Importance and examples of fungal disease

Question 1

Describe the main features of fungi

Answer 1

eukaryotic
Can grow in 3 forms:
- yeast - unicellular
- mould - multicellular-filamentous hyphae
- both - dimorphic fungi
Digest food externally and absorb food through cell wall (saprophytic)
Complex cell walls consisting of chitin and other polysaccharides
Reproduction by spore formation

Question 2

Do normal animals carry fungi on their skin?

Answer 2

Yes, animals harbour saprophytic fungi (yeasts & moulds)

Some are transient contaminants, while others (e.g. Malassezia yeasts) are part of normal flora

Question 3

What are transient contaminants?

Answer 3

Microorganisms temporarily present on animal’s skin, often from environment, but not part of normal flora

May cause disease under certain conditions

Question 4

What are dermatophytes?

Answer 4

Fungal pathogens that cause ringworm

Can be cultured from normal animals but are typically transient contaminants from environment & are never commensals

Question 5

What are the classifications of fungi based on their normal habitat?

Answer 5

Geophilic - found in soil/environment; only occasionally infect animals/man

Zoophilic - found on animals; occasionally transmitted to man

Anthropophilic - found on man; occasionally transmitted to animals

Question 6

What are the mechanisms by which fungi cause disease?

Answer 6

Tissue invasion (mycosis) - most common
Toxin production (mycotoxicosis)
Induction of hypersensitivity

Question 7

What are the categories of mycosis (fungal tissue invasion) and where are the sites of the lesions?

Answer 7

Question 8

What factors predispose animals to tissue invasion by fungi (mycosis)?

Answer 8

Immunological deficits
Immunosuppression, including corticosteroid treatment
Immaturity, ageing, malnutrition
Prolonged antibiotic therapy
Exposure to heavy challenge of fungal spores
Traumatised tissues
Persistent moisture on skin surface

Question 9

Give examples of diseases that cause disease via superficial mycosis

Answer 9

Dermatophytosis - caused by dermatophytes
Dermatomycosis - disease caused by non-dermatophytic fungi

Question 10

Describe dermatophytosis

Answer 10

Superficial mycosis caused by dermatophytes e.g. Microsporum spp, Trichophyton spp

Pathogenic fungi, invading keratinised structures
Highly contagious
Zoonotic potential

Commonly affect many species:
Cat, cattle, horse

Question 11

Describe dermatomycosis

Answer 11

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

Question 12

What is Malassezia dermatitis?

Answer 12

Very common dysbiosis of dogs (+- cats) often occurring secondary to other skin disease - affecting skin and ears

Type of dermatomycosis

Question 13

What is Candidiasis, and what does it cause?

Answer 13

Opportunistic dermatomycosis caused by Candida albicans in immunosuppressed animals

Can cause mycotic stomatitis (puppies, kittens, foals) & thrush (oesophagus/crop in young chickens)

Question 14

What are subcutaneous (deep) mycoses, and how do they develop?

Answer 14

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

Question 15

Define chromomycosis

Answer 15

lesion caused by pigmented fungus

Question 16

Define hyalohyphomycosis

Answer 16

lesion caused by non-pigmented fungi

Question 17

Define mycetmoa

Answer 17

organism in granules/grains within lesion, associated with swelling and draining sinus

Eumycotic mycetoma - organism is fungal
Actinomycotic mycetoma - organism is bacterial (e.g. Actinomyces, Nocardia))

Question 18

Define pseudomycetomas

Answer 18

have different granule formation to mycetoma
Can be dermatophytic (caused by dermatophytes) or bacterial

Question 19

What is systemic mycosis?

Answer 19

Fungal infection that spreads throughout internal organs, usually via inhalation of fungal spores

Question 20

What is cryptococcosis and how is it transmitted?

Answer 20

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

Question 21

What species are affected by cryptococcosis, and what signs does it cause?

Answer 21

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

Question 22

What is mycotoxicosis, and what causes it?

Answer 22

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)

Question 23

What are the effects of mycotoxicosis?

Answer 23

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

Question 24

How do fungi induce hypersensitivity?

Answer 24

Fungal spores can act as allergens, causing chronic pulmonary disease in cattle/horses & conditions like recurrent airway obstruction (RAO) in horses

Question 25

Give examples of fungi that cause hypersensitivity in small animals

Answer 25

Malassezia hypersensitivity in dogs & cats, leading to skin conditions

Saprophytic fungi can act as allergens, contributing to canine atopic dermatitis