Rabies virus causes rabies, the most fatal infection in humans. No specific antirabies agents are useful, once clinical signs or symptoms develop.
In general, four stages of rabies are recognized in humans. These are (i) incubation, (ii) prodromal period, (iii) acute neuro-logic period, and (iv) coma; which subsequently lead to death.
Incubation period: The average period of incubation is20–90 days. Rarely, incubation lasts as long as 19 years. In more than 90% of cases, incubation is less than 1 year. During the incubation period, the virus travels from peripheral areas to the CNS. The patients remain asymptomatic during the period. The incubation period is less than 50 days if the patient is bitten on the head or neck or if a heavy inoculum is trans-ferred through multiple bites, deep wounds, or large wounds. A person with a scratch on the hand may take longer to develop symptoms of rabies than a person who receives a bite on the head. The rabies virus is protected from the immune system during this period and no antibody response is observed.
Prodromal period: The virus enters the CNS during the pro-dromal period. The duration of this period is 2–10 days. The period is characterized by nonspecific symptoms and sign Paresthesia or pain develops at the inoculation site and is pathognomonic for rabies. Paresthesia occurs in 50% of cases during this phase and may be the patient’s only presenting sign. Other symptoms may include malaise, anorexia, head-ache, fever, chills, pharyngitis, nausea, emesis, diarrhea, anxiety, agitation, insomnia, and depression.
Acute neurologic period: This period is associated with objec-tive signs of developing CNS disease. The duration is 2–7 days. Furthermore, it presents with the following conditions:
Furious rabies:Patients develop agitation, hyperactivity, restless-ness, thrashing, biting, confusion, or hallucinations. After several hours to days, this becomes episodic and interspersed with calm, cooperative, lucid periods. Furious episodes last for less than 5 minutes. Episodes may be triggered by visual, auditory, or tac-tile stimuli, or they may be spontaneous. Seizures may occur.
Autonomic instability is observed, including fever, tachycardia, hypertension, hyperventilation, drooling, anisocoria, mydriasis, lacrimation, salivation, perspiration, and postural hypotension. Other neurologic signs include cranial nerve involvement with diplopia, facial palsy, and optic neuritis. This phase may either end in cardiorespiratory arrest or progress to paralysis.
Paralytic rabies:It is also known as dumb rabies or apatheticrabies, because the patient is relatively quiet compared to a per-son with the furious form. Paralysis develops from the outset. Fever, headache, and nuchal rigidity are prominent. Paralysis is symmetric and may be either generalized or ascending and may be mistaken for Guillain–Barré syndrome. Calmness and clar-ity gradually deteriorates to delirium, stupor, and then coma.
Coma: The patient may go into coma within 10 days of onset;but duration is variable. Coma leads to respiratory failure within a week of neurologic symptoms. Hypoventilation and metabolic acidosis predominate. Acute respiratory distress syn-drome is common. Without intensive supportive care, respi-ratory depression, arrest, and death occur shortly after coma. Most cases result in death within 14 days because of compli-cations, despite intensive supportive care. Recovery is very unlikely. A few reports indicate that those patients who sur-vived had pre-exposure or postexposure prophylaxis supported by most advanced life-support system.