Fungal disease Flashcards
What are the three main growth forms of fungi?
Yeast (unicellular), mould (multicellular-filamentous hyphae) & dimorphic fungi (both forms)
How do fungi obtain nutrients?
Digest food externally & absorb nutrients through their cell walls
What are the components of fungal cell walls?
Chitin & other polysaccharides
How do fungi reproduce?
By spore formation, which can be asexual or sexual
What is the predominant nature of most fungi?
Most fungi are saprophytic (live on dead organic material) though some cause opportunistic infections
Do normal animals carry fungi on their skin?
Yes, animals harbour saprophytic fungi (yeasts & moulds)
Some are transient contaminants, while others (e.g. Malassezia yeasts) are part of normal flora
What are transient contaminants?
Microorganisms temporarily present on animal’s skin, often from environment, but not part of normal flora
May cause disease under certain conditions
What are dermatophytes?
Fungal pathogens that cause ringworm
Can be cultured from normal animals but are typically transient contaminants from environment & are never commensals
How are fungi classified based on habitat?
Geophilic (soil/environment)
Zoophilic (on animals)
Anthropophilic (on humans)
What are the three main mechanisms by which fungi cause disease?
Tissue invasion (mycosis) (most common)
Toxin production (mycotoxicosis)
Induction of hypersensitivity
How can mycosis (tissue invasion) be classified & what tissues do they affect?
What factors predispose an animal to mycosis (tissue invasion by fungi)?
Immunological deficits
Immunosuppression (including corticosteroids)
Immature or aging immune system
Malnutrition
Prolonged antibiotic use
High fungal spore exposure
Tissue trauma
Persistent moisture on skin surface
What are the 2 most common superficial mycosis?
Dermatophytosis (caused by dermatophytes)
Dermatomycosis (caused by non-dermatophytic fungi)
Describe dermatophytosis
Superficial mycosis caused by dermatophytes (Microsporum & Trichophyton spp.)
Invade keratinised structures (skin, hair, nails), are highly contagious, & have zoonotic potential
Commonly affected species:
- Cats, cattle, and horses
Describe dermatomycosis
Superficial mycosis caused by non-dermatophytic fungi (e.g. Malassezia & Candida
Normal skin commensals that cause disease when they overgrow due to underlying condition
Infection is not contagious
What is Malassezia dermatitis?
Very common dysbiosis of dogs (+- cats) often occurring secondary to other skin disease - affecting skin and ears
Type of dermatomycosis
What is Candidiasis, and what does it cause?
Opportunistic dermatomycosis caused by Candida albicans in immunosuppressed animals
Can cause mycotic stomatitis (puppies, kittens, foals) & thrush (oesophagus/crop in young chickens)
What are subcutaneous (deep) mycoses, and how do they develop?
Fungal infections that invade dermis or subcutaneous tissues, often following foreign body penetration that introduces environmental saprophyte
Cause chronic localised lesions (unless immunocompromised)
e.g. Sporotrichosis (Sporothrix schenkii)
- Zoonotic but rare in UK
What is chromomycosis, and how does it differ from hyalohyphomycosis?
Chromomycosis is lesion caused by pigmented fungi, while hyalohyphomycosis is caused by non-pigmented fungi
(Pic is Chromomycosis)
What is a mycetoma, and what are its two main types?
Organism forming granules/grains in lesions, associated with swelling & draining sinuses
Eumycotic mycetoma: organism is fungal
Actinomycotic mycetoma: organism is bacterial (e.g. Actinomyces, Nocardia)
What are pseudomycetomas, and how do they differ from true mycetomas?
Resemble mycetomas but have different granule formation
Can be caused by dermatophytes (dermatophytic pseudomycetomas) or bacteria (bacterial pseudomycetomas)
What is systemic mycosis?
Fungal infection that spreads throughout internal organs, usually via inhalation of fungal spores
What is cryptococcosis and how is it transmitted?
Systemic fungal infection caused by Cryptococcus neoformans & Cryptococcus gattii
Associated with pigeon droppings & cause opportunistic infections via inhalation of fungus in dust –> nasal infection –> invasion via cribriform plate to CNS or spread through haematogenous/lymphatic routes
What species are affected by cryptococcosis, and what signs does it cause?
Cats: Respiratory, cutaneous, neural & ocular infections
Dogs: Disseminated disease with neural & ocular signs
Cattle (rare): Mastitis, nasal granulomas
Horses (rare): Nasal granulomas, sinusitis, cutaneous lesions, pneumonia, meningoencephalomyelitis, abortion
What is mycotoxicosis, and what causes it?
Mycotoxicosis is poisoning caused by fungal mycotoxins present in contaminated crops, pasture, or stored feed
Common fungi involved include Penicillium, Aspergillus (aflatoxicosis), Fusarium & Claviceps (ergotism)
What are the effects of mycotoxicosis?
Mycotoxicosis can result from acute or chronic ingestion, leading to wide range of clinical signs depending on toxin
In food-producing animals, mycotoxins can accumulate in tissues, posing risk of human exposure
How do fungi induce hypersensitivity?
Fungal spores can act as allergens, causing chronic pulmonary disease in cattle/horses & conditions like recurrent airway obstruction (RAO) in horses
Give examples of fungi that cause hypersensitivity in small animals
Malassezia hypersensitivity in dogs & cats, leading to skin conditions
Saprophytic fungi can act as allergens, contributing to canine atopic dermatitis
What are the three main pathological changes seen in fungal infections?
