Clinicopathologic variation in raccoons infected with different street rabies virus isolates

Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square 19348, USA.

Ten raccoons were divided into two random groups (groups 1 and 2) of five animals each. Group 1 raccoons were inoculated intramuscularly in the masseter muscle with a raccoon rabies virus isolate obtained from a natural case of raccoon rabies from the northeastern USA. Group 2 raccoons were infected by a similar route with a Latin American canine isolate of rabies virus. Raccoons either died suddenly or developed neurologic signs compatible with rabies. Clinical signs of rabies in group 1 raccoons were more severe than in group 2. Raccoons in group 1 either died acutely or were euthanized within 25 days (mean +/- SD = 20.6 +/- 2.7 days) postinfection, whereas all group 2 raccoons showed neurologic signs and were euthanized within 17 days (14.2 +/- 2.2 days) postinfection. Light microscopic findings revealed extensive nonsuppurative encephalitis predominantly located in the cerebrum and brain stem of raccoons in group 1, whereas in group 2 raccoons the lesions were confined to the brain stem regions. In group 1 raccoons, Negri bodies were commonly seen on hematoxylin and eosin (HE)-stained sections of brain and in ganglion cells of 5 other tissues (trigeminal nerve, salivary glands, duodenum, pancreas, adrenal gland). Negri bodies, however, were either absent or were only occasionally observed in corresponding tissues of raccoons infected with the canine strain (group 2). Paraffin-embedded tissue sections were also examined for Negri bodies by an immunoperoxidase test, which revealed results similar to the HE findings. Results of this study are compared with histopathologic and immunohistochemical findings in raccoons naturally infected with rabies.