Mad Dogs and Friendly Skunks - Controlling Rabies

by Food and Drug Administration (FDA)

One of humanity's oldest and most feared diseases is a form of viral encephalitis transmitted by the saliva of infected animals. First described in a Mesopotamian tract dating from 1800 B.C., the illness was known to the ancient Greeks as lyssa, meaning "frenzy." But the Romans, adapting the Latin word that means "to rage," gave us the name by which the disease is known today: rabies.

For at least 38 centuries, and probably much longer, the threat of rabies has terrified people throughout the world. The cry "mad dog" has long been a warning literally to run for your life, because mad dog meant rabies, and rabies meant death.

A combination of innovative research and determined public health measures has all but eradicated human rabies in the United States and many other countries. On average, one case a year was reported to the U.S. Centers for Disease Control in the 1980s, down from an average of 11 cases a year in the 1950s.

Rabies is a "reportable" disease, which means state health authorities must notify CDC of every case in humans or animals. And where rabies is concerned, "case" is virtually synonymous with death. Only three people who showed clear evidence of rabies are known to have survived the illness, and all three suffered permanent nervous system damage resulting in physical or psychological aftereffects.

Dogs Not Only Villains

Rabies is a disease primarily of mammals, including both wild and domestic animals and human beings. Although people usually associate rabies with dogs, among domesticated animals in the United States, rabies today is more likely to be found in cats.

Cats, dogs and cattle account for nearly 90 percent of rabies cases in domestic animals, with horses, mules, sheep, goats, swine, and ferrets making up the remaining cases. Among wild animals, the disease is most often reported in skunks and raccoons. Other wild species in this country in which rabies is commonly found include bats, foxes, mongooses (only in Puerto Rico), groundhogs, and rodents.

The rabies virus, present in the saliva of an infected animal, is usually spread by a bite or scratch that punctures the victim's skin. The virus has a strong affinity for cells of the nervous system. It enters nerve cells at the site of the wound, travels to the brain, and then follows other nerve pathways to muscles and organs that are especially affected by rabies.

There are at least two other ways in which humans are known to have contracted rabies, both extremely rare. Two people were exposed by breathing the air in caves inhabited by rabid bats. And six people contracted rabies following implants of corneas from donors who had undiagnosed rabies.

The virus concentrates in the salivary glands, which explains why it is usually spread by bites. It also invades and damages the muscles involved in drinking and swallowing. Most human victims, and apparently lower animals as well, suffer excruciating pain on swallowing liquids. Though they suffer from thirst, animal and human rabies victims can be terrified by the sight of water — hence another name for the disease: hydrophobia.

Symptoms usually develop between 20 and 60 days after exposure. Rabid animals may become aggressive, combative, and highly sensitive to touch and other kinds of stimulation. And they can be vicious. This is the "furious" form of rabies, the kind traditionally associated with mad dogs.

But there is also a "dumb" form of the disease in which the animal is lethargic, weak in one or more limbs, and unable to raise its head or make sounds because its throat and neck muscles are paralyzed. In both kinds of animal rabies, death occurs a few days after symptoms appear, usually from respiratory failure.

In humans the course is similar. After a symptom-free incubation period that ranges from 10 days to a year or longer (the average is 30 to 50 days), the patient complains of malaise, loss of appetite, fatigue, headache, and fever. Over half of all patients have pain (sometimes itching) or numbness at the site of exposure. Patients become excited, anxious, and irritable. They may complain of insomnia or depression. Two to 10 days later, signs of nervous system damage appear — hyperactivity and hypersensitivity, disorientation, hallucinations, seizures, and paralysis. Death may be sudden — due to cardiac or respiratory arrest — or follow a period of coma that can last for months with the aid of life-support measures.

Aristotle Got It Wrong

The ancients were certainly familiar with rabies; Homer refers to "canine madness" in the Iliad. But they had some major misconceptions about it. Aristotle, for example, said that lyssa (rabies) would afflict any animal bitten by a mad dog, except humans.

A Roman scholar, Celsus, apparently more perceptive than Aristotle, recognized that people were susceptible to the disease. But, like others before and after him, Celsus thought human rabies could be cured, and he offered advice on how to treat it. Among other things, Celsus advocated holding victims under water both to relieve thirst and overcome hydrophobia. Chances are the hapless beneficiaries of this treatment who didn't drown died of rabies.

In the 16th century, an Italian physician and scientist, Fracastorius, called rabies "the incurable wound," and said it was invariably fatal. His great reputation ensured that his opinion was well received by scholars of the day. But within a generation the misguided, if more hopeful, view of rabies treatment resurfaced, and dangerous, ineffective "cures" were again widely employed, among them the water treatment, in use as late as the 18th century.

Prevention Is Key

The advent of scientific medicine made rabies control possible — not by cure, but by prevention. A series of observations and experiments led to Pasteur's development of a rabies vaccine in 1885. (See accompanying article, page 26.) Within a few years, the Pasteur treatment was being administered to thousands of potential rabies victims.

Unlike other immunizations, the rabies vaccine is administered after exposure to the virus. This unusual technique is successful because, as Pasteur showed, the rabies virus takes a comparatively long time to induce disease — a minimum of 10 days, and in rare cases up to a year. The length of the incubation period apparently depends on both the location of the wound — the farther from the brain, the longer the incubation — and the dose of virus received. Thus, without treatment, severe bites on the head or upper body might lead to the appearance of symptoms earlier than would a superficial bite or scratch on the ankle.

About the Author

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