Week 11

• Fungus

  • Widely distributed organism without chlorophyll, with nucleus, producing spores; reproducing sexually or asexually
  • With filamentous and branching structures evolved with cell walls containing chitin (or cellulose or both)
  • Unicellular or multicellular eukaryotes
  • Non-photosynthetic
  • Heterotrophic: produce exoenzymes and obtain nutrients by absorption
  • Saprophytes (majority!), mutualistic symbionts, parasites
  • Grow aerobically (many strict anaerobes), optimum 25-37 °C
  • Tolerate high osmotic pressures and acidic environments (pH 5.0)
• Fungal structure

  • Rigid cell wall
  • Mannan, chitin, cellulose, glucan and chitosan
  • Chitin (polysaccharide) provides rigidity and structural support
  • Ergosterol is the dominant sterol
  • Nucleus, mitochondria ribosomes (80S)
  • Microtubules
• Fungal forms

  • Molds (multicellular filaments)
  • Yeasts (unicellular spheres)
  • Dimorphic fungi (can switch between mold and yeast forms)
• Yeasts

  Oval, spherical or elongated single cells, 3-5 μm, reproduce by budding or both budding and spore formation

  Candida, Cryptococcus neoformans, Malassezia pachydermatis
• Molds

  Filamentous with branching filaments or hyphae, 2-10 μm in diameter, mycelium is the filamentous mass of hyphae

  Penicillium spp. Aspergillus spp.
• Mold structure

  • Vegetative mycelium (develops inside substrate, provides support and absorbs nutrients)
  • Aerial hyphae (vertically growing)
  • Reproductive mycelium (differentiation of aerial hyphae to support fruiting bodies)
  • Septate hyphae (septa divide hyphae into compartments)
  • Non-septate or coenocytic hyphae
• Dimorphic fungi

  Can change from mycelial form (at room temperature) to yeast form (at 37°C or in tissues of animals)

  Change is regulated by factors such as temperature, CO2 concentration, pH

  Coccidioides immitis, Sporothrix schenckii, Histoplasma spp. Blastomyces dermatitidis
• Asexual fungal reproduction

  • Fission of somatic cell: dividsion of nuclei by myotosis (Saccharomyces spp.)
  • Budding: cell wall bulge out and daughter nucleus migrates into bud (yeasts)
  • Fragmentation of hyphae: each disjointed hyphae becomes a new organism (Coccidioides immitis)
  • Sporulation followed by germination of spores (Aspergillus)
• Asexual spore types

  • Arthroconidia (formed by hyphal fragmentation)
  • Blastoconidia (produced by budding)
  • Chlamydoconidia (thick-walled resistant spores)
  • Macroconidia (large multi-celled conidia)
  • Microconidia (small conidia)
  • Phialoconidia (produced from phialides)
  • Sporangiospores (released from ruptured sporangium)
• Sexual spore types

  • Zygospores (formed in zygosporangium)
  • Basidiospores (produced on basidia)
  • Ascospores (develop in asci)
• Predisposing factors to fungal tissue invasion

  • Immunosuppression
  • Prolonged antibiotic therapy
  • Immunological defects
  • Immaturity, ageing
  • Malnutrition
  • Exposure to heavy challenge of fungal spores
  • Traumatized tissue
  • Persistent moisture on skin surface
  • Some neoplastic conditions
• Mechanisms of fungal diseases

  • Mycosis (tissue invasion)
  • Mycotoxicosis (toxin production)
  • Induction of hypersensitivity
• Classes of fungal infections

  • Superficial or cutaneous mycoses (epidermis, keratinized structures, mucus membranes)
  • Subcutaneous mycoses (subcutaneous tissues)
  • Systemic mycoses (respiratory/digestive tracts, disseminated from lungs)
• Dermatophytosis

  Infection caused by dermatophyte or 'ringworm fungi' in keratinized tissues (hair, feathers, skin, nails, claws, horns)

  Common zoonotic disease

  Arthrospores (arthroconidia) are the infectious forms
• Dermatophytes

  • Geophilic (inhabit and replicate in soil with decomposing keratin)
  • Zoophilic (obligate pathogens of animals, host-specific)
  • Anthropophilic (obligate pathogens of humans)
• Microsporum

  M. canis (most common dermatophyte of domestic animals), M. gypseum, M. manum, M. gallinae

  Invade hair and skin, produce thick-walled multiseptated macroconidia

  Microconidia are sessile or stalked, calvate, and arrange singly along hyphae
• Trichophyton


  T. mentagrophytes, T. equinum, T. verrucosum

  Invade hair, skin, nails, horns, claws

  Thin-walled smooth macroconidia

  Macroconidia are cylindrical, clavate to cigar-shaped
• Dermatophytosis clinical signs

  • Alopecia, erythema, scaling, crusting, annular-ringed lesions, vesicles or papules
  • Trichophyton infections more severe then Microsporum infections due to associated inflammation
• Dermatophytosis transmission

  • Arthrospores shed by infected animal, viable for months to years in environment
  • Direct contact with infected animal
  • Exposure to arthrospores in environment or fomites
• Dermatophytosis pathogenesis

  1. Spores enter stratum corneum

  2. Release keratinase, protease, elastase

  3. Inflammatory reaction

  4. Movement away from site of infection to next hair follicle

  5. Central healing with classical ringed lesion
• Canine ringworm causative agents

  • Microsporum canis
  • Microsporum gypseum
  • Trichophyton erinacei
  • Trichophyton mentagrophytes
• Canine ringworm clinical signs

