This form of tropical ringworm is chiefly found in the islands of the South Pacific and in the Malay Archipelago. It is also found in Southern China and quite recently has extended to Southern India and Ceylon. Recently it has been reported from Brazil.
On account of the disease having been carried from the Tokelau Group to Samoa it is often designated tokelau.
Manson was the first to recognize the affection as due to a fungus which he demonstrated microscopically in the scales.
He was also able to transmit the disease by inoculation experiments and found that after about ten days a raised, brownish spot appeared at the site of inoculation. This spot increased in size until when about ¼ inch in diameter its central portion became detached, thus giving rise to several thin, rosette-like scales, free at the center but still attached peripherally. The fungus advances peripherally, leaving a smooth surface within. Again there is a similar process developing in the original central spot to again form a circle of scales within the older and more peripheral circle. The process is repeated until several rings of scales are formed each originating from the central focus as concentric ripples form on water from the fall of a pebble.
These scale circles are from ⅛ to ½ inch apart and give a festooned appearance to the affected skin. It was formerly supposed that the causative fungus was Aspergillus concentricus but Castellani has demonstrated that fungi of this genus, when present, are merely accidental. He has isolated in cultures what he considers the causative fungus, Endodermophyton concentricum. He treated scales for ten minutes with absolute alcohol and then placed single scales in a series of tubes of maltose bouillon.
The fungus grows between the rete malpighii and the external epidermal layers forming a network of mycelial threads, about 3 microns broad.
Another fungus cultured from tinea imbricata scales is Endodermophyton indicum. Inoculation of this organism in pure culture produced the disease.
The characteristics of the genus Endodermophyton are: The growth of a mycelial network between the rete malpighii and the superficial epidermal layers; in cultures only mycelial filaments are found; there are no conidia-bearing hyphae.
The fungus is also called Trichophyton concentricum.