Which test is used to diagnose distemper by detecting the viral antigen using immunocytology?

• A. Clinical signs
• B. Cytoplasmic inclusions on blood smear
• C. CSF IgG versus serum IgG
• D. Immunocytology of antigen

Answer: (D)

The main idea is detecting the virus directly in cells using antibody-based staining. In canine distemper, you can collect cells from mucosal surfaces or tissues and apply an immunocytology test that uses labeled antibodies to bind to distemper viral antigen. If the virus is present, the antibodies bind and produce a visible signal under the microscope, giving direct evidence of infection. This approach is more specific and timely than waiting for clinical signs alone or for indirect indicators like antibody levels in CSF versus serum, which reflect the immune response rather than the actual presence of the virus. Cytoplasmic inclusions on a blood smear may occur but are not reliably diagnostic for distemper, so they are less useful. Thus, immunocytology of antigen best fits because it directly demonstrates the viral antigen in cells.