JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

October 2026

P-P17

SLIDE A

HISTOPATHOLOGIC DESCRIPTION

Gill: Multifocally affecting 15% of the gill architecture, primary lamellae and secondary lamellae are expanded up to 4 times normal by variably-sized, up to 300µm diameter, irregularly round to oval microsporidian xenomas. These xenomas have a thin eosinophilic wall and contain numerous eosinophilic, 2-4 µm, 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 lamellae adjacent to microsporidian xenomas.

SLIDE B

HISTOPATHOLGIC DESCRIPTION

Affecting 80% of the gill parenchyma, primary lamellae and to a lesser extent secondary lamellae are expanded up to 10 times normal by numerous foamy macrophages, fewer lymphocytes, and scattered hemorrhage, fibrin, edema, and necrotic cellular debris. Multifocally, there are intralamellar myxozoan cysts up to 200 µm in diameter that contain numerous 2 x 10 µm, fusiform, eosinophilic spores. There is necrosis and loss of lamellar cartilage, and adjacent secondary lamellae have moderate epithelial and mucous cell hyperplasia with marked lamellar fusion. Multifocally separating secondary lamellae are cross sections of 100 µm diameter adult trematodes characterized by a 5 µm thick tegument, filled with a parenchyma, reproductive organs, vitellaria, and poorly discernible paired caeca (trematodes not present in all slides).

GENERAL DISCUSSION

• Microsporidia: Obligate intracellular parasite (fungi) of invertebrates and many poikilothermic and endothermic vertebrates (primarily fish); most are specific to a fish species or closely related group, but few have a broad range (including Loma salmonae)
  • Loma salmonae classically causes gill xenomas on salmonids +/- mortality
• Myxozoa: Obligate parasite of invertebrates (primarily annelid worms) and poikilothermic vertebrates (primarily fish and amphibians); most are species and organ specific, though a few have a broader range; many are incidental or minimally pathogenic
  • Henneguya ictaluri causes necrotizing branchitis and dermatitis in farmed, channel catfish +/- mortality; economically significant cause of parasitic gill disease (PGD) with high mortality rates (Stilwell et al., Vet Pathol 2024)

PATHOGENESIS/LIFE CYCLE

• Microsporidia:
  • Direct life cycle: Spore ingested > sporoplasm discharged via polar filament extrusion > migrates to & invades endothelial cells of gills > undergoes asexual reproduction (merogony), forming many meronts > give rise to sporonts, which undergo sporogony to produce spores within membrane-bound packets (sporophorous vesicles) > cell ruptures, spores released into water
  • In general, microsporidians are cell type specific, but can infect many organs if that cell type is widespread; how they spread in the body is unknown; few species (including Loma salmonae) can transmit vertically (inside the egg)
  • Microsporidians are often categorized by their ability to induce xenoma formation, which is a markedly hypertrophied host-cell (cyst-like) filled with microsporidian stages and compressing surrounding tissue forming a compression capsule; Loma salmonae is a xenoma forming microsporidian
• Myxozoa:
  • Complex life cycle, alternating between vertebrate (intermediate) and invertebrate (definitive) hosts (alternating bisporogony); two different forms, depending on species/genus: histozoic form: resides in intercellular spaces, blood vessels, or intracellular OR coelozoic form: resides in the gall bladder, swim bladder, or urinary bladder
  • Henneguya ictaluri: Histozoic type; oligochaete worm (Deuterocohnia digitata) contains infective actinospore (Aurantiactinomyxon ictaluri) > actinospore released into water > infects catfish through the skin or orally using extrudable polar filament > actinospore matures to myxospore (Henneguya ictaluri) and replicate asexually in the gill > cell ruptures, spores released into water

TYPICAL CLINICAL FINDINGS

• Loma salmonae: Wild-caught or pond-raised salmonids with variously sized white cyst-like nodules (xenomas) on the gills that enlarge slowly
• Henneguya ictaluri: Pond-raised channel catfish with, respiratory impairment and dyspnea; epidemics occur most commonly at 16-20° C, especially in the spring (April-May), and are associated with new ponds

TYPICAL GROSS FINDINGS

• Loma salmonae: White-yellowish variably sized nodules (xenomas) on gills or other infected tissue possibly becoming large masses
• Henneguya ictaluri: Gills start as pale and swollen and progress to grossly thickened/blunted (“clubbed”) or broken gill lamellae that bleed easily (“hamburger gills”)

TYPICAL LIGHT MICROSCOPIC FINDINGS

• Loma salmonae:
  • Gill filaments, primary/secondary lamella, bone, and cartilage expanded by xenomas (200-300µm thin walled cysts filled with numerous 2-4µm round to elliptical, birefringent eosinophilic spores with prominent posterior vacuole) +/- granulomatous inflammation; focal atrophy, fusion, loss of secondary lamellae
  • Gill epithelial & mucus cell hyperplasia
• Henneguya ictaluri:
  • Gill filaments, primary/secondary gill lamella, gill bone, gill cartilage: expanded by myxozoan cysts (up to 200µm thin-walled cysts filled with numerous 2-10µm fusiform eosinophilic spores) with severe epithelial hyperplasia, lamellar fusion, and granulomatous bronchitis with nodules surrounding cysts containing cartilage necrosis, tissue liquefactive necrosis, and parasites
  • Cysts may be present in the liver, spleen, kidney, and brain with little tissue reaction; with healing, see chondroplasia without cysts

