Canine distemper is usually an acute, febrile disease, especially of young dogs, although older unprotected dogs are also susceptible. The first clinical manifestation of distemper is a diphasic febrile response. The first response may be overlooked, but the second generally occurs 2 - 3 days later in conjunction with other clinical signs, which initially include congested conjunctiva and nasal mucosa with subsequent serous to mucopurulent discharges. Pneumonia, depression, anorexia, vomiting, and diarrhea usually follow. Neurologic disturbances, such as neuromuscular tics, "chewing gum" seizures, and paresis are frequent sequelae in dogs that recover from acute disease.
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Hyperkeratosis of the nose and digital pads ("hard pad") develops in some cases. Pustular dermatitis may be seen affecting the abdomen of puppies.
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Gross necropsy lesions characteristic of pneumonia and enteritis may be present. Thymic atrophy may be noted in young dogs. Microscopic lesions are widespread in visceral organs and the brain and characteristic viral inclusion bodies are commonly found in brain, lung, stomach, and urinary bladder.
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Dogs that recover may years later develop what has been called "old dog encephalitis" as a result of a persistent infection. This manifestation is usually recurrent, with a few to several episodes of neurological manifestations within weeks to months that usually end with death of the dog.