  • Brittle hair, dry and scaly skin, crusts and scabs
  • Kerion (T. mentagrophytes infection): intense inflammation, swelling, ulceration, purulent exudate
• Microsporum gypseum is associated with compulsive burying of objects in soil, Trichophyton mentagrophytes with good rat catching ability, and Trichophyton erinacei with avid 'hedgehog-worrying'
• Feline ringworm

  • M. canis (cats serve as primary reservoir)
  • Often asymptomatic -> public health risk!
  • Lesions: circular areas of stubbed hair, alopecia, mild scaling and folliculitis at the head
  • Most common in kittens with immature immune system and adults with immune deficiency
  • Feline otitis: persistent waxy, ceruminous otic discharge when caused by M. canis
• Bovine ringworm

  • Trichophyton verrucosum
  • Calves are more susceptible. Incidence higher in winter
  • Lesions: circular, scattered, accompanied by skin scaling and alopecia; large plaques may develop with the formation of thick scabs and crusts
  • Severe inflammation, pruritis (secondary bacterial infection?). Spontaneous resolution after this stage.
• Porcine ringworm

  • Microsporum nanum, (M. gypseum, M. canis,T. mentagrophytes)
  • Common disease, affecting large breeds
  • Higher incidence with high density and humidity, and poor sanitation
  • Lesions: circular, roughened, mildly inflamed; anywhere on the body
• Equine ringworm

  • Trichophyton equinum, M. gypseum
  • Lesions: multiple, dry, scaly raised lesions on any body part; inflammation and exudates cause hair to mat together and enlarged lesions create a moth-eaten appearance
  • Infections often become chronic and subclinical, but recur under stress
• Avian ringworm = Favus = Whitecomb


  • M. gallinae
  • Lesions: white patches on the comb of infected male birds
  • Occasionally, disease may extend into feathers
• Dermatophytosis: diagnosis

  1. Wood's lamp examination of lesions (50-60% will test positive)

  2. Direct microscopic examination (use 10-20% KOH)

  3. Culturing (Dermatophyte test medium)
• Dermatophytosis: direct microscopic examination

  • Examine hairs from lesions and scales
  • Fine, hyaline, septate hyphae in keratin scales and in hair shafts
  • Arthroconidia on hair
• Dermatophytosis: diagnosis

  • Microsporum canis culture: spreading whitish, cottony surface growth with a golden-yellow reverse pigment
  • Microscopic morphology (lactophenol cotton blue stain): typical macroconidia structure
  • Physiological tests: temperature tolerance, urease production, in vitro hair perforation test for Trichophyton spp. because they develop few macroconidia
• Dermatophytosis: treatment

  1. Clipping, if long-haired

  2. Shampoos, dips, sprays

  3. Topical antifungal agents

  4. Systemic therapy (Griseofulvin, Azoles, Terbinafine)

  5. Treat until 3 negative cultures
• Dermatophytosis: control and prevention

  1. Clipping, topicals, sprays, etc.

  2. Clean up environment and fomite sources

  3. Vacuum carpeted areas

  4. Wash bedding

  5. Disinfect collars, leashes, tack, brushes, clippers, etc.

  6. 10% bleach solution

  7. Vaccination: effective strategy for control of bovine ringworm
• Dermatophytosis is a zoonosis: Microsporum canis and Trichophyton verrucosum
• Dermatomycoses: general characteristics

  • Yeasts and normally saprophytic filamentous fungi causing cutaneous infections resembling dermatophytosis
  • Predisposing factors: High humidity and excessive wax accumulation, Hairy and pendulous ears, Neoplasm, Allergies, Change in quality or quantity of sebum, Presence of other dermatoses, Recent antibiotic or glucocorticoid therapy, Trauma
• Dermatomycoses: Malassezia spp.

  • M. furfur, M. pachydermatis, M. sympodialis, M. globosa, M. obtuse, M. restrica, M. dermatis, M. nana, M. slooffiae
  • Lipophilic yeasts, opportunistic pathogens
  • Members of the normal cutaneous flora: skin, lips, anus, vagina, anal sacs and external ear canal of dogs
• Otitis externa

  • M. pachydermatis
  • Clinical signs: head shaking, pruritis, offensive odor, chronic otitis
  • Diagnosis: otoscopic examination, cytological examination for bacteria, yeasts and mites
  • Treatment: identify and eliminate predisposing factors, topical antifungals and systemic antimicrobials (middle ear infection)
• Seborrheic dermatitis

  • M. pachydermatis
  • Superficial dermatitis as regionalized disease or generalized disorder
  • Clinical signs: Face rubbing, foot licking, erythematous and scaly skin, alopecia, hyperpigmentation and lichenification
  • Breed predisposition: poodles, cocker spaniels, chihuahuas,German shepherds, boxers, basset hounds
  • Predisposing factors: allergies, seborrhea (cfr. factors for otitis externa)
  • Diagnosis: History of poor response to antibiotics, glucocorticoids and immunotherapy & demonstration of yeast cells on skin scrapings or swabs
  • Treatment: Removal of predisposing factors; shampoos, creams or dips with selenium sulfide, miconazole, ketoconazole, chlorhexidine; oral ketoconazole for systemic disease
• Most useful diagnostic tool for Malassezia infection: cytologic exam of affected areas (impression smears): slightly elongated oval (2.5-5.5 x 3.0 – 6.5 um) broad-based yeast cells (flask-shaped or peanut-shaped cells)
• Trichosporon spp.

  • Nasal mass in cats
  • Occludes naris
  • Surgical excision
  • Follow up with parenteral administration of ketoconazole