Rabies virus — Questions

Histological feature: Negri bodies: eosinophilic cytoplasmic inclusions of viral nucleoprotein in Purkinje cells, hippocampal and brainstem neurons. Pathognomonic but not fully sensitive; absence does not exclude rabies.

Virus: Rabies virus (genus Lyssavirus, family Rhabdoviridae): a bullet-shaped, enveloped, negative-sense RNA virus (Baltimore class V), ~12 kb genome encoding N, P, M, G and L.

Pre-exposure prophylaxis (PrEP): vaccine on days 0 and 7 (intramuscular or intradermal) for high-risk groups (laboratory, veterinary and animal workers, bat handlers, and travellers to endemic areas with poor PEP access). Immunocompromised patients receive a third dose at day 21 to 28.

Post-exposure prophylaxis (PEP), Category III exposure: wash the wound for 15 minutes, antibiotics/tetanus vaccine as indicated, vaccine on days 0, 3, 7 and 14 to 28 plus rabies immunoglobulin (20 IU/kg HRIG or 40 IU/kg ERIG) infiltrated into the wound within 7 days of the first dose. Previously vaccinated patients: two doses (days 0 and 3), no RIG.

Five lyssaviruses besides classical rabies virus (RABV) have been detected in South Africa:

• Duvenhage virus (DUVV): Phylogroup 1; insectivorous bats; rare fatal human encephalitis.
• Mokola virus (MOKV): Phylogroup 2; not cross-protected by vaccine or RIG.
• Lagos bat virus (LBV): Phylogroup 2; African fruit bats; no human cases.
• Matlo bat lyssavirus (MBLV): recently described, highly divergent.
• Phala bat lyssavirus (PBLV): recently described in a Schlieffen’s bat.

Standard vaccine and RIG cross-protect only within Phylogroup 1, so any bat contact is managed as Category III regardless.

A complete answer follows the virus from the bite to the brain and out to the saliva, stressing that spread is neural and that there is no viraemia.

Local replication. After inoculation in saliva, rabies virus replicates in striated muscle near the neuromuscular junction.

Neural entry. It binds the nicotinic acetylcholine receptor at the motor end-plate (NCAM and p75NTR also implicated) and enters peripheral nerve terminals.

Centripetal transport. Virus travels by retrograde fast axonal transport to the spinal cord and brain. There is no viraemia, so it is shielded from circulating antibody, which is why post-exposure prophylaxis works.

CNS injury. The virus causes neuronal dysfunction rather than destruction, with a striking paucity of histological damage; eosinophilic cytoplasmic Negri bodies are pathognomonic, and the P protein blunts interferon signalling.

Centrifugal spread. From the CNS the virus passes outward along autonomic and sensory nerves to the salivary glands (high-titre shedding closes the transmission cycle) and skin. Once symptomatic, disease is effectively 100% fatal.

After CNS replication, rabies virus spreads centrifugally along sensory and autonomic nerves to multiple peripheral tissues, including the heart, adrenal medulla, gastrointestinal tract and skin.

The most clinically important destination is the salivary glands, where the virus replicates to high titres and is shed in saliva. This is the basis for bite transmission to the next host, closing the zoonotic cycle. Centrifugal spread to skin underlies the diagnostic value of the nuchal skin biopsy (virus in nerve fibres around hair follicles).

Rabies virus binds the nicotinic acetylcholine receptor (nAChR) at the postsynaptic membrane of the neuromuscular junction. Two additional neuronal receptors, neural cell adhesion molecule (NCAM) and p75 neurotrophin receptor (p75NTR), are also implicated in cell-to-cell spread within the nervous system.

The glycoprotein G is the viral attachment protein and mediates pH-dependent fusion after endocytosis. Entry is the rate-limiting step at the periphery; once the virus has entered an axon and begun retrograde transport, it is no longer accessible to neutralising antibody.

Negri bodies are eosinophilic cytoplasmic inclusion bodies composed predominantly of rabies viral nucleoprotein (N). They are classically described in the cytoplasm of Purkinje cells of the cerebellum, hippocampal pyramidal neurons, and brainstem neurons.

They are pathognomonic for rabies when present, but not 100% sensitive: their absence does not exclude infection. Direct fluorescent antibody (DFA) testing on brain tissue is more sensitive and is the post-mortem gold standard.

Rabies virus is the type species of the genus Lyssavirus, family Rhabdoviridae, order Mononegavirales. The genus name derives from the Greek lyssa, “frenzy”.

Other Rhabdoviridae genera include Vesiculovirus (vesicular stomatitis virus, mostly veterinary) and several plant and invertebrate viruses; Lyssavirus is the genus of medical importance, containing classical rabies virus and the rabies-related lyssaviruses (Duvenhage, Mokola, Australian bat lyssavirus and others).

