by Laura Vasquez, DVM, MS, DACVIM (Neurology)
Canine distemper virus (CDV) is a highly contagious disease spread through aerosol or droplets. The virus spreads from the respiratory epithelium hematogenously to other organs and is in the nervous system by day 8. In the acute phase, CDV is shed through all body fluids. If no immune response occurs within 14 days of infection, dogs will be unable to clear the infection and develop severe signs. A 50% mortality rate is reported in dogs with neurologic signs, partly due to a high rate of symptomatic recurrence months or years after initial infection.
CDV must be on the list of differentials in dogs presenting with neurologic signs, especially if they are young, unvaccinated, recently adopted, or with unknown pasts. Recent histories of diarrhea, cough, oculonasal discharge, lethargy, or fever are red flags. Other symptoms may include vomiting, keratoconjunctiva sicca, blindness, hyperkeratosis, enamel hypoplasia, and skin pustules. Neurologic symptoms may take from 3 weeks to a few months to develop from time of infection.
Acute neurologic signs commonly localize as intracranial and include vestibular syndrome, seizures, altered mentation, and cerebellar ataxia. Myelopathy signs such as hyperesthesia, paresis and proprioceptive ataxia may also be observed. Optic neuritis is seen when vision deficits are found. If dogs survive, they commonly develop life-long myoclonus. Chronic distemper encephalitis, also called “old dog encephalitis,” can occur because of virus particles persistently in the CNS causing inflammation and neurodegeneration.
Diagnosis is primarily based on PCR on whole blood, conjunctival swabs, or urine. PCR testing is 30% more sensitive that IFA. IFA can be performed on conjunctival, tonsillar and genital swabs as well as urine. IFA, however, is only positive for the first three weeks of infection. Antibody titers only determine level of protection. IgG titers are non-specific and may indicate current infection, previous infection or post-vaccination. IgM titers, likewise, can be present for up to several weeks post infection but may also be present post vaccination. Other supportive testing includes identifying severe leukopenia on a CBC or pneumonia on thoracic radiographs. In neurologic dogs, antibody titers can be checked on CSF.
There is no specific treatment for CDV. Most therapy aims are supportive. Respiratory care may include IV fluid therapy, nebulization, oxygen therapy, and antibiotics for secondary infections. For neurologic symptoms, anti-inflammatory steroids and anti-seizure medications are used. There is no known cure for myoclonus although several medications have been described to lessen the severity, including valproic acid, mexiletine, and botulinum toxin. Care must be taken when considering some of these treatments as side effects can be severe.
Given how contagious CDV is, distemper patients must be isolated from other patients or housemates and rigorous PPE protocols employed. The virus can be shed for up to 2 weeks in the acute phase and much longer in neurologic dogs.