Retrospective studies which cover a long time span and include large numbers of animals are of great value to obtain information about the temporal occurrence of diseases including host-pathogen interactions and transmission routes. This can contribute to a better understanding of the epidemiology of infectious agents.

The archived necropsy reports of all dogs examined at the Department of Pathology, University of Veterinary Medicine Hannover, between 1962 and 2022 were evaluated regarding the macroscopic and histologic findings in the central nervous system (CNS). Cases of non-suppurative inflammation were re-evaluated using haematoxylin and eosin-stained sections. Whenever a viral etiology was suspected, immunohistochemistry was performed for specific pathogens.

During the investigated time period 20117 dogs were sent for necropsy, of which 2646 (13%) had neuropathological changes. In 776 cases, non-suppurative encephalitis and/or myelitis was found. 186 (24%) of these cases were linked to Morbillivirus canis (CDV) infections, an agent frequently detected until the early 2000s. Varicellovirus suidalpha1 (SHV1) infection was diagnosed in 119 dogs (16%), mainly from the 1970s to the 1990s. Other infectious agents were detected sporadically. The remaining cases couldn't be etiologically determined.

Distemper remains the most frequent cause of non-suppurative encephalitis in dogs. However, only sporadic cases were documented since 2005. After a transient high prevalence, SHV1 was rarely observed in the past 30 years. Summarized, this retrospective study shows the value of archived material for further pathogenetic investigations, although the number of unresolved cases demonstrates the challenges in etiologic clarification of non-suppurative encephalitis.