Canine distemper virus infection of vaccinal origin in a 14-week-old puppy

The body of a 14-wk-old puppy (Canis familiaris) was submitted to the Animal Health Laboratory, University of Guelph, Ontario for postmortem examination following a history of intermittent anorexia and lethargy progressing to pyrexia, pruritic skin rash, mucoid nasal discharge, decreased mentation, dysphagia, muscle twitches, and focal seizures. Gross examination revealed rhinitis and pulmonary edema. Histologically, there was fibrinonecrotizing bronchopneumonia, tracheitis, and neutrophilic and lymphohistiocytic rhinitis; rarely within the cortical gray and white matter of the brain were small clusters of glial cells, with rare individual neutrophils in the choroid plexus. Although canine distemper was suspected, none of the usual supportive histologic lesions of distinct syncytial cells, viral inclusion bodies, or demyelinating leukoencephalitis were observed. Lung and brain tissues were PCR-positive for canine distemper virus (CDV), and CDV was detected immunohistochemically in the brain. The agent from the PCR-positive sample from the brain was genotyped and was a 99.9% match to the CDV Rockborn strain, indicating that the disease agent in our case was vaccinal in origin. Our unusual case highlights the possibility of reversion to virulence in a modified-live virus vaccine, and the occurrence of a disease in the absence of a full complement of the usual and compatible histologic lesions.