JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL SYSTEM

April 2025

M-P06 (NP)

 

Signalment (JPC# 3121691): Adult female guppy (Poecilia reticulata) from a pet store

HISTORY: The fish was euthanized following a prolonged period of anorexia and weight loss.

HISTOPATHOLOGIC DESCRIPTION: Whole body, parasagittal section: Extending through the multifocally eroded and ulcerated scaled skin, further extending through the dermis, infiltrating subjacent skeletal muscle, and expanding the submesothelial space of the coelomic cavity by up to 300µm, are numerous ciliate trophozoites. These ciliate trophozoites are 30 to 40 µm diameter, ovoid, and have a thin cell wall lined circumferentially by cilia of equal length and distribution (holotrich), cytoplasm that contains variably sized eosinophilic globules, one irregularly oval to elongate deeply basophilic macronucleus, and occasionally a contractile vacuole. Trophozoites are frequently admixed with a small amount of eosinophilic cellular and karyorrhectic debris (necrosis), minimal fibrin, edema, and rare hemorrhage as well as few granulocytes and lymphocytes (most significant in the areas of cutaneous ulceration/erosion). Individual rhabdomyocytes are surrounded, separated, and rarely replaced by ciliate trophozoites, and are often characterized by one of the following: shrunken with irregularly sized sarcoplasm (atrophy); pale, swollen, vacuolated sarcoplasm with loss of cross striations (degenerate); or hypereosinophilic, fragmented sarcoplasm with nuclear pyknosis, karyolysis, or karyorrhexis (necrotic).

Diffusely affecting 80% of the liver, hepatocytes are moderately expanded up to twice normal size by small, discrete, clear cytoplasmic vacuoles (lipid).

Within the lumen of one cross section of orad intestine is a cross section of a 350µm diameter adult cestode with a 3µm brightly eosinophilic tegument bounding a spongy parenchyma containing basophilic calcareous corpuscles and a uterus with many 10-20 µm thin walled eggs.

MORPHOLOGIC DIAGNOSIS: Skin, skeletal muscle, and coelomic cavity: Ciliated trophozoites, numerous, with mild lymphohistiocytic inflammation, rhabdomyocyte degeneration and necrosis, and erosive to ulcerative dermatitis, guppy, (Poecilia reticulata), Cyprinodontiform.

ETIOLOGIC DIAGNOSIS: Tetrahymenal coelomitis, myositis, and dermatitis

CAUSE: Tetrahymena corlissi

SYNONYMS: Guppy disease; spectacle eye; tetrahymeniosis; “tet disease”

GENERAL DISCUSSION:

• Tetrahymenids are common, typically free-living, saprozoic, freshwater ciliate protozoans but can be highly lethal fish pathogens and may cause systemic disease
• Tetrahymeniosis causes severe economic losses in commercial guppy farms worldwide
• Called “guppy disease” because of its predilection for guppies (including mortality up to 100%), but can infect other livebearers, cichlids, tetras, golden perch, laboratory zebrafish, and even freshwater leeches
• Reported to infect amphibians

PATHOGENESIS:

• Environmental contamination à direct skin invasion à reproduction by binary fission forming small reproductive cysts containing 2-8 tomites à may cause systemic infection which overwhelms the host
• Stress, such as poor water quality or concurrent disease, predisposes fish to disease
  • Poor water quality: e.g. high ammonia content, high organic load, extreme water temperature

TYPICAL CLINICAL FINDINGS:

• Nonspecific, including anorexia, weight loss, abnormal swimming in water column; often no premonitory signs.
• Acute death, with mortality between 87-100% in guppies; variable mortality in other species

TYPICAL GROSS FINDINGS:

• Muscular swelling may be evident
• White patches on skin due to high numbers of ciliates in copious mucus
• A mass of ciliates can form a rim around the orbit (“spectacle eye”)

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• 30-50um ciliate protozoan with a thin cell wall and circumferential surface cilia of equal length and distribution (“holotrich”)
• In heavy infections, parasite can be found within the meninges and brain of the fish, with a distinct lack of host immune response
• Can invade viscera (muscle, kidney, brain), epithelium, causing necrosis and degeneration; penetration into muscle and deep tissues is common
• Minimal/mild granulocytic inflammation

