Rabies Virus and Infection

Snapshot

• A 10-year-old boy presents to the emergency room after being bit by an animal in a cave. His parents report that they live near some caves that the boy frequently explores. While they are not sure if it was a bat, they are concerned about rabies. On physical exam, there is a small excoriation on his left leg. Otherwise, physical exam is within normal limits. The boy is given the rabies vaccine and immunoglobulin.

Introduction

• Classification
  • rabies virus 
    • linear negative single-stranded RNA virus
    • rhabdovirus
    • helical capsid and bullet-shaped
  • transmission
    • via bites from infected animals
      • developed countries
        • bats > raccoons and skunks > dogs
      • developing countries
        • dogs > others
    • via aerosol transmission (rare)
      • bats
• Epidemiology
  • demographics
    • more common in children
    • majority of cases are in Asia and Africa
  • risk factors
    • interaction with non-domestic animals
• Pathogenesis
  • long incubation period (weeks to months)
  • replicates in muscle and binds to the nicotinic acetylcholine receptor
    • retrograde migration to central nervous system via dynein motors 
• Associated conditions
  • encephalitic rabies (more common)
  • paralytic rabies
• Prevention
  • whole virus inactivated vaccine
    • active immunization
  • rabies immunoglobulin
    • passive immunization
• Prognosis
• most cases, without early intervention, lead to coma and death within weeks

Presentation

• Symptoms
  • nonspecific prodrome
    • malaise
    • fever
    • headache
• Physical exam
  • fever
  • encephalitic rabies
    • hydrophobia
      • muscle spasms when offered water
      • leads to “foaming at the mouth”
    • photophobia
    • autonomic dysfunction
      • excess sweating
      • piloerection
      • hypersalivation
    • agitation
    • seizures
  • paralytic rabies
    • quadriparesis
    • bilateral facial weakness
  • late-stage disease
    • paralysis
    • coma
• death

Studies

• Labs
  • viral studies
    • reverse transcription polymerase chain reaction (RT-PCR)
    • isolation of virus
• Histology 
  • brain biopsy
    • performed post-mortem
    • Negri bodies 
      • eosinophilic inclusion bodies in cerebellar Purkinje cells and hippocampal neurons
      • pathognomonic
• Making the diagnosis
• based on clinical presentation

Differential

• Botulism 
  • distinguishing factor
    • flaccid paralysis without other signs of encephalopathy
• Temporal lobe (herpes simplex virus-1) encephalitis 
  • distinguishing factor
• disinhibited behaviors such as hyperphagia, hypersexuality, and hyperorality

Treatment

• Management approach
  • give post-exposure prophylaxis immediately 
    • when bitten by rabid animal
    • when bite cannot be ruled out
  • once patients are symptomatic, there is no treatment and patients should receive supportive care
    • pain management
    • sedation
• Conservative
  • wound cleaning
    • indication
      • post-exposure
• Medical
  • rabies immunoglobulin
    • indication
• post-exposure prophylaxis

Complications

• Respiratory failure leading to death