DL: Treatment of fungal skin disease

Question 1

What steps would you take to show that lesions are due to dermatophytosis? 4

Answer 1

• Wood’s Lamp Exam
• Microscopy (skin scrapes and hair plucks to look for hyphae and arthroconidia)
• Fungal culture (Sabaroud’s material preferable to Dermatophyte growth medium)
• Skin biopsies

Question 2

What advice would you give to an owner about M.canis infection?

Answer 2

• zoonotic
• minimise contact
• wash hands and arms thoroughly
• inform of human CS and go to doctor
• bedding hygiene
• vacuum and disinfection (dilute bleach works wells)

Question 3

What the the possible treatments for M.canis

Answer 3

• TOPICAL - shampoo, targets stratum corneum but not hair invasion
• SYSTEMIC - itraconazole, needed to clear infection (along with cell mediated immunity as topical therapy isn’t sufficient)

Question 4

Give a treatment protocol for a cat with M.canis

Answer 4

• ITRACONAZOLE (systemic) - 3 cycles of therapy, one week on one week off (as cumulatese in stratum corneum and keratinised tissue so administration every week isn’t necessary_
• MICONAZOLE AND CHLORHEXIDINE shampoo (topical, 2% each)
• clip hair around lesion, across whole body if widespread
• (Griseofulvin no longer available)

Question 5

Why are antifungals generally less effective and more toxic than antibacterials?

Answer 5

Fungi like mammals are eukaryotes (unlike bacteria) therefore more similar to human/host cells than bacteria so host cells may well be targeted with the therapy as well as the fungus itself

Question 6

How do polyenes such as amphotericin B and nystatin work?

Answer 6

bind to sterols and form hydrophobic channels. Binds to ergosterol (fungi) and cholesterol (mammal) and therefore causes adverse effects to the fungi and the host.

Question 7

Are imidazole and triazole antifungals more or less useful that polyenes and 5-fluorocytosine?

Answer 7

Imidazole and triazole because they have a wider spectrum of activity, there is no prevalent resistance against them, they have fewer side effects. Also polyenes are poorly absorbed from the GIT.

Question 8

What is the mode of action of imidazoles?

Answer 8

Alter cell membrane permeability (by blocking the synthesis of ergosterol the primary cell sterol of fungi).

Question 9

How does f-fluorocytosine work?

Answer 9

incorporated into fungal DNA after conversion from 5-FU. Human/pet not affected by this but the problem with its use relates to fungal resistance to it.

Question 10

Outline a treatment protocol for Malassezia dermatitis

Answer 10

• 2% miconazole/ 2% chlorhexidine (every 3 days for 3 weeks until controlled). Good evidence when given twice weekly.
• systemic - possible but more expensive, ketoconazole or itraconazole. Only fair EBVM for these though.

Question 11

What is an appropriate treatment for Histoplasma infection (a dimorphic yeast) in a dog with signs of wasting and lung infection.

Answer 11

Systemic antifungal (itraconazole) with amphotericin B if there is systemic spread.