JPC SYSTEMIC PATHOLOGY
Signalment (JPC #1910090): Juvenile Chinook salmon (Oncorhynchus tshawytscha)
HISTORY: Several fish had difficulty swimming, decreased growth rate, and increased mortality due to starvation and predation
HISTOPATHOLOGIC DESCRIPTION: Gill: Multifocally there is distortion of lamellar architecture with primary lamellae and secondary lamellae expanded up to 4 times normal by variably-sized, up to 300 um diameter, irregularly round to oval microsporidian xenomas, with a thin eosinophilic wall, and numerous eosinophilic, 2-4 um oval spores. Multifocally primary and secondary lamellae are mildly expanded by low numbers of macrophages, lymphocytes, and necrotic cellular debris. There is multifocal atrophy, blunting, fusion and loss of secondary lamellae with branchial epithelial and mucous cell hyperplasia in adjacent lamellae.
MORPHOLOGIC DIAGNOSIS: Gill: Branchitis, granulomatous, subacute, multifocal, mild, with lamellar fusion and atrophy, and many intra-lamellar microsporidian xenomas, Chinook salmon, piscine
ETIOLOGIC DIAGNOSIS: Branchial microsporidiosis
CAUSE: Loma salmonae
SYNONYMS: Microsporidial gill disease
Signalment (JPC #1901138): Fingerling channel catfish (Ictalurus punctatus)
HISTORY: Several fish were gasping at the water surface and succumbed to exertion
HISTOPATHOLGIC DESCRIPTION: Multifocally, primary lamellae expanded up to 10 times normal width by numerous foamy macrophages, fewer lymphocytes, and scattered hemorrhage and edema admixed with necrotic cellular debris. Within primary and secondary lamellae, there are few viable and degenerate microsporidian xenomas that are irregularly round, 20-50 um, and contain numerous 2-4 um, eosinophilic oval spores, frequently surrounded by palisading macrophages. Multifocally, there are intralamellar myxozoan cysts up to 200 um in diameter that contain numerous, 2 x 10 um, fusiform, eosinophilic spores. There is necrosis and loss of lamellar cartilage. Adjacent secondary lamellae have moderate epithelial and mucous cell hyperplasia with marked lamellar fusion. Multifocally separating secondary lamellae are cross sections of 100 um diameter adult trematodes characterized by a 5 um thick tegument, filled with a parenchyma, reproductive organs, vitellaria, and poorly discernible paired caeca (trematodes not present in all slides).
MORPHOLOGIC DIAGNOSIS: 1. Gill: Branchitis, proliferative and necrotizing, subacute, diffuse, moderate with lamellar blunting, fusion, and atrophy, lamellar cartilage degeneration, and intralamellar myxosporan cysts, channel catfish, piscine
- Gill: Branchitis, subacute, multifocal, mild, with intralamellar microsporidian xenomas
- Gill: Trematodes, multiple
ETIOLOGIC DIAGNOSIS: Branchial myxosporidiosis
CAUSE: Henneguya spp
SYNONYMS: Proliferative gill disease, hamburger gill disease
- Microsporidia: obligate intracellular, unicellular eukaryotes,
- Considered atypical fungi without mitochondria
- Uniquely, reproduction and maturation occurs in host cell cytoplasm
- Exist extracellularly as small, thick-walled spores with a coiled polar filament
- Low pathogenicity, opportunistic pathogen, latent or mild symptoms in mammals
- Primarily infect fish and arthropods; rarely infect reptiles and amphibians
- Encephalitozoon cuniculi: most relevant microsporidia species in mammals
- Loma salmonae infects the gills of salmonid fish
- Myxosporea: metazoan parasites that mainly infect fish
- Henneguya ictalur: most commonly in farmed catfish; necrotizing branchitis and dermatitis
- Previously considered protozoan, reclassified as metazoan
- Microsporidia (Loma salmonae):
- Direct life cycle: Spore ingested > sporoplasm discharged through tubular extrusion apparatus (polar tubule/tube, polar filament) > migrates to the gills > asexual reproduction > forms numerous meronts > gives rise to sporonts > sporogony > mature spores > liberated from tissues of live / dead host
