Rabies is a viral disease transmitted to humans primarily through the bite or scratch of an infected mammal. Caused by the rabies virus, this infection attacks the central nervous system, leading to progressive and fatal inflammation of the brain and spinal cord (encephalomyelitis). While rare in countries with widespread domestic animal vaccination, rabies remains a global public health concern. Once clinical symptoms appear, the disease is virtually 100% fatal.
How Rabies Attacks the Nervous System
The journey of the rabies virus from the entry point to the brain dictates the long incubation period. Following a bite, the virus is deposited in the wound and initially replicates within the surrounding muscle tissue. This phase may last days or weeks, depending on the amount of virus and the wound’s proximity to the central nervous system (CNS).
The virus then uses the neuromuscular junction to access the peripheral nervous system. Once inside a peripheral nerve, the virus travels toward the spinal cord and brain using retrograde axonal transport. This movement relies on the host cell’s internal transport machinery, specifically the motor protein dynein, allowing the virus to travel at approximately 3 millimeters per hour.
The time required to reach the CNS can range from a few weeks to over a year. When the virus finally reaches the brain, it multiplies rapidly, causing widespread infection of neurons and the onset of neurological symptoms. From the brain, the virus spreads outward through other nerves to infect organs like the salivary glands, completing the transmission cycle.
The Stages of Fatal Symptoms
Once symptoms begin, the disease progresses through three distinct phases leading to death.
The first is the prodromal phase, which lasts between two and ten days and often mimics a non-specific viral illness with symptoms like fever, headache, and general malaise. A specific sign is an unusual tingling, burning, or pain sensation (paresthesia) at the original bite site.
The prodromal phase is followed by the acute neurological period, where the disease manifests in one of two forms: furious or paralytic rabies. Furious rabies, accounting for about 80% of human cases, is characterized by hyperactivity, anxiety, and agitation. Classic symptoms include hydrophobia (fear of water) and aerophobia (fear of drafts), which occur due to painful spasms of the throat and diaphragm muscles.
Paralytic rabies is less common and runs a longer course, often making it more difficult to diagnose. This form is marked by muscle weakness and flaccid paralysis that typically begins at the bite site and gradually spreads throughout the body. The final stage involves rapid deterioration into a coma, with death typically occurring within days due to cardiorespiratory arrest.
Why Rabies is Nearly Always Fatal
The fatality of symptomatic rabies results directly from the virus’s ability to cause profound and irreversible damage to the central nervous system (CNS). Once established in the brain, the virus triggers severe inflammation known as encephalitis. Current medical science has no effective antiviral treatment to clear the virus from the brain once symptoms appear.
The rabies virus is adept at evading the body’s immune response within the nervous system. It does not significantly compromise the blood-brain barrier, which shields it from systemic immune cells and antibodies. Because the virus is protected, brain damage progresses unchecked, leading to a breakdown in the neural control of vital bodily functions.
The damage affects the brainstem, which regulates autonomic functions such as breathing and heart rate. The widespread injury and dysfunction inflicted by the rabies virus are permanent. This destructive process, coupled with the lack of immune clearance in the brain, means that once a person shows clinical signs, the prognosis is poor.
Preventing Rabies Death
Avoiding a fatal outcome from rabies relies entirely on immediate action taken before symptoms develop. Post-Exposure Prophylaxis (PEP) is a highly effective, life-saving measure recommended immediately following potential exposure. The first step is thorough wound cleansing with soap and water for about 15 minutes to physically wash away the virus.
PEP involves two distinct components: passive and active immunization. The passive component is Human Rabies Immune Globulin (HRIG), which provides immediate, pre-formed antibodies infiltrated into and around the wound site. This neutralizes the virus before it can enter the nerves, allowing time for the body’s own immune system to respond.
The active component is a series of rabies vaccinations, typically four doses given on days 0, 3, 7, and 14 after exposure. This vaccine stimulates the person’s immune system to produce its own long-lasting antibodies against the virus. When administered promptly and correctly, PEP is nearly 100% effective in preventing the disease. Pre-exposure vaccination is also available for individuals at high risk, such as veterinarians or travelers, which simplifies the PEP regimen if exposure occurs later.