Acute suppurative inflammation
Chronic inflammation (pyogranulomatous/ granulomatous)
Necrosis
What characterises acute suppurative inflammation in fungal infections?
It is a neutrophilic inflammation that can lead to micro-abscess formation with pus (suppuration)
How do fungal infections lead to chronic inflammation and granuloma formation?
Fungal cell walls are complex & not fully degraded by neutrophils
When phagocytes die, fungal material is released, recruiting macrophages, which produce chemokines & cytokines to recruit more macrophages
This repetitive cycle results in granuloma formation
Granulomas consist mainly of macrophages, while pyogranulomas contain both macrophages & neutrophils
What is necrosis in fungal infections, and what causes it?
Necrosis occurs when fungi invade blood vessels, leading to infarction (lack of blood supply), causing tissue death
What are common clinical signs of superficial mycoses?
Dermatophytosis: Alopecia, claw disease, papules
Malassezia: Erythema, scaling, hair loss, otitis
Candida: Ulcers, erosions with yellow/grey exudate.
What are common clinical signs of subcutaneous mycoses?
Cutaneous papules, subcutaneous nodules, ulceration, discharging tracts & regional lymphadenopathy
Usually due to traumatic implantation of fungus - most commonly on feet/limbs or head
- Occasionally disseminate to other organs
What are common clinical signs of systemic mycoses?
Granulomas, pyogranulomas, necrosis in affected organs & systemic illness
What is Aspergillosis, and what causes it?
Fungal infection caused by Aspergillus spp., commonly A. fumigatus, a soil saprophyte with occasional pathogenic effects
What are the main routes of Aspergillus infection, and what diseases do they cause?
Inhalation (most common):
- Local respiratory infection (esp. birds)
- Guttural pouch mycosis (horses)
- Nasal aspergillosis (dolichocephalic dogs)
Local inoculation (rare):
- Keratitis (horses)
- Mastitis (cattle, via contaminated intra-mammary tubes)
Haematogenous spread from GI tract:
- Mycotic placentitis/abortion (cattle)
What are the common diagnostic techniques for fungal infections?
Direct microscopic examination
Fungal culture
Histopathology
Other tests
- PCR & Wood’s lamp for dermatophytosis
- Latex agglutination test for cryptococcal capsular antigen in serum/CSF/urine
- ELISA for Sporothrix schenkii antibodies
Give examples of direct microscopic examination methods for fungal infections
Examine hair plucks/scale
- e.g. for dermatophytosis
Cytology stained (e.g. with Diff Quick):
- e.g.: Malassezia - stained direct/indirect impression smears or acetate tape strips
- e.g. Cryptococcus spp - see yeasts in CSF or aspirates/direct smears of cutaneous lesions/nasal exudate
What samples are used for fungal culture?
Hair, dry skin scrapings & coat brushings (for dermatophytes)
Tissue culture (from biopsy or post-mortem material) for subcutaneous/deep mycoses
What medium is commonly used for fungal culture?
Sabouraud dextrose agar is used for most fungi, though some require specialized or enriched media
How are fungi speciated in the lab?
Based on:
Asexual spore type.
Colony appearance.
Features of vegetative hyphae
What is histopathology used for in fungal diagnosis?
Primarily used for diagnosing subcutaneous & systemic fungal infections through biopsy or post-mortem material
How does histopathology confirm fungal infections?
Demonstrates fungi within tissues, distinguishing infection from carriage or contamination
What special stains are used in histopathology for fungi?
Periodic acid-Schiff (PAS) stain.
Grocott-Gomori methenamine silver (GMS) stain
What factors influence the clinical presentation of mycotoxicosis?
Type of mycotoxin produced
Amount of toxin ingested & duration of exposure
What is aflatoxicosis, what causes it, and what are its effects?
Caused by aflatoxins from Aspergillus spp., found in maize, stored grain & soybean
Affects cattle, pigs, poultry, dogs & trout
Causes hepatotoxicity, immunosuppression, mutagenesis/teratogenesis & ill-thrift
What is ergotism, what causes it, and what are its effects?
Caused by ergotamine toxins from Claviceps spp., found in ryegrass & other grasses/cereals
Affects cattle, pigs, poultry, deer, sheep & horses
Causes neurotoxicity & perinatal deaths (lambs, calves)
What are the epidemiological features of mycotoxicosis?
Outbreaks usually seasonal & sporadic
May be associated with certain batches of food
What are the clinical signs of mycotoxicosis?
Often ill-defined
e.g. Immunosuppression may present as increased susceptibility to infectious disease or higher rates of vaccination failure
How is mycotoxicosis confirmed?
By demonstrating mycotoxins in feed or tissues of infected animals
What are common hypersensitivity reactions to fungi?
Type 1 hypersensitivity to fungi/moulds can cause chronic pulmonary disease/recurrent airway obstruction, canine atopic dermatitis & Malassezia-associated pruritus
What are the clinical signs of fungal hypersensitivity in the respiratory system & how is it diagnosed?
Chronic cough, nasal discharge & occasional respiratory distress
Diagnosis based on history & physical examination
What are the clinical signs and diagnostic methods for fungal hypersensitivity in the skin?
Pruritus, often associated with environmental fungi in canine atopic dermatitis & Malassezia
Diagnosis can be confirmed using IgE serology or intradermal testing to investigate causal allergens
