Sore mouth is caused by a virus. It commonly affects nursing or recently weaned lambs, but sheep of all ages can be affected. The disease is characterized by the formation of lesions, which progress into thick crust or scabs, on the lips. Often, an entire group of lambs will become infected. However, mortality is low.

Sore mouth is spread by direct contact. The incubation period depends on the amount of virus present and varies from 8 to 10 days. Lesions begin as small red spots on the lips, and scabs develop two or three days later. Occasionally, the nostrils, eyelids, and mouth also are involved. When the scabs are prominent on the lips, lambs may eat less because eating is painful. Loss of body weight can be noticeable. Occasionally, a lamb might die from starvation or secondary pneumonia induced by sore mouth. Nursing lambs may spread the infection to the teats of the ewes. When this occurs, it is not unlikely for mastitis to follow.

Use extreme care and sanitation in treating infected sheep because the virus can affect humans. Lesions are sometimes found on the hands and faces of people working with infected sheep.

Treating sheep with sore mouth has not proven very effective. However, applying a topical antibiotic ointment may help reduce the potential for secondary infection. Commercially available vaccines also can be used on infected premises or in feedlots to prevent sore mouth. Apply vaccines as directed on the label. Generally, the application entails creating a small lesion (scratch) on the inside of the thigh, and applying the vaccine to the lesion. Vaccination of lambs at tail-docking time is a good management procedure. Vaccinate show flocks at least one month before the beginning of the show season.