Fungi and Fungal Pathogens Quiz

Study Notes

Fungi and Fungal Pathogens

Fungi are eukaryotic organisms, often classified based on morphology (structures and spores) and pigmentation.
Most fungi are harmless saprophytes (decomposers). Only a small percentage are pathogenic to animals and humans (148,000 species known, with only about 400 being pathogenic).
Fungi are classified traditionally into four classes based on the presence or absence and type of sexual spores (ascospores, basidiospores, zygospores, and chytridiomycota).
Phylum Ascomycota: Includes septate hyphae (hyphae with cross-walls); examples include Microsporum spp., Trichophyton spp., Candida albicans, and Aspergillus spp.
Phylum Basidiomycota: Includes septate hyphae; examples include Cryptococcus spp and Malassezia pachydermatis.
Phylum Zygomycota: Includes non-septate hyphae (hyphae without cross-walls); examples include Mucor spp and Mortierella spp.
Phylum Chytridiomycota: Includes non-septate hyphae; an example is Batrachochytrium dendrobatidis, a major pathogen of amphibians.
Fungi are typically aerobic and grow optimally between 20-30°C. A characteristic of potential pathogens is their ability to grow at 37°C.
Fungi are broadly categorized as yeasts, molds, and dimorphic species that change form depending on the conditions.
Yeasts: Are unicellular, larger than bacteria (3-10 µm), typically oval or round, and gram-stain positive. They reproduce by budding. Example: Candida.
Molds: Filamentous or hyphae, ranging from 2-10 µm in diameter that grow parallel to the surface, forming branched interconnected mycelia. May be septate (including cross walls) or non-septate.
Dimorphism: Some fungi can exist as either a yeast or mold form, depending on environmental conditions (eg., temperature, presence of nutrients).
Cell Structure: Fungal cell walls contain chitin, mannoproteins, and glucans; they have a plasma membrane with a significant amount of ergosterol (a sterol not found in animal or plant cells). Ergosterol is a common target for antifungal drugs.

Types of Diseases Caused by Fungi in Veterinary Medicine

Contagious cutaneous diseases (ringworm): Caused by dermatophytes.
Cutaneous or mucosal infections.
Systemic or tissue infections: Caused by Sporothrix schenckii, Blastomyces dermatitidis, Histoplasma capsulatum, and Coccidioides immitis.
Sensitization (allergic reactions): To fungal products
Mycotoxic disease: Ingestion of preformed toxic fungal metabolic products

Dermatophytes (Ringworm)

Highly contagious, transmitted via conidia to other animals.
Grow in the non-living keratin layer of skin/fur.
Host specificity varies; some infect a narrow range of hosts.
Lesions result both from fungal activity and host immune response.

Yeasts and Yeast-like Infections (e.g., Candida albicans)

Candida albicans is frequently found in animals and is commonly associated with oral thrush (pyogenic lesions, mouth, intestine or genitalia).

Systemic or Tissue Mycoses

Many species have differing survival modes across different temperatures, either in the environment or inside animals.

Fungal Species and Their Impacts

Specific fungal species, including examples like Cryptococcus neoformans, Malassezia pachydermatis, and Aspergillus spp. can cause a wide array of diseases in animals.

Mycotoxicosis

Acute or chronic intoxications from toxins produced by certain fungi, especially those growing on crops or stored feed.
Mycotoxins are metabolites and low-molecular-weight compounds, effects range by toxicity.