Growth appears on culture medium after as little as 4-6 days incubation. Surface is white and silky at the center and bright yellow at the periphery. The reverse side of the plate is bright yellow or orange.
Microscopic appearance: macroconidia of all Microsporum spp are spindle-shaped; microconidia relatively few or absent.
Taxonomy
A dermatophyte.
Tolerances
Temperature
Incubated at 25°C for 3 weeks.
Ultraviolet
Inhibited by high doses of ultraviolet radiation.
Some isolates of M. canis (50%) produce a green fluorescence under Wood's ultraviolet light (wavelength 366 nm), due to a tryptophan metabolite.
Development
Growth
Utilize keratin for growth.
Reproduction
In the non-parasitic state, including culture, asexual reproductive units are macroconidia and microconidia. In the parasitic state, asexual reproductive units are arthroconidia (Indistinguishable from species to species except in size ranges).
Sexual spores (ascospores) are absent in the parasitic phase.
Longevity
Viable in infected feline hair at room temperature for up to 18 months.
Dermatophytes may be geophilic (soil reservoir), zoophilic (animal reservoir) or anthropophilic (human reservoir).
Transmission
Most commonly direct contact; also fomites.
Source most commonly an infected cat.
Pathological effects
Dermatophytes are able to hydrolyze keratin and cause some damage to the epidermis and hair follicle. A hypersensitivity reaction is then mounted and the fungus moves away from the site of inflammation to normal skin. This causes the classic ringworm circular lesion with healing at the center and inflammation at the edge.
Lesions may occur anywhere on the body.
In the cat: often subclinical in adults, generally non-inflammatory except in young kittens. May become generalized in debilitated kittens.
In the dog: typically non-inflammatory scaly patches with alopecia. May be kerion.
Other Host Effects
Can cause subclinical or inapparent infection in the host animal; it may cause persistent sub-clinical infection in long-haired cats.
Control
Control via chemotherapies
Topical: clotrimazole , tolnaftate, natamycin , ketoconazole . Lime sulfur (Lym dip) is a very effective topical.
Oral: griseofulvin + ketonazole (Nizoral tablets), itraconazole (Spornox capsules or Itrafungol oral solution) or fluconazole (Diflucan tablets).
Control via environment
In infected cat colonies, in-contact cats should also be treated.
Twice weekly chlorhexideine-miconazole shampoos may help to reduce spread to other cats and human contacts.
Vaccination
It does prevent development of lesions after challenge.
Other countermeasures
Hair should be clipped from affected areas.
Twice weekly chlorhexideine-miconazole shampoos may be a useful adjunct to systemic therapy.
Prepare area with alcohol to kill saprophytic fungi. Hairs should be plucked rather than cut with scissors as the base of the hair contains most fungal material.
The outer edge of the scab should be collected by scraping with a blunt scalpel until blood is just drawn; this is the site where the dermatophyte grows. The skin or hairs can be collected directly into an envelope (this keeps them dry) then further wrapped to send to laboratory.
Laboratory diagnosis 50% M. canis infections (and other Microsporum spp) produce metabolites which fluoresce green under the ultraviolet light of a Wood's lamp . This is a useful preliminary examination of the whole animal, skin scrapings or hair from the affected areas, and is particularly useful for suspected inapparent infections of kittens.
Direct microscopy of a KOH preparation to visualize arthroconidia and hyphae. Arthroconidia of M. canis surround the hair (ectothrix)
Lactophenol cotton blue preparation of the culture shows spindle-shaped macroconidia. Identification of dermatophyte is important to determine source of infection.
Culture on Sabouraud's dextrose agar and examination of lactophenol cotton blue wet preparation of colonies, together with clinical picture, is sufficient for identification.
Sparkes A H, Robinson A, Mackay A D & Shau S E (2000) A study of the efficacy of topical and systemic therapy for the treatment of feline Microsporum canis infection.J Feline Med and Surg2 (3), 135-142.
Mancianti F, Pedonese F, Millanta F & Guarnieri L (1999) Efficacy of oral terbinafine in feline dermatophytosis due to Microsporum canis.JESFM1, 37-41.
Mignon B R et al (1999) Histopathological pattern and humoral immune response to a crude exo-antigen and purified keratinase of Microsporum canis in symptomatic and asymptomatic infected cats.Med Mycol37 (1), 1-9.
Pier A C et al (1998) Parasitic relationship between Microsporum canis and the cat.Med Mycol36 (Suppl 1), 271-275.
Mancianti F et al (1998) Efficacy of oral administration of itraconazole to cats with dermatophytosis caused by Microsporum canis.JAVMA213 (7), 993-995.
Morielio K A & DeBoer D J (1995) Feline dermatophytosis - recent advances and recommendations for therapy.Vet Clin North Am Small Anim Pract25 (4), 901-921.
Vetstream contributor(s)
Dr Rosanna Marsella DVM DACVD, PO BOX 100126, SACS, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610-0126, USA.
50% M. canis infections (and other Microsporum spp) produce metabolites which fluoresce green under the ultraviolet light of a Wood's lamp 
. This is a useful preliminary examination of the whole animal, skin scrapings or hair from the affected areas, and is particularly useful for suspected inapparent infections of kittens.
