Rabies

Rabies Overview

  • Name of Disease: Rabies (also known as Hydrophobia)

  • Etiological Agent: Rabies virus (a rhabdovirus)

Disease Characteristics

  • Primarily a viral infection of non-human carnivores; transmission to humans is rare, typically through bites.

  • Incubation Period: Highly variable; common range is 30-90 days, can be 10-240 days.

  • Fear of rabies is instinctual, often due to the high fatality rate once symptoms develop.

Pathology

  • Transmission: Occurs via the saliva of an infected animal, introducing the virus to muscle and nerve tissues.

  • Virus Spread:

    • Limited reproduction makes the virus travel along nerves to the CNS.

    • Higher fatality rates observed with bites to areas rich in sensory nerve endings such as the face and neck.

  • Neurotropism:

    • Replicates within CNS neurons, travels centrifugally to invade organs and tissues.

  • Negri Bodies:

    • Characteristic cytoplasmic inclusion bodies in neurons, pathognomonic for rabies.

    • Most common in the hippocampus and cerebellum; absence does not exclude the disease.

  • Histopathological Changes:

    • Edema, hemorrhage, congestion, necrosis in ganglia, neuronophagia, with severe changes in the pons and medulla.

Diagnosis

  • Clinical diagnosis typically relies on a bite history and symptom presentation.

  • Laboratory tests necessary when a history is lacking to confirm exposure.

Clinical Phases of Rabies

  1. Prodromal Phase:

    • Lasts 2-4 days, with nonspecific symptoms: low fever, malaise, headache, nausea, etc.

    • Significant Symptoms:

      • Abnormal sensations at the bite site (80% of cases) including pain, coldness, pruritus.

  2. Excitation Phase:

    • Symptoms intensify with increased anxiety and muscle tone.

    • Cranial Nerve Malfunctions:

      • Ocular palsies (e.g., strabismus), pupil dilation/constriction, absence of corneal reflexes.

    • Hydrophobia: Characterized by painful spasms when swallowing liquids.

  3. Paralytic Phase:

    • Flaccid paralysis develops; respiratory failure and death ensue.

Definitive Diagnosis Procedures

  1. Isolation of the virus from saliva, CSF, urine, or nerve tissue.

  2. Fluorescent rabies antibody test on brain tissue.

  3. Identification of Negri bodies.

Prognosis

  • Recovery from rabies is exceptionally rare (only 6 noted cases); common CNS sequelae.

Treatment

  1. Immediate wound treatment:

    • Wash thoroughly with soap and water.

    • Apply 1% quaternary ammonium compounds.

    • Carefully apply antirabies serum and do not suture the wound.

  2. Initiate tetanus procedures and rabies vaccination based on animal autopsy results.

  3. Supportive care if symptoms develop:

    • Tracheostomy, oxygen therapy, control brain pressure, treat seizures.

Vaccines Available

  1. Rabies Vaccine Options:

    • Semple (not available in U.S.)

    • Duck embryo vaccine (risk for hypersensitivity).

    • Attenuated virus vaccine grown in human diploid cells (5 injections).

    • Recombinant vaccine (1 vaccination).

  2. Antirabies Antiserum:

    • Heterologous (equine origin) with serum sickness risk.

    • Homologous (human origin) with fewer side effects but higher cost.