Introduction
Fungal morphology is a foundational pillar in veterinary mycology, enabling identification and classification of fungi of clinical and zoonotic importance. Morphological structures reflect reproduction, tissue colonization capacity, and pathogenic potential across different hosts.
Morphologic study remains essential in veterinary diagnosis—especially in cultures, skin scrapings, cytology, histopathology, and direct KOH examination—where observing hyphae, yeasts, conidia, and reproductive structures is indispensable to guide diagnosis and treatment.
Main groups of fungi by morphology
From a morphological standpoint, fungi of veterinary relevance fall into three major categories: yeasts (unicellular, budding cells), filamentous fungi (mycelium with septate or coenocytic hyphae), and dimorphic fungi (filamentous form in the environment and yeast-like form in tissues). This classification facilitates understanding of biology, transmission, and pathogenicity.
General morphology of yeasts
Yeasts are unicellular fungi that reproduce mainly by budding. They have oval or spherical cells with a cell wall rich in glucans and mannans, and a membrane containing ergosterol. Some species (e.g., Candida) form pseudohyphae that facilitate tissue invasion.
In veterinary medicine, Malassezia pachydermatis (lipophilic yeast) is part of the normal skin and ear microbiota of dogs but can proliferate under humidity, allergies, or skin alterations, causing dermatitis and otitis externa. The “peanut” or “bottle-shaped” morphology and unipolar budding are frequent diagnostic features.
General morphology of filamentous fungi
Filamentous fungi are characterized by hyphae forming a mycelium. Hyphae may be septate (Ascomycota and Basidiomycota) or coenocytic (Mucorales). Mycelial arrangement, conidia and sporangia morphology, and colony texture are key diagnostic features.
In veterinary dermatology, identifying dermatophytes (e.g., Microsporum canis and Trichophyton mentagrophytes) relies on observing fusiform macroconidia, microconidia in clusters or aligned, and hyphae with arthroconidia attached to hair shafts. In the respiratory tract, Aspergillus fumigatus shows conidiophores with a terminal vesicle and chains of conidia.
Morphology of dimorphic fungi
Dimorphic fungi exhibit a filamentous form at environmental temperature (25–30°C) and a yeast-like form in tissues/host (35–37°C). Relevant examples: Histoplasma capsulatum (small intracellular yeasts in macrophages) and Sporothrix schenckii (cigar-shaped yeasts in subcutaneous lesions).
This temperature- and environment-dependent morphological shift is a key pathogenic factor and must be considered when interpreting cultures and clinical samples, especially in systemic and subcutaneous mycoses of domestic and wild animals.
Cell wall and capsule: composition and diagnostic relevance
The fungal cell wall contains glucans (β-1,3 and β-1,6), chitin, and mannans, providing rigidity and protection; ergosterol in the membrane confers susceptibility to azole and polyene antifungals. Some yeasts (e.g., Cryptococcus neoformans) have a polysaccharide capsule visible with India ink staining, important in neurological infections.
Reproductive structures and diagnostic features
Differentiating conidia (asexual), sporangia (Mucorales), macroconidia and microconidia (dermatophytes), as well as the organization of conidiophores, phialides, and metulae in Aspergillus, guides identification at genus and species level. The presence of chlamydoconidia in Candida may be observed under specific culture conditions.
Morphologic diagnostic techniques: direct, culture, and histology
- Direct exam (KOH 10–20%): dissolves keratin and clears the sample, allowing visualization of hyphae, arthroconidia, and yeasts in hair, scales, and nails.
- Culture on selective media (e.g., Sabouraud, DTM for dermatophytes): assesses colony morphology (color, texture, pigmentation) and microscopic structures with lactophenol cotton blue.
- Histopathology with PAS and Grocott-GMS: highlights fungal walls in tissues, useful for systemic mycoses and granulomatous lesions. India ink demonstrates capsules in Cryptococcus.
Fungal structures: composition, function, and examples
| Structure | Group | Function | Examples |
|---|---|---|---|
| Septate hyphae | Filamentous fungi | Colonization and growth | Aspergillus, Microsporum, Trichophyton |
| Coenocytic hyphae | Mucorales | Tissue invasion and rapid expansion | Mucor, Rhizopus, Absidia |
| Pseudohyphae | Yeasts | Tissue invasion | Candida albicans |
| Conidiophore + vesicle | Filamentous fungi | Conidia production | Aspergillus fumigatus, A. niger |
| Macroconidia | Dermatophytes | Dissemination and diagnosis | Microsporum canis, Trichophyton |
| Microconidia | Dermatophytes | Asexual reproduction | Trichophyton mentagrophytes |
| Sporangia | Mucorales | Spore production | Mucor, Rhizopus |
| Arthroconidia | Dermatophytes | Hair adhesion and transmission | Microsporum, Trichophyton |
| Polysaccharide capsule | Yeasts | Immune evasion | Cryptococcus neoformans |
| Dimorphic form | Dimorphic fungi | Adaptation to environment/host | Histoplasma, Sporothrix |
Clinical considerations and morphologic interpretation
Morphologic interpretation must be correlated with clinical context and sampling site. The presence of hyphae and arthroconidia on hair shafts suggests dermatophytosis; conidiophores in culture indicate Aspergillus; capsules visible with India ink suggest cryptococcosis. Environmental contamination should be ruled out by repeating samples and culture.
Conclusion
Fungal morphology is essential to understand the biology and pathogenicity of fungi of veterinary importance. Recognizing structures such as hyphae, conidia, sporangia, capsules, and dimorphic forms—along with techniques like KOH, culture, and histopathology—enables more precise diagnosis and control of mycoses in animals and public health.
References
De Hoog, G. S., et al. Atlas of Clinical Fungi. CBS-KNAW Fungal Biodiversity Centre.
Quinn, P. J., et al. Veterinary Microbiology and Microbial Disease. Wiley-Blackwell.
Samson, R. A., & Varga, J. Aspergillus Systematics in the 21st Century. CRC Press.
Chermette, R., et al. Dermatophytoses in Animals. Mycopathologia.
Larone, D. H. Medically Important Fungi: A Guide to Identification. ASM Press.