Adult female cardinal tetra fish (Paracheirodon axelrodi)This animal was one of 15 cardinal tetra (Paracheirodon axelrodi) in a 220 gallon heated freshwater tank. The tank contained a total of 29 species of tropical fish. Additional species included firehead tetra (Hemigrammus bleheri), rummy nose tetra (Hemigrammus rhodostomus), yellow phantom tetra (Hyphessobrycon roseus), serpae tetra (Hyphessobrycon eques), red phantom tetra (Hyphessobrycon sweglesi), bleeding heart tetra (Hyphessobrycon erythrostigma), Savanna tetra (Hyphessobrycon stegemanii), blue emperor tetra (Inpaichthys kerri), silver tetra (Ctenobrycon spilurus), white finned rainbowfish (Bedotia leucopteron), snakeskin barb (Puntius rhomboocellatus), cherry barb (Puntius titteya), three-striped glass catfish (Eutropiellus debauwi), glass catfish (Kryptopterus bicirrhis), diagonal-stripe catfish (Corydoras melini), slant bar catfish (Corydoras loxozonus), Napo catfish (Corydoras napoensis), eartheater (Geophagus megasema), Leopolds angelfish (Pterophyllum leopoldi), freshwater angelfish (Pterophyllum scalare), blue butterfly cichlid (Mikrogeophagus ramirezi), Bolivian ram (Mikrogeophagus altispinosus), slender hemiodus (Hemiodopsis gracilis), green knifefish (Eigenmannia virescens), festivum (Mesonauta mirificus), spotted platyfish (Xiphophorus maculatus), chessboard cichlid (Crenicara filamentosa) and panchax (Pachypanchax spp.). 

Gross Description:  

Similar lesions to those noted clinically were apparent on both of the euthanized animals. Numerous (between 10 and 20) 2 to 3 mm diameter transparent raised nodules were present on the skin surface. Each contained a prominent coiled to elongate approximately 2 mm long and 0.1 mm diameter white vermiform structure. Both animals were in otherwise good body condition and were reproductively active based on gonad development. Cytology findings are presented below. 

Histopathologic Description:

Histologically, the dermis is multifocally expanded and the overlying scales are elevated in multiple locations by 100 to 200 um diameter by 1000 to 2000 um long tubular mesomycetozoal cysts that have a 1 to 2 um thick eosinophilic laminated wall and contain innumerable 3 to 7 um diameter round spores. Individual spores have a prominent large eosinophilic central to eccentric vacuole (retractile body) that is surrounded by a rim of weakly eosinophilic to clear cytoplasm containing a peripheralized deeply basophilic nucleus. There is a mild to moderate associated inflammatory infiltrate within the dermis composed primarily of granulocytes and macrophages. Cysts are occasionally captured as they penetrate the overlying epidermis and protrude from the epidermal surface. At the base of the anal fin, where the inflammatory cell infiltrate is most severe, moderate numbers of pyriform to ovoid holotrich ciliates (approximately 40 x 80 um) infiltrate the dermis and are admixed with the inflammatory cells and mesomycetozoal cysts and spores. 

Morphologic Diagnosis:  

1. Skin, dorsum, ventrum, base of anal and caudal fins: Dermatitis, granulocytic, histiocytic, multifocal, subacute, mild to moderate with multiple intralesional mesomycetozoal cysts and luminal spores (Dermocystidium spp.)

2. Skin, base of anal fin: Dermatitis, granulocytic, histiocytic, focal, subacute, moderate with intralesional protozoa (Tetrahymena spp.)

Lab Results:  

Skin scrapings and wet mounts from both clinical and necropsy cases contained numerous elongate mesomycetozoal cysts that were approximately 100 um diameter and tapered at one end to a 5 um diameter thin projection. The cysts were often ruptured at one end and were filled with innumerable approximately 5 to 7 um diameter spores. Individual spores had a prominent large central to eccentric refractile vacuole (refractile body). These findings were consistent with Dermocystidium spp. infection. Skin scrapings from the representative animals of each of the 28 other species in the tank were negative for mesomycetozoal cysts. One frozen tetra was submitted for 18s small subunit rRNA PCR to confirm the pathomorphologic diagnosis and provide species identification but results are not available at this time.



Contributor Comment:  

Dermocystidium is a member of the class Mesomycetozoea (previously known as the DRIP clade), which contains organisms that lie at the boundary between animals and fungi. 