- Myxosporea (Henneguya ictaluri):
- Complex life cycle: often alternates between vertebrate and invertebrate hosts with sporogony in each (i.e. alternating bisporogony)
- Fish host infected with actinosporean by direct penetration or ingesting intermediate host (worm) > actinosporean grows into a plasmodium > cells within plasmodium begin spore formation, divide > mature spore > clinically sick fish > spores released from live or dead fish > worm ingests mature spore
TYPICAL CLINICAL FINDINGS:
- Asymptomatic to death
- Proliferative Gill Disease (Hamburger gill disease); primarily a catfish disease, often times it occurs in new ponds, fish have respiratory impairment
- Most of the species that form visible cysts on the integument and gills cause little to no clinical signs; release of enzymes by the parasites once the host dies may cause massive muscle liquefaction (significantly reducing carcass value)
TYPICAL GROSS FINDINGS:
- White specks on gills or other infected tissue possibly becoming large masses
- Grossly similar to other diseases that cause focal masses, including microsporea
- Proliferative Gill Disease (Hamburger gill disease): Gills are swollen mottled red and white with clubbed to broken lamellae; advanced stages, gill filaments do not lie flat; gill architecture is obscured
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Xenomas: hypertrophic host cell with numerous microsporidian spores
- Fusion and distortion of primary lamellae; mild graulomatous inflammation
- Gills have severe granulocytic inflammation, nodular epithelial hyperplasia; occasional small parasitic myxozoan cysts, which may occlude lamellar troughs
- Multicellular fusiform spores within the cysts; multiple polar capsules in spores; all non-sexual stages are multinucleate with enveloping (primary) and enveloped (secondary) cells
- Moderate to severe cartilaginous necrosis within the nodule
- Cysts may be seen in the liver, spleen, kidney, and brain with limited inflammation (cysts described as small blue clusters of grapes)
- Spores are spherical, oval or cylindrical up to 10 um in length
- The single polar filament is coiled around the single uninucleate sporoplasm
- Spores are variable in size and shape depending on species
- Spores contain two or more polar filaments that are each coiled inside separate polar capsules
- Binucleate sporoplasm
ADDITIONAL DIAGNOSTIC TESTS:
- Wet mounts from fresh tissue can be effective at identifying spores
- PCR, high sensitivity and specificity
- Histochemical stains:
- Modified trichrome stain: best observation of polar tube, polaroplast, and posterior vacuole in rabbits infected with Encephalitozoon cuniculi
- Luna stain (for microsporidia)
- Periodic acid–Schiff (PAS): PAS-positive granules at the anterior end of spore
- Giemsa: light to dark blue with darkly stained nucleus; highlights polar capsule in myxosporea
- Grocott methenamine silver (GMS) or Warthin-Starry (WS)
- Partially acid-fast with Ziehl-Neelsen or Kinyoun
Microsporea and Myxosporea grossly produce masses
- Ichthyophthirius multifilis (“ich”): ciliated protozoan, horseshoe shaped macronucleus, trophozoite feeding stage causes formation of small (1 mm diameter) white nodules protruding from skin and gills; “salt-like” dusting of the skin
- Epitheliocystis: Chlamydia-like bacteria; hypertrophied lamellar epithelium with large, intracellular, spherical, membrane bound, basophilic granular inclusions composed of the bacteria
- Dermal metacercariae: Digenean trematode infection; white, yellow, or black, flat to raised nodules 1 – 4 mm in diameter
