Diagnosis is based on history, signalment, physical examination, and laboratory findings.  Because CDV infection causes nonspecific signs in a broad spectrum of organ systems it can often be confounded with other respiratory and enteric diseases.  Laboratory diagnosis is based on culture, identification of inclusion bodies via microscopy or of virus based on electron microscopy, immunofloresence or immunoperoxidase tests, ELISA or seroneutralization assays, or by the reverse transcriptase polymerase chain reaction (RT-PCR).

History
Contact with other dogs or areas where other dogs have urinated or deficated.  Presumptive diagnosis can be made from the history of clinical signs in many cases.

Signalment
Puppies 3-6 months old are more suseptible than adult dogs but adult dogs can still be infected (Greene, 2006).  Unvaccinated, or dogs whoes vaccinations are not current are far more suseptible than properly vaccinated dogs.

Physical Examination
Physical examination reveling clinical signs such as ocular or nasal discharge, respiratory distress, vomiting, diarrhea, hyperkeratosis of the foot pads or planum nasale, and neurological signs.  An ocular exam my reveal optic neuritis, chorioretinitis and retinal detachment (Nelson and Couto, 2003).  A CBC my reveal a leukopenia.  In neurological dogs a CSF tap may show the presence of mononuclear cells, and an increase protein concentration due to increased CDV antibodies and inflammation (Nelson and Couto, 2003).  Neonatally infected animals may have a marked hypoglobulinemia caused by the virus (Greene, 2006).  Thoracic radiographs may show an interstitial pneumonia pattern early in the disease or a bronchopneumonia pattern late in the disease due to secondary bacterial infections (Greene, 2006)

Culture
Isolation and identification of CDV by using samples from infected animals to inoculated dog or ferret culture cells.

Microscopy/Electron Microscopy
Distemper inclusions can be seen in early infection on stained blood films in low numbers in lymphocytes, monocytes, neutrophils and erythrocytes (Greene, 2006)
Serum Antibody Testing
High titres of IgM neutralizing antibodies can be measured and indicate either recent immunization or infection.

Immunofluorescent/Immunohistochemistry
Immunoflourescent techniques using anti-CDV antibodies can detect virus in tissue.  Early in infection a positive detection can be made from a conjunctival scraping or from respiratory or genital epithelium.  Later diagnosis can be made by detecting infected macrophages from a tracheal wash.  Late in the disease biopsy samples from the skin, and foot pads can be used because the virus persists here for up to sixty days (Nelson and Couto, 2006).  Almost any sample can be used for immunoflourescent identification of CDV but  it requires special equipment at equipped laboratories.  Immunohistochemistry detection of CDV antigen in the nasal mucosa, footpad, and haired skin can even be used to detect infection antemortem (Greene, 2006)

ELISA
ELISA has been used to detect viral antigen in serum or CSF (Greene, 2006)

RT-PCR
Reverse transcriptase polymerase chain reaction is a rapid, sensitive, and specific technique for identifying the presence of the CDV genome in an infected animal or post-mortem animal.  The downside to this test is that it is technically demanding and not offered by all laboratories. A positive test is highly specific for the disease but so far the test is not widely available (Greene, 2006).

Hemagglutination
Presence of virus can be demonstrated by viral hemagglutination of red blood cells.  This technique can be used for detection and quantification of viral infection.