📚 Central Nervous System Special Senses Module L5 Rabies

📚 Lecture Overview

Rabies is an acute, almost universally fatal infection of the central nervous system caused by a negative-sense single-stranded RNA virus of the family Rhabdoviridae. Transmission occurs primarily through the saliva of infected animals via bites or wound contamination. Because clinical rabies is non-treatable once symptoms develop, prompt recognition, preventive vaccination, and immediate post-exposure prophylaxis (PEP) are critical to preventing mortality.

🎯 Key Concepts & Definitions

• Rhabdoviridae: The virus family characterized by rod- or bullet-shaped, enveloped particles containing single-stranded negative-sense RNA.
• Hydrophobia: Involuntary, painful pharyngeal spasms triggered by trying to swallow liquids, highly specific to furious rabies.
• Aerophobia: Severe aversion or fear triggered by drafts or fresh air, a pathognomonic clinical feature of furious rabies.
• Furious Rabies: The most common form of rabies, characterized by central nervous system hyperactivity, agitation, autonomic dysfunction, and seizures.
• Paralytic Rabies: A less common (20%) clinical presentation marked by ascending muscle paralysis, a longer disease course, and frequent misdiagnosis.
• Post-Exposure Prophylaxis (PEP): A combination of immediate local wound care, passive antibody administration (HRIG), and active vaccination (HDCV/PCEC) given immediately after exposure.

📖 Main Content

1. Virology & Viral Structure

• Classification: Family Rhabdoviridae, Genotype 1 (responsible for the vast majority of worldwide cases).
• Morphology: Rod- or bullet-shaped enveloped particles with protruding glycoprotein spikes surrounding an inner ribonucleocapsid.
• Genome: Single-stranded, non-segmented negative-sense RNA.
• Host Range: Extremely broad; affects all warm-blooded animals (mammalian species vary in susceptibility).

2. Transmission & Sources

• Primary Source: Saliva of infected animals. Dogs and cats shed the virus in saliva 3 to 6 days before clinical symptoms appear and continue shedding until death.
• Modes of Transmission:
• Animal bites: Most frequent route.
• Non-bite exposures: Contamination of scratches, abrasions, or open wounds with infectious saliva.
• Inhalation: Aerosolized virus in laboratory settings or bat caves.
• Human-to-human: Extremely rare; reported only in recipients of infected donor corneal transplants.

3. Clinical Progression & Manifestations

A. Incubation Period

• Typically 3 to 8 weeks (range: 1 week to >1 year).
• Factors influencing duration:
  1. Bite site location (shorter if closer to CNS or in heavily innervated areas).
  2. Severity of the wound.
  3. Amount of virus inoculated.
  4. Protection provided by clothing.

B. Prodromal Phase (2–10 Days)

• Non-specific symptoms: Fever, malaise, anorexia, headache, photophobia, nausea, vomiting.
• Pathognomonic feature: Severe local pruritus (itching) and paresthesia at the bite site.

C. Neurological Disease Courses

4. Laboratory Diagnosis

• Human Diagnostic Limitations: There are NO diagnostic tests to detect rabies in humans before clinical symptoms appear.
• Postmortem Diagnostic Gold Standard: Detection of rabies virus antigen by Fluorescent Antibody Test (FAT) on brain tissue specimens.
• Premortem Diagnostic Tests in Humans:
• Skin Biopsy: Taken from the nape of the neck to detect viral antigen in cutaneous nerves near hair follicles.
• Saliva: Detection of viral RNA using RT-PCR.
• CSF & Serum: Neutralizing antibody detection via indirect immunofluorescence or virus neutralization (antibodies in CSF strongly indicate infection).
• Animal Management: Suspected rabid animals must be sacrificed immediately for neural tissue examination, or quarantined for 10 days if under observation.

5. Prevention & Post-Exposure Prophylaxis (PEP)

Rabies is a preventable but non-treatable disease.

       [ Exposure Incident ]
                 │
                 ▼
    [ Immediate Wound Washing ]
       (Soap & Water 15 min)
                 │
                 ▼
     [ Local HRIG Infiltration ]
      (Half at wound, half IM)
                 │
                 ▼
  [ 5-Dose HDCV / PCEC Vaccine ]
    (Days 0, 3, 7, 14, and 28)

Vaccines

• All human vaccines contain inactivated virus.
  1. Human Diploid Cell Vaccine (HDCV): Vaccine of choice; grown in human diploid cells and inactivated with $\beta$-propiolactone.
  2. Purified Chick Embryo Cell Vaccine (PCEC): Grown in chicken fibroblasts.
• Animal Vaccine: Recombinant vaccinia virus carrying the rabies glycoprotein gene (administered orally to wildlife and domestic animals).

Immunoglobulins

• Human Rabies Immunoglobulin (HRIG): Derived from hyperimmunized human plasma.
• Equine Rabies Immunoglobulin (ERIG): Derived from immunized horses (used if HRIG is unavailable).

