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Viral Disease - Rabies Virus

Synonyms: Lyssa, Hydrphobia

Definition and Cause

Rabies is an invariably fatal infection resulting in encephalitis in most mammals.

Although now rare in domestic animals, the zoonotic potential of this fatal disease makes it crucial that every veterinarian understand the disease.

Etiology

Rabies virus (RV) is an SS-RNA virus if the family Rhabdoviridae.

Pathophysiology

Epidemiological findings

RV is maintained in wildlife reservoirs worldwide (e.g., fox, skunk, bat, raccoon).

Different wildlife reservoirs predominate in certain geographic areas and carry different RV variants.

Cats have a higher incidence of RV infection than dogs in the United States.

Transmission

It occurs primarily through contact with infectious particles in saliva via bite wounds or contamination of mucous membranes or open wounds.

Other minor sources include other body fl uids, particularly urine, airborne virus, or ingestion.

Rabies in dogs and cats is usually contagious only if a bite exposure occurs.

Pathogenesis

The virus replicates locally and then spreads along the peripheral nervous system axons to the CNS and back down the peripheral nervous system and cranial nerves to the salivary glands.

Inoculation at the head often leads to signs of encephalitis more rapidly than inoculation elsewhere on the body.

Viral shedding via saliva typically does not begin until 5 to 10 days before the onset of clinical signs.

Clinical Signs

The onset of CNS signs is usually with in 2 to 24 weeks, but depends on the viral load inoculated, the degree of innervation at the site, the proximity of the site to the CNS, and the strain of RV.

Classic clinical presentation is either "furious" or "paralytic," but not all cases fi t these categories, and a combination of signs is possible.

With the furious form of rabies, the disease course lasts only days.

  • » Behavioral changes, hyperexcitability, hyperesthesia, and aggression occur.
  • » Ataxia and seizures develop and are followed by death.

With the paralytic form, the disease course typically lasts about 1 week.

  • » Progressive ascending lower motor neuron paralysis begins at the inoculation site and leads to dysphagia, masticatory muscle paralysis, and laryngeal and pharyngeal dysfunction, with classic signs of salivation.
  • » Death is caused by respiratory paralysis.

Diagnosis

History of a recent bite wound and classic clinical presentation may be helpful.

Clinical pathologic fi ndings are nonspecific.

Antemortem testing may confi rm RV infection, but can not adequately rule it out.

Owing to the serious public health implications, euthanasia of animals suspected of RV infection is urgently recommended and often required by law.

Postmortem testing confi rms or denies the diagnosis.

  • » Submit the brain-chilled but not frozen-as soon as possible to a state diagnostic laboratory.
  • » Direct FA testing of the brain is the diagnostic method of choice.
  • » Additional tests on brain specimens include PCR for viral particles, histopathologic evaluation (intracellular inclusions or Negri bodies), and mouse inoculation
  • » RV strains may be identifi ed to help determine the source of exposure.

Differential Diagnosis

Dogs: CDV, acute polyradiculoneuritis, botulism, tick paralysis, pseudorabies, neoplasia, organophosphate intoxication, other intoxications, and other causes of meningitis and encephalitis

Cats: toxoplasmosis, feline infectious peritonitis (FIP), feline immunodefi ciency virus (FIV), feline leukemia virus (FeLV), pseudorabies, thiamine defi ciency, intoxications, and other causes of meningitis and encephalitis

Treatment

Treatment is unsuccessful and RV infection is invariably fatal.

Because of the public health risk, animals with suspected RV infection are euthanized.

Monitoring and Prevention

Vaccination provides excellent protection against the disease.

  • » Vaccination in most states is initiated by a killed vaccine at 12 to 16 weeks of age, with a booster vaccination at 1 year of age followed by triennial boosters.
  • » Some states require yearly vaccination; some vaccines are approved for yearly use only (e.g., canary poxvectored vaccines).
  • » The use of rabies vaccine has been associated with injection-site sarcomas in cats.

In most municipalities, rabies vaccination for dogs and cats is required by law.

Recommendations for preexposure vaccination and management and postexposure management are available yearly from the American Veterinary Medical Association and from the Centers for Disease Control and Prevention

Use extreme care when handling an animal or its bodily fluids if RV infection is suspected.

RV is susceptible to most disinfectants and does not survive well in the environment.


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