Rabies is a viral disease that attacks the central nervous system of mammals, including humans, causing acute inflammation of the brain and spinal cord. While the dramatic image of “foaming at the mouth” is common in popular culture, the clinical reality is more accurately described as excessive, thick saliva collecting in the mouth in the final stages of the illness. This accumulation of saliva is a direct consequence of neurological damage, signaling that the disease has become nearly universally fatal.
The Truth About Excessive Saliva
The appearance of “foaming” is not due to the production of actual foam, but rather a visual effect created by the body’s inability to clear thick saliva. Saliva production is greatly increased as the virus spreads to the salivary glands, increasing the viral load in the fluid. This thick, copious saliva then mixes with air as the person breathes or struggles, creating the froth or “foam.” This excessive salivation, or hypersalivation, is a late-stage symptom that facilitates viral transmission through a bite.
The Neurological Cause of Hypersalivation
The underlying mechanism for the collection of saliva is the virus’s destructive impact on the parts of the brain that control swallowing and muscle coordination. Rabies targets the nervous system, including the brainstem where the swallowing reflex is coordinated. The damage disrupts the delicate timing of muscle contractions necessary for a normal swallow.
This impairment results in dysphagia, the difficulty or inability to swallow, which prevents the individual from clearing the accumulating saliva. The muscle spasms in the throat and larynx can be painful, which is why an attempt to drink water often triggers violent, involuntary contractions. This reaction is known as hydrophobia, or “fear of water.”
The mere suggestion or sight of water can cause these painful throat spasms, leading the person to avoid drinking. A similar phenomenon, aerophobia, involves spasms triggered by a gust of wind or air blown on the face. The combination of increased saliva production and the inability to swallow results in the characteristic drooling and frothing seen in the final stages of the disease.
Transmission and Stages of Rabies in Humans
Rabies is most commonly transmitted to humans through the bite of an infected animal, which sheds the virus in its saliva. The virus cannot penetrate intact skin and must enter the body through a break in the skin, such as a bite wound or contact with mucous membranes. Worldwide, dogs are the most common source of transmission, while bats, raccoons, skunks, and foxes are primary carriers in the United States.
Following exposure, the virus enters the peripheral nervous system and travels slowly along the nerve pathways toward the central nervous system. The incubation period, the time between exposure and the appearance of the first symptoms, is highly variable, often ranging from one to three months but potentially from less than a week to more than a year.
The disease progression is generally divided into three stages: the incubation period, the prodromal phase, and the acute neurological phase. The prodromal phase begins when the virus enters the central nervous system, causing non-specific, flu-like symptoms such as fever, headache, and malaise. This is followed by the acute neurological phase, where the virus causes inflammation of the brain, leading to severe symptoms like agitation, anxiety, confusion, hypersalivation, and hydrophobia.
Prevention and Urgent Care
Because rabies is virtually always fatal once neurological symptoms appear, prevention and immediate medical intervention are necessary after any potential exposure. The first step is thorough wound care, which involves gently washing the bite or scratch with soap and water for at least 15 minutes to reduce the viral load at the site of entry. This simple action is highly effective in reducing the likelihood of infection.
Following wound cleansing, the definitive treatment is Post-Exposure Prophylaxis (PEP), which must be initiated as soon as possible. For individuals who have never been vaccinated against rabies, PEP consists of two components: Human Rabies Immune Globulin (HRIG) and a series of rabies vaccines.
The HRIG provides immediate, passive immunity by delivering antibodies directly to the body, with most of the dose infiltrated into and around the wound site. The rabies vaccine is then administered to prompt the body to create its own active, long-lasting immunity.
PEP Schedule
If the exposed person has never been vaccinated, the standard schedule involves four doses of the vaccine:
- Day 0
- Day 3
- Day 7
- Day 14
If the exposed person has previously been vaccinated, they only require two vaccine booster doses on days 0 and 3, and do not receive HRIG. The speed and effectiveness of PEP mean that rabies is entirely preventable if treatment is sought promptly after exposure.