After inoculation, rabies virus binds receptors at neuromuscular junctions (nicotinic acetylcholine receptor, NCAM, p75NTR) and undergoes retrograde fast axonal transport along peripheral nerves to the spinal cord and brain. There is no detectable viraemia during the incubation period, so the virus is sequestered in muscle and nerve.

This is why post-exposure prophylaxis works: while the virus is still on its way to the CNS, neutralising antibody (vaccine-induced or passive RIG) can reach it and neutralise it. Once the virus enters the CNS, antibody is excluded by the blood-brain barrier and PEP is too late.

The genus Lyssavirus is split into three antigenic phylogroups. Standard rabies vaccines and RIG cross-protect within Phylogroup 1, which includes the classical rabies virus, Duvenhage virus (SA-relevant), Australian bat lyssavirus, European bat lyssaviruses 1 and 2, and Irkut, Aravan and Khujand viruses.

Phylogroup 2 (Mokola, Lagos bat, Shimoni bat) is antigenically distinct, and current vaccines provide no reliable cross-protection. Phylogroup 3 (West Caucasian bat, Ikoma, Matlo bat) is similarly poorly protected.

Standard PEP works for dog rabies and most bat lyssavirus exposures, but provides no proven protection against Mokola virus.

Approximately 59,000 human deaths from rabies occur each year, almost certainly an underestimate given limited surveillance in endemic settings. Over 99% of cases are caused by domestic dog exposure. Asia and Africa carry the largest share of the burden.

The WHO’s stated target is zero human dog-mediated rabies deaths by 2030, pursued through mass dog vaccination and improved access to post-exposure prophylaxis. Bat-mediated rabies is the dominant terrestrial cycle in the Americas but is a much smaller contributor to global mortality.

Louis Pasteur administered the first successful rabies vaccine on 6 July 1885 to Joseph Meister, a nine-year-old severely bitten by a rabid dog. Pasteur had developed an attenuated preparation through serial passage of rabies virus in rabbit spinal cord. Meister survived. The achievement is regarded as opening the modern era of vaccinology, predating any understanding of viruses or even the germ theory in its mature form.

Modern rabies vaccines (HDCV, PCEC, Verorab) are inactivated whole-virus preparations grown in cell culture rather than animal nervous tissue, with dramatically improved safety and efficacy.

The Milwaukee protocol was the regimen used in the 2004 Wisconsin survivor (Jeanna Giese, bat-virus exposure, no prior vaccine). Subsequent attempts have failed in over 30 documented cases, with no proven viral clearance, no neuroprotection at autopsy in treated patients, and ketamine has been shown to be ineffective in cultured rabies-infected neurons. The Milwaukee protocol has no role in current management.

Symptomatic rabies remains effectively 100% fatal in the unvaccinated. Care is palliative. The handful of true survivors (mostly with prior partial vaccination, mostly with bat-variant exposure) reflects a less neurovirulent virus and pre-existing immunity, not the intervention.

Rabies virus is Baltimore class V: a non-segmented, negative-sense single-stranded RNA virus of approximately 12 kb. The genome is organised 3’-N-P-M-G-L-5’ and encodes five proteins:

• N: nucleoprotein
• P: phosphoprotein (polymerase cofactor; also blunts innate immunity)
• M: matrix protein
• G: glycoprotein (the major neutralising antigen and the target of all current vaccines)
• L: large polymerase (the RNA-dependent RNA polymerase)

This non-segmented negative-sense architecture is characteristic of the order Mononegavirales, shared with the paramyxoviruses, filoviruses and bornaviruses.

The typical incubation period is 1 to 3 months, but the range is very wide, from a few days to over a year, with rare documented cases at six years or more.

Incubation length depends on:

• Distance from inoculation to the central nervous system (proximal head and face bites are shortest).
• Inoculum size (larger viral dose, shorter incubation).
• Innervation of the bite site (richly innervated sites are shorter).

Post-exposure prophylaxis is effective if started promptly after exposure, because the long incubation gives vaccine-induced immunity time to develop before the virus reaches the CNS.

Approximately 80% of human cases present as the furious (encephalitic) form: episodes of hyperexcitability, agitation, aggression, hallucinations and confusion separated by lucid intervals, with autonomic dysfunction (hypersalivation, sweating, arrhythmias).

Hydrophobia, painful spasms of the diaphragm and inspiratory muscles triggered by swallowing or even the sight or mention of water, is pathognomonic but more common with canine than bat exposure. Aerophobia, the same spasms triggered by air movement, is equally characteristic.

The remaining 20% present as paralytic (dumb) rabies: ascending flaccid paralysis from the bitten limb, sphincter involvement, slower progression, hydrophobia rare. Both forms progress to coma and death.

The current ICTV-ratified species name is Lyssavirus rabies, under the binomial mandate that requires every virus species to be named “genus name + a single-word epithet”.

The previous name was Rabies lyssavirus (a “common-name first” format that has now been retired in favour of the genus-first binomial). The everyday name “rabies virus” remains correct and unaffected in clinical and laboratory practice.