ADDITIONAL DIAGNOSTIC TESTS:

• Ciliate can be found on wet mount of skin, gills, internal organs; ciliate has circumferential, evenly distributed cilia with one long caudal cilium (present in only some species of Tetrahymena)

DIFFERENTIAL DIAGNOSIS:

• Icthyophthirius multifiliis: similar gross appearance, different parasite morphology ( multifiliis is much larger with a macronucleus)
• Uronema marinum and Philasterides dicentrarchi appear similar histologically; they have eccentric macronuclei, abundant foamy basophilic cytoplasm with ingested host erythrocytes, and a single long caudal cilium; speciation based on histologic appearance is difficult
• Paramecium and other free-living, non-pathogenic ciliates: similar parasite morphology, lacks pathogenic lesions (i.e. incidental finding if present in section)

COMPARATIVE PATHOLOGY:

• Few ciliated protozoa are pathogenic; the organisms are usually incidental findings within the intestinal tract or occasionally as post mortem reflux into the respiratory tract
• Pathogenic ciliates include:
• Balantidium are common ciliates of pigs, rodents, and non-human primates; rarely associated with mild to severe enteritis and dysentery
• Tetrahymena pyriformis: Pathogenic ciliate affecting internal organs and musculature of common carp, catfish, and rainbow trout
• Other Tetrahymena : Deep ulcerative dermatitis of Atlantic salmon in fresh water; can affect some crustaceans and turbellarians
• Icthyophthirius multifiliis (see I-P16) and Cryptocaryon irritans are the freshwater and marine (respectively) ciliated protozoa which cause white punctuate lesions in the integument of fish; these are predominantly non-pathogenic, however can result in severe lesions, invasion of the meninges and brain, and death in crowded environments causing stress and immunosuppression in aquaculture environments
• Scuticociliata (histophagous ciliated parasites), including Uronema marinum, are free-living holotrich ciliates that seem to be the marine counterpart to Tetrahymena ; they initially cause epidermal pallor and ulceration, then necrosis, hemorrhage, and edema in skeletal muscle and systemic spread with minimal inflammatory response
• Other holotrichs: Philasterides dicentrarchi (another scuticocilata) causes disease in leafy and weedy sea dragons and several species of sharks, Pseudocohnilembus persalinus causes disease in sea bass and turbot
• Trichophrya piscium is a suctorian ciliate commonly encountered on the gills and epidermis of fresh-water fish; generally considered a commensal organism, but in times of high stress they can cause excess mucus production and a bluish film on the epidermis

REFERENCES:

1. Frasca S, Wolf JC, Kinsel MJ et al. Osteichthyes. In: Terio KA et al, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 990 – 991.
2. Gardiner CH, Fayer R, Dubey JP. An Atlas of Protozoan Parasites in Animal Tissues. 2nd ed. Washington, DC: Armed Forces Institute of Pathology; 1998:16-17.
3. Leibowitz MP, Zilberg D. Tetrahymena infection in guppies, Poecilia reticulata Peters: parasite characterization and pathology of infected fish. J Fish Dis. 2009:32(10):845-855.
4. Noga EJ. Fish Disease: Diagnosis and Treatment. 2^nd Ames, IA: Wiley-Blackwell; 2010:140-141, 142- 143.
5. Roberts RJ. The parasitology of teleosts. In: Roberts RJ ed. Fish Pathology. 4th ed. Philadelphia, PA: W.B. Saunders; 2012:311-312.
6. Sharon G, Pimenta Leibowitz MP, Kumar Chettri J, Isakov N, Zilberg D. Comparative study of infection with Tetrahymena of different ornamental fish species. J Comp Pathol. 2014; 150:316-324.
7. Speare D, Frasca S. Nervous System. In: Ferguson HW ed. Systematic Pathology of Fish. 2^nd London, UK: Scotian Press; 2006: 237, 304.

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