ULTRASTRUCTURAL FINDINGS

• Microsporidia (general): Spores: spherical, electron-dense exospore, electron-lucent endospore, faint nucleus, and single coiled polar tubule; NO polar capsule (unlike Myxozoa) or mitochondria
• Myxozoa (general): Spores: two or more coiled polar tubules contained inside separate polar capsules; multinucleate sporoplasm; shell with two to six valves

ADDITIONAL DIAGNOSTIC TESTS

• PCR: High sensitivity and specificity
• Wet mount of affected tissue effective for identifying spores; with Henneguya ictaluri, also see focal areas of cleared cartilage (strongly suggestive, even without spores)
• Microsporidia (general):
  • Modified trichrome stain: Best observation of polar tube, polaroplast, and posterior vacuole
  • Luna stain
  • Periodic acid–Schiff (PAS): PAS-positive granules at the anterior end of spore
  • Gram positive
  • Chitin-binding fluorochromes (e.g. Fungi-Fluor, Calcofluor) highly sensitive (spores contain chitin)
• Myxozoa (general):
  • Luna stain
  • Periodic acid–Schiff (PAS): PAS-positive granules at the anterior end of spore
  • Ziehl-Neelsen or Kinyoun: Partially acid fast
  • Silver stains: GMS or Warthin-Starry
  • Giemsa: Highlights polar capsule

DIFFERENTIAL DIAGNOSIS (FOR GROSSLY EVIDENT GILL MASSES)

• Henneguya exilis (“lamellar disease”): Related species that causes severe gill pathology in fish
• Desmozoon lepeophtherii: Microsporidian associated with gill disease in farmed salmon; difficult to detect on HE section, need IHC or calcofluor white (Herrero, et. al J Vet Diagn Invest 2020)
• Ichthyophthirius multifilis (“ich;” I-P16): 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
• Lymphocystis (I-V15): Lymphocystis disease virus (an iridovirus) causes white-pink masses on skin, gills and other surfaces due to hypertrophic fibroblasts (lymphocysts) filled with basophilic intracytoplasmic inclusions
• Epitheliocystis: Chlamydia-like bacteria; causes lamellar epithelial hypertrophy with large spherical, membrane bound, basophilic granular bacterial inclusions
• 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

COMPARATIVE PATHOLOGY

• Microsporidia (general):
  • Zebrafish: Pseudoloma neurophilia: Xenomas in various tissues; tropism for CNS; spore coats are partially birefringent under polarized light
  • Rabbits, rodents, primates: Encephalitozoon cuniculi: most relevant microsporidian species; affects primarily the CNS; severe disease; rare unless immunocompromised
  • Pigeons, budgerigars, finches, canaries, hummingbirds, parrots: Encephalitozoon hellum; chronic proliferative enteritis
  • Canids: Encephalitozoon spp with microgranulomas in brain, kidney and other organs; reported in captive African wild dogs and a litter of Arctic foxes
  • Amphibians: Pleistophora myotrophica causes necrotizing myositis; Alloglugea bufonis causes intestinal xenomas
  • Reptiles: Encephalitozoon pogonae associated with paratesticular and ovarian granulomas, intrapericardial arteritis and hemopericardium in bearded dragons; Microsporidiosis recently reported in Hermann’s tortoises characterized by presence of basophilic spores, and granulomatous and necrotizing hepatitis and pneumonia; single case report in crocodilia
  • Stingrays: Dasyatispora levantinae causes spindle-shaped, raised white lesions filled with thick, caseous material parallel to muscle fibers
  • Invertebrates: Common cause of xenomas; in honey bees, Nosema apis and Nosema ceranae are a problem (tropism for ventricular epithelium) (Biganski et. Al, J Vet Diagn Invest 2023)
  • Gilthead Sea Bream: Enterospora nucleophila cause of emaciative disease, intranuclear development, infects the GI tract and hematopoietic organs
• Myxozoa (general):
  • Amphibians: Cystodiscus: develop spore-forming plasmodia in the gallbladder of anurans, caudates, and caecilians; usually incidental, except C. australis and C. axonis also develop plasmodia surrounded by gliosis, hemorrhage and necrosis within the CNS/PNS of tadpoles and post-metamorphic frogs, as well as biliary hyperplasia, portal fibrosis, and lymphoplasmacytic portal hepatitis; plasmodia are 5-25 um with numerous small basophilic nuclei; Chloromyxum, Sphaerospora, Hofereullus: usually incidentally infect renal tubules and glomeruli of anurans and caudates; C. careni may be associated with mononuclear interstitial nephritis; H. anurae (frog kidney enlargement disease) causes enlarged kidneys with cysts
  • Goldfish and Koi: Myxobolus koi: causes proliferative branchitis
  • Salmonids: Myxobolus cerebralis (M-P05): “whirling disease”; affects cartilage and causes skeletal deformities
  • Salmon: Tetracapsuloides bryosalmonae; proliferative kidney disease
  • Chinook Salmon: Ceratomyxa shasta: infects intestinal epithelium

REFERENCES

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