(See also Overview of Brain Infections.)
Rabies is a viral infection of the brain that is transmitted by animals and that causes inflammation of the brain and spinal cord. Once the virus reaches the spinal cord and brain, rabies is almost always fatal.
The virus is usually transmitted when people are bitten by an infected animal, usually a bat in the United States or a dog in countries where dogs are not routinely vaccinated against rabies.
Rabies can cause restlessness and confusion or paralysis.
A skin biopsy can detect the virus.
Infection can be prevented by immediately cleaning the wound and by injecting rabies vaccine and immune globulin.
The virus is transmitted in the saliva of an infected animal. From the point of entry (usually a bite), the rabies virus travels along nerves to the spinal cord and then to the brain, where it multiplies. From there, it travels along other nerves to the salivary glands and into the saliva. Once the rabies virus reaches the spinal cord and brain, rabies is almost always fatal. However, the virus typically takes at least 10 days—usually 30 to 50 days—to reach the brain (how long depends on the bite’s location). During that interval, measures can be taken to stop the virus and help prevent death. Rarely, rabies develops months or years after an animal bite.
Rabies causes at least 55,000 deaths worldwide each year. Most deaths occur in rural areas of Latin America, Asia, and Africa. In the United States, only a few people die each year.
The rabies virus is present in many species of wild and domestic animals throughout most of the world. Animals with rabies may be sick for several weeks before they die. During that time, they often spread the disease.
The rabies virus, which is present in the saliva of a rabid animal, is transmitted when the animal bites or, very rarely, licks another animal or a person. The virus cannot pass through intact skin. It can enter the body only through a puncture or another break in the skin or through the nose or mouth when many airborne droplets containing the virus are inhaled (as can occur in a cave that contains infected bats).
Many different mammals—such as dogs, cats, bats, raccoons, skunks, and foxes—can transmit rabies to people. In the United States, most cases of rabies are thought to be transmitted by bats.
Rabies rarely affects rodents (such as hamsters, guinea pigs, gerbils, squirrels, chipmunks, rats, and mice), rabbits, or hares. In the United States, these animals have not been known to cause rabies among people. Rabies does not affect birds and reptiles.
In the United States, vaccination has largely eliminated rabies in dogs, and the source of rabies is almost always wild animals, usually bats but also foxes, skunks, or racoons. In many cases, the bat bites are unnoticed. Most deaths due to rabies result from being bitten by an infected bat.
In countries where dogs are not routinely vaccinated against rabies (including most countries in Latin America, Africa, Asia, and the Middle East), most deaths due to rabies result from being bitten by an infected dog. A few cases result from being bitten by other animals including monkeys, which are sometimes kept as pets.
The wound from the bite may be painful or numb. Bat bites typically cause no symptoms.
Rabies symptoms appear when the rabies virus reaches the brain or spinal cord, usually 30 to 50 days after a person is bitten. However, this interval can vary from 10 days to more than a year. The closer the bite to the brain (for example, on the face), the more quickly symptoms appear.
Rabies may begin with a fever, headache, and a general feeling of illness (malaise). Most people become restless, confused, and uncontrollably excited. Their behavior may be bizarre. They may hallucinate and have insomnia. Saliva production greatly increases. Spasms of the muscles in the throat and larynx occur because rabies affects the area in the brain that controls swallowing, speaking, and breathing. The spasms can be excruciatingly painful. A slight breeze or an attempt to drink water can trigger the spasms. Thus, people with rabies cannot drink. For this reason, the disease is sometimes called hydrophobia (fear of water).
As the disease spreads through the brain, people become more confused and agitated. Eventually, coma and death result. The cause of death can be blockage of airways, seizures, exhaustion, or widespread paralysis.
In 20% of people, rabies begins with tingling or paralysis of the limb that was bitten. The paralysis then moves through the body. In these people, thinking is typically unaffected, and most of the other symptoms of rabies do not develop.
Doctors suspect rabies when people have a headache, confusion, and other symptoms of the disease, especially if people have been bitten by an animal or exposed to bats (for example, if they were exploring a cave). However, many people with rabies are unaware of having been bitten by an animal or exposed to bats.
A sample of skin is taken (usually from the neck) and examined under a microscope (skin biopsy) to determine whether the virus is present. Samples of saliva are also examined to check for the virus. A spinal tap (lumbar puncture) is done to obtain a sample of cerebrospinal fluid (the fluid that flows through the tissues that cover the brain and spinal cord). This sample is also examined.
A variation of the polymerase chain reaction (PCR) technique, which produces many copies of a gene, is often used to identify the virus's unique DNA sequence in a sample of skin, cerebrospinal fluid, or saliva. Several samples of the fluids, taken at different times, are tested to increase the chances of detecting the virus.
Avoiding being bitten by animals, especially wild animals, is best. Pets that are not known and wild animals should not be approached. Signs of rabies in wild animals may be subtle, but their behavior is typically abnormal, as in the following:
An animal that may be rabid should not be picked up to try to help it. A sick animal often bites. If an animal appears sick, people should call local health authorities, who can help remove it.
The rabies vaccine should be given to people who are likely to be exposed to the rabies virus before exposure. Such people include the following:
Three doses of the vaccine are injected into a muscle. The first dose is given right away (called day 0). The other injections are given on day 7 and between days 21 and 28. The injection site may be painful and swollen but usually only slightly. Serious allergic reactions are rare.
Vaccination protects most people to some degree for the rest of their life. However, protection decreases with time, and if exposure is likely to continue, people are tested periodically, and if the levels of protective antibodies are low, they are given a booster dose of vaccine.
Immediately after being bitten, people should clean the wound thoroughly with soap and water. Deep puncture wounds are flushed out with running water. Then people should see a doctor. Doctors clean the wound further with an antiseptic called benzalkonium chloride. They may trim ragged edges of the wound.
Doctors also try to determine the likelihood that rabies was transmitted. Early determination is essential because rabies can usually be prevented if appropriate measures are taken promptly.
Immediately after an animal bite, no test can determine whether the rabies virus has been transmitted. Thus, people who have been bitten may be given rabies immune globulin and vaccine by injection to prevent rabies. Rabies immune globulin, which consists of antibodies to the virus, provides protection immediately but only for a short time. The rabies vaccine stimulates the body to produce antibodies to the virus. The vaccine provides protection that begins more gradually but that lasts for a much longer time.
Whether vaccine and immune globulin are needed depends on whether people have been previously immunized with rabies vaccine and what the type and status of animal are. For example, doctors determine the following:
If people have been bitten, need preventive treatment, and have not been immunized previously, they are given rabies immune globulin and rabies vaccine right away (on day 0). Immune globulin is injected around the wound if possible. They are given three more injections of the vaccine: on days 3, 7, and 14. People with a weakened immune system (by a disorder such as AIDS or a drug) are given an additional injection on day 28.
If people have already been vaccinated, the risk of developing rabies is reduced. However, the wound must be cleaned promptly, and an injection of rabies vaccine is given immediately and on day 3.