Epidemiology and Routes of Transmission
The infection is distributed globally, with the exception of specific island nations enforcing strict quarantine measures.
Viral reservoirs are divided into two types of foci:
- Sylvan (wildlife rabies): Maintained by wild animals such as foxes, arctic foxes, wolves, raccoon dogs, jackals, bats (including vampire bats), and rodents.
- Urban rabies: The primary hazard comes from infected stray or domestic dogs and cats.
Transmission occurs most frequently via the contact route—through bites or when infected saliva contaminates broken skin. The virus accumulates in the salivary glands of the sick animal, becoming contagious 2 to 10 days before symptom onset (in humans, the virus appears in saliva up to 8 days before clinical signs).
In specific scenarios (e.g., caves inhabited by infected bat colonies), airborne transmission is possible. Alimentation (consuming meat from an infected animal) and transplantation (corneal grafts from donors with rabies) are extremely rare.
Pathogenesis: How the Virus Invades the Body
Disease progression follows several key stages:
- Inoculation: The virus enters a wound via saliva. Primary replication and persistence occur at the bite site.
- Centripetal Spread: The pathogen travels toward the central nervous system strictly along peripheral nerve axons, reaching the spinal cord and brain.
- CNS Infection: The virus actively replicates in neurons, with a predilection for the hippocampus. Specific intracellular inclusions—Negri bodies—form within the cytoplasm. Severe dystrophic, inflammatory, and degenerative changes develop in the nervous tissue.
- Centrifugal Spread: From the brain, the virus disseminates along neural pathways to other organs. Its primary destination is the salivary glands, ensuring continued transmission.
Clinical Presentation: From Anxiety to Paralysis
The incubation period varies significantly. Factors include wound location (bites to the head and neck shorten incubation; extremities lengthen it) and viral load. If contracted from a bat, symptoms can manifest in as little to 3–4 weeks.
The classic disease course comprises three stages:
- Prodromal stage: Begins with general malaise, a sense of unexplainable fear, anxiety, and persistent insomnia.
- Excitation period: Marked by pronounced reflex hyperexcitability. Convulsions, pharyngeal and laryngeal muscle spasms, and labored breathing occur. Hallucinations and pathognomonic phobias emerge, triggering new convulsive episodes: hydrophobia (fear of the sight of water), aerophobia (reaction to drafts of air), photophobia (fear of bright light), and acoustophobia (intolerance to noise).
- Paralytic stage: The final stage, occurring on days 3–7. Characterized by limb paralysis and respiratory arrest due to involvement of the respiratory center.
An atypical variant exists known as "dumb rabies". It lacks the excitation and hydrophobia stages, immediately manifesting with hypersalivation and paralysis.
Clinical Features in Animals
In dogs, the incubation period typically lasts 14–16 days. Typical symptoms include excitation, excessive salivation, hydrophobia, and vomiting. A hallmark sign is the dog persistently chewing at the bite site.
Animal behavior changes drastically: it becomes extremely aggressive, attacking humans, other animals, and even inanimate objects. Paralysis and death ensue 1–3 days after symptom onset.