Rabies

While many animal diseases cannot be passed from animal to man, rabies has long been known as an easy traveler from one species to the next. The disease was known among ancient people. The very name rabies, Latin for "rage" or "madness," suggests the fear with which early men and women must have viewed the disease. For centuries there was no treatment, and the disease was left to