Test 3- Intro To Fungi and Funigal Infections Flashcards
Taxonomy of Fungi
• About 1.5 million species in the Kingdom Fungi
- >80,000 described
- ~400 are known human/animal pathogens
General Characteristics of Fungi
• Eukaryotic!- so are we- have to be careful when treating
• Non‐photosynthetic
- Heterotrophic
- General taxonomy is constantly in flux
- Several Phyla
General Characteristics
Non‐motile (usually)
Aerobic (usually)
• Some yeast forms may ferment anaerobically
Spore‐bearing
Cell Wall
- Chitin, cellulose, proteins, and lipids (ergosterol- sterol that we use as a target for drugs)
- Provides rigidity and osmotic stability
Capsule- can be a defining characterisitic
• Polysaccharides
- Greatly variable in thickness
- May be antigenic and/or antiphagocytic
Unicellular species or forms
• YEAST
Multicellular species or forms
• MOLDS and MUSHROOMS
Yeast
- Oval or spherical cells
- 3‐5 um in diameter
- Unicellular
• Some variability in form
———–Some form chains known as pseudohyphae
Molds
Molds
- Main element (tubular structure) of the vegetative or growing form is the HYPHAE
- Can be divided by cross‐walls called septa
—————-Septate vs. non‐septate (coenocytic)
• Pigmentation
———-Dark pigment (Dematiaceous)
——— Non‐pigmented (Hyaline)
Dimorphic Fungi
• Change from mycelial form (at room temperature) to yeast (at 37°C or in the tissues of animals)
Fungal Reproduction
Sexual: NOT COMMON
- Only demonstrated in a few fungi
- Fusion of two haploid nuclei followed by meiotic division of the diploid nucleus
Asexual:
• Very common and effective
- Division of nuclei by mitosis
- Three mechanisms:
- Sporulation followed by germination of the spores
——————– Aspergillus and Penicillium
• Fragmentation of hyphae
————-Coccidioides immitis
• Budding of yeast cells
————Candida and Cryptococcus
Fungal Infections
• Rarely cause disease in healthy, immunocompetent animals
————-Except dermatophytes(ring worm)
Factors which may predispose to fungal invasion of tissues:
- Immunosuppression
- Prolonged antibiotic therapy
- Immunological defects
- Immaturity, ageing and malnutrition
- Exposure to heavy challenge of fungal spores
- Traumatized tissues
- Persistent moisture on skin surface
- Some neoplastic conditions
Immunity to Fungal Infections
• Immunity is mainly cell‐mediated
—————–Antibodies are usually produced (Except with superficial dermatophyte infections)
———————– Antibodies are not protective
• Most lesions are granulomatous
————— Resemble mycobacteriosis and other diseases caused by facultative intracellular bacteria
Isolation of Fungal Organisms
• Most grow in common media at 20‐25°C
———————- Sabouraud’s dextrose (SabDex) agar
• Some grow in Blood agar in the yeast phase at 37°C
Dermatophytes
- Molds capable of parasitizing only keratinized epidermal structures
- Superficial skin, hair, feathers, horns, hooves, claws and nails
loss of hair around the eyes, nose; not itchy in dogs and cats; only in humans
• Dermatophyte infections are called dermatophytosis or ringworm
• ZOONOSIS
• Several causative agents:
• Microsporum
• Trichophyton
• Epidermophyton
Ecology
Anthropophillic- HUMANS- not transmitted to animals
Zoophilic- Animals- can be transmitted to humans
M. canis
Geophilic- can be transmitted to humans
M. Gypseum
General Characteristics of Dermatophytes
- Phylum Ascomycota
- Grow slowly on SabDex
——–Produce pigmented colonies( white, yellow, or tan)
• Septate hyphae
General Characteristics
• Produce macroconidia and microconidia in culture(not if you take a hair from a cat or a fresh sample)
- Macroconidia are most useful for identification
- Microsporum: large, septate, rough, thick‐walled, fusiform or obovate macroconidia
• Trichophyton: cylindrical or cigar‐shaped, smooth, thin‐ or thick‐walled macroconidia(can be confused with demodex)
Virulence and Pathogenesis
- Proteolytic enzymes (e.g. keratinase, elastase and collagenases)
- Synergistic action with some bacteria (e.g. Staphylococcus spp.)- make sure you get the staff under control
- Infective arthrospores adhere to keratinized structures
- Favorable environmental conditions (trauma, moisture, warmth) encourage germination
- Hyphae grow centrifugally producing the typical ‘ringworm’ lesion depending on the local inflammatory response
• Transmission is mainly through direct contact
• Indirect by fomites or environmental contamination may also occur
• In the parasitic state only hyphae and arthroconidia (arthrospores) are seen
Pathology
• Initial phase: subclinical or mild host response
• Hypertrophy of the stratum corneum with accelerated keratinization and exfoliation
• Scruffy appearance and some hair loss (alopecia)
• Later phase: more pronounced inflammatory response
- Ranges from erythema to vesiculopustular reactions and suppuration
- Roughly circular pattern of the lesions and their inflamed margins suggested the terms ringworm and tinea
- Ringworm usually regresses spontaneously within a few weeks or months
Laboratory Diagnosis
Initially based on assessment of source (host animal, lesion appearance, etc.)
In cats and dogs, examination of lesions with a Wood’s lamp is helpful
• Filtered UV light
• M. canis often will fluoresce green due to a tryptophan metabolite
• 50‐60% of infected animals will be positive- if you don’t see the flourescence, it doesn’t mean that is isn’t there
Skin scraping and hair examination
• Observe for hyphae and arthroconidia
ECTOTHRIX(hang on to the hair shaft)
Laboratory Diagnosis
- Include material from the margins of any lesion- WANT THE ACTIVE EDGE OF THE LESION
- 10‐20% potassium hydroxide and heated gently
———— Treatment clears sample and allows better microscopic examination (100x)
• Stains may improve visualization
- ————– Permanent ink, lactophenol cotton blue, dimethylsulfoxide
- —————-Calcofluor white reagents imparts fluorescence to fungal structures
Culture
• Sabouraud’s dextrose (SabDex) agar at 25°C for up to 3 weeks
———–Cycloheximide, tetracycline or chloramphenicol can be added
• Dermatophyte Test Medium (DTM)
- ———Phenol Red indicator
- ——– Beware of false positives
- Rapid Sporulation Medium (RSM)
- (Molecular techniques)
Culture
Treatment and Control
• Isolate infected animals
• Systemic therapies:
- Itraconazole
- Griseofulvin
- Terbinafine
- Fluconazole
————— beware of teratogenicity in the systemic therapies
Treatment and Control
• Lime sulfur or miconazole shampoo
• Clip hair, especially for extensive lesions
————–Dispose of hair carefully because of infective arthrospores
- Contaminated bedding should be discarded (burned) and grooming equipment disinfected (0.5% sodium hypochlorite)
- Vaccines have not been effective in dogs/cats
Dermatophytosis in Other Species
Cattle
• T. verrucosum
• Usually affects calves
• Commercial vaccines used successfully in Europe and Russia
Horses
• T. equinum, M. canis (=M. equinum); rarely M. gypseum, T.
mentagrophytes, and T. verrucosum
Young horses, associated with tack and grooming gear
Limited vaccination in Europe
Pigs
• M. nanum (uncommon)
• No age predilection, no current economic significance
Poultry
• M. gallinae
- Avian ringworm or favus
- Often affects the comb and wattles, but may invade feather follicles
Treatment of Fungal Infections
Systemic antifungals- Side Effects!!!
Griseofulvin- don’t use in preganant animals
