Canine Rabies Study Guide

Overview and Clinical Importance

Rabies is a fatal viral zoonosis caused by neurotropic viruses in the genus Lyssavirus (family Rhabdoviridae). The disease causes acute encephalitis in all warm-blooded mammals and is virtually 100% fatal once clinical signs appear. Rabies remains a critical public health concern worldwide, with dogs serving as the primary reservoir in endemic regions. Understanding rabies diagnosis, management of suspected cases, and public health protocols is essential for NAVLE success and clinical practice.

Etiology

Viral Characteristics

Rabies virus is a bullet-shaped, single-stranded, negative-sense RNA virus belonging to the genus Lyssavirus. The viral genome encodes five structural proteins: nucleoprotein (N), phosphoprotein (P), matrix protein (M), glycoprotein (G), and RNA-dependent RNA polymerase (L). The glycoprotein G is responsible for viral attachment to host cell receptors and is the primary target of neutralizing antibodies.

Rabies Virus Properties

Epidemiology

Reservoir Species and Geographic Distribution

In the United States, wild carnivores and bats serve as primary reservoirs. The canine rabies virus variant was eliminated from the US by 2008 through vaccination programs. However, domestic dogs remain at risk from spillover infections from wildlife reservoirs including raccoons, skunks, foxes, and bats. Globally, domestic dogs remain the most important source of human rabies, causing approximately 99% of human cases in endemic regions of Asia and Africa.

Major Wildlife Reservoirs in the United States

Transmission and Pathogenesis

Routes of Transmission

Rabies transmission occurs primarily through bite wounds when virus-laden saliva is deposited into tissues. The virus does not penetrate intact skin. Less common routes include contamination of open wounds or mucous membranes with infected saliva or neural tissue. Aerosol transmission has been documented in laboratory settings and bat caves but is extremely rare in natural conditions.

Pathogenesis Stages

Stage 1 - Local Replication: Following inoculation, rabies virus replicates locally in muscle tissue near the bite wound. The virus may remain at the inoculation site for days to weeks before entering peripheral nerves.

Stage 2 - Centripetal Spread: The virus enters peripheral nerves and travels via retrograde axonal transport toward the central nervous system at a rate of approximately 15-100 mm per day. During this phase, the virus is essentially "hidden" from the immune system within nervous tissue.

Stage 3 - CNS Infection: Upon reaching the brain, the virus replicates extensively in neurons, particularly in the limbic system (explaining behavioral changes), cerebellum, and brainstem. This stage correlates with onset of clinical signs.

Stage 4 - Centrifugal Spread: The virus spreads centrifugally from the CNS to multiple organs via peripheral nerves, including salivary glands (enabling transmission), cornea, and skin. Virus shedding in saliva may begin several days BEFORE clinical signs appear.

Clinical Signs

Clinical rabies in dogs progresses through three recognized phases, though considerable overlap occurs and not all animals exhibit all phases. The disease is invariably fatal once clinical signs appear, with death typically occurring within 7-10 days of symptom onset.

Phase 1: Prodromal Phase (1-3 Days)

The prodromal phase is characterized by vague, nonspecific signs that rapidly intensify. Clinical findings may include:

• Behavioral changes - normally aggressive dogs may become docile; friendly dogs may become nervous or shy
• Slight fever (may be absent)
• Apprehension, nervousness, or anxiety
• Pupil dilation
• Licking or scratching at the bite site (paresthesia)

Phase 2: Excitatory/Furious Phase (2-4 Days)

The furious form is classically described as "mad dog syndrome" and is characterized by:

• Extreme aggression - dogs may attack without provocation, biting at objects, other animals, or people
• Hyperreactivity to stimuli (sounds, light, touch)
• Restlessness and roaming - rabid dogs may travel long distances
• Pica - eating unusual objects (straw, sticks, stones, feces)
• Self-mutilation
• Altered vocalization (characteristic abnormal bark)
• Seizures

Phase 3: Paralytic/Dumb Phase (2-4 Days)

The paralytic form (dumb rabies) is actually MORE COMMON in dogs than the furious form, despite being less dramatic:

• "Dropped jaw" (pathognomonic) - paralysis of masseter muscles causes the mouth to hang open
• Inability to swallow - leads to drooling and "foaming at the mouth"
• Facial distortion from facial nerve paralysis
• Progressive ascending paralysis affecting limbs
• Choking sounds - owners may think the dog has something stuck in throat
• Depression and lethargy
• Death from respiratory paralysis

Comparison of Rabies Forms in Dogs

Diagnosis

CRITICAL: There is NO approved diagnostic test for rabies in living animals. Definitive diagnosis requires post-mortem examination of brain tissue. Clinical diagnosis based on signs alone is unreliable and should never be used for public health decisions.

