JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

January 2026

N-F02B

Signalment (JPC #1505161): 4-year-old female dog

HISTORY: The dog was slightly dehydrated, had a left head tilt, and forelimb paresis.

HISTOPATHOLOGIC DESCRIPTION: Slide A: Brain stem: Effacing approximately 30% of the brainstem, affecting both gray and white matter, are multifocal to coalescing granulomas up to 1.5 mm in diameter composed of a central area of liquefactive necrosis surrounded by epithelioid macrophages, lymphocytes, plasma cells, few neutrophils, and occasional multinucleated foreign-body type giant cells. Within granulomas there are numerous extracellular and intrahistiocytic, 5-15µm diameter, round, pale basophilic, yeasts that have a 1 µm thin wall surrounded by a 2-8 um thick, amphophilic, mucinous capsule and occasionally exhibit narrow-based budding. There are a few small areas of liquefactive necrosis centered on previously described yeasts. The neuroparenchyma adjacent to the granulomas is multifocally infiltrated by low numbers of lymphocytes and there are increased numbers of glial cells (gliosis). There is multifocal vacuolation and few swollen dilated myelin sheaths with eosinophilic and swollen axons (spheroids). Multifocally, there is expansion of the Virchow-Robin space by moderate numbers of previously described inflammatory cells (perivascular cuffing), hemorrhage, fibrin, and/or edema. Multifocally, there is expansion of the leptomeninges by moderate numbers of lymphocytes, plasma cells, and macrophages.

Slide B: Mucicarmine stain: Yeast capsules are diffusely carminophilic, which multifocally highlights yeast capsule that exhibit narrow-based budding.

MORPHOLOGIC DIAGNOSIS: Brain stem: Granulomas, multifocal to coalescing, marked, with mild lymphohistiocytic meningitis, perivascular cuffing, and numerous extracellular and intrahistiocytic carminophilic yeast, breed not specified, canine.

ETIOLOGIC DIAGNOSIS: Cryptococcal meningoencephalitis

CAUSE: Cryptococcus neoformans var. neoformans

CONDITION: Cryptococcosis

SYNONYMS: European blastomycosis (torulosis)

GENERAL DISCUSSION:

• Dimorphic, saprophytic, yeast-like fungus with worldwide distribution, predilection for respiratory and central nervous systems
• Infection is neither contagious nor zoonotic
• Is the most common systemic mycotic disease in cats, also affects many other species to include dogs, horses, cattle, and humans
• Most cases caused by one of two species:
• C. neoformans: Worldwide distribution; usually infects immunocompromised (T cell deficient) hosts; pigeons regarded as the most important vector; the pigeon’s high body temperature may protect it from infection; found in pigeon or other avian guano and soil; affects primarily immunocompetent cats, and less likely dogs, horses, cattle, goats, NHPs, ferrets, guinea pigs, various exotic species, and immunocompromised humans; two varieties:
  • C. neoformans var. grubii: The most common isolate worldwide from humans and animals with cryptococcosis
  • C. neoformans var. neoformans
• C. gattii (formerly C. neoformans var. gattii): Historically restricted to tropical/subtropical climates; cause of an ongoing outbreak in the North American Pacific Northwest involving humans, cats, dogs, and other animals; occurs most often in immunocompetent hosts; several species of eucalyptus trees provide an environmental niche; koalas are sentinel hosts; more frequent in cats than dogs; more likely to be a primary pathogen without underlying immune deficiencies
• The only pathogenic fungus that secretes a polysaccharide capsule

PATHOGENESIS:

