Feline Cryptococcosis and Other Fungal Respiratory Diseases – NAVLE Study Guide

Overview and Clinical Importance

Fungal respiratory diseases represent an important category of infectious disease in cats, with cryptococcosis being the most common systemic mycosis of cats worldwide. Understanding the clinical presentations, diagnostic approaches, and treatment protocols for these infections is essential for the NAVLE examination and clinical practice.

This study guide covers the major fungal pathogens affecting the feline respiratory system, including Cryptococcus, Histoplasma, Blastomyces, Coccidioides, Aspergillus, and Sporothrix species. Each organism has unique geographic distribution, clinical presentations, and treatment considerations that are frequently tested on board examinations.

Cryptococcosis

Etiology and Epidemiology

Cryptococcus neoformans and Cryptococcus gattii are the primary species causing feline cryptococcosis. These are encapsulated basidiomycetous yeasts that infect cats following inhalation of basidiospores from the environment. The polysaccharide capsule is the major virulence factor, inhibiting phagocytosis and providing protection against host immune defenses.

Environmental sources: C. neoformans is commonly found in pigeon droppings and avian guano. C. gattii is associated with certain tree species, particularly Eucalyptus. Both species are found in decaying vegetation and soil worldwide.

Clinical Forms of Feline Cryptococcosis

Diagnosis of Cryptococcosis

Cytology

Cytologic examination is the most rapid diagnostic method. Samples can be obtained from nasal exudate, skin exudate, CSF, or fine needle aspirates. Cryptococcus organisms appear as round to oval yeast cells (4-20 micrometers in diameter) surrounded by a thick, non-staining mucopolysaccharide capsule. The characteristic narrow-based budding distinguishes Cryptococcus from Blastomyces (which has broad-based budding).

Staining Methods for Cryptococcus

Latex Agglutination Cryptococcal Antigen Test (LCAT)

The Latex Agglutination Cryptococcal Antigen Test (LCAT) detects cryptococcal capsular polysaccharide antigen in serum, urine, or CSF. This is the most reliable non-invasive test for cryptococcosis diagnosis.

• Sensitivity: 91% in dogs, 98% in cats; Specificity: approximately 100%
• Titer of 1:2 or greater is considered positive
• False negatives may occur with localized disease (nasal, ocular) or capsule-deficient strains
• Titers can be used to monitor treatment response; should decline 2-4 fold per month during successful therapy
• CSF cryptococcosis virtually always produces a positive serum titer

Treatment of Feline Cryptococcosis

Treatment Duration: Continue treatment until the LCAT titer becomes negative, or for at least 2-4 months beyond resolution of clinical signs. Typical treatment duration is 6-10 months for nasal/cutaneous forms; CNS disease may require longer treatment.

Prognosis

Prognosis for nasal and cutaneous cryptococcosis is generally favorable with appropriate treatment. CNS and disseminated disease carry a guarded prognosis. A 10-fold drop in LCAT titer after 2 months of treatment indicates a favorable response.

Histoplasmosis

Etiology and Epidemiology

Histoplasma capsulatum is a dimorphic fungus that causes the second most common systemic fungal disease in cats. Despite its name, the organism does NOT have a true capsule.

• Geographic distribution: Mississippi, Ohio, and Missouri River valleys (endemic regions in USA); also found worldwide
• Environmental source: Soil contaminated with bat and bird feces; high nitrogen content favors fungal growth
• Transmission: Inhalation of microconidia from environment; can affect indoor cats through potted plants, unfinished basements
• Risk factors: FeLV-positive cats may have increased susceptibility

Clinical Presentation

Clinical signs of feline histoplasmosis are often nonspecific and reflect systemic involvement:

Diagnosis

Cytology: Histoplasma organisms appear as small (1-4 micrometers), round to oval intracellular yeast cells with a basophilic center surrounded by a light (clear) halo. Organisms are found within macrophages and occasionally neutrophils. The halo represents the cell wall, NOT a true capsule.

Best samples: Lung FNA, bronchoalveolar lavage, bone marrow aspirate, lymph node aspirate

Antigen testing: Galactomannan antigen ELISA (urine preferred over serum); high cross-reactivity with Blastomyces

Treatment

Itraconazole (10 mg/kg PO q24h) is the treatment of choice for cats with histoplasmosis. Treatment duration averages 6 months minimum. Amphotericin B may be used in severe cases or if GI function is compromised (prevents absorption of oral azoles).

Blastomycosis

Blastomyces dermatitidis causes blastomycosis, which affects dogs more commonly than cats. In cats, the disease is rare but presents similarly to other systemic mycoses.

• Geographic distribution: Ohio River Valley (east of Mississippi River); Great Lakes region; endemic to North America
• Environmental source: Moist, acidic soil near water; sandy soils with decaying organic matter
• Clinical signs: Respiratory signs (cough, dyspnea), skin lesions, ocular disease, CNS involvement, bone involvement

Cytology: Blastomyces appears as large (8-25 micrometers), thick-walled yeast with characteristic BROAD-BASED budding. NO capsule is present (unlike Cryptococcus).

Treatment: Itraconazole (10 mg/kg PO q24h) is treatment of choice. Fluconazole for CNS involvement. Treatment typically continues for several months beyond resolution of clinical signs.

Coccidioidomycosis (Valley Fever)

Coccidioides immitis and C. posadasii cause coccidioidomycosis (Valley Fever). This disease is less common in cats than dogs but occurs in endemic regions.