Direct Fluorescent Antibody Test (DFA)

The DFA test is the GOLD STANDARD for rabies diagnosis and is recommended by WHO and CDC. This test detects rabies viral antigen in brain tissue impressions using fluorescein-labeled anti-rabies antibodies.

Negri Bodies (Histopathology)

Negri bodies are eosinophilic intracytoplasmic inclusion bodies (2-10 micrometers) found in neurons, particularly in the hippocampal pyramidal cells and cerebellar Purkinje cells. While historically important and pathognomonic when present, Negri bodies are found in only approximately 50% of rabies cases and are NO LONGER recommended for primary diagnosis due to lower sensitivity compared to DFA.

Other Diagnostic Methods

• Direct Rapid Immunohistochemistry Test (dRIT): Alternative to DFA with comparable sensitivity/specificity; uses light microscopy instead of fluorescence; suitable for field conditions
• RT-PCR: Highly sensitive; can detect viral RNA in decomposed samples; used for confirmatory testing and genotyping
• Virus Isolation: Mouse inoculation or cell culture; time-consuming (14-21 days); rarely used for routine diagnosis

Differential Diagnosis

Clinical signs of rabies may mimic other conditions affecting the central nervous system. Rabies should be considered in any dog presenting with unexplained neurological disease or acute behavioral changes, especially if vaccination status is unknown or the dog has potential wildlife exposure.

Management of Suspected Rabies Cases

CRITICAL: There is NO TREATMENT for clinical rabies in animals. Once neurological signs appear, the disease is invariably fatal. Management focuses on public health protection, appropriate quarantine, and definitive diagnosis.

Animals Showing Clinical Signs Consistent with Rabies

• Immediately isolate the animal in a secure enclosure to prevent contact with people and other animals
• Report to local public health authorities - rabies is a reportable disease
• Recommend euthanasia and laboratory testing - the animal should be humanely euthanized and brain tissue submitted for DFA testing
• Use appropriate PPE when handling: gloves, eye protection, face shield; avoid direct contact with saliva
• Preserve the brain - do NOT damage the brain during euthanasia; include brainstem and cerebellum

Quarantine Protocols

10-Day Quarantine: Dog/Cat That Bites a Person

Any healthy dog, cat, or ferret (regardless of vaccination status) that bites or scratches a person must be confined and observed for 10 days. The rationale: if a dog was shedding virus in its saliva at the time of the bite, it will develop clinical signs and die within this period.

Post-Exposure Management: Dog Exposed to Rabid Animal

Rabies Vaccination

Rabies vaccination is legally required in most US states and is considered a CORE vaccine for dogs. All licensed rabies vaccines for dogs in the United States are inactivated (killed) vaccines.

Vaccination Protocol

Public Health and Zoonotic Considerations

Rabies is one of the most important zoonotic diseases because of its virtually 100% fatality rate once clinical signs develop. Approximately 59,000 people die from rabies annually worldwide, primarily in Asia and Africa where dog vaccination programs are inadequate.

Veterinary Personnel Protection

• Pre-exposure vaccination is recommended for all veterinary personnel working with susceptible animals
• Use appropriate PPE: gloves, eye protection, face shields when handling suspected rabid animals
• Avoid contact with saliva and neural tissue from suspected cases
• Serologic titer testing every 2 years for those with ongoing risk; booster if titer falls below 0.5 IU/mL

Human Post-Exposure Prophylaxis (PEP)

While veterinarians do not administer human PEP, understanding the protocol helps guide bite victim counseling. Human PEP includes:

• Wound care: Immediate thorough washing with soap and water for at least 15 minutes
• Rabies immune globulin (RIG): Infiltrated into and around the wound
• Rabies vaccine: Series of 4-5 doses (unvaccinated individuals)

Memory Aids and Board Tips

"RABIES" Mnemonic for Clinical Signs

R - Restlessness and behavioral changes

A - Aggression (furious form) or Ataxia (paralytic form)

B - Bizarre behavior, biting, roaming

I - Inability to swallow (dysphagia) → drooling

E - Encephalitis leading to seizures and death

S - Salivation (hypersalivation) and paralysis

Key Numbers to Memorize

10 days = Quarantine for dog/cat that BITES a person

28 days = Time to be considered immunized after initial vaccine

45 days = Observation for VACCINATED dog exposed to rabid animal

4 months = Strict quarantine for UNVACCINATED dog/cat exposed to rabid animal

7-10 days = Time from onset of signs to death