Rabies

Rabies is a fatal neurotropic viral infection caused by the rabies virus (genus Lyssavirus, family Rhabdoviridae), transmitted primarily through saliva-contaminated bite wounds or mucous membrane exposure from infected animals. It remains one of the most lethal infectious diseases known to humankind, with a case fatality rate exceeding 99% once clinical symptoms develop. Globally, approximately 59,000 deaths occur annually, predominantly in Asia and Africa, with the vast majority of deaths occurring in low-resource settings where post-exposure prophylaxis (PEP) is unavailable. The disease is clinically significant because it is almost invariably fatal after symptom onset but is entirely preventable with appropriate post-exposure vaccination and immunoglobulin administration. Medical students and residents must recognize exposure risk, understand the critical importance of timely prophylaxis, and be familiar with both classic and variant presentations that may present diagnostic challenges.

The pathophysiology of rabies involves a well-characterized sequence of viral replication, neuronal dissemination, and central nervous system (CNS) invasion resulting in encephalitis and brainstem dysfunction.

The rabies virus, a single-stranded, negative-sense RNA virus, initially replicates at the inoculation site (dermal wound) within myocytes and fibroblasts for days to weeks before neuronal invasion occurs. This eclipse period is critical—viral particles are predominantly contained at the local site. The virus then infects sensory nerve terminals at the bite wound through interaction of viral glycoprotein (G) with nicotinic acetylcholine receptors and other cellular receptors (phosphoprotein P interacts with interferon antagonism pathways). The receptor-mediated endocytosis allows viral entry; the virus then undergoes uncoating in endocytic vesicles, releasing the ribonucleoprotein complex into the cytoplasm. Local replication within motor and sensory nerve terminals is followed by retrograde axonal transport along microtubules—the virus hitchhikes within vesicles along the axon toward the spinal cord at rates of approximately 50-100 mm/day, explaining why proximal bite wounds (head, neck) lead to shorter incubation periods than distal bites (extremities).

Once the virus reaches the dorsal root ganglia and spinal cord, it replicates rapidly in neurons, establishing widespread CNS infection. The virus exhibits remarkable neurotropism and neuropathogenesis despite minimal direct cytopathic effect in many regions. Viral glycoproteins are incorporated into synaptic vesicles, and the virus preferentially spreads via trans-synaptic anterograde and retrograde transmission, moving bidirectionally from neuron to neuron through synaptic connections. This dissemination pattern explains the pattern of CNS involvement and progressive encephalitis. The virus crosses the blood-brain barrier not through direct viral crossing but by infected leukocyte infiltration and neuronal infection originating from spinal cord involvement. Viral ribonucleoprotein complexes are transported centrifugally along peripheral nerves, including the vagus nerve, to innervate organ systems and ultimately reaching the salivary glands, cornea, and skin—which become sources for viral shedding and diagnostic sampling.

Despite producing potent innate immune responses (interferon production), the rabies virus has evolved sophisticated immunoevasion mechanisms. The viral phosphoprotein (P) antagonizes interferon signaling by blocking STAT1 and STAT2 phosphorylation, effectively silencing the interferon response and preventing the development of robust antiviral immunity during the critical early phase. The virus replicates efficiently while remaining largely "invisible" to early immune detection. Paradoxically, when adaptive immunity finally develops, the CNS inflammatory response itself contributes to neuronal death—CD8+ T lymphocytes and microglial activation in response to high viral loads cause synaptic dysfunction and neuronal death through bystander damage, excessive cytokine production (IL-6, TNF-α, IL-1β), and complement activation. The encephalitis is particularly severe in the brainstem, limbic system, and hippocampus, explaining the autonomic dysregulation, behavioral changes, fear of water (hydrophobia), and fatal arrhythmias.

The medulla oblongata shows particularly severe pathological changes (Negri bodies—eosinophilic cytoplasmic inclusions containing viral ribonucleoproteins—are pathognomonic but found in only ~80% of cases). Neuronal loss and inflammation in the medullary centers controlling respiration and cardiovascular function lead to apnea, cardiac arrhythmias, hypotension, and autonomic storm. Damage to the nucleus ambiguus and dorsal motor nucleus of the vagus results in loss of vagal parasympathetic tone, producing tachycardia and hypertension initially, followed by bradycardia and sudden asystole. The hypothalamus is damaged, producing fever, diaphoresis, and altered thermoregulation. Hydrophobia results from damage to brainstem centers controlling swallowing and increased sensitivity to external stimuli; aspiration of saliva is prevented by reflex inhibition of swallowing triggered by attempted water ingestion or even the sight/sound of water.

Notably, histologic examination of rabid brains shows surprisingly modest inflammatory infiltrate despite severe clinical encephalitis, contributing to diagnostic delays. The inflammation is primarily composed of microglial nodules and perivascular lymphocytic cuffs without extensive neuronal necrosis—the disease manifests more as functional neuronal dysfunction from viral replication than from massive inflammatory cell death.

The primary etiology is bite or scratch exposure to a rabid animal. The principal rabies reservoir varies geographically: in North America and Western Europe, raccoons, skunks, foxes, and bats account for the majority of cases; in Latin America and the Caribbean, vampire bats and dogs predominate; in Asia and Africa, dogs are responsible for up to 95% of human rabies deaths (post-bite fatality rate if untreated approaching 100%). The probability of viral transmission depends on the animal species involved—bat exposure carries inherently higher transmission risk per exposure due to higher viral loads, though the absolute number of exposures to dogs is vastly greater. Bites to the head, face, neck, and upper extremities pose higher risk than bites to the legs because they shorten the incubation period (proximity to the CNS). Multiple bite wounds carry higher viral load exposure.