• Organism is unencapsulated in the environment (present in soil and bird [especially pigeon] guano) > aerosolization of basidiospores or desiccated yeast cells > infection (usually via inhalation):
  • Nasal cavity is primary site of infection in cats, dogs, koalas, and psittacines; infection may spread locally from the nasal cavity to involve the skin, oral mucosa, eyes or brain
  • Lungs are primary site of infection in humans but pulmonary involvement usually mild/asymptomatic
  • In the host body, organism regains capsule > may disseminate hematogenously or via lymphatics or extend locally from the nasal cavity to the CNS. Also spread via yeast infected macrophages
  • Multifocal cutaneous involvement results from hematogenous dissemination in most cases; rarely may develop localized cutaneous disease due to penetrating skin injury
• Virulence factors:
  • Mucopolysaccharide (glucuronoxylomannan) capsule:
    • Prevents phagocytosis by macrophages and inhibits leukocyte migration and recruitment of inflammatory cells
    • Phenotypic switching in size and structure of capsular polysaccharides may enhance immune response evasion
  • Melanin: acts as an antioxidant and eliminates reactive oxygen species
  • Enzymes:
    • Laccase (major virulence factor) is the enzyme responsible for making melanin and interferes with oxidative burst of macrophages
    • Urease expression may promote sequestration in microcapillaries, a critical step in dissemination to the brain
• Immunity is cell-mediated and dependent on delayed-type hypersensitivity reactions: IFN-y and other cytokines elicit and activate macrophage +/- neutrophils to kill the yeast by using reactive nitrogen and oxygen intermediates
• Immune suppression of cell-mediated immunity (FIV or FeLV in cats, Ehrlichia canis or long-term glucocorticoid therapy in dogs) appears to increase susceptibility

TYPICAL CLINICAL FINDINGS:

• Most infected animals do not develop clinical disease
• Majority of clinical signs are attributable to infection of the primary organ:
  • CNS: Depression, seizures, circling, head pressing, paresis, blindness (encephalitis)
  • Skin: Multifocal papules and nodules ranging from 1-25 mm in diameter, +/- ulceration
  • Upper respiratory tract: Sneezing, snuffling, mucopurulent to hemorrhagic chronic nasal discharge; +/- swelling over bridge of nose that may progress to facial deformity; +/- otitis interna, dyspnea, mandibular lymphadenomegaly
  • Eyes: Peripheral blindness with unresponsive pupils (chorioretinitis or panopthalmitis)

TYPICAL GROSS FINDINGS:

• CNS:
  • Multiple small “cysts” (“cryptococcomas”) with a viscous, gelatinous appearance (not true cysts – are filled with microbes and the faint gray appearance is caused by the mucinous capsules)
  • Large gelatinous lesions (torulomas,) caused by unrestricted growth
  • Increased meningeal opacity with edema and congestion
• Skin: Lesions (mainly of nose and head) are usually small, firm nodules that ulcerate, exude a small amount of serous exudate, then heal
• Upper respiratory tract: Granulomatous rhinitis +/- facial deformity, swelling over nasal plane
• Lungs: Mycotic pneumonia
• Eyes: Optic neuritis, chorioretinitis
• Bone: lesions seen maxilla (nasal turbinates, sinuses, and cribriform plate)
• Other: Renal granulomas, lymphadenomegaly with gelatinous appearance, otitis interna (C. neoformans var. grubii), mastitis, placentitis with abortion and infection of the fetus

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Organism:
  • Pleomorphic, round or oval, thin‑walled cells (2-10 µm diameter) surrounded by a thick mucopolysaccharide capsule (1-30 µm thick) that forms a clear or refractile halo
  • Organisms with unstained capsules create “soap-bubble” appearance
  • Reproduce by narrow-based budding
• Leptomeninges: loosely organized, lacy appearance with often myriad organisms
• Inflammation usually scant (especially in cats), but may be mild to moderate and consist of macrophages with engulfed organisms and variable numbers of neutrophils, eosinophils, and mononuclear cells; leptomeningeal inflammation can extend along the roots of cranial nerves; inflammatory response is often more intense in the dog
• Rare granulomas with occasional multinucleate giant cell macrophages
• Ocular involvement: Organisms between the choroid and retina

ULTRASTRUCTURAL FINDINGS:

