Rabies Virus (RABV)
Enveloped negative-sense single-stranded RNA virus with a characteristic bullet morphology and a 5-gene genome (N-P-M-G-L). G protein binding to nAChR, NCAM, and p75NTR at the neuromuscular junction initiates dynein-mediated retrograde axonal transport, carrying the nucleocapsid to the CNS at ~2–3 mm/day. The virus then spreads trans-synaptically through limbic circuits, producing a fulminant encephalomyelitis. Pathognomonic Negri bodies (eosinophilic intracytoplasmic inclusions of N-protein aggregates) appear in hippocampal neurons. Clinical disease is encephalitic (~80%) or paralytic (~20%). No proven cure exists; palliative care and the anecdotal Milwaukee Protocol are the only options once symptoms begin. Post-exposure prophylaxis (wound wash + RIG + 4-dose vaccine) is essentially 100% effective when initiated before symptom onset. ~59,000 deaths per year globally, predominantly in Asia and Africa from dog bites.
Genome and Structural Proteins
| Genome | ~12 kb negative-sense ssRNA; single-stranded, non-segmented; 5 genes in obligate order: 3′-N-P-M-G-L-5′ |
| N (nucleoprotein) | Encapsidates RNA in a helical ribonucleoprotein (RNP) complex; protects genome from nuclease degradation; primary target for diagnostic DFA (direct fluorescent antibody) and RT-PCR |
| P (phosphoprotein) | Essential polymerase cofactor (L subunit co-factor); also an interferon antagonist — blocks nuclear import of phospho-STAT1 and STAT2, preventing IFN-stimulated gene transcription in peripheral nerve terminals before CNS entry |
| M (matrix protein) | Structural link between nucleocapsid and envelope; critical determinant of bullet-shaped morphology; drives budding from plasma membrane |
| G (glycoprotein) | Homotrimeric surface spike; sole target of virus-neutralising antibodies; mediates receptor binding and low-pH-triggered membrane fusion; Arg333 critical for full neurotropism and pathogenicity (Arg→Gln attenuates) |
| L (large protein) | RNA-dependent RNA polymerase (RdRp) and mRNA capping enzyme; responsible for transcription of 5 mRNAs and genome replication via full-length antigenome intermediate |
| Particle shape | Distinctive asymmetric bullet: flat at one end (L face, 3′ genome terminus), rounded at the other (S face, 5′ terminus); 75 nm diameter × 180 nm length; unique among human-pathogenic viruses |
Entry — Neuromuscular Junction and Three Receptors
Receptor 1 — Nicotinic Acetylcholine Receptor (nAChR)
RABV G protein contains a sequence homologous to the alpha-neurotoxin binding region of snake venom curaremimetic toxins. This region binds the acetylcholine binding site on the alpha subunit of nAChR at the postsynaptic NMJ membrane with moderate affinity. The high density of nAChR at NMJ folds makes this the predominant initial binding site. G protein binding may also partially block neuromuscular transmission — a mechanism possibly contributing to functional neurological deficits disproportionate to the modest neuronal cell death observed at autopsy.
Receptor 2 — Neural Cell Adhesion Molecule (NCAM / CD56)
NCAM is expressed broadly on neurons, muscle cells, and Schwann cells. It functions as an alternative receptor for RABV G protein, facilitating entry at non-NMJ sites. NCAM-mediated entry may account for infection of sensory neurons and Schwann cells, contributing to the sensory prodrome (paraesthesias at the bite site) that is pathognomonic when present.
Receptor 3 — p75 Neurotrophin Receptor (p75NTR / LNGFR)
The low-affinity nerve growth factor receptor p75NTR is expressed on peripheral neurons and is the receptor for which G protein position-333 arginine (Arg333) is most critical. p75NTR binds RABV G protein at a site distinct from the NGF binding site. Arg333 → Gln or Gly mutations at this position severely attenuate neuroinvasiveness, which is why all attenuated vaccine strains carry this mutation. Following G-protein binding at any receptor, RABV is internalised by clathrin-mediated endocytosis; endosomal acidification (pH ~5.5) triggers G protein refolding and membrane fusion, releasing nucleocapsid into the cytoplasm.
