- The bite of rabid animal is the usual portal of entry which contains the infected saliva that is introduced into the depths of the wound.
- The virus may gain access through a pre-existing scratch or abrasion occasionally if this has been contaminated with the saliva.
- The virus multiplies locally in the tissues, invades adjacent nerve fibers, and spreads centrally to infect the neurons in the brain and spinal cord.
- It is also thought that the virus may enter and travel along the neurons of peripheral nerves.
- Bite wounds are taken as usual route of entry but sometimes it may enter through unusual route.
- Rare examples of infection have been reported like by inhalation in bat infested caves where there was bat-rabies and also by ingestion through eating of rabies infected cattle.
- Fox and coyote are highly susceptible that have been experimentally infected by inhalation from exposures in caves.
- It varies from 10 days to 2 years after the rabid animals have bitten to the person.
- It depends on the distance of the viruses from the brain where the viruses have been introduced.
- Incubation period might be 60 days for bite on the legs, 40 days on the arm, 30 days on the head region.
- This period is shorter in children as compared to adult.
- Incubation period is short in the condition where severely lacerated wounds of the heads and shoulders have been heavily infected.
- The virus is carried to the brain via the bloodstream in this case.
- Degeneration of nerve cells occurs in cortex, mid-brain, basal ganglia, pons and medulla.
- Histopathological examination of brain shows Negri inclusion bodies in the neurons of cerebellum and hippocampus.
- In addition to the above, a polio-encephalitis affecting usually the brain stem and spinal cord characterized by perivascular cuffing with lymphocytes and plasma cells, parenchymal microglial response and sometimes neuronophagia may be seen.
- The inflammatory reaction appears early whereas the Negri bodies are more commonly found, in cases with a long illness.
- From the neurons which show little or no cytopathic effect, the virus may spread to other tissues including the salivary glands.
- In the tissues, the virus multiplies in acinar cells, the adrenals, kidneys, pancreas and myocardium.
- Sometime, conjunctiva and cornea are also affected due to which the examination of exudate or corneal impression smears may be useful in immune-fluorescence for diagnostic procedure.
- Sub-clinical disease is not known to occur in human.
- Headache, fever, a profound sense of apprehension, and a feeling of irritation at the site of bite are the initial symptoms.
- Patient will have dry throat and thirst, but they won’t drink.
- High fever, difficulty in swallowing and the consequent fear of water become the dominant symptoms.
- Due to this the disease is also called hydrophobia.
- The attempts of drinking water after thirst provoke violent contraction of diaphragm and inspiratory muscles.
- Thereafter, mere site or sound of water precipitates distressing muscular spasm.
- The patient develops focal and generalized seizures, disorientation and delirium as the disease progresses.
- 10- 20% of patients exhibit flaccid paralysis.
- The patients become comatose later, which lasts for 2-10 days.
- Death results from neurological and pulmonary failure.
Pathogenesis of Rhabdo (rabies) virus