- Bacterial granulomas: Especially mycobacteriosis and nocardiosis; cutaneous and visceral, focal and coalescing, yellow-white nodular masses
- Pseudoloma neurophilia: Xenomas in various tissues; tropism for CNS; most common in zebra fish; spore coats partially birefringent under polarized light
- Encephalitozoon cuniculi: most relevant microsporidia species; affects primarily the CNS in rabbits, rodents, insectivores, carnivores, and primates,
- Less common in lizards, skinks, and bearded dragons
- Severe disease rare unless immunocompromised
- Pleistophora hyphessobryconis: “neon tetra disease”, loss of color, white patches under skin, body is contorted
- Heterosporis (Pleistophora) anguillarum: reported in garter snake (Thamnophis sirtalis) and Japanese eel (Anguilla japonica); does not form xenomas,
- Often infects skeletal myocytes
- Lacks staining with Grocott’s methenamine silver and no PAS reactions
- Nosema apis: significant problem in honey bees
- Encephalitozoon hellum: affects pigeons, budgerigars and parrots
- Infect invertebrates and poikilothermic vertebrates (primarily fish)
- Species specific for the host organism and tissue
- Henneguya exilis “lamellar disease”: displacement of the lamellae, epithelial atrophy and hyperplasia with inflammation that progresses to granulomatous
- Myxobolus koi (goldfish and koi): causes proliferative branchitis
- Myxobolus cerebralis [VSPO M-P05]: “whirling disease” in salmonids, affects cartilage and causes skeletal deformities
- Tetracapsuloides bryosalmonae (salmon): proliferative kidney disease
- Hofferellus carassii “kidney bloater disease”: Necrogranulomatous inflammation of abdominal cavity, kidney, heart, brai; parasites very small (2 – 4 um), slightly eccentric nuclei
- Kudoa spp. (Myxozoa: Multivalvulida) affect marine and estuarine teleosts (rays)
- Ceratomyxa shasta: infects intestinal epithelium of Chinook salmon
- Cali A, Neafie RC, Takvorian PM. Chapter 14: Microsporidiosis. In: Meyers W, Firpo A, Wear DJ, eds. Topics on the Pathology of Protozoan and Invasive Arthropod Diseases. Washington, DC: Armed Forces Institute of Pathology; 2011: 200-223.
- Cavin JM, Donahoe SL, Frasca S, et al. Myxobolus albi infection in cartilage of captive lumpfish (Cyclopterus lumpus). J Vet Diagn Invest. 2012;24(3):516-524.
- Eydner M, Donhauser J, Beineke A, et al. Microsporidiosis in four tortoises (Testudo hermanni boettgeri). Vet Pathol. 2017;54(4):704-709.
- Gardiner CH, Fayer R, Dubey JP. Microspora and Myxozoa: An Atlas of Protozoan Parasites in Animal Tissues. 2nd ed. Washington, DC: Armed Forces Institute of Pathology, American Registry of Pathology; 1998:12-15.
- Hadfield CA, Poynton SL, Clayton LA, et al. Kudoa (Myxozoa: Multivalvulida) in skeletal muscle of captive bullnose eagle rays, Myliobatis freminvillei (Rajiformes: Myliobatidae). J Zoo Wildl Med. 2014;45(4):896-905.
- Kent ML, Benda S, St-Hilaire S, et al. Sensitivity and specificity of histology for diagnoses of four common pathogens and detection of nontarget pathogens in adult Chinook salmon (Oncorhynchus tshawytscha) in fresh water. J Vet Diagn Invest. 2013;25(3):341-351.
- Noga EJ. Problems 58 through 76: Diagnoses made by necropsy of the viscera and examination of wet mounts or histopathology of internal organs. In: Noga EJ, ed. Fish Disease: Diagnosis and Treatment. Ames, IA: Wiley-Blackwell; 2010: 229-236, 247-253.
- Nolan MW, Roberts HE, Zimmerman KL, et al. Pathology in Practice: Myxosporidia. J Amer Vet Med Assoc. 2010; 236(6): 631-633.
- Richter B, Csokai J, Graner I, et al. Encephalitozoonosis in two inland bearded dragons (Pogona vitticeps). J Comp Pathol. 2013;143(2-3):278-282.
- Richter B, Graner I, Csokai J. Heterosporis anguillarum infection in garter snake (Thamnophis sirtalis). J Comp Pathol. 2014;150(2-3):332-335.