Many strains and varieties of T. rubrum have been described and opinion differs between mycologists as to the exact validity of many of these. For practical purposes two types may be distinguished: T. rubrum downy type and T. rubrum granular type.
Microscopically, the downy type is characterized by the production of scanty to moderate numbers of slender clavate microconidia and no macroconidia.
Typical slender clavate microconidia of T. rubrum downy type.
Microscopically, the granular type is characterized by the production of moderate to abundant numbers of clavate to pyriform microconidia and moderate to abundant numbers of thin-walled, cigar-shaped macroconidia. The macroconidia may or may not have terminal appendages.
Typical cigar shaped macroconidia of T. rubrum granular type.
Superimposed on these two microscopic morphologies are culture characteristics such as pigment production and surface typography.
Once again, Kaminski's dermatophyte identification scheme of 6 different media is invaluable in the identification and differentiation of the varieties of T. rubrum [see description for Trichophyton sp.]. . However, it must be stressed that it is the combination of characters (macroscopic and/or microscopic) from each medium that leads to a precise identification. Remember, no one single test is infallible. T. rubrum is a very variable organism and many characteristics either overlap or are inconsistent.
Trichophyton rubrum is an anthropophilic dermatophyte. The downy strain has become the most widely distributed dermatophyte of man. It frequently causes chronic infections of skin, nails and rarely scalp. The granular strain is a frequent cause of tinea corporis in South East Asia and in Aborigines living in the Northern Territory of Australia. However, since the Vietman War, it has been spread throughout the world, especially to those countries with returning troops or to those receiving refugees, where it has often been described as a new species.
The granular strain represents the parent strain of the downy type; the later evolved by establishing a niche in feet (tinea pedis) when the former was imported into Europe around the turn of the century. It should be stressed that intermediate strains between the two types do occur and that many culture and morphological characteristics overlap.
Rebell, G., and D. Taplin. 1970. The Dermatophytes. 2nd. revised edition. University of Miami Press, Coral Gables, Florida. USA.
Rippon, J.W. 1988. Medical Mycology. 3rd Edition. W.B. Saunders Co., Philadelphia, USA.