical signs

Typically, this disease presents as a relatively superficial, cottony/woolly (floccous), white growth on the skin, or gills, or on fish eggs when in water. The color of these lesions often varies because the mycelium tends to entrap sediment particles or algae and may appear to be red, brown, or green. Out of the water, the floccous appearance is difficult to appreciate because the mycelium collapses into a slimy, matted mass when fishh is out of water. Initial lesions are often focal, small, and inconspicuous, but these can rapidly enlarge because of the rapid development of the mycelium over a short period of time. If the infection is on a scaled surface, the scales of the fish are usually elevated from the body. Although superficial (epidermal) initially, these lesions may extend into dermis and musculature with time. These early lesions are usually pale surrounded by a ring of erythema. Sloughing of the epidermis ensues. Thus, chronic infections invariably result in erosions or superficial ulcerations, although the defects in most instances are covered by the fungal mat. Some saprolegnian infections have been reported to extend from the gills into the pharynx and esophagus, whereas others have been known to infect other tissues including the liver, spleen, kidney, swim bladder, eye, and brain.

Saprolegniasis typically is seen as cottony white, gray, brown, red, or greenish masses on skin or gills of freshwater or brackish fish. Initial affected areas may be small; however, the fungi can spread rapidly and cover most of the body. Occurrence may be acute, but duration is often chronic.

Saprolegnia causes superficial disease with a mild inflammatory response. Lesions rarely go beyond superficial musculature. Epithelial disruption results in osmoregulatory compromise with potential for severe dilution of bodily fluids and infection by other pathogens.

Biology

Although the life cycle is complex (including sexual [oospore] and asexual reproduction [zoospore]), the zoospore, produced at the tips of the hyphae in zoosporangia, is probably the most important means of reproduction and spread of disease. Primary zoospores emerge from the zoosporangia, encyst, germinate, and release secondary zoospores, the main motile and dispersive stage. The secondary zoospores are believed to be attracted chemically to fish eggs and also have the ability to re-encyst repeatedly, if initial hosts are unsuitable.

Diagnosis

Tentative identification of Saprolegnia and related species can be made by microscopic examination of wet mounts of infected tissues and observation of branching, filamentous, aseptate (not separated by cross-walls, known as septa) fungal hyphae comprising the vegetative mass known as a mycelium. Definitive diagnosis requires culture, visualization, and identification of sexual or asexual reproductive stages.

Treatment and control

Treatment of infected populations can be difficult. Anatomic location of initial lesions and severity seem to determine likelihood for survival, as do ability to determine and eliminate predisposing factors after onset. Acute temperature drops, handling, and poor water quality are among the most important factors, as described previously. Fish that are infected primarily on the distal areas of fins have a better chance than those that are infected on the main trunk. Less severely infected fish can recover if provided with good water quality.

Increasing salinity to freshwater fish assists with osmoregulatory balance and appears to affect fungi to some degree. Most freshwater species of fish can tolerate 1 to 3 g/L of salt indefinitely; some species can tolerate even more. Unfortunately, as much as 10 to 30 g/L may be required to achieve significant inhibition and control. Once Saprolegnia has taken hold of a population, however, prognosis can be poor.

Other chemotherapeutics have demonstrated limited efficacy when used as bath treatments for large populations. Malachite green is considered the most effective agent, although its use in food fish is prohibited because of carcinogenicity and teratogenicity. Malachite green alone, or in conjunction with formalin, can be highly toxic to some fish species, especially scaleless. Other agents with some promise include hydrogen peroxide and formalin, labeled by the US Food and Drug Administration for use as an antifungal.

For aquarium ornamental fish, more aggressive therapy, including use of combination antibacterial/ antifungal ointments, can increase survival significantly if only one or a small number of fish are affected. Increasing salinity to upper tolerance levels for the fish species also is recommended in these instances.

Saprolegnia in PubMed

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Aphanomyces invadans

Taxonomy

cellular organisms - Eukaryota - Stramenopiles - Oomycetes - Saprolegniales - Saprolegniaceae - Aphanomyces - Aphanomyces invadans

Brief facts

Over the past few decades, Aphanomyces invadans (believed to be the same species as A. invaderis and A. piscicida, or very closely related) has become recognized as the primary agent associated with epizootics of ulcerative disease in cultured and wild fish around the world. Other names for this disease: Mycotic Granulomatosis (MG) (Japan), Red Spot Disease (RSD) (Australia), Epizootic Ulcerative Syndrome (EUS) (Asia), and Ulcerative Mycosis (United States). Aphanomyces invadans outbreaks are severe in naive populations and can result in high mortalities. Predisposing factors include lower temperatures (less than 25°C, 77°F), and reduced salinity in brackish water systems, presumably suppressing the immune system or primary defenses (mucus production, epithelial integrity) against invasion.

Pathology

Aphanomyces is unlike the more common water molds (Oomycetes) such as Saprolegnia in its pathology. Its zoospores are able to germinate, infect susceptible fish species, and cause deep ulcerative, granulomatous lesions with necrosis of skin and muscle with potential for invasion into the body cavity and internal organs, including the anterior and posterior kidney, spleen, gastrointestinal (GI) tract, liver, pancreas, spinal cord, and gonads.

Deep ulcerative lesions are the hallmark of the chronic stage of an Aphanomyces infection. Early lesions may present as small erythematous loci on the skin that progress to a necrotizing dermatitis and ultimately resulting in a deep, dermal ulcer.

