What is the diagnostic sign for rabies?

The presence of Negri bodies is the diagnostic sign for rabies. Negri bodies are distinct, pathognomonic structures found within the cytoplasm of nerve cells in animals and humans that have been infected with the rabies virus. They appear as eosinophilic (pink-staining), round or oval inclusions and are typically located in the hippocampus and cerebellum of the brain.

The identification of Negri bodies is critical in confirming a diagnosis of rabies, particularly when examining brain tissue post-mortem or in biopsies from symptomatic individuals. Their presence indicates active viral replication and is a key component in the laboratory diagnosis of rabies, as tissues from infected animals often show these characteristic inclusions.

In contrast, Aschoff's bodies are associated with rheumatic fever, Reed-Sternberg cells are indicative of Hodgkin's lymphoma, and the Schilling test is a clinical assessment for vitamin B12 absorption. Each of these points to various conditions unrelated to rabies, highlighting the specificity and importance of Negri bodies in the context of rabies diagnosis.