A 35-year-old man from rural Uttar Pradesh presents with a 6-week history of fever, agitation, and hydrophobia following a dog bite to his hand. He deteriorates rapidly and dies despite supportive care. Postmortem brain biopsy is performed. The structure marked **A** in the diagram is identified within pyramidal neurons of the hippocampus. What is the diagnostic significance of this finding?

Explanation

## Why "Confirms rabies virus infection and is pathognomonic for the disease" is right Negri bodies (marked **A**) are eosinophilic cytoplasmic inclusions found specifically in neurons infected with rabies virus, particularly within Purkinje cells of the cerebellum and pyramidal neurons of the hippocampus. When present on histopathology, they confirm rabies and are considered pathognomonic for the disease. According to Murray 9e and Park 26e, the presence of Negri bodies in the appropriate neuronal locations is diagnostic of rabies encephalitis. In this case, the identification of Negri bodies in hippocampal pyramidal neurons confirms the diagnosis of rabies postmortem. ## Why each distractor is wrong - **"Indicates a poor prognosis but is not diagnostic without immunofluorescence confirmation"**: While rabies does carry a uniformly fatal prognosis once symptoms develop, Negri bodies ARE diagnostic when present. Immunofluorescence (DFA) and PCR are additional confirmatory methods, but Negri bodies themselves are pathognomonic and do not require further confirmation for diagnosis. - **"Suggests an alternative diagnosis such as Japanese encephalitis rather than rabies"**: Negri bodies are specific to rabies virus (Lyssavirus, Rhabdoviridae family) and are not found in Japanese encephalitis or other viral encephalitides. Their presence definitively points to rabies, not alternative diagnoses. - **"Proves the patient received inadequate post-exposure prophylaxis"**: The presence or absence of Negri bodies has no bearing on whether PEP was given. Negri bodies are a pathological finding in established rabies infection; their presence indicates the disease developed, but does not reflect the adequacy of prior prophylaxis. PEP failure can occur even with appropriate administration if initiated too late. **High-Yield:** Negri bodies in hippocampal pyramidal neurons or cerebellar Purkinje cells = pathognomonic for rabies on histopathology, but absent in ~50% of cases; postmortem DFA/PCR is the gold standard when Negri bodies are not found. [cite: Murray 9e; Harrison 21e Ch 200; Park 26e]