Orf, an exanthemous disease, is caused by a parapox virus primarily occurring in sheep and goats. It has many other names including thistle disease, scabby mouth, and sheep pox. It can infect humans as well. It is a zoonotic disease meaning that humans contract the disease through direct contact with infected animals.

Generally there are no systemic symptoms. The finger, hand, arm, face, and genitals can be infected locations. Due to this it is important to practice proper hygiene and wear gloves around infected animals. The condition is rare and hard to diagnose. Normally the disease is benign and self-limiting; however, it can progress quickly and even become life-threatening to an immune-compromised host. Topical cidofovir has successfully been used in some patients. The eye can take serious damage if infected with Orf.

Spread through formites and direct contact the disease has been recorded since the late 19th century in most sheep and goat raising areas. Thistles pose a major threat as scratches from them can cause infection. Symptoms include papules and pustules on the lips and muzzle. On rare occasion the infection is extensive and persistent when an animal does not produce an immune response.

There is a live virus vaccine that is administered to ewes at the age of two months but only where there is an outbreak. This vaccine can cause the disease in humans. Lesions amongst sheep and goat usually appear on the hairline and elsewhere on the lips and muzzle. In rare cases young lambs will develop lesions on the tongue, gums, roof of the mouth and the esophagus.