In the present report, a member of a group of RNA South American viruses found in Brazil, that causes febrile disease in humans and encephalitis in neonate and adult murine models, was selected as a model to study encephalitis outcomes in adult albino Swiss mice. In mice housed under standard conditions with free access to water and food, we induced viral encephalitis by intranasal inoculation of Piry virus–infected brain homogenate and correlated neuropathological features. We quantified the cellular inflammatory response in the septal region using a stereologically based unbiased method with clinical signs and neuroinvasion, comparing the outcomes with those of animals inoculated with uninfected brain homogenate. Three-month-old female mice maintained in standard environment received an equal volume of Piry virus infected or normal brain homogenates into the nostrils. From the 1st to 8th days post-instillation (dpi), five subjects from the infected colony were fixed and processed to detect viral antigens and microglia. Control subjects were sacrificed in the 5th dpi and processed for the same markers. After Piry virus encephalitis induced microglial activation and neuroinvasion of glial cells and neurons mainly in the olfactory pathways early in the disease (2 – 4 dpi), but also included hippocampus, cerebellum and brain stem nuclei later on (5 - 8 dpi). The correlation of the host cellular inflammatory quantitative response in the septal area with clinical signs and neuroinvasion, revealed that the number and the morphology of microglias changed early in the disease before neuroinvasion had reached the septal region and clinical signs had appeared. Great variability in clinical symptoms intensity and survival rate were found in the outbred albino Swiss mice strain as compared with previous report in the inbred C57Bl6 strain suggesting less isogenic background. Taken together, our previous and present report dedicated to investigate Piry virus encephalitis progression in the outbred albino Swiss mice strain may open a new field of investigation of the genetics, anatomical and immune substrates of tropical sublethal arbovirus encepahlitis.
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