Histopathology of fungus and it's types in cats in photos under microscope

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Cryptococcus histopathology microscopy cat feline

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Aspergillus Sporothrix Dermatophyte Histoplasma fungal infection histopathology microscopy PAS GMS stain

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Fungal Infections in Cats — Histopathology & Types Under the Microscope

Cats are susceptible to a range of fungal infections. Below is a systematic breakdown of the major types, their microscopic/histopathological features, staining characteristics, and representative photomicrographs.

Special Stains Used in Fungal Histopathology

Stain	What it Highlights
H&E	General tissue architecture; fungi may be visible but poorly defined
PAS (Periodic Acid–Schiff)	Fungal cell walls stain magenta/red
GMS (Gomori Methenamine Silver)	Fungal walls stain black — most sensitive stain
Mucicarmine	Cryptococcus capsule stains red (unique)
India Ink	Cryptococcus capsule seen as halo (wet prep)

1. Cryptococcosis (Cryptococcus neoformans / C. gattii)

Most common systemic fungal infection in cats. Often affects nasal passages, CNS, skin, and eyes.

Histopathology:

Thick polysaccharide capsule creates a clear "halo" around each yeast cell — the defining feature
Yeast cells: 5–20 µm, round, narrow-based budding
Tissue reaction varies from minimal (gelatinous/mucoid response) with few inflammatory cells, to granulomatous with epithelioid macrophages and multinucleated giant cells
"Soap-bubble" lesions in CNS tissue due to capsule accumulation
GMS: black-staining yeast; Mucicarmine: capsule stains bright red; PAS: yeast walls magenta

Cryptococcus neoformans — PAS (c) and GMS (d) staining showing thick-capsulated yeast with narrow-based budding

Cryptococcus neoformans: (c) PAS stain showing encapsulated round yeasts with clear halos; (d) GMS stain highlighting narrow-based budding yeast cells — patterns identical in feline tissue.

2. Sporotrichosis (Sporothrix schenckii)

Cats (especially in Brazil) are a major reservoir and source of human transmission via scratches.

Histopathology:

Mixed granulomatous + suppurative reaction
Neutrophilic microabscesses surrounded by granulomatous inflammation (epithelioid macrophages, giant cells, lymphocytes)
Organisms: 3–5 µm cigar-shaped or oval yeast cells — sparse in most hosts, but abundant in cats (unlike dogs/humans)
Pathognomonic: asteroid body — eosinophilic Splendore-Hoeppli material surrounding the yeast
PAS/GMS needed to identify sparse organisms; in cats, direct smear often sufficient due to heavy fungal burden

— Fitzpatrick's Dermatology, p. 2995

3. Dermatophytosis (Microsporum canis, M. gypseum, Trichophyton spp.)

M. canis is the most common cause of ringworm in cats.

Histopathology:

Hyphae and arthroconidia within/surrounding hair shafts (ectothrix or endothrix pattern)
Perifollicular lymphoplasmacytic infiltrate; in severe cases (kerion): suppurative folliculitis and furunculosis
Organisms: 2–4 µm chains of spores; septate hyphae in stratum corneum
PAS/GMS highlights hyphae and spores surrounding hair shafts
Feline favus (M. canis): arthroconidia form "sheaths" around hairs (ectothrix)

4. Aspergillosis (Aspergillus fumigatus, A. niger)

Primarily sino-nasal or disseminated in immunocompromised cats.

Histopathology:

Septate hyphae, 3–6 µm wide, with dichotomous (Y-shaped) branching at 45° — hallmark feature
Angioinvasion: hyphae penetrate vessel walls → thrombosis, hemorrhagic infarction
Acute angle branching distinguishes from Mucor (90° branching)
Conidiophores with fruiting heads (vesicles + phialides) may be seen in cavitary lesions
GMS reveals black septate hyphae; H&E shows parallel walls

5. Histoplasmosis (Histoplasma capsulatum)

Rare in cats; when present, usually disseminated (lung, liver, spleen, intestine, bone marrow).

