Fungal causes of skin disease

Question 1

What are the different types of fungi that can be found on the skin?

Answer 1

skin commensals (e.g. Malassezia) - colonisation

transient contaminants - carriage
e.g environmental saprophytes, occasionally dermatophytes

active pathogens (e.g. dermatophytes) – clinical disease

Question 2

What skin diseases do fungi cause?

Answer 2

Tissue mycosis:
- superficial
- subcutaneous
- deep
Hypersensitisation (uncommon)

Question 3

Give an example of fungi cause hypersensitisation

Answer 3

Atopic dogs may become sensitised to fungi (e.g. saprophytic fungi, Malassezia)

Question 4

What are the 3 classifications of dermatophytes?

Answer 4

Geophilic - adapted for environment
Zoophilic - adapted for mammals - most common source of animal disease
Anthropophilic - adapted for man

Question 5

What are the stages of fungal infection?

Answer 5

Skin microtrauma & moisture => inoculation and germination of infective arthrospores
=> penetrate stratum corneum => growth of fungal hyphae => lesions (foci within 5-7 days)

Question 6

What must occur for fungi to cause clinical skin disease?

Answer 6

immune system or skin barriers compromised

Question 7

How are fungi transmitted?

Answer 7

direct contact
indirect (fomites)

Question 8

What is the most common manifestation of fungal skin disease?

Answer 8

infection of hair/surface keratin

Question 9

What are the clinical features of fungal infection of hair/surface keratin?

Answer 9

folliculitis => alopecia - usually fairly well-demarcated
adherent scale
+/- erythema, crust, comedones
occasional pustules
variable pruritus
Lesions usually asymmetric at outset
Haired skin nearly always affected

Question 10

What are the possible clinical presentations of fungal skin disease?

Answer 10

Infection of hair/surface keratin
Furunculosis
Fungal kerion
Pseudomycetoma
Onychomycosis (claw disease)

Question 11

What are the clinical features of fungal furunculosis?

Answer 11

Looks similar to deep pyoderma - nodular swelling +/- draining sinus

Question 12

What are the clinical features of fungal kerion

Answer 12

Nodular inflammatory mass due to inoculation of fungus into dermis
Especially hunting/working dogs

Question 13

What are the clinical features of pseudomycetoma?

Answer 13

Esp in Persian cats, Yorkshire Terriers
Subcutaneous/deep mycosis => nodular granulomas
+/- ulceration and draining tracts with grains

Question 14

How is dermatophytosis diagnosed?

Answer 14

Direct examination of hair/scales
wood’s lamp
Fungal culture
Fungal PCR
Skin biopsy - histopathology

Question 15

Describe the diagnosis of fungal skin disease via direct examination of hair/scales

Answer 15

Gentle skin scrape/pluck hair from lesion margin (select with Wood’s lamp if possible)
Place in liquid paraffin on microscope slide, add coverslip
Examine with microscope
x4-10 objective - infected hairs look distorted, paler/wider than normal hairs
x40 objective - arthrospores around hair +/or hyphae within hair shaft

Question 16

Describe the use of a Wood’s lamp to diagnose fungal skin disease

Answer 16

Used to find abnormal hairs for direct examination/culture
Allow time for your eyes to adapt to light
Hold 2-4cm from skin – look for apple green fluorescence of hairs
Only detectsMicrosporum canis (most important dermatophyte in dog/cat) – 90+% untreated cases fluoresce!

Question 17

Describe the use of fungal culture to diagnose fungal skin disease

Answer 17

Sample using new toothbrush in wrapping (McKenzie toothbrush technique)
Can be in-house (using dermatophyte test-medium (DTM)) or at external lab (usually Sabouraud’s medium)
dermatophytes have pale colonies with red colour change of surrounding medium
Colony characteristics and microscopy of macroconidia needed to identify dermatophyte
Negative if no growth at 14 days
Cannot distinguish between carriage and disease

Question 18

Describe the use of fungal PCR to diagnose fungal skin disease

Answer 18

Very sensitive test
Results available faster than with culture
Submit samples from lesion only
Include crust, scale and minimum 20 hairs with bulbs
Cannot distinguish between carriage and disease

Question 19

When should histopathology of skin biopsies be used to diagnose fungal skin disease?

Answer 19

If deep infection suspected
If unsure if dermatophyte detected by other means is due to carriage or active infection

Question 20

What host factors impact malassezia on the skin’s ability to cause skin disease?

Answer 20

Anatomical features e.g., skin folds, Other skin disease, Systemic disease => increased malessezia numbers +/- decreased species diversity (dysbiosis), shift towards more pathogenic strains => clinical disease

Question 21

Describe the features of malassezia dermatitis/otitis

Answer 21

Almost always a secondary infection
Not contagious
Potentially zoonotic
Very common in dog

Question 22

Describe the clinical presentation of malassesia dermatitis/otitis

Answer 22

Pruritus
Erythema, grease, scale, crust => chronic changes (lichenification, hyperpigmentation)
Often malodourous (smells)
Focal/multifocal/generalised
Diffuse/well-demarcated

Commonly affected areas ears, lips, muzzle, interdigital skin, flexor surfaces/medial aspects of limbs, ventral neck/body, axillae, perineum
Also paronychia (claw fold infection)
Erythro-ceruminous otitis

Question 23

How is malassezia infection diagnosed?

Answer 23

Identify lesions
Identify Malassezia at these sites with cytology:
- Stained acetate tape strip
- Stained impression smear - direct or indirect
If present, assess response to antifungal therapy (decreased number and improve clinical sigs)

Question 24

What does malassezia look like under a microscope?

Answer 24

See peanuts/snowmen/footprints/Russian dolls using x40- x100 (oil immersion) objective