Introduction

• Cause: viral disease of the central nervous system (CNS).
• Signs: 'furious ' and 'dumb' forms.
• All mammals susceptible (to a variable extent). Birds less susceptible.
• Reservoir hosts vary with country, but mainly include dogs, cats, foxes, bats, raccoons and wolves.
• Treatment: none.
• Diagnosis: signs, history of exposure to the virus, confirmation on examination of brain at histopathology.
• Prognosis: grave/hopeless.

Pathogenesis

Etiology

• Lyssavirus Lyssavirus (rabies) strain variation: antigenic variation and antigenic drift.

Predisposing factors

General

• All mammals susceptible to infection (to a variable extent). To a lesser degree birds can be susceptible too.

Specific

• Bite/scratch from infected animal.
• Contact with saliva from infected animal, especially if skin damaged.

Pathophysiology

• Transmission by bite/scratch or accidental break in skin.
• Primary replication in muscle fibers at site of inoculation → virus aggregates around proprioreceptor nerve endings → virus binds to nicotinic acetylcholine receptors and enters axon sheath of afferent nerve within 5 hours → virus migrates (retrograde axoplasmic flow) to spinal ganglion at up to 3 mm/h → multiplies in spinal ganglion (or hippocampus) → virus enters dorsal horn of spinal cord → ependymal cells → brain → centrifugal spread down cranial nerves → salivary glands (and lungs/kidneys) → multiplication → excretion in saliva in 50% of infected dogs (others die before this stage).
• Combined active and passive immunization shortly after exposure eliminates virus before it reaches CNS.
• Some exposed dogs do not develop disease (immunity?).

Timecourse

• Incubation period depends on:
  • Distance of site of inoculation from CNS.
  • Severity of bite.
  • Dose of virus inoculated.
• Incubation 5 days-12 months (usually less than 3 months).
• Death within 5-15 days of onset of signs.

Epidemiology

• Dog requires high virus dose for infection ('intermediate susceptibility'), puppies more susceptible than adults.
• Infection by inhalation very rare.
• Reservoir host varies with location: include Canidae, Mustelidae and Chiroptera (bats).
• Endemic in racoons, skunks (USA), vampire bats (Central America), mongoose (S. Africa - parts of Caribbean), red fox (Europe), Jackals (Africa).
• Dog rabies is most significant in zoonosis.
• Bats may have subclinical infection with persistent viremia.
• Skunks and foxes may recover from infection.
• Some dogs have mild symptoms, recover, and remain as persistent excretors.
• Some countries free of rabies (see above).
• Virus survives only when numerous susceptible hosts available in close contact.

Diagnosis

Treatment

Prevention

Outcomes