is a highly fatal form of BVD that may be acute or chronic and is seen infrequently in persistently infected cattle. Mucosal disease is induced when persistently infected cattle become superinfected with cytopathic BVDV. The origin of the cytopathic BVDV is usually internal, resulting from a mutation of the resident persistent, noncytopathic BVDV. In those cases, the cytopathic virus is antigenically similar to the resident noncytopathic virus. External origins for cytopathic BVDV include other cattle and modified live virus vaccines. Cattle that develop mucosal disease due to exposure to a cytopathic virus of external origin often produce antiviral antibody. Prevalence of persistent infection usually is low, and many persistently infected cattle do not develop mucosal disease, regardless of exposure. Acute mucosal disease is characterized by fever, leukopenia, dysenteric diarrhea, inappetence, dehydration, erosive lesions of the nares and mouth, and death within a few days of onset. At necropsy, erosions and ulcerations may be found throughout the GI tract. The mucosa over Peyers patches may be hemorrhagic and necrotic. Extensive necrosis of lymphoid tissues, especially gut-associated lymphoid tissue, is seen on microscopic examination. Clinical signs of chronic mucosal disease may last several weeks to months and are less severe than those of acute mucosal disease. Intermittent diarrhea and gradual wasting are common. Coronitis and eruptive lesions on the skin of the interdigital cleft cause lameness in some cattle. Lesions found at necropsy are less pronounced than, but similar to, those seen in acute mucosal disease. Often, the only gross lesions seen are focal ulcerations in the mucosa of the cecum, proximal colon, or rectum, and the mucosa over, Peyers patches of the small intestine may appear sunken.