Increasingly recognized is transmission through bat contact without documented bite, including aerosol exposure in caves inhabited by infected bats and contact of bat saliva with mucous membranes or minor skin breaks. Several cases of rabies in the United States have resulted from bat exposure where the patient had no recalled bite. The natural history of bat rabies often produces a slower, paralytic ("dumb rabies") variant rather than the furious form. Occupational exposure (spelunking in bat-inhabited caves) and travel to endemic areas increase risk.

Organ transplantation from donors with undiagnosed rabies has caused cluster cases; this represents a critical epidemiologic concern in developed countries where wildlife rabies is uncommon, and exposure history may be overlooked. Non-bite exposures include corneal transplants, neural tissue grafts, and other tissues. Rarely, mucous membrane contact with saliva from a rabid animal (particularly in laboratory settings) may transmit infection.

While rabies can infect immunocompromised hosts, the disease course may be altered—specifically, the incubation period may be prolonged, and the presentation may be atypical. Patients on immunosuppressive therapy post-transplant who acquire rabies may show delayed symptom onset and modified immune responses.

Living or working in endemic areas (veterinarians, wildlife handlers, laboratory personnel working with the virus, spelunkers) increases exposure risk. Recent travel to rabies-endemic regions without appropriate post-exposure prophylaxis after animal exposure.

The clinical manifestations of rabies follow a stereotyped sequence despite individual variation in incubation period and presentation style.

The disease begins with nonspecific constitutional symptoms mimicking viral illness: fever (often low-grade), malaise, fatigue, headache, and myalgias. Critically, paresthesias or pruritus at or near the bite site may occur and is highly suggestive of rabies encephalitis; this is thought to result from viral replication in sensory ganglia and manifests as pain, tingling, or itching often burning in character. The prodrome may last 2-10 days before progression to neurological phase.

This classic presentation includes progressive behavioral changes and autonomic hyperactivity:

This variant, more commonly associated with bat-transmitted rabies, presents with ascending paralysis often mimicking Guillain-Barré syndrome:

Some patients may present with:

The triad of hydrophobia, aerophobia, and encephalitis is highly specific for rabies when present, but absence of these features does not exclude the disease. Paralytic rabies presentations are frequently initially misdiagnosed as Guillain-Barré syndrome, resulting in critical delays in recognition.

Diagnosis of rabies is challenging because the disease is rare in developed countries, no single rapid point-of-care test is universally available early in the disease course, and the virus is difficult to culture safely. A high index of suspicion based on exposure history is critical.

A detailed travel history, occupational exposure, and specific questioning about animal contact (bite, scratch, mucous membrane contact, or bat exposure) must be elicited. The specific animal species, geographic location, whether the animal is available for testing, and the interval since exposure should be documented. Key questions include: "Have you been bitten or scratched by any animal?" "Any contact with bats, particularly in caves or enclosed spaces?" "Have you had any animal saliva contact with broken skin or mucous membranes?" "Organ or tissue transplant?" The latency period—time from exposure to symptom onset—is typically 1-3 months but ranges from 7 days to over 1 year. Shorter intervals suggest more proximal bites (head/neck); longer intervals suggest distal extremity bites.

A punch biopsy (3 mm) from the nape of the neck is taken and stained with fluorescent anti-rabies nucleoprotein antibodies. This test achieves sensitivity of 50-80% when positive (highly specific, ~99%), but negative results do not exclude rabies. DFA can be performed urgently (within hours) in centers with expertise. Viral antigen is found in nerve fibers at the base of hair follicles; therefore, biopsy should include hair follicles.

The gold standard diagnostic test involves examination of brain tissue post-mortem (typically medulla oblongata, hippocampus, cerebral cortex) using fluorescent anti-rabies monoclonal antibodies. Sensitivity approaches 100% but obviously provides only post-mortem confirmation. Negri bodies—pathognomonic cytoplasmic inclusions—are present in ~80% of cases but are not required for diagnosis.

Real-time RT-PCR of cerebrospinal fluid (CSF) and saliva is increasingly available and achieves high sensitivity (80-90%) and specificity (>95%) when positive. This is becoming the preferred diagnostic modality in developed countries due to rapid turnaround (hours to 24 hours) and high sensitivity in ante-mortem diagnosis. Multiple samples (CSF, saliva, serum, urine) increase diagnostic yield.

Attempting to culture rabies virus is dangerous due to biosafety concerns and is restricted to reference laboratories with BSL-4 facilities. Intracerebral inoculation of mouse brain or cell culture (neuroblastoma, baby hamster kidney cells) can demonstrate viral growth, but this requires days to weeks and is impractical for acute diagnosis.

Findings are often normal or nonspecific. When abnormalities are present:

Imaging is useful primarily to exclude mimics (mass lesions, subdural hematoma, other defined encephalitides).

CSF findings are nonspecific:

Contraindications and pitfalls: there is no contraindication to PEP — pregnancy, infancy, and immunosuppression are not reasons to withhold it. Avoid corticosteroids/immunosuppressants during PEP, and do not exceed the 20 IU/kg RIG dose.