• Geographic distribution: Lower Sonoran Life Zone - Arizona, California, southwestern Texas, New Mexico, Nevada, Utah; recently identified in Washington state
• Environmental factors: Infection follows soil disturbance (dust storms, earthquakes, construction); rainy season followed by drought increases risk
• Clinical presentation: Dermatologic signs most common in cats (draining tracts, abscesses); respiratory signs less common than in dogs; approximately 50% have disseminated disease at diagnosis
• Lung lesions: Found in greater than 80% of cats that died of Valley Fever, even without respiratory signs

Diagnosis

Serology: Agar gel immunodiffusion (AGID) test for anti-coccidioidal antibodies is the basis for diagnosis in cats; sensitivity approximately 83%. Antibody titers help monitor treatment response.

Cytology/Histopathology: Coccidioides appears as large spherules (20-200 micrometers) containing endospores. Spherules are diagnostic when identified but are not commonly found on cytology.

Treatment

Fluconazole is the treatment of choice in cats (cats tolerate high doses well). Treatment duration is typically greater than 1 year; lifelong therapy may be required. Itraconazole is an alternative. Monitor antibody titers every 3-4 months until negative or stable.

Prognosis: Disseminated disease carries a guarded to grave prognosis. Overall recovery reported at 60-90%. Relapses are common.

Aspergillosis

Aspergillus species cause opportunistic fungal infections in cats. The disease occurs in two main forms in cats: sino-nasal aspergillosis (SNA) and sino-orbital aspergillosis (SOA).

• Distribution: Worldwide; found in soil and decaying vegetation
• Predisposition: Brachycephalic cats (Persian, Himalayan) appear predisposed
• Sino-Nasal form: Chronic nasal discharge, sneezing; confined to nasal cavity and sinuses
• Sino-Orbital form: Extension into orbit causing exophthalmos, facial deformity; more invasive; associated with immunocompromise

Diagnosis

Definitive diagnosis: Cytologic or histologic detection of branched, septate hyphae (2-5 micrometers diameter) from rhinoscopy samples, nasal lavage, or biopsy of orbital masses.

CT imaging: Identifies extent of nasal/sinus/orbital involvement; cannot distinguish from neoplasia without tissue sampling

Treatment

Sino-Nasal Aspergillosis: Combination of extended oral antifungal therapy (itraconazole, voriconazole) plus topical antifungal infusion (clotrimazole, enilconazole) under general anesthesia

Sino-Orbital Aspergillosis: Radical surgery may be required; voriconazole or posaconazole for systemic involvement. Prognosis is guarded.

Sporotrichosis

Sporothrix schenckii complex (particularly S. brasiliensis) causes sporotrichosis. This is primarily a cutaneous/subcutaneous infection but can involve the respiratory system. IMPORTANT: This is a significant ZOONOTIC disease - cats can transmit infection to humans through scratches and bites.

Epidemiology and Zoonotic Risk

• Geographic distribution: Tropical and subtropical areas; major epidemic in Brazil with S. brasiliensis; sporadic cases in North America (recently identified in Kansas, Oklahoma)
• Environmental source: Soil, plants, sphagnum moss; transmission via traumatic inoculation (scratches, bites, wounds)
• ZOONOTIC RISK: HIGH - cats have high fungal burden in lesions and can transmit to humans through scratches/bites. Cat caretakers are 4x more likely to become infected.

Clinical Presentation

Cutaneous form (most common): Multiple ulcerated nodules, especially on head (bridge of nose, pinnae), face, and limbs; draining tracts; non-healing wounds

Respiratory signs: Occur in approximately one-third of cases; sneezing, nasal discharge; fungus can be isolated from nasal mucosa and bronchoalveolar lavage

Lymphocutaneous form: Nodules along lymphatic chains draining the primary lesion

Disseminated form: Systemic involvement; may be associated with FIV infection

Diagnosis

Cytology: Sporothrix organisms appear as cigar-shaped or oval yeast cells (2-10 micrometers). In cats, organisms are often numerous and readily visualized (unlike dogs/humans where organisms are sparse). Look for pyogranulomatous inflammation with intracellular yeasts.

Culture: CONFIRMATORY but requires biosafety precautions (laboratory hazard). Grows slowly (several weeks).

PCR: Rapid results (within days); useful alternative to culture for species identification

Treatment

Itraconazole: First-line treatment (10 mg/kg PO q24h). Treatment should continue for at least 1-2 months beyond resolution of all clinical signs. Cats often require greater than 12 months of treatment; treatment failures occur.

Potassium iodide: May be added to itraconazole therapy

Monitoring: Monitor liver enzymes (itraconazole hepatotoxicity); appetite assessment. Treatment abandonment is common (30-40%) and leads to disease recurrence.

Comparative Summary of Feline Fungal Respiratory Diseases

Memory Aids for Board Exams

"CRYPTO = CAPSULE" Mnemonic: Cryptococcus is the only fungal pathogen with a prominent polysaccharide CAPSULE (visible as clear halo). Remember: Crypto-Capsule, Clear halo, Cribriform plate (CNS spread).

Budding Pattern Mnemonic: "Crypto Needs a Narrow Neck; Blasto has a Broad Base" - Cryptococcus = narrow-based budding; Blastomyces = broad-based budding

Size Mnemonic: "Histo is HIDDEN (small, intracellular 1-4 um); Blasto is BIG (8-25 um); Cocci makes COLOSSAL spherules (20-200 um)"

Geographic Distribution - "River Valleys": Histoplasma = Mississippi/Ohio/Missouri Rivers. Blastomyces = Ohio River (east of Mississippi). Coccidioides = Southwest desert (think Arizona).

Drug Selection: "FLU for FLUID compartments" - FLUconazole penetrates CNS and eye. Itraconazole for everything else (but poor CNS penetration). Amphotericin B for severe/refractory cases.