Histopathology:

Intracellular small yeasts (2–4 µm) packed within macrophages/histiocytes
Pyogranulomatous inflammation with epithelioid macrophages, multinucleated giant cells
In disseminated disease: macrophage-laden sinusoids in liver and spleen
Organisms may be mistaken for Leishmania; distinguished by GMS/PAS positivity and absence of kinetoplast
GMS: tiny black yeasts in clusters within macrophages

Disseminated fungal granulomas in spleen — H&E showing spherules (Coccidioides) surrounded by epithelioid granulomas with giant cells

Splenic granulomas with central fungal spherules (epithelioid macrophages + giant cells) — pattern seen in disseminated systemic mycoses.

6. Coccidioidomycosis (Coccidioides immitis / C. posadasii)

Rare in cats; endemic in arid southwestern USA, Mexico, Central/South America.

Histopathology:

Large spherules (20–200 µm), thick-walled, filled with endospores (2–5 µm) — pathognomonic
When spherules rupture, released endospores trigger intense suppurative reaction
Granulomatous inflammation: epithelioid macrophages, Langhans giant cells, lymphocytes
PAS: endospores stain pink; GMS: spherule walls stain black; H&E: spherules visible as large round structures

Coccidioides spherules — H&E showing large thick-walled spherules containing endospores within granulomatous inflammation

Coccidioides immitis: large endosporulating spherules (20–100 µm) surrounded by multinucleated giant cells and granulomatous inflammation — identical morphology in feline tissue.

7. Blastomycosis (Blastomyces dermatitidis)

Rare in cats; sporadic in endemic North American river valleys.

Histopathology:

Large round yeasts (8–15 µm) with thick, refractile double-contour wall
Broad-based budding — single bud, wide attachment point (key distinguishing feature from Cryptococcus's narrow base)
Pyogranulomatous reaction: neutrophilic abscesses surrounded by granulomatous tissue
GMS: thick-walled yeasts with broad base clearly visible

8. Mycetoma / Phaeohyphomycosis (Pigmented Molds)

Caused by dematiaceous (melanin-producing) fungi (Alternaria, Curvularia, Exophiala, etc.).

Histopathology:

Brown-pigmented hyphae in tissue — visible on H&E due to melanin content
Suppurative granulomatous reaction with chronic fibrosis
Eumycetoma: fungal grains (compact colonies) within purulent abscesses
Fontana-Masson stain highlights melanin pigment

Summary Table

Fungus	Microscopic Form in Tissue	Size	Key Histologic Pattern	Best Stain
Cryptococcus	Encapsulated yeast, narrow-based budding	5–20 µm	Mucoid/gelatinous, minimal inflammation	Mucicarmine, GMS
Sporothrix	Cigar-shaped yeast; asteroid body	3–5 µm	Mixed granulomatous + suppurative	PAS, GMS
Microsporum/Trichophyton	Ectothrix hyphae + arthroconidia on hair	2–4 µm	Folliculitis, furunculosis	PAS, GMS
Aspergillus	Septate hyphae, 45° branching	3–6 µm wide	Angioinvasive, necrosis	GMS
Histoplasma	Small intracellular yeast in macrophages	2–4 µm	Pyogranulomatous, macrophage-packed	GMS, PAS
Coccidioides	Spherules with endospores	20–200 µm	Granulomatous, spherule rupture	GMS, PAS
Blastomyces	Broad-based budding yeast	8–15 µm	Pyogranulomatous	GMS
Dematiaceous molds	Brown septate hyphae	Variable	Suppurative granuloma + fibrosis	H&E (melanin)

Key clinical notes for cats specifically:

Cats with sporotrichosis have an extremely high organism burden — making cytology from exudates highly diagnostic and making them a zoonotic risk to veterinarians and owners
Cryptococcosis has a predilection for the nasal planum and CNS in cats; the mucinous capsule suppresses host immunity
M. canis dermatophytosis is the most common feline skin fungal infection overall
Immunosuppression (FIV, FeLV, corticosteroids) predisposes to disseminated infections

— Fitzpatrick's Dermatology, p. 2995; Medical Microbiology 9e

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