The epidermis soon undergoes proliferation, and becomes covered with crusts, which adhere to the hairs, gluing them together, and finally causing them to be shed, leaving bare patches the size of a shilling or a florin. The lesion extends in an ever-widening circle, until it attains, perhaps, the dimensions of a five-shilling piece or more.
The affected hairs break off level with the free surface of the skin, rendering the patches more apparent. White hairs are less affected, and some always remain projecting above the crusts, causing the patches, when on a white skin, to retain a certain amount of covering.
At first the crust is closely adherent to the skin, and, if forcibly detached, exposes the dermis, which is swollen and bleeding. Gradually the centre becomes detached, whilst the periphery, representing a more recent lesion, continues to adhere. The crusts then rest on a thin layer of pus, and the dermis, whilst still inflamed, is punctuated with numerous minute apertures, representing the roots of the detached hairs. The pus lifts the crust; gradually it dries up and forms superposed layers, which may or may not prove adherent to the parasitic products, and which form a new crust. The latter is purely inflammatory in character, and is left after the fall of the first. It no longer contains any parasites, at least within its deeper layers.
This second crust dries up in its turn, falls away or breaks up, leaving a smooth spot, over which the hairs again appear, either at once, or at least after a short period of desquamation.
The disease is accompanied by well-marked pruritus, more marked at the commencement and towards the end than during the intermediate period, but, nevertheless, much less acute than in scabies.
Ringworm may undergo spontaneous cure in from six weeks to three months. It is more obstinate in calves than in adults, and the want of grooming tends to increase its duration. If it extends over a large part of the body the disease may seriously affect the animal’s health, and the cases described by Macorps prove that where pruritus is violent it seriously affects the animal’s general condition.
The patches may finally become confluent and the disease extend over the whole of the neck, shoulder and back, or it may attack the entire body, leaving it practically hairless.
When the hair has been shed, the crusts and discharge seen at the outset disappear, and the bare spots are covered with a scaly coating, due to excessive production of epidermic cells.
According to Gerlach, such crusts are thicker where the skin is black, and often exhibit a greyish-white, fibrous, starchy appearance. On unpigmented portions of the skin, which are usually thinner, the crust is less dense, and is slightly yellowish. Gerlach failed to reinoculate the bare patches of skin left after a primary eruption of ringworm. Where the hair had again grown an eruption could again be produced, though it was usually of a feeble character.
In a second form of the disease, the spots may be of very small dimensions. The hair falls away, but there is no exudative inflammation, and no formation of crusts. In this second form the animals simply show characteristic circular bare spots about the head, neck, or shoulders.