• Characteristic filamentous capsule: thin filaments are anchored in the fungal cell wall and radiate peripherally, where they are thickened at the periphery by interstitial protein
• Other features: distinct nucleus, prominent nucleolus, ribosomes, dense granule, mitochondria, laminated cell wall with an electron lucent zone and reticulated capsule
• Microbody (cytoplasmic organelle): a modified peroxisome involved in xanthine and urate metabolism; may underlie propensity to live in urate-rich bird guano

ADDITIONAL DIAGNOSTIC TESTS:

• Histochemical stains:
  • Mucopolysaccharide capsule stains positively by Mayer’s mucicarmine or Alcian blue; negative staining of wide capsular zone with India ink
  • Yeasts stain with PAS or GMS
• Latex cryptococcal agglutination test (LCAT): Measures cryptococcal polysaccharide capsular antigen in serum or cerebrospinal fluid; rapid and very sensitive
• Fungal culture (may take up to 6 weeks) is required for definitive diagnosis
• Cytology🡪 extremely pleomorphic yeast ranging from spherical to fusiform, yeast stains pink to blue-purple and may demonstrate narrow based budding
• PCR: Detection of CAP59 gene
• IHC: Positive results are only suggestive, and should be supported by other tests
• CT/MRI: Findings may be suggestive, e.g. case report in a koala with atypical presentation (Martinez-Nevado, Jour Wildlife Zoo Med 2017)

DIFFERENTIAL DIAGNOSIS:

For microscopic findings: Poorly encapsulated C. neoformans is difficult to differentiate from other unencapsulated fungi

• Blastomyces dermatitidis (I-F06, P-F05, S-F02): lacks a thick capsule; exhibits broad-based budding
• Histoplasma capsulatum var. capsulatum (D-F01, P-F02, N-F01): smaller (2-5 um); usually within macrophages
• Coccidioides immitis (C-F01, P-F03): much larger (5-50 um); reproduces by endosporulation rather than budding
• Candida parapsilosis (cat): granulomatous rhinitis; yeast and hyphal forms in tissue

COMPARATIVE PATHOLOGY:

• Cats: In a recent study, all cats with Cryptococcus sp. CNS infections exhibited mild to moderate pyogranulomatous optic neuritis and/or chorioretinitis (Wronski et al, 2023)
• Horses: Cryptococcosis may cause rhinitis, pneumonia, meningitis, encephalitis, abortion with mycotic placentitis, fetal pneumonia, and disseminated infection
• Donkey: Skin lesions
• Goats: Mastitis, pulmonary infection, meningoencephalitis
• Cattle: Mastitis, pulmonary infections, meningoencephalitis (var. grubii)
• Sheep: Mastitis and rhinitis
• Water buffalo: Mastitis
• Guinea pigs (laboratory): Multifocal granulomatous lesions in the meninges with fibrosis; focal cutaneous lesions
• Cheetahs, free-ranging and captive: Ulcerative skin nodules and gelatinous foci in several tissues (C. gattii)
• Apes and gorillas: Affected with both C. neoformans and C. gattii; cloudy meninges and cystic degeneration of the periventricular white matter
• Captive callitrichids: affecting the lungs, brain, lymph nodes, larynx, salivary gland, and ears
• Koalas: Sentinel animals for C. gattii; primarily targets lower respiratory tract but can be disseminated to the CNS
• Kangaroo: Case report of granulomatous pneumonia and severe bilateral glomerular and tubulointerstitial amyloidosis inducing protein-losing nephropathy
• Dolphins/porpoises: Progressive disease starting as mild lymphoplasmacytic inflammation in the lungs, followed by nodular to diffuse granulomatous inflammation, and ending with multisystemic fungemia
• Birds: infrequently causes disease in birds due to high body temperature. Can respiratory tract infection (nasal sinuses, lungs, and air sacs) due to lower body temperature. Can disseminate to the CNS. Lesions are similar to the cat with a myxomatous gelatinous material.

REFERENCES:

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