Update on canine viral diseases: Distemper and parvovirus (Proceedings)
Distemper is very contagious disease of dogs that may involve the gastrointestinal, respiratory and/or neurologic systems. Although widespread vaccination of the dog population has reduced the incidence of this disease, distemper still occurs sporadically, even in vaccinated dog populations. There is some suggestion that disease may be re-emerging. Because some dogs may be show signs of respiratory disease alone, canine distemper is an important differential for kennel cough.
In vaccinated dog populations, as is typical in developed countries, distemper is most common in dogs aged 3 to 6 months of age, as maternal antibody wanes. However, in poorly vaccinated dog populations, dogs of all ages may be affected.About 50% of dogs develop neurologic signs, but development of these signs is unpredictable. Dogs developing nasal and digital hyperkeratosis usually have neurologic complications, and dogs that develop impetiginous dermatitis (a measles-like rash), considered a favorable prognostic sign, rarely have CNS disease. Once neurologic signs occur, they are generally progressive despite treatment.
Occasionally, distemper inclusions may be seen on close examination of peripheral blood smears. These can occur as blue, round, eccentrically placed inclusions within red blood cells; or gray oval structures within lymphocytes and less commonly neutrophils and monocytes. Determining the anti-CDV IgG level in CSF can also be helpful. Comparing the distemper IgG titre in CSF with that in serum and calculating the C coefficient (see below) can also be helpful to predict whether local antibody production has occurred in the CSF.
C coefficient = (CSF anti-CDV IgG/serum anti-CDV IgG) x (serum anti-ICH IgG/CSF anti-ICH IgG). C > 1 suggests local antibody production.
In dogs with acute (usually first week or two of signs - rarely more chronic) distemper, scrapings of the conjunctival sac may show inclusions within conjunctival epithelial cells. Sensitivity is increased using immunofluorescence can be on these smears. IFA techniques can also be used on cells in CSF, blood, bone marrow, and urine sediment, and on tissue samples at postmortem to confirm the diagnosis. It can also be applied to frozen sections of hyperkeratotic pads. RT-PCR assays for detection of viral nucleic acid have been shown to be very sensitive and specific for antmortem diagnosis of distemper using serum, whole blood and CSF.
Treatment is symptomatic. Some clinicians have had success treating myoclonus with procainamide (10-20 mg/kg q12h), which seems to inhibit the firing of the motor neurons. Anticonvulsants such as phenobarbital may also be helpful.
Current vaccine recommendations are to vaccinate every 3-4 weeks between 6 and 16 weeks of age. Two doses of vaccine are necessary for adequate protection in puppies presenting when they are 8-10 weeks or older. Thereafter, a booster is recommended at one year then every 3 years. All commercially-available canine CDV vaccines are MLV vaccines, as killed whole virus vaccines do not produce sufficient protection. However, a recombinant canarypox-based CDV vaccine is commercially available in the US.