Prophylaxis Protocols

• Pre-exposure Prophylaxis:
• Indications: High-risk groups (vets, animal handlers, lab workers).
• Regimen: 3 IM injections on Days 0, 7, and 21 (booster every 2 years if risk persists).
• Note: Pre-exposure prophylaxis does not eliminate the need for prompt PEP following a new exposure.
• Post-exposure Prophylaxis (PEP) Regimen:
  1. Wound Care: Immediate, thorough washing with soap and water; apply virucidal agent (povidone-iodine). DO NOT SUTURE THE WOUND (avoids exposing nerve endings to virus).
  2. Passive Immunization: Administer HRIG once on Day 0. Infiltrate half the dose into/around the wound site; inject the remainder IM at a distant site (anterolateral thigh).
  3. Active Immunization: 5 IM doses of HDCV or PCEC administered in the deltoid region on Days 0, 3, 7, 14, and 28.

Treatment

• No specific effective antiviral therapy exists (Interferon and Ribavirin have no beneficial effect).
• Supportive care may prolong survival, but outcomes are almost 100% fatal once clinical symptoms appear.

📊 Visual Learning

Diagram 1: Clinical Progression

Diagram 2: Post-Exposure Prophylaxis Steps

Diagram 3: Primary Routes of Infection

💡 Important Points to Remember

• Rabies virus is bullet-shaped with a negative-sense single-stranded RNA genome (family Rhabdoviridae).
• Dogs and cats shed the virus in their saliva 3–6 days BEFORE clinical symptoms appear.
• Hydrophobia and aerophobia are pathognomonic symptoms of furious rabies.
• Paresthesia and intense itching at the bite site during the prodrome phase is a key diagnostic clue.
• Human rabies CANNOT be diagnosed prior to symptom onset; there are no presymptomatic screening tests.
• NEVER suture a wound from a potentially rabid animal because it forces viral particles deeper into proximity with nerve endings.
• Direct Fluorescent Antibody Test (FAT) on brain tissue is the postmortem diagnostic gold standard.
• Pre-exposure prophylaxis requires 3 doses (Days 0, 7, 21), but individuals STILL REQUIRE PEP if exposed later.
• Post-exposure active vaccination requires 5 doses (Days 0, 3, 7, 14, 28) given in the deltoid muscle.
• HRIG is given only ONCE: half around the bite wound and half in the anterolateral thigh (never in the same deltoid muscle as the vaccine).
• Antiviral drugs like Ribavirin and Interferon are ineffective against clinical rabies.

⚠️ Common Exam Questions & Traps

Exam Tricks & MCQs

1. The Wound Suturing Trap:
   - Question Scenario: A patient arrives with a deep, bleeding bite from a stray dog. Option choices include cleansing, giving HRIG, giving vaccine, and primary wound closure/suturing.
   - Examiner Trick: Students often select primary wound closure to stop bleeding or promote healing.
   - Correct Action: Never suture the wound. Suturing traps virus particles and exposes disrupted local nerve endings to the virus.

2. The "Vaccinated Professional" Trap:
   - Question Scenario: A veterinarian who received a complete 3-dose pre-exposure vaccination series 1 year ago is bitten by a rabid animal.
   - Examiner Trick: "Does this patient need post-exposure prophylaxis?"
   - Correct Action: YES. Pre-exposure prophylaxis does NOT eliminate the need for PEP; it only simplifies/modifies subsequent management.

3. Presymptomatic Screening Confusion:
   - Question Scenario: Asking for the best lab test to screen an asymptomatic patient 2 days after a bat exposure.
   - Examiner Trick: Offering RT-PCR, blood cultures, or antibody titers as presymptomatic options.
   - Correct Action: Select "No test is available." Rabies cannot be diagnosed in humans before clinical symptoms appear.

4. The Paralytic Rabies Misdiagnosis:
   - Question Scenario: A patient presents with ascending weakness and paralysis weeks after an uncharacterized exposure.
   - Examiner Trick: Steering students toward Guillain-Barré Syndrome or spinal cord lesions.
   - Correct Action: Look for history of animal contact or bite site paresthesia; 20% of rabies cases present as paralytic rabies.

5. HRIG Inoculation Site Error:
   - Question Scenario: Asking where to inject HRIG in relation to the vaccine.
   - Examiner Trick: Suggesting mixing HRIG with the vaccine or administering both in the same deltoid.
   - Correct Action: Infiltrate half around the wound and half in the anterolateral thigh. Never inject HRIG into the same anatomical site as the vaccine.

📝 Quick Review Checklist

I can state the viral family (Rhabdoviridae), morphology (bullet-shaped), and genome type (ssRNA negative-sense).
I know how many days before clinical onset dogs and cats shed virus in their saliva (3–6 days).
I can list the key clinical differences between furious rabies and paralytic rabies.
I understand why rabies cannot be diagnosed in humans during the asymptomatic incubation period.
I know the postmortem diagnostic gold standard (Fluorescent Antibody Test on brain tissue).
I can list the exact days for the 5-dose PEP vaccine schedule (Days 0, 3, 7, 14, 28).
I know the rules for HRIG administration (given once, half around wound, half in thigh).
I remember why wounds from rabid animals must never be sutured.