Causation. The disease is due to the growth of germs on the skin of animals which are in a receptive condition. The parasite thereafter develops in the hairs, the hair follicles and the epidermis, causing lesions which vary according to the species.
At the present time three groups of ringworm are recognised as occurring on animals, Trichophytic, Microsporous, and Favus.
Clinically the trichophytons are divided into the T. ectothrix, which lives outside the hair, and both outside and inside the hair follicles; T. endothrix, which penetrates the thickness of the hair itself, rendering it brittle and easily destroyed; and T. endo-ectothrix, which both surrounds and invades the hair.
The study of artificial growths of these fungi will probably afford valuable information on the above points. In the ox the particular parasite is invariably the Trichophyton mentagrophytes, whatever may be the characteristics of the clinical lesion.
In France ringworm is particularly common in Auvergne and Normandy, where hygienic precautions are neglected, but cases may also be found throughout the country.
After affecting cows throughout the winter, the disease often disappears in the spring. Throughout the winter cows are kept in dark and often filthy sheds, where the parasite propagates rapidly, whilst in spring they are sent to grass, where the conditions are inimical to contagion.
Diagnosis. The diagnosis of ringworm seldom presents any difficulty. The appearance of the lesions (Fig. 266), their particular tendency to spread and contagious character, facilitate the diagnosis. They entirely differ from those of eczema or mange, and should any doubt exist, the slightest microscopic examination is sufficient to dissipate it. In ringworm in the ox the base of the hairs is covered with enormous numbers of spore chains, which do not extend into the depths.
To detect the parasites it is best to shave off a thin fragment of skin from the periphery of the patch, place it on a slide with a drop or two of 30 per cent. caustic potash solution, and heat it for a few seconds almost to boiling point. By applying a cover glass with firm pressure the epidermal cells are spread out and the parasites can be seen, especially around the roots of the hairs. Sometimes they form little dirty-yellow masses, consisting almost exclusively of spores.
When ringworm has attacked the entire surface of the body, it is much more difficult to distinguish from sebaceous eczema, and a microscopic examination or experimental inoculation may become necessary. Inoculation with ringworm material always succeeds with calves.
Prognosis. Ringworm is not dangerous in itself. In time it may disappear spontaneously, but when it extends over the whole body it may be dangerous. Owing to their bare condition the animals easily catch cold, while the epidermic proliferation is very great, and makes great demands on the animal’s bodily powers. Under such circumstances it is better to slaughter early.