The colony grows rapidly on Sabouraud agar with a central umbo, finely granular surface and fringed subsurface border. A brilliant yellow pigment is produced in the medium. Microscopically, microconidia are numerous, macroconidia are often formed and intermediate forms are prominent. No spiral hyphae are produced. The optimum temperature is 35° C. and growth is inhibited by pH 4.0.

Incidence of Infection
Strains of Trichophyton mentagrophytes were isolated from 51 (44.7%) of 114 hedgehogs. These "hedgehog" strains were readily recognisable by their rapid growth, finely granular surface with a central elevation and fringed sub-surface border (fig. 2). All produced a brilliant yellow pigment which diffused into the medium. They were obtained most frequently from the nose and face but in many animals all areas of the body yielded positive cultures.
There was considerable evidence that the extent of infection was associated with density of the ectoparasitic mite Caparinia tripilis. The part played by mites in the transmission of this fungus is under investigation in this laboratory. Preliminary results indicate:-
(1) Heavy mite infestation is associated with widespread fungous infection. Animals with few or no mites can rarely be shown to carry a Trichophyton. Mites were found in 24 (60%) of the 40 animals specifically investigated for their presence. Brockie (1958) recorded mite infestation in 41.6% of 180 animals, but this lower incidence may be due to examination with the naked eye, rather than with the microscope. The presence of both mites and dermatophyte was demonstrated in 19 (47.5%) of the animals, a few (5%) apparently carried only fungus, and in others (12.5%) only mites were encountered. It is at present impossible to determine which is the primary invader.
(2) Isolates of T. mentagrophytes identical with those recovered from hedgehog inocula have been recovered from mites treated in the following ways:- removed from the host, washed three times in saline; washed in 1/1000 phenyl mercuric nitrate, then rinsed in two changes of distilled water. In each case almost every acarine removed from a dermatophyte-carrying host yielded T. mentagrophytes.
(3) The dermatophyte has been grown from acarine droppings after they have been separated from the mite.
(4) Washed mites placed on chloramphenicol-cycloheximide medium have been examined microscopically in a KOH wet preparation after 48 hours incubation. Hyphal structures have been observed both within the acarine and emerging from various levels of the body, although no such structures were visible before incubation.
These findings suggest that the intra-dermal fungal structures are not destroyed by the digestive processes of the mite and it seems possible that transmission of Caparinia tripilis to a new host is frequently accompanied by transmission of T. mentagrophytes.
Experimental transmission of fungous infection by means of the mites presents certain difficulties. All attempts to rear the mite in an artificial environment have been unsuccessful. Further work on these problems is contemplated. It should be noted that during its journey from Britain to New Zealand, the hedgehog lost its flea (Archaeopsylla erinacei) so that this ectoparasite, so conspicuous on animals living in Europe, is absent from hedgehogs in this country.

Direct Examinations
Scrapings from a limited number of animals exhibiting obvious scaling were examined in 20% KOH. These lesions were occasionally associated with a fungus alone, but mites and bacterial pathogens have also been isolated from them. (Smith, unpublished). Positive animals were usually heavily infected and in most cases the hyphae were completely fragmented into arthrospores of varying size and shape (figs 10, 11). Hair invasion was rare.

IN VITRO CHARACTERISTICS OF HEDGEHOG ISOLATES OF T. MENTAGROPHYTES
In table 2 [not provided] a number of characteristics of hedgehog isolates is compared with those of granular and downy varieties of Trichophyton mentagrophytes.

Morphology
In Sabouraud's dextrose agar slide culture (figs. 3, 4, 5) hedgehog isolates produce numerous elongated microconidia attached along the sides of hyphae as well as in clumps. Macroconidia are somewhat irregular in shape and size and have smooth thin walls and 2 to 6 septa. Spores intermediate between macro- and microconidia are frequently observed. Spirals are absent. Hedgehog strains can be distinguished microscopically from other granular isolates by the irregular macroconidia, elongated microconidia, abundance of intermediate spore forms, and absence of spirals.

