Negri bodies are distinct structures that appear within the cytoplasm of nerve cells. These formations are eosinophilic, meaning they stain pink with eosin dye. Their presence is a strong indicator of a severe viral infection affecting the central nervous system, and their identification is a histological proof of rabies.
The Link to the Rabies Virus
The appearance of Negri bodies in nerve cells is considered a pathognomonic sign of rabies, meaning their presence confirms the disease. While distributed throughout the brain, they are most commonly located in specific types of neurons. These include the large pyramidal neurons in the hippocampus and the Purkinje cells of the cerebellar cortex.
The rabies virus travels from the point of entry along nerves to the central nervous system, where it replicates within the gray matter of the brain. This replication process leads to the formation of Negri bodies in various parts of the brain, including the brainstem and hypothalamus. The consistent association of these bodies with rabies was first observed by Adelchi Negri in 1903.
Formation and Composition
Negri bodies function as viral factories within the cytoplasm of infected neurons and are the primary sites of rabies virus transcription and replication. During the infection cycle, these spherical bodies, which initially appear as one or two per cell, grow in size as the virus multiplies. They are accumulations of viral components.
The composition of these bodies is primarily viral nucleoprotein and viral RNA, along with aggregates of the rabies virus N and P proteins. The eosinophilic nature of Negri bodies, which gives them their characteristic pink appearance after staining, is due to this high concentration of protein. These sharply outlined inclusions measure between 2 and 10 micrometers in diameter.
Role in Rabies Diagnosis
The detection of Negri bodies is a method for the postmortem diagnosis of rabies. This is achieved through the microscopic examination of brain tissue from the suspected case. To make the inclusions visible, technicians use histological staining techniques. One common method uses hematoxylin and eosin, which stains the Negri bodies a distinct pink or red color.
The presence of Negri bodies in the brain tissue of an animal or person confirms a rabies diagnosis. However, their absence does not completely rule out the possibility of the disease. Negri bodies are found in approximately 75% of rabies cases, meaning a significant portion of infected individuals may not have them. A negative result requires other laboratory tests, such as the fluorescent rabies antibody (FRA) test, for a definitive diagnosis.