Pemphigus foliaceus is an autoimmune disease. In other words, it is caused by the animal's own immune system. In this case, the body recognizes a glycoprotein, desmoglein I, found in the skin as foreign and mounts an antibody response against it. This glycoprotein is involved in the attachment of one skin cell to another within the outer layer of the skin (epidermis).

The result of the antibody binding to desmoglein I is a loss of attachment leading to a cleft within the epidermis. This cleft fills with fluid and white blood cells called neutrophils, which is the pustule that we see. Cells that are losing their ability to attach to one another round up and float into the middle of the fluid. These large, round cells are called acanthocytes. The pustules are very fragile and rupture easily leaving a lesion in the epidermis that is covered by a crust.

This disease tends to begin on the bridge of the nose and the ears and progress to the feet, including the footpads, and legs. Eventually the skin of the trunk and the rest of the body are affected. Often the lesions will cross from the skin on the bridge of the nose to the nose itself causing a loss of the normal cobblestone appearance and loss of pigmentation. Skin lesions that involve the nose are seen almost exclusively with autoimmune skin diseases. Animals with pemphigus foliaceus may have a fever and refuse to eat.

Other diseases that may be confused with pemphigus foliaceus are those that also have pustules as their major lesion. These include:

Pyoderma (bacterial infection of the skin)

Pemphigus erythematosus also begins on the head but fails to progress any further. Animals with pemphigus erythematosus are not usually as physically sick as those with pemphigus foliaceus.

Pemphigus vulgaris and pemphigus vegetans are also part of the pemphigus complex and are quite rare. The difference between the various pemphigus diseases is the depth of the ulcers. Pemphigus foliaceus has the most superficial ulcers. Pemphigus vulgaris has the deepest ulcers.

Discoid lupus erythematosus is an autoimmune disease that is often limited to the nose. Rarely, lesions may be seen on the ears, lips, and skin of the groin. This is a much less severe disease than pemphigus foliaceus.

Demodicosis, a skin problem caused by infestation with Demodex mites, often begins on the head and spreads to the legs and trunk. Secondary infection with bacteria can lead to pustules and make this disease look like pemphigus. There is no involvement of the nose with this disease.

Dermatophytosis (ringworm) is a fungal disease of the skin that can present as a pustular disease.