What they are:
Nodules are referred to as simply nodules or as pressure bumps or sweat bumps. In medical terms they are referred to as eosinophilic granulomas with collagen degeneration, nodular collagenolytic granulomas or nodular necrobiosis. They are distinct, firm nodules about the size of a R2 coin, usually found on the neck, withers or back. Several small nodules can cluster together in what appears to be a larger lump. The skin above the nodule appears normal with no hair loss, and the nodule does not contain any infectious material. These bumps can occur in horses of any age, breed or gender. They are usually not painful or itchy unless they are found in an area in which they get rubbed. Over time these nodules can calcify, which makes them feel harder.
The nodules are caused by the breakdown of collagen, a protein found in the middle layer of the skin. The mechanism by which these nodules form has not been fully understood, but the precipitating event is believed to be hypersensitivity when the immune system overreacts to the presence of an allergen and creates a mass to contain it. Allergic reactions to insect bites are therefore thought to be the most common cause of these nodules, but injections and minor scratches from trauma can be the cause in some cases.
Nodules are harmless unless they cause the horse discomfort because they are in an area that gets rubbed, or because they continue to grow in size. If treatment is felt to be necessary, your vet can inject steroids into the nodule to shrink it. Sometimes a course of these injections is required and in other cases, if the nodule has become calcified, it may require surgical removal.