Epidemiology and transmission routes
The infection is prevalent almost worldwide. The only exceptions are certain island nations where strict quarantine restrictions are maintained.
Sources of the virus are divided into two types of foci:
- Natural (sylvatic rabies): wild animals serve as the reservoir—foxes, arctic foxes, wolves, raccoon dogs, jackals, bats (including hematophagous species), and rodents.
- Anthropurgic (urban rabies): the primary danger is posed by infected domestic or stray dogs and cats.
Transmission most commonly occurs via the contact route—through a bite or when infected saliva comes into contact with damaged skin. The virus accumulates in the salivary glands of the sick animal, and the animal becomes contagious even during the incubation period, 2–10 days before the onset of symptoms (in humans, the virus appears in saliva 8 days before clinical signs).
In special cases (e.g., in caves inhabited by colonies of infected bats), airborne transmission is possible. Alimentary transmission (eating meat from an infected animal) and transplantation transmission (corneal transplantation from a donor with rabies) are extremely rare.
Pathogenesis: how the virus invades the body
The development of the disease goes through several key stages:
- Entry: the virus enters the wound with saliva. Primary replication and persistence occur at the bite site.
- Centripetal spread: the pathogen travels toward the central nervous system strictly along the axons of peripheral nerves, reaching the spinal cord and brain.
- CNS involvement: the virus actively replicates in neurons, predominantly affecting the hippocampus. Specific inclusion bodies—Negri bodies—form in the cytoplasm of the cells. Severe dystrophic, inflammatory, and degenerative changes in the nervous tissue develop.
- Centrifugal spread: from the brain, the virus is disseminated via neural pathways to other organs. Its main target is the salivary glands, ensuring the further transmission of the infection.
Clinical presentation: from anxiety to paralysis
The duration of the incubation period varies greatly. It is influenced by the location of the wound (bites to the head and neck shorten the incubation period, while bites to the extremities lengthen it) and the number of bites. If the infection is acquired from a bat, symptoms may appear within 3–4 weeks.
The classic course of the disease includes three stages:
- Prodromal stage: begins with general malaise, a sense of inexplicable fear, anxiety, and persistent insomnia.
- Excitation stage: marked reflex excitability comes to the forefront. Convulsions, spasms of the pharyngeal and laryngeal muscles occur, and breathing becomes noisy. Hallucinations and specific phobias appear, triggering new convulsive attacks: hydrophobia (fear of the sight of water), aerophobia (reaction to a draft of air), photophobia (fear of bright light), and acoustophobia (intolerance to noise).
- Paralytic stage: the final stage, setting in on days 3–7. It is characterized by paralysis of the limb muscles and respiratory arrest due to damage to the respiratory center.
There is also an atypical form—"quiet rabies". In this form, the excitation and hydrophobia stages are absent, and the disease immediately manifests with hypersalivation and paralysis.
How do animals contract the disease?
In dogs, the incubation period typically lasts 14–16 days. Typical symptoms include excitation, profuse salivation, hydrophobia, and vomiting. A characteristic sign is that the dog constantly chews on the site where it was bitten.
The animal's behavior changes dramatically: it becomes extremely aggressive, attacks people, other animals, and even inanimate objects. Paralysis and death occur 1–3 days after the appearance of the first symptoms.
Briefly: answers to topical questions
What factors determine the duration of the incubation period in human rabies?
The duration of the incubation period in rabies depends on the location of the bite, the multiplicity of lesions, the viral load, and the source of infection.
- Wound localization—for bites to the head, face, neck, hands, fingers, and genitals, the pathway of the virus to the CNS is physically shortened, which sharply reduces the incubation period. Bites to the extremities lengthen the period.
- Multiplicity of bites—the presence of multiple wounds accelerates the development of the disease.
- Viral load—the amount of virus entering the wound.
- Source of infection—when the virus is transmitted from bats, the incubation period is significantly shorter (no more than 3–4 weeks).
What symptoms are characteristic of the prodromal stage of rabies in humans?
The prodromal stage (stage of precursors) is characterized by a combination of local, general, and neurological symptoms.
- Local symptoms—pain along the nerves, hyperemia, edema, itching, muscle fasciculations, and paresthesias (tingling, pricking, burning) occur in the area of the bite wound.
- General infectious manifestations—elevated temperature (fever), malaise, flu-like and dyspeptic syndromes.
- Neurological syndrome—a progressive sense of causeless fear, anxiety, restlessness, and insomnia (sleep disorders).
Which organs and tissues, apart from the salivary glands, are affected by the virus during its centrifugal spread?
During centrifugal spread from the CNS along the efferent fibers of peripheral nerves, the rabies virus affects a wide range of internal organs and tissues. In addition to the salivary glands, targets for viral replication include:
- cornea;
- hair follicles;
- regional lymph nodes (cervical, axillary);
- lungs;
- adrenal glands;
- kidneys;
- skeletal muscles;
- heart;
- pancreas;
- mammary glands (the virus may be excreted in milk during lactation).
What is the mechanism of death in the paralytic stage of rabies?
Death in the paralytic stage of rabies occurs as a result of severe damage to the central nervous system and vital centers. The mechanism of a fatal outcome is caused by the degenerative transformation of peripheral motor neurons of the brainstem and spinal cord. The immediate causes of the patient's death are:
- paralysis of the respiratory center (respiratory arrest);
- cardiac paralysis (acute heart failure);
- cerebral edema leading to coma.
Death is usually recorded 12–20 hours after the development of paralysis.
What laboratory diagnostic methods are used for intravital confirmation of rabies in humans?
For intravital confirmation of rabies, a combination of virological, molecular-genetic, and serological methods is used.
- Direct fluorescent antibody (DFA) test—detection of viral antigens in corneal imprints and skin biopsies (from the back of the head or scalp).
- Viral RNA detection — in skin biopsies, corneal impressions, saliva, urine, and tear fluid.
- Biological assay — isolation of the virus from saliva, tear fluid, or cerebrospinal fluid via intracerebral inoculation of suckling mice.
- Serological diagnosis — detection of antibodies in the cerebrospinal fluid and a rising titer in blood serum (in unvaccinated individuals) using ELISA and complement fixation test (CFT).
Rabies phobias can be remembered using the "Four Elements and Senses" rule: Water (hydrophobia), Air (aerophobia), Light (photophobia), and Sound (acustophobia). Any of these stimuli triggers convulsions.
Class and exam questions
What determines the duration of the incubation period in humans?
From the site of the bite (closer to the head means faster), the multiplicity of injuries, and the source of infection (incubation is shorter with bats).
What are Babes–Negri bodies?
These are specific intracellular inclusions that form in the cytoplasm of affected neurons, especially in the hippocampus.
What else to study on this topic
- Morphology and Antigenic Structure of the Rabies Virus
- Laboratory Diagnostic Methods and Detection of Babes–Negri Bodies
- Specific Prophylaxis: Vaccines and Rabies Immunoglobulin
- Features of the Immune Response During Rabies Virus Infection
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