In the past, distemper was a major cause of neurological disease in dogs. Although widespread use of effective vaccines has markedly reduced the incidence, outbreaks still occur and neurological manifestations are by no means rare.
The signs associated with infection vary with the viral strain and age and immune response of the dog. Dogs that develop an early, effective immune response eliminate the virus and recover with mild or no clinical signs. This happens 25 to 75% of the time. Dogs that are unable to mount an immune response suffer severe systemic illness including acute encephalitis (inflammation of the brain), usually leading to death within about 3 weeks of exposure. Dogs with a delayed immunologic response don't develop acute illness but may develop a chronic, persistent infection, characterized by chronic encephalitis.
Two common clinical forms of distemper encephalitis have been recognized; acute encephalitis in young dogs and chronic encephalitis in mature dogs.
Immature dogs with distemper encephalitis typically suffer a rapid onset of systemic illness characterized by nasal discharge, cough, vomiting and diarrhea. Neurologic dysfunction can occur during or after the systemic illness. Seizures, especially "chewing-gum fits", a type of focal seizure characterized by rhythmic contractions of the masticatory muscles, are common.
Mature dogs are more likely to develop chronic encephalitis. Many of these dogs have an adequate vaccination history. Signs of systemic illness are often absent or transient. These patients often have slowly progressive gait deficits or incoordination. Seizures can occur but are uncommon in this form of distemper.
In puppies with acute distemper infection, diagnosis can often be based on the typical systemic and neurological signs. Diagnosis of distemper encephalomyelitis in mature dogs can be difficult because concurrent systemic signs may be absent and laboratory tests are often non-specific. Fluorescent staining of a swab taken from the eyelid tissue to detect distemper virus antigen is positive in about 50% of patients. Blood tests to detect antibodies to canine distemper virus are often performed but results can be difficult to interpret. Immature dogs with acute distemper encephalitis often do not develop serum antibodies. Mature dogs with chronic distemper encephalomyelitis usually have detectable serum antibodies, but since many of these dogs have been vaccinated the diagnostic usefulness of these results is limited.
Analysis of spinal fluid is probably the most helpful diagnostic procedure. The presence of antibody to canine distemper virus in the spinal fluid is probably the most reliable indicator of infection. However, false positive and false negative results can occur. Computed tomography and magnetic resonance imaging are also useful.
Dogs with distemper encephalomyelitis will occasionally recover, so treatment for at least one to two weeks should be attempted. The prognosis is poor for puppies with severe systemic and neurological signs. Since there is no specific treatment, supportive care and symptomatic treatment are important. Antibiotics are indicated because of the immunosuppressive nature of the virus. Anti-seizure medication should be administered in patients with seizures and may be necessary even after recovery from other signs. Neurologic signs will often improve, at least temporarily, with corticosteroid administration.
Tipold A, Vandevelde M, Jaggy A: Neurological manifestations of canine distemper infection. J Small Anim Pract 33:466-470, 1992
Thomas WB, Sorjonen DC, Steiss JE: A retrospective evaluation of 38 cases of canine distemper encephalomyelitis. J Amer Anim Hosp Assoc 29:23-28, 1993
WB Thomas DVM