Rabies Clinical Presentation

Updated: Jun 21, 2019
  • Author: Sandra G Gompf, MD, FACP, FIDSA; Chief Editor: Michael Stuart Bronze, MD  more...
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Presentation

History

Identify the following in any suspected case of rabies virus exposure:

  • The nature of the interaction with the animal (Recall that "provocation" is not an indication of rabies risk, since humans may not understand what is provocative to a wild animal.)

  • Strange animal behavior (eg, nocturnal animal out during the daytime)

  • Vaccination status of the animal for rabies [13]

  • Availability of the animal for testing

Incubation period

The rabies virus is segregated from the immune system during this period, and no antibody response is observed. The infected individual remains asymptomatic during this period. The average duration of incubation is 20-90 days. In more than 90% of cases, incubation is less than 1 year. A person whose inoculum occurs with a scratch on the hand may take longer to develop symptoms of rabies than a person who receives a bite to the head. The incubation period is less than 50 days if the patient is bitten on the head or neck or if a heavy inoculum is transferred through multiple bites, deep wounds, or large wounds. Infected patients may not recall exposure because of the prolonged incubation period.

Rarely, incubation has been reported many years following exposure. In cases of salivary exposure, incubation has been estimated based on molecular, phylogenetic, and epidemiologic evidence to be greater than 6-8 years. [22, 23, 24, 25] A suspected prolonged incubation of 25 years was reported in India with no identifiable risk factors other than a dog bite occurring in a time frame and location coincident with the likelihood of rabies in local dogs. [6]

Prodromal period

The virus enters the CNS. The duration of this period is 2-10 days. Nonspecific symptoms and signs develop, including fevers and flulike illness. Paresthesia, pain, or intense itching at the inoculation site is pathognomonic for rabies and occurs in 30% of canine-associated rabies cases and 70% of bat-associated rabies cases during this phase; this may be the individual’s only presenting sign. Symptoms may include the following:

  • Malaise
  • Anorexia
  • Headaches
  • Fever
  • Chills
  • Pharyngitis
  • Nausea
  • Emesis
  • Diarrhea
  • Anxiety
  • Agitation
  • Insomnia
  • Depression

Acute neurologic period

This period is associated with objective signs of developing CNS disease; however, consciousness remains unaffected until the onset of coma. The duration is 2-7 days. Symptoms include muscle fasciculations, priapism, and focal or generalized convulsions. Patients may die immediately or may progress to paralysis, which may be present only in the bitten limb at first but usually becomes diffuse.

Two-thirds of human rabies cases acquired from dogs manifest as "furious rabies." Patients develop agitation, hyperactivity, restlessness, thrashing, biting, confusion, or hallucinations. After several hours to days, this becomes episodic and interspersed with calm, cooperative, lucid periods. Furious episodes last less than 5 minutes. Episodes may be triggered by visual, auditory, or tactile stimuli or may be spontaneous. Seizures may occur. This phase may end in cardiorespiratory arrest or may progress to paralysis.

The remaining one-third of patients with rabies develop paralytic rabies, also known as dumb rabies or apathetic rabies, because the patient is relatively quiet compared with a person with the furious form. Paralysis occurs from the outset, and fever and headache are prominent. Paralytic rabies may initially mimic Guillain-Barré syndrome (GBS) with ascending lower–motor-neuron weakness unpreceded by classic "mad" signs, and rabies should be considered in the differential diagnosis of GBS. [26]

Bat-associated rabies often manifests with more atypical findings.

Coma

This begins within 10 days of onset, and the duration varies. Without intensive supportive care, respiratory depression, arrest, and death occur shortly after coma.

Next:

Physical Examination

Neurologic period

With furious rabies, patients present with episodic delirium, psychosis, restlessness, thrashing, muscular fasciculations, seizures, and aphasia. Hydrophobia and aerophobia are pathognomonic for rabies and occur in 50% of patients. Attempting to drink or having air blown in the face produces severe laryngeal or diaphragmatic spasms and a sensation of asphyxia. This may be related to a violent response of the airway irritant mechanisms. Even the suggestion of drinking may induce hydrophobic spasm.

Autonomic instability is observed with furious rabies, with symptoms that include the following:

  • Fever

  • Tachycardia

  • Hypertension

  • Hyperventilation

  • Anisocoria, fixed pupillary dilation (“blown pupil”), optic neuritis (may falsely suggest brain death)

  • Facial palsy

  • Mydriasis

  • Lacrimation

  • Excessive salivation

  • Perspiration

  • Postural hypotension

In patients with paralytic rabies, fever and nuchal rigidity may occur. Paralysis is symmetrical and may be either generalized or ascending and may be mistaken for Guillain-Barré syndrome. The sensory system is usually spared. Calm clarity gradually progresses to delirium, stupor, and then coma.

Coma

Respiratory failure occurs within 1 week of neurologic symptoms. Hypoventilation and metabolic acidosis predominate. Acute respiratory distress syndrome is common. Wide variations in blood pressure, cardiac arrhythmias, and hypothermia ensue. Bradycardia and cardiac arrest occur. With intensive support, life may be extended for 3 or 4 months; however, death is usually the outcome.

Death

It is important to determine brain death by brain biopsy or absence of cerebral arterial flow, because some of the neurologic signs may falsely suggest brain death (see above).

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