Retrograde Axonal Transport — The Incubation Window
RABIES AXONAL TRANSPORT AND CNS INVASION
Bite (infected saliva deposited into muscle/SC tissue)
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v
Virus replicates locally in muscle cells (brief period)
OR enters motor/sensory nerve terminals directly
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v
G protein binds nAChR / NCAM / p75NTR at NMJ
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v
Clathrin-mediated endocytosis --> endosomal acidification
--> G protein fusogenic conformational change
--> nucleocapsid released into axon cytoplasm
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v
RETROGRADE AXONAL TRANSPORT
Nucleocapsid (RNP) associated with dynein/dynactin motor complex
on microtubule tracks (moves toward neuronal cell body, soma)
Rate: ~2-3 mm/day in motor axons (slower ~1mm/day in sensory)
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[INCUBATION PERIOD -- length determines by bite location]
Face/neck bite: 2-3 cm to brainstem --> ~1-2 weeks
Arm bite: 50-80 cm to spinal cord --> 1-3 months
Foot bite: 100-120 cm to spinal cord --> 1-6+ months
(range: days to years, median 1-3 months)
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v
Cell body in dorsal root ganglion / anterior horn
--> virus replicates --> new virions produced
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v
TRANS-SYNAPTIC SPREAD (centripetal)
New virions released at synapses infect post-synaptic neurons
Brainstem --> cerebellum --> hippocampus --> limbic cortex
--> neocortex (ascending spread through limbic system)
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v
CENTRIFUGAL SPREAD (after CNS infection established)
Along autonomic/motor efferents to:
- Salivary glands (enables transmission via bite)
- Cornea, skin (enables antemortem diagnosis by DFA)
- Heart, adrenal glands, retina
PEP WINDOW:
Post-exposure prophylaxis MUST be initiated before the
virus reaches the CNS. Once CNS penetration occurs and
encephalitis begins, PEP cannot prevent death.
The axonal transport window IS the therapeutic window.
Immune Evasion — Exploiting Neuronal Privilege
IFN Antagonism by P protein
The phosphoprotein (P) directly blocks nuclear import of phosphorylated STAT1 (pSTAT1) and STAT2, preventing IFN-stimulated gene (ISG) transcription. This evasion is active in peripheral nerve terminals during the retrograde transport phase — the period when the immune system has the best chance of intercepting the virus. Without robust ISG responses, RABV traverses peripheral nerves undetected.
Minimal Neuronal Apoptosis
RABV suppresses neuronal apoptosis during active replication — maintaining neuronal viability is essential for sustained trans-synaptic spread. This contrasts with many other neurotropic viruses (e.g., herpes, encephalitic flaviviruses) that rapidly kill neurons. The clinical severity of rabies encephalitis is therefore largely functional — disruption of ion channel function, synaptic transmission, and neurotransmitter release — rather than structural neuronal destruction.
Blood-Brain Barrier and Immune Exclusion
Neurons in the CNS express low levels of MHC-I, are relatively inaccessible to CTL killing, and are protected by the blood-brain barrier from circulating antibodies and immune cells. RABV exploits this immune privilege: by the time adaptive immunity generates RABV-neutralising antibodies, the virus is replicating in an immunologically shielded compartment. This is why even high neutralising antibody titres (induced by PEP) cannot rescue patients once clinical encephalitis is established.
Clinical Disease — Stages and Forms
| Stage | Duration | Hallmarks |
|---|---|---|
| Incubation | Days to years (median 1–3 months) | Completely asymptomatic; virus confined to peripheral nervous system during retrograde transport; PEP fully effective here |
| Prodrome | 2–10 days | Non-specific: fever, malaise, headache, nausea; pathognomonic when present: paraesthesias, pain, or itching at the healed bite site (centrifugal spread to sensory nerves from spinal cord) |
| Acute neurological — Encephalitic (furious) form | 2–7 days | ~80% of cases. Hydrophobia (water triggers laryngeal/pharyngeal spasms: inspiratory spasms on swallowing; RABV may suppress central swallowing inhibition); aerophobia (air currents trigger same); agitation, hyperexcitability, autonomic instability (tachycardia, hypersalivation, piloerection); fluctuating consciousness; preserved lucid intervals initially |
| Acute neurological — Paralytic (dumb) form | 2–14 days | ~20% of cases. Ascending flaccid paralysis mimicking Guillain-Barré syndrome; less dramatic hydrophobia/aerophobia; brainstem and respiratory failure. More often misdiagnosed; associated with bat exposures in some series |
| Coma and death | Days | Progressive brainstem failure; central apnoea; cardiac arrhythmias; multiple organ failure; death near-universal without aggressive ICU support |
Neuropathology
Negri bodies: Eosinophilic intracytoplasmic inclusion bodies in pyramidal neurons of the hippocampus (Sommer sector, CA3) and cerebellar Purkinje cells — pathognomonic of rabies. Represent phase-separated condensates (membrane-less organelles) of N-protein and viral RNA. DFA against N-protein on brain touch impressions or frozen sections is the reference diagnostic standard (sensitivity ~99% in brain, lower in ante-mortem biopsies). RT-PCR (saliva, CSF, nuchal skin biopsy) is preferred for ante-mortem diagnosis.
Babes nodules: Small perivascular glial and lymphocytic aggregates throughout brainstem and spinal cord; non-specific but supportive of diagnosis in context.
