Background Gross Pathology Histopathology & Immunohistochemistry

BACKGROUND AND CLINICAL INFORMATION: Head

Pathogen: Rabies virus is a rhabdovirus with single strand of RNA. [Click here to see a case]

Route of infection: Virus is introduced from infected saliva via bites, open wounds and, rarely, by inspiration of infected feces in bat- infested caves.

Pathogenesis:

Virus replicates in myocytes and spread to neuromuscular and neurotendinal spindles and then go to the spinal cord via the axons.
The virus spread from the periphery to the brain and then to the retina, cornea and the nerve endings of the skin, salivary gland and lacrimal gland. Viral antigens are also detectable in the blood.

Clinical type: Rabies may present as furious form (80%) or paralytic form (20%).

Incubation period: 15 days to 1 year, mean is 40 days.

Cause of death: Patients are usually died of myocarditis and heart failure.

GROSS PATHOLOGY: Head

The brain can be grossly unremarkable.

HISTOPATHOLOGY AND IMMUNOHISTOCHEMISTRY: Head

Anatomical distribution of lesions:

Hippocampus, cerebellum, thalamus, basal ganglia, cerebral cortex.
Spinal cord motor neurons and dorsal root ganglion in paralytic form. This form is less common.
Brain stem, especially gray matter of the pons and medulla.

Histology:

It is basically a picture of polioencephalomyelitis with inflammatory infiltration variation from scanty to florid.
There is very little destruction of neurons and very mild inflammatory cell infiltration.
Paralytic form: changes may be severe in the spinal cord and DRG may also be involved.

Negri body:

Negri body is pathognomonic. They are oval eosinophilic neuronal cytoplasmic inclusion measuring 1-7 micron in diameter. They are most consistenetly seen in the pyramidal cells of hippocampus and Purkinje cells of cerebellum. They are also seen in cortex, spinal cord, brain stem. Negri body may not be seen in every case.
Negri body may very big when they are found in motor neurons of spinal cord. Negri body is not found in every case.
Wide distribution of viral antigen can be demonstrated immunohistochemically in neurons, oligodendrocytes, peripheral nerves, nerves of hair follicles, corneal cells, retina, and salivary glands.

PNS changes include demyelination and remyelination, also inflammatory infiltration.