Discus fish (Symphysodon aequifasciata) adult femaleCommunity tank with other discus and tropical fish. Zoo owned fish held in a community fish tank with other discus and tropical fish. This individual had been identified with an increased respiratory rate for approximately 18 months but problems with isolation of this fish meant that it was left on display. Other individuals in tank had a much lower respiration rate and were clinically normal. This female fish was submitted live for necropsy.

Gross Description:  

No gross abnormalities identified at necropsy.

Histopathologic Description:

Multifocally, the gills are moderately thickened by oedema and show mild hyperplasia with stunted, fused or complete lack of secondary lamellae. Gill interstitial tissue is multifocally infiltrated by moderate to severe numbers of inflammatory cells, predominantly lymphocytes with lesser numbers of eosinophilic granulocytes. There is multifocal moderate external haemorrhage and in between gill filaments are numerous (50 x 300um) multicellular parasites showing a thin (~2um) eosinophilic tegument, poorly discernable basophilic parenchyma and occasionally an oral sucker by which they are attached to the gill epithelium (trematodes). Multifocally, at the base of gill filaments, arterial vascular walls are moderately to severely thickened and show hyalinisation (fibrinoid necrosis) and infiltration by mild to moderate numbers of degenerate and viable leucocytes with much cell debris. Occasional clusters of basophilic, finely granular material are present in the secondary lamellae (bacterial colonies). 

Morphologic Diagnosis:  

Gill multifocal moderate to severe gill inflammation with oedema, mild hyperplasia, a leukocytoclastic fibrinonecrotic vasculitis and epithelial attached adult trematodes. Discus fish, aetiology consistent with Dactylogyrus sp.

Lab Results:  

Wet preparation of gills: Numerous motile parasites attached to the gill epithelium. Appearance was consistent with Dactylogyrus species, monogenetic gill trematodes, due to its shape being flattened and leaf-like, with four anterior eyespots; a cephalic end which was scalloped and had an attachment organ (haptor). 


Dactylogyrus sp.

Contributor Comment:  

Although monogenean gill flukes are commonly found on wild fish, they are rarely a direct cause of disease or death in free-ranging populations. In captivity, epidemics can occur with significant morbidity and mortality especially in cultured fish with excessive parasite loads under conditions of overcrowding, inadequate sanitation and poor water quality.1,2 Under these circumstances, the parasites rapidly multiply. Gill fluke infection affects many fresh and saltwater species across a variety of temperatures, but is especially common in carp, goldfish and discus fish.4 There is a diverse range of fluke species, most are host- and site-specific, requiring only one host to complete an entire life cycle.5

Freshwater fish infected with heavy gill infestations result in respiratory disease. Clinical signs can include opaque mucus covering the gills, protrusion of the gill filaments from under the gill covers and gills may be swollen and pale. Infected fish are less tolerant of low oxygen conditions and have an increased respiratory rate with gulping of air at the water surface. Fish become anoxic with flaring of the gill opercula.1 At a very advanced stage, the fish will isolate itself and spend long periods lying on the bottom with its fins clamped to its body. Acute infections are characterized by a short period of dyspnoea followed by sudden death.1

Most monogenean flukes are browsers, moving about the body surface and feeding on dermal mucus and gill debris. Monogeneans have a series of hooks that enable them to attach while feeding.2 Flukes anchoring to the gills induce a variety of lesions, depending on the density and species of parasite. Lesions can range from excessive mucous secretions, hyperplasia of gill epithelium with fusion of secondary lamellae to the presence of haemorrhage, necrosis / ulceration and inflammation.3,6 Secondary infection by bacteria and fungi are commonly established at damaged epithelial sites.4

In this case, there is a widespread arterial vasculitis, which we propose may be mediated by immune complexes. Other discus fish in the collection are almost certainly affected by flukes but the involvement of a vasculitis, in other cases, remains to be determined.

The Dactylogyrus gill fluke life cycle is direct, not requiring intermediate hosts. The adults are oviparous and produce eggs with long filaments. The eggs are usually attached to the gills and develop into a free-swimming onchomiricidium, which then locates and attaches to the fish within a few hours.5

JPC Diagnosis:  

1. Gill: Branchitis, lymphocytic and granulocytic, multifocal, moderate with blunting, fusion, and loss of lamellae, mild epithelial hyperplasia and adult trematodes (Fig. 1-1), Discus fish (Symphysodon aequifasciata), Perciformes.
2. Gill: Vasculitis, necrotizing, multifocal, moderate with edema and hemorrhage (Fig. 1-2).

Conference Comment:  

The contributor gives an excellent overview of Dactylogyrus sp. gill infections. Gills are composed of two sets of four holobranchs that are located on either side of the pharynx. Each holobranch is composed of two hemibranchs that project from the posterior edge of the branchial arch. These hemibranchs contain numerous primary lamella and their secondary lamella.4,8 Cells on the primary and secondary lamellae include melanocytes, lymphocytes, macrophages, endothelial cells, mucous cells, rodlet cells, and chloride cells.

Conference participants noted a microsporidian-like organism within some of the sections examined. Coinfections are not uncommon within compromised gill epithelium.7

Other diseases of importance that may affect the gills include the following list adapted from Moeller4 and Wootten8


2. Ferguson HW: Gills and pseudobranchs. In: Systematic Pathology of Fish, eds. Herman RL, Meade JW, pp. 11-40. Iowa State University Press, Ames, IA, 1989
3. Herbert BW, Shaharom FM, Anderson IG: Histopathology of cultured sea bass (Lates calcarifer) (Centropomidae) infected with Cruoricola lates (Trematoda: Sanguinicolidae) from Pulau Ketam, Malaysia. Int J Parasitol 25:3-13, 1995
4. Moeller RB: Diseases of Fish. found at, 2001
5. Roberts RJ: The pathophysiology and systemic pathology of teleosts. In: Fish Pathology, ed. Roberts RJ, pp. 55-132. WB Saunders, London, 2001
6. Roubal FR: Microhabitats, attachment of eggs and histopathology by the monogenean Allomurraytrema robustum on Acanthopagrus australis (Pisces: Sparidae). Int J Parasitol 25:293-298, 1995
7. Smith SA, Noga EJ: General parasitology. In: Fish Medicine, ed. Stoskopf MK, pp. 132-148. W.B. Saunders Company, Philadelphia, PA, 1993
8. Wooten R: The parasitology of teleosts. In: Fish Pathology, ed. Roberts RJ, pp. 242-288. WB Saunders, London, 2001

A virtual slide is not available for this case.

(Fig 1-1) Gill

(Fig 1-2) Gill

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