Paradox of minimal cell death: Neuronal loss is relatively modest despite overwhelming encephalitis severity. This supports the hypothesis that RABV causes primarily functional neurological disruption — via inhibition of synaptic proteins, alteration of ion channels (GABAA, nAChR), and neurotransmitter release — rather than the structural neuronal destruction seen in HSV or Japanese encephalitis virus.
Post-Exposure Prophylaxis (PEP) — The Axonal Transport Window
- Immediate wound care: Vigorous washing with soap and water for ≥15 minutes; povidone-iodine application; debridement if indicated. This step alone may mechanically remove virus. Do not suture unless haemostasis requires it (suturing seals virus in).
- Rabies Immunoglobulin (RIG): Human RIG (HRIG) 20 IU/kg, or equine RIG (ERIG) 40 IU/kg for Day 0 only. Infiltrate as much as anatomically possible into and around the wound; remainder given IM at a distant site. RIG provides immediate passive antibody until the vaccine generates active immunity. NOT given to previously vaccinated individuals.
- Rabies vaccine — 4-dose series (CDC/WHO recommended for unvaccinated): Days 0, 3, 7, 14 IM (deltoid in adults; anterolateral thigh in children). HDCV (Human Diploid Cell Vaccine) or PCECV (Purified Chick Embryo Cell Vaccine) are the primary modern vaccines. Previously vaccinated: 2 doses only (Days 0, 3); no RIG required.
- Category-based risk stratification (WHO): Category I (touching/feeding animals, licks on intact skin): no PEP. Category II (nibbling unbroken skin, minor scratches): vaccine alone. Category III (transdermal bites/scratches, licks on broken skin, bat contact): full RIG + vaccine. All Category III require immediate action.
Milwaukee Protocol — Experimental Management of Established Rabies
In 2004, a 15-year-old Wisconsin patient (Jeanna Giese) survived clinical rabies following treatment with therapeutic coma induction (ketamine, midazolam), antivirals (ribavirin, amantadine), and intensive supportive care. A formal protocol (the Milwaukee Protocol) was developed and applied in >40 subsequent cases globally, with only 5–6 additional survivors. The original rationale was that coma would reduce metabolic demand while host immunity cleared the virus. Subsequent analysis suggests survival correlated with early onset of virus-neutralising antibodies and a degree of immune-mediated viral clearance — not with the pharmacological protocol itself. The Milwaukee Protocol is not recommended outside of experimental ethics-approved settings.
Vaccines — Pre-Exposure Prophylaxis (PrEP)
| Vaccine | Type | Schedule (PrEP) | Key notes |
|---|---|---|---|
| HDCV (Imovax Rabies) | Inactivated virus, human diploid cell grown | Days 0, 7, 21 or 28 IM; booster every 2–3 years or per titre testing | Reference standard; highest immunogenicity; recommended for high-risk occupations (veterinarians, lab workers, travellers to enzootic areas) |
| PCECV (RabAvert) | Inactivated virus, purified chick embryo cell grown | Same as HDCV | Equally immunogenic; lower cost; widely available globally |
| PVRV (Verorab) | Inactivated, Vero cell | Days 0, 7, 21 or 28 | WHO prequalified; extensively used in Asia and Africa; cost-effective for global programs |
| mRNA vaccines (experimental) | Lipid nanoparticle-encapsulated mRNA encoding G protein | Under clinical investigation (phase I/II) | Potential for low-cost, thermostable manufacture; accelerated antibody response; not yet approved |
PrEP does not eliminate the need for post-exposure care after a potential exposure, but simplifies it: 2 booster doses only (Days 0 and 3), no RIG required. This is critical in low-resource settings where RIG is often unavailable. RIG unavailability is the most significant barrier to effective PEP globally.
Cross-Atlas Connections
References
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- Lafon M. Rabies virus receptors. J Neurovirol. 2005;11(1):82-87. doi:10.1080/13550280590900427 · PubMed 15804965
- Rupprecht CE, Briggs D, Brown CM, et al. Use of a reduced (4-dose) vaccine schedule for postexposure prophylaxis to prevent human rabies. MMWR Recomm Rep. 2010;59(RR-2):1-9. PubMed 20300058
- Jackson AC, Warrell MJ, Rupprecht CE, et al. Management of rabies in humans. Clin Infect Dis. 2003;36(1):60-63. doi:10.1086/344905 · PubMed 12491202
- Willoughby RE Jr, Tieves KS, Hoffman GM, et al. Survival after treatment of rabies with induction of coma. N Engl J Med. 2005;352(24):2508-14. doi:10.1056/NEJMoa050382 · PubMed 15958806
- Bennett JE, Dolin R, Blaser MJ. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Elsevier; 2020.
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This entry covers RABV genome, axonal transport, clinical stages, PEP protocol, and vaccine types. Planned expansions: RABV molecular neuroscience, bat reservoir ecology, global elimination targets (WHO 2030), and novel mRNA vaccine platforms.