Phylogenetic analysis of the 18s small subunit rDNA genes indicates this class contains 10 different genera of parasitic and saprophytic microbes including: Amoebidium, Anurofeca, Dermocystidium, Ichthyophonus, Pseudoperkinsus, Psoros-permium, Rhinosporidium, Sphaerosoma and two currently unnamed agents clone LKM51 and rosette agent.(2)

The majority of mesomycetozoea are pathogens of aquatic species, specifically fishes and invertebrates with Rhinosporidium a noteable exception. The life cycles of these organisms have not been completely documented. In vitro, both Dermocystidium and the rosette agent develop uniflagellated zoospores which could serve as a method of transmission and infection.(2) Waterborne transmission of Dermocystidium has been documented.(4)

There are currently 14 recognized species of Dermocystidium which all cause pathogenic infection in fishes and aquatic invertebrates.(7) The skin and gills are the primary sites of infection, though visceral lesions have also been reported.(5) The most diagnostic feature on cytology and histology is the presence of cysts (sporocysts) containing the characteristic spherical spore (endospore) stage with a large central vacuole (refractile body).(3) There are four previous reports of Dermocystidium infection in Paracheirodon genus fishes (cardinal tetra, Paracheirodon axelrodi and neon tetra, P. innesi) with two providing histopathology similar to what is presented in this case.(1,6) In the two most recent reports the agent was confirmed by 18S rDNA PCR to be Dermocystidium salmonis, a pathogen previously reported in Pacific salmon species. Both reported lesions to be predominantly epidermal and most severe along the anterior body and fins.(1,6) The histologic lesions and distribution of the infection in the fish presented here are similar to these two previous reports, which suggests Dermocystidium salmonis infection in this group of cardinal tetra. One cardinal tetra from this group was submitted for 18S rDNA PCR and diagnostic confirmation; however, results were not available at the time of case submission. 

In recent case reports as well as in this case infection occurred in mixed species tanks, but only Paracheirodon genus fishes were affected. This may indicate a sensitivity of this genus to Dermocystidium infection. A source for infection was not clear in the current case. There were no recent changes in water source and there was no prior history of this infection in other tanks. There had been recent (6 weeks prior) addition of other genera of fishes to the impacted tank following 30 day quarantine, however there had not been a recent introduction of Paracheirodon spp. It is not clear from the current literature if a carrier state in other fishes is possible. The focal Tetrahymena infection in this case was believed to be opportunistic and incidental. 

JPC Diagnosis:  

Skin: Dermatitis, ulcerative and granulocytic, subacute, multifocal, moderate with multiple mesomycetozoan cysts and ciliates.

Conference Comment:  

The conference histologic description was aligned very closely with the contributors description above. Participants commented on the excellent quality of the digital slide and parasagittal section. Participants were careful to point out the shape of the cysts located dorsally as being elongated, versus the cysts located ventrally which are round to ovoid; this is an important component to the description and can aid in identification of the organism. The contributor provided excellent gross images and the moderator led a discussion regarding the difficulty in identifying the mesomycetozoan tubular, opaque cysts grossly due to their resemblance to nematodes. Another important point raised by the moderator included recognizing the fish contains abundant eggs, which is indicative of good body condition and lack of debilitation. 

Some conference participants identified the ciliate as a trichodinid due to its similar size compared with a tetrahymenid, but as the moderator pointed out, Trichodina spp. dont penetrate the skin whereas tetrahyminids do. Trichodinids, while also ciliates, generally cause a relatively mild disease in healthy fish but can result in significant losses in young fish, particularly when secondary bacterial infections are present or in cases of debilitation due to other causes. Tetrahymenids are often opportunistic invaders, as seen in this case, but can also invade internal organs and result in lethal infections in severe cases. Infection has been referred to as guppy disease due to its preference for infecting guppies. Tetrahymena sp. can also cause disease in catfish, common carp, and rainbow trout secondary to skin damage and invasion of internal organs.(3)


1. Langenmayer MC, Lewisch E, Gotesman M, et al. Cutaneous infection with Dermocystidium salmonis in cardinal tetra, Paracheirodon axelrodi (Schultz, 1956). J Fish Dis. 2014; 38(5):503-506.

2. Mendoza L, Taylor JW, and Ajello L. The Class Mesomycetozoea: Heterogeneous group of microorganisms at the animal-fungal boundary. Ann Rev Microbiol. 2002; 56:315-355.

3. Noga EJ. Fish Disease: Diagnosis and treatment. 2nd ed. Ames, IA: Wiley-Blackwell; 2010: 137-141, 174-175.

4. Olson RE, Dungan CF, and Hold RA Water-borne transmission of Dermocystidium salmonis in the laboratory. Dis Aquat Org. 1991; 12:41-49.

5. Roberts RJ. The mycology of teleosts. In: Roberts RJ, ed. Fish Pathology. Chicheste, UKr: Wiley-Blackwell; 2012: 383-401.

6. Westmoreland LSH, Hadfield CA, Clayton LA, et al. Mesomycetozoea in cardinal tetras (Paracheirodon axelrodi) and green neon tetras (Paracheirodon simulans). In: Proceedings of the IAAAM, 46th Annual Conference. Chicago: April 6-10, 2015.

7. Index Fungorum; Accessed January 13, 2016.

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1-5. Scaled skin

1-6. Scaled skin

1-7. Scaled skin

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