Microsporum canis is a zoophilic dermatophyte of worldwide distribution and is a frequent cause of ringworm in humans, especially children. Invades hair, skin and rarely nails. Cats and dogs are the main sources of infection. Invaded hairs show an ectothrix infection and fluoresce a bright greenish-yellow under Wood’s ultra-violet light.
Morphological Description: Colonies are flat, spreading, white to cream-coloured, with a dense cottony surface which may show some radial grooves. Colonies usually have a bright golden yellow to brownish yellow reverse pigment, but non-pigmented strains may also occur. Macroconidia are typically spindle-shaped with 5-15 cells, verrucose, thick-walled and often have a terminal knob, 35-110 x 12-25 µm. A few pyriform to clavate microconidia are also present. Macroconidia and/or microconidia are often not produced on primary isolation media and it is recommended that subcultures be made onto lactritmel agar and/or boiled polished rice grains to stimulate sporulation.
Growth on Rice Grains: good growth of white aerial mycelium with production of yellow pigment. Microscopy reveals numerous macroconidia and microconidia similar to those described above.
Reverse Pigment on Potato Dextrose Agar: Bright yellow (both M. audouinii and M. canis var. equinum are salmon to pinkish-brown).
Vitamin Free Agar (Trichophyton Agar No.1): Good growth indicating no special nutritional requirements. Cultures are flat, white, suede-like to downy, with a yellow to pale yellow-brown reverse.
Hair Perforation Test: Positive at 14 days.
Key Features: Distinctive macroconidia and culture characteristics. Abundant growth and sporulation on polished rice grains and in vitro perforation of hair.
Culture of Microsporum canis.
Macroconidia of Microsporum canis are typically spindle-shaped with 5-15 cells, verrucose, thick-walled and often have a terminal knob. A few pyriform to clavate microconidia are also present.
Macroconidia and/or microconidia of Microsporum canis are often not produced on primary isolation media and it is recommended that sub-cultures be made onto boiled polished rice grains to stimulate sporulation.
Microsporum canis dysgonic strains are rare but may also occur. These dysgonic strains typically have a heaped and folded, yellow-brown thallus and macroconidia are usually absent. However, typical colonies and macroconidia of M. canis are usually produced by this variant when subcultured onto polished rice grains. Note: The dysgonic type colony of M. canis is similar to that of Microsporum ferrugineum.
Supplementary description for Microsporum canis var. distortum, a dysgonic variant of M. canis with distinctive distorted macroconidia. Abundant growth and sporulation on rice grains.
Microsporum canis var. distortum is a zoophilic fungus known to cause infections in cats, dogs and other animals. It is a rare cause of tinea capitis in New Zealand, Australia and North America. Clinical disease is similar to M. canis. Invaded hairs show an ectothrix infection and fluoresce a bright greenish-yellow under Wood’s ultra-violet light.
Microsporum canis var. distortum culture and distorted macroconidia
Microsporum canis var. equinum is now considered to be a genotypic synonym of Microsporum canis (de Hoog et al. 2000). This variant of M. canis is a rare cause of ringworm of horses. Invaded hairs show an ectothrix infection and fluoresce a bright greenish-yellow under Wood’s ultra-violet light. Rarely infects man or other animal species. Reported from Australia, Europe and North America.
Microsporum canis var. equinum colonies are pale buff to pale salmon with, a buff to pinkish-buff to yellow-brown reverse. Macroconidia are small, broad, irregular, spindle-shaped, 18-60 x 5-15 µm with rough thick walls and few septa. Microconidia are pyriform to clavate in shape, 3-9 x 1.5-3.5 µm, but are rarely produced.