Microsporum spp

From Dog
Mycelia of Microsporum canis under lght microscopy

Microsporum spp are a saphrophytic zoonotic fungus which commonly causes ringworm in dogs.

These dermatophytes (skin-loving fungi) commonly cause limited disease, that is more annoyance than pathology, in dogs.

Species which are pathogenic to dogs include:

  • Microsporum canis[1]
  • Microsporum gypseum
  • Microsporum persicolor[2] (Hunting dogs over-represented)

Concomitant infections with other fungi and parasites such as Demodex spp are not uncommon.

Infection is usually by direct contact, although fomites, mechanical transmission and aerosol are other means of transmission. Microsporum persicolor is a zoophilic (associated primarily with animals) fungus which may also be geophilic (reservoir in soil and may infect humans or animals)[3]. It is a natural resident and occasional pathogen in small rodents such as voles, hamsters, and field mice[4].

These dermatophytes reside entirely on the skin surface, feeding on stratum corneal cells. However, the presence of the fungus and its metabolic products usually induces an allergic and inflammatory eczematous response in the host. The type and severity of the host response is often related to the species and strain of dermatophyte causing the infection. Secondly, the dermatophytes are the only fungi that have evolved a dependency on human or animal infection for the survival and dissemination of their species[5]. Proteolytic enzymes have been postulated to be key factors involved in the invasion of the stratum corneum and keratinized epidermal structures. Among these proteases, the secreted subtilisin protease Sub3 was found to be required for adherence of M. canis arthroconidia to feline epidermis[6].

These properties of dermatophytes distinguish them from saprophytic and opportunistic dematiaceous fungi which usually gain entry to the dermis and subcutaneous tissue via wounds, grass seed-penetrating injuries, etc., and result in phaeohyphomycosis, leading to subcutaneous and systemic diseases and often death.

Dermatophytes must also be distinguished from other zoophilic organisms such as yeast, especially Malassezia spp, which is also common in dogs and reside superficially on the skin. Yeasts cause more generalised dermatological disease (otitis externa, seborrhea, paronychia), often covering the entire skin of the host, whereas dermatophytes are often localised to the head and extremities of the limbs.

Dermatophytes normally reside in wood, soil and plant debris and are found world-wide. There appears to be no seasonal regularity to outbreaks in animals and have been reported year round[7]. The first infection step consists of adherence of arthroconidia to the stratum corneum. After germination of the arthroconidia, dermatophytes invade keratinised structures that have to be digested into short peptides and amino acids to be assimilated.

References

  1. Sakae H et al (2011) Analysis of 25 cases of microsporum canis infection encountered at a dermatology clinic in Kumamoto during a recent 3-year period. Med Mycol J 52(2):139-144
  2. Muller A et al (2011) Dermatophytosis due to Microsporum persicolor: a retrospective study of 16 cases. Can Vet J 52(4):385-388
  3. Scott DW et al (1995) Muller and Kirk’s Small Animal Dermatology. 5th ed. Philadelphia: WB Saunders. pp:332–350
  4. Contet-Audonneau N & Percebois G (1986) Microsporum persicolor : isolement du sol. Bull Soc Fr Mycol Med 15:193–196
  5. Sparkes AH, Robinson A, MacKay AD and Shaw SE (2001) A study of the efficacy of topical and systemic therapy for the treatment of feline Microsporum canis infection. Journal of Veterinary Medicine and Surgery 2:135
  6. Baldo A et al (2012) Inhibition of the keratinolytic subtilisin protease Sub3 from Microsporum canis by its propeptide (proSub3) and evaluation of the capacity of proSub3 to inhibit fungal adherence to feline epidermis. Vet Microbiol May 8
  7. August, JR (2006) Consultations in feline internal medicine. Vol. 5 Elsevier, Saunders