Diagnosis

A presumptive diagnosis is based on the presence broad (<7 pm), aseptate hyphae together with macrophages and smaller numbers of lymphocytes and eosinophilic granular cells on skin scrapes and crush muscle preparations of the lesions. Culture is required for positive identification.

Treatment and control

Aphanomyces species were much more sensitive to malachite green (0.5 ppm), hydrogen peroxide (100 ppm to 500 ppm), and sodium chloride (10 ppm to 20 ppm) following 1-hour treatments, than Saprolegnia species. These studies were done in vitro, however, and treatments of active cases most likely will differ, depending upon species and extent and progression of disease, and tolerance of each fish species to chemotherapeutic and concentration.

Aphanomyces epizootic ulcerative syndrome.jpg
Fish with typical epizootic ulcerative syndrome (EUS) clinical signs collected from Mangalore, India. (A) Terapon spp. (B) Cynoglossus spp. (C) Sillago spp. (D) Mugil spp. (E) Channa spp.
Development and standardization of a monoclonal antibody-based rapid flow-through immunoassay for the detection of Aphanomyces invadans in the field. Adil B et al. J Vet Sci. Dec 2013; 14(4): 413–419.

Aphanomyces invadans in PubMed

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Branchiomyces spp. (freshwater gill rot)

Taxonomy

cellular organisms - Eukaryota - Stramenopiles - Oomycetes - Saprolegniales

Brief facts

Also known as gill rot, branchiomycosis is an acute, localized fungal disease of the gill affecting a wide variety of freshwater fish. It has a fairly broad geographical range, with cases reported in the United States, Japan, Indochina, Israel, and Europe, where it is considered to be a major problem in commercial fish production. Because it causes a respiratory insult, the fish with subacute to acute infections most often present with respiratory distress and/or lethargy. Epizootics usually occur when water temperatures are above 20°C and mortalities can be as high as 50%. Morbidity may reach as high as 100%, but not all species of fish present in an affected pond may be susceptible. The course of infection is usually rapid and is exacerbated by crowding, algal blooms, or increased levels of un-ionized ammonia.

Pathology

Grossly, in acute cases of branchiomycosis, the gills have a marbled appearance because of the hemorrhage and infarcted tissue. The fungal mycelia obstruct blood flow within the vessels causing thrombosis. This corresponds to the dark red brown areas in the gills, which contrasts with light grayish-white ischemic zones.

Treatment and control

Several chemicals have been suggested for the treatment of branchiomycosis. These include malachite green, benzalkonium chloride, copper sulfate, formalin and feeding methylene blue. The disease also has an acute onset and rapid progression, which makes prevention rather than treatment the key to controlling this disease. Both quick lime (to raise the pH and clarify the water) and copper sulfate have been suggested as prophylactic measures and to prevent recurrence. Improving water quality (e.g., reduced feeding and stocking density) may also be key in the prevention and treatment of the disease. Removing dead fish during the epizootic stage is also important in reducing the source point of infection.

Branchiomyces in PubMed

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Microsporidia

Microsporidians are a diverse group of unicellular organisms that live as obligate intracellular parasites in many invertebrates and are reported from all classes of vertebrate hosts. Many microsporidian species are widely distributed in teleosts in freshwater, brackish and marine habitats. Once considered as the most primitive eukaryotes, they are now regarded as highly specialized fungi with simple life cycles consisting of growth and sporulation stages. Microsporidians are single walled spores with length of 2–10 mm, mostly ellipsoidal or egg shaped and lack mitochondria. The spores contain extrusive apparatus capable of injecting sporoplasm into host cell via extrudable polar tube.

Loma salmonae spp. (freshwater gill rot, microsporidial gill disease of salmonids, MGDS)

Taxonomy

cellular organisms - Eukaryota - Opisthokonta - Fungi - Microsporidia - Pansporoblastina - Glugeidae - Loma - Loma salmonae

Brief facts

Loma salmonae causes Microsporidial Gill Disease (MGD) in seven salmonid species of the genus Oncorhynchus, brook trout (Salvelinus fontinalis), and brown trout (Salmo trutta). The typical clinical signs include pale gills with petechial hemorrhages, hyperplasia, and white cysts called xenomas. Fish with high numbers of xenomas in their gills suffer respiratory distress, secondary infection and high mortality rates. Laboratory studies show that L. salmonae can be transmitted directly via ingestion of spores by fish. Briefly, the spore enters the gut and injects sporoplasm into an epithelial cell, it migrates to the heart, then to the gill, and forms a spore-laden xenoma within the gill filaments. The rupture of xenoma causes proliferative branchitis and massive number of spores release.

Treatment and control

There are no reported effective treatments for microsporidians.

Loma salmonae in PubMed

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Pleistophora spp. (freshwater gill rot)

Taxonomy

cellular organisms - Eukaryota - Opisthokonta - Fungi - Microsporidia - Pansporoblastina - Pleistophoridae - Pleistophora

Brief facts

Pleistophora is the etiologic agent of Neon Tetra Disease, causing white cysts (xenomas) to develop in the muscle and other tissues. These cysts may be visible below the skin. The xenomas can cause severe deformity of the tissues. Rupture of the xenoma releases spores that are released and ingested by new hosts. Pleistophora also affects angelfish, rasboras, barbs, and tetras. Clinical signs include muscle wasting, erratic swimming behavior, color loss, lethargy, secondary bacterial infections at the site of cyst rupture, and deaths.

Treatment

There are no reported effective treatments for microsporidians.

Pleistophora in PubMed

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References

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