Negri bodies (NBs) are cytoplasmic inclusions associated with rabies virus infection and are a characteristic histopathologic feature of rabies. Although their morphology has been described in humans and experimental animal models, detailed characterization in naturally infected dogs remains limited. The objective of this study was to characterize the histopathologic, immunophenotypic, and ultrastructural features of NBs in naturally infected dogs from the Philippines. Brain tissues from 24 dogs were examined by routine histology, special stains, immunohistochemistry, and indirect immunofluorescence. Transmission electron microscopy was performed in 6 selected cases. In the cerebrum, hippocampus, and cerebellum, NBs were predominantly composed of abundant matrix material and occasionally contained inner bodies (classic NBs), with variable association with bullet-shaped virions and ribosomes. These inclusions were immunoreactive for rabies virus nucleoprotein and phosphoprotein and stained positively with luxol fast blue. Inner bodies were immunoreactive for glycoprotein and matrix protein and were positive with silver impregnation, Nissl, and Bodian stains. In contrast, small inclusions (lyssa bodies) identified in the thalamus and brainstem consisted mainly of immature viral particles with minimal matrix material. Collectively, these findings indicate region-dependent ultrastructural variation in rabies virus-associated inclusions in naturally infected dogs, while supporting a shared viral origin among the different morphologic forms. This characterization further supports the diagnostic value of these inclusions in veterinary neuropathology.
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