Mechanism of persistence with canine distemper virus: difference between a laboratory strain and an isolate from a dog with chronic neurological disease.

The mechanism of persistence in culture was different with canine distemper virus (CDV) isolated from a dog with chronic neurological disease (ODE; old dog encephalitis) compared to a laboratory strain (CDV/Ond). CDV/Ond persistence was achieved after seven undiluted passages while CDV/ODE-8, the recent isolate, showed immediate persistence in Vero cells. Both persistent infections resisted challenge with lytic CDV/Ond but not with unrelated vesicular stomatitis virus. The medium from CDV/Ond infection showed interference, whereas the medium from CDV/ODE-8 infection did not. Ultracentrifugation of the CDV/Ond supernatant effectively removed the defective interfering (DI) particles. Fluorescent microscopy showed the presence of CDV antigen in the cytoplasm of both types of persistently infected cells. By electron microscopy, 1% of the CDV/Ond cells and 23% of the CDV/ODE-8 cells showed distemper viral nucleocapsids. Persistence of CDV/Ond appears to be due to classical DI particles, whereas persistence of CDV/ODE-8 appears to be due to cell-associated particles. The persistence of CDV in dogs with ODE may be due to this cell-associated phenomenon.