Introduction
Canine distemper virus (CDV) infects and causes disease in domestic dogs and many wild carnivores, including coyotes, ferrets, foxes, lions, mink, raccoons, and skunks. Although sustained vaccination of domestic dog populations has greatly reduced the incidence of canine distemper, the disease is still prevalent worldwide and now occurs in sporadic outbreaks. Young puppies between 3 and 6 months of age are more susceptible to infection and undergo more severe disease than adult dogs. However, non-vaccinated older dogs are also highly susceptible to infection and disease.
The ante-mortem diagnosis of canine distemper based on clinical signs is challenging because of the many other diseases that mimic the clinic signs. Laboratory testing is required to confirm a suspicion of canine distemper. Submission of the optimal samples assures that the laboratory can produce reliable results for CDV testing.
What samples should I submit to the laboratory?
- EDTA-blood? Yes! The distemper virus has a high tropism for white blood cells. Therefore, buffy coat cells are a very good sample for CDV diagnosis. Based on our experience, buffy coat cells separated from EDTA-blood are often an excellent sample for distemper diagnosis. The submission of EDTA-blood is strongly recommended.
- Conjunctival smears? No. Most textbooks recommend the submission of conjunctival smears for fluorescent antibody (FA) staining. Our experience over the years has been that submitted conjunctival smears are often full of mucus but contain few or no cells. Therefore, conjunctival smears are not recommended. However, nasal swabs may be useful for PCR testing and virus isolation (see table of available tests on next page).
- Serum? No. Detection of CDV antibodies in serum is not useful for diagnosis, except in unvaccinated dogs older than 4 months. Even in this group, paired serum samples need to be submitted – one collected at presentation and one collected 2-3 weeks later. A four-fold or greater rise in antibody titer between the “acute” and “convalescent” serum samples confirms the diagnosis of CDV. A positive antibody titer on a single sample from an unvaccinated dog confirms CDV infection but does not determine when it occurred. Serum antibody titers are also useful for ascertaining the vaccination status of a dog.
- Cerebrospinal fluid? Yes! Antibody molecules are normally too large to penetrate the blood-brain barrier so antibodies resulting from vaccination or non-CNS infection will not be detectable in CSF. Positive antibody titers in CSF can confirm the diagnosis of cerebral distemper. The virus can also be detected in CSF using PCR. If cerebral distemper is suspected, always consider submitting a CSF sample along with serum (clotted blood) for distemper serology (serum neutralization test). However, there are two caveats for interpreting antibody titers in CSF: 1) If there is extensive inflammation in the animal linked to other causes, circulating antibodies from vaccination or subclinical infection may be present in the CSF and 2) the CSF tap needs to be clean without contaminating peripheral blood.
When should I collect samples?
For antigen detection (FA, PCR, or virus isolation), the optimum time for sample collection is during the height of the febrile response, when nasal discharge is still serous or sero-mucoid. This is also the best time to collect the “acute” sample if paired serology is envisaged. Convalescent serum samples should be collected 2-3 weeks following the acute sample and CSF samples can be collected whenever CNS signs are observed.
What tests should I request?
The choice of test will depend on the sample submitted, the sensitivity of the test, the condition of the sample, and the desired turn-around time. The table above summarizes the tests available, the costs, and turn-around times at the Athens Veterinary Diagnostic Laboratory.
What is the take-home message?
In our experience, buffy coat cells are the most rewarding specimen for ante-mortem diagnosis of canine distemper. Submit about 6 ml of EDTA-blood (purple top tube) from suspect distemper dogs. If cerebral distemper is suspected, a CSF sample submitted along with serum (red top tube/clotted blood) can lead to a definitive diagnosis.
