The usefulness of single serologic samples for diagnosis of canine distemper is limited by viral immunosuppression and interference due to maternal or vaccine-induced antibodies. A single high IgM titer or a 3 to 4-fold increase in the IgG titer in acute and convalescent sera (2–4 weeks apart) may help to confirm distemper infection. Ideally, paired sera should be submitted together and assayed at the same time. Acute serum can be frozen and submitted to the laboratory along with the convalescent sample.

The test code for canine distemper IgG and IgM titers is T555.

During the acute stages of infection, fluorescent antibody (FA) testing may detect distemper viral antigen in cells from conjunctiva, blood, respiratory tract, or urinary bladder. These tests are very specific (if the test is positive, the dog has canine distemper virus), but lack sensitivity (many false negatives). On rare occasions, recent vaccination may cause a false positive result.

Submit air-dried, unstained smears from conjunctival scrapings, buffy coat preparations, respiratory tract secretions (including transtracheal wash or bronchioalveolar lavage fluid), or urine sediment. The test code for Distemper antigen IFA testing is S16250.

A recent study found that detection of canine distemper viral antigen in 1 cm skin biopsies from normal haired skin of the dorsal neck is a reliable (sensitive and specific) ante-mortem test for distemper infection. The effect of vaccination on this test is uncertain and it is probably less reliable in the advanced neurologic form of the disease.

The test code for immunohistochemistry is 16141 (sent to Cornell). Submit biopsies in formalin.

CSF analysis from dogs with distemper may show increased levels of protein and cell count (predominantly lymphocytes), or may be normal. However, these CSF changes are not specific for distemper virus infection. Detection of anti-distemper IgG in CSF, in the absence of blood contamination, is considered confirmatory evidence of distemper infection. If the CSF is contaminated with peripheral blood or there is significant blood-brain barrier disruption, CSF antibody may be artifactually increased. To confirm CNS antibody production, the ratio of CSF to serum distemper antibody titer can be compared to the ratio of CSF to serum parvovirus antibody titer. If the distemper antibody in the CSF is secondary to blood contamination or blood-brain barrier disruption, the ratio should be similar to that for parvovirus antibody, whereas the ratio will be greater if there is local CNS production of distemper antibody. Regardless, a negative CSF distemper titer does not exclude a diagnosis of distemper.

The test code for CSF analysis is "CSF" (use the Pathology/Cytology Test Request Form) and requires 0.5 mL of CSF fluid in both a RTT and LTT. The test code for distemper IgG measurement in CSF is 573 for East Coast and Test-Express clients, and 5273 for West Coast clients.

Limited clinical studies have shown PCR detection of distemper viral nucleic acid in urine sediment to be a specific and sensitive (80-90%) test for distemper. Other samples that can be tested include nasal epithelial swabs, BAL, preps, serum, buffy coat preps, and CSF. Recent vaccination (within 3 weeks) may cause false-positive results.

The test code for distemper PCR testing is 16501. Submit urine sediment resuspended in 1-2 mL of saline, blood in a LTT, CSF or other fluids in either RTT or LTT. If submitting swabs, use a Culturette-type swab. If using Copan swabs, do not insert the swab back i nto the transport gel.

Ante-mortem diagnosis of canine distemper can be elusive. Histopathologic lesions consistent with canine distemper virus infection can be found in the respiratory tract, urinary bladder, liver, intestine, CNS (especially the cerebellum), skin (if gross lesions are present) and the eye. Distemper viral antigen may be detected by immunohistochemistry if histopathology is not definitive.