Nutritional Requirements
None of the strains of T. mentagrophytes exhibited an absolute requirement for the vitamins tested. Hedgehog isolates therefore appear to resemble the other varieties of T. mentagrophytes. Pigment production of hedgehog strains was stimulated by the thiamine supplement. The granular and downy strains of T. mentagrophytes failed to grow on the basal medium containing ammonium nitrate with and without its histidine supplement. Hedgehog strains showed a limited growth on these media but the colony was open and thread-like and did not develop a powdery surface.

Temperature Range
Table 3 lists the size of the colonies of the different strains after eight days incubation at different temperatures. None of the varieties was able to grow at 10° C. and all grew well over a range of 27° C. to 35° C. Hedgehog isolates appeared to have a higher optimal temperature than did the other varieties since the largest colonies of all six strains were produced at 35° C. Only two of the strains were able to grow at 40° C. Ability to produce small colonies after eight days incubation at 40° C. was shown by three other organisms, isolated from spontaneous human infections contracted from hedgehogs, but not included in the test series. Plates incubated at 40° C. were transferred after eight days to a 27° C. incubator, to test the viability of the inoculum. Table 3 includes the results of this test. It will be seen that all hedgehog isolates, three granular and one downy strain survived a temperature of 40° C. for eight days. All strains incubated at 10° C. for eight days remained viable and produced normal colonies when transferred to the 27° C. incubator.

pH Tolerance
All hedgehog strains grew very poorly on media of pH 4.0. The growth of both granular and downy strains was less inhibited by the acid environment (fig. 9) although the colonies were not as large as those on less acid media. All strains grew well at pH 9.6. Microscopically, the hedgehog strains grown at pH 4.0 consisted almost entirely of chlamydospores.

Growth in Liquid Media
There were marked differences in the morphological characters of hedgehog, granular and downy strains when grown in liquid media. Both hedgehog and downy strains grew poorly in the form of pellets at the bottom of the flasks. Downy strains produced rather larger and more fluffy pellets than did those of the hedgehog isolates. Microscopically the growth of both varieties consisted of distorted hyphae often fragmented into chlamydospores of irregular shapes. Hedgehog isolates produced a brilliant yellow pigment which could be observed after four days incubation and which reached maximal levels after seven days. Downy strains produced very little pigment of a dirty buff colour.
Granular strains of T. mentagrophytes grew far more luxuriantly in the liquid medium both as a subsurface mass and a surface pellicle. Microscopically the hyphae appeared normal with little obvious chlamydospore production. All strains rendered the media alkaline, the final pH ranging from 7.9 to 8.3.

Growth on Sterile Soil Medium
All strains grew well on the sterile soil medium but showed considerable variation in both macroscopic and microscopic characters. After eight days incubation, the colonies of the hedgehog strains had an average diameter of 15 mm. There was a smooth central umbo from which radiated mainly sub-surface hyphae. These at intervals produced aerial branches. Microscopically, microconidia were only produced by these aerial branches and were far from numerous. No macroconidia or spirals were produced.
Granular strains produced somewhat larger colonies with an average diameter of 20 mm. They also had a central knob with a granular surface and radiating subsurface hyphae, but these at intervals produced on the surface pseudo-cleistothecia packed with microconidia. Spiral hyphae were very numerous in these structures.
Downy strains produced colonies of 15 mm. average diameter consisting of a thick fluffy aerial mycelium in which many pseudo-cleistothecia were embedded. Spiral hyphae were prominent at the periphery of these structures. Figures 6 to 8 illustrate the macroscopic character of the different varieties.

Growth on Keratin Medium
Hedgehog quills provided the best substrate for all three varieties, while hedgehog, guinea-pig and cow hair supported a somewhat more luxuriant growth than did dog or human hair. Microscopically the variants had characters very similar to those produced on sterile soil, both granular and downy strains producing pseudo-cleistothecia when grown on hedgehog quills. None of the strains showed any growth on the control plates of washed sterile agar.

Animal Pathogenicity Tests
The result of experimental infection of guinea-pigs with the granular and downy strains was identical with that described by Georg (1954). The hedgehog isolate appeared as chains of arthrospores in the skin and outside the hair. Larger irregular spores inside the hair were also present. The infection cleared spontaneously after about three weeks.
Georg (1954) has shown that downy strains will revert to granular after repeated animal transfer. A series of five repeated infections of guinea-pigs did not bring about any obvious change in the morphological characters of the hedgehog variety.