Mycology: Fungal Pathogens and Host Interactions
What you must know
Fungal questions emphasize morphology (yeasts, molds, dimorphic fungi), reproduction (budding, hyphae, conidia, arthroconidia), and host defenses (innate immunity, especially neutrophils and macrophages, matters more than antibody). Know the major veterinary pathogens: Malassezia (lipophilic yeast, otitis and dermatitis), Candida (opportunistic yeast, mucosal disease), Aspergillus (mold, respiratory and sinonasal disease), Cryptococcus (encapsulated yeast, systemic disease, often neurologic), Blastomyces, Histoplasma, and Coccidioides (dimorphic systemic fungi), and Dermatophytes (Microsporum, Trichophyton) causing ringworm. Understand dimorphism (mold in environment, yeast in tissue), capsule (Cryptococcus, India ink or latex agglutination historically), and dermatophyte diagnostics (Wood's lamp, KOH preparation, culture).
Traps
Questions test that systemic dimorphic fungi are not contagious from animal to animal - infection is acquired from the environment. Another trap is confusing yeast vs. mold forms of the same organism (dimorphism) or attributing Cryptococcus capsule to another yeast. Do not assume all fungi are susceptible to antibacterial drugs; fungi are eukaryotes, so antibacterial antibiotics do not treat fungal disease. Watch for questions that treat dermatophytosis as a systemic disease rather than a superficial keratin infection.
Rule of thumb
If the stem describes a capsulated yeast with neurologic signs, think Cryptococcus; if it describes mold in tissue with hyphae, think Aspergillus or a similar opportunistic mold.
Worked example
A question asks which fungal organism is acquired from the environment and produces a thick capsule visible with special stains. The answer is Cryptococcus, which is environmental (often pigeon-associated) and encapsulated. The discriminator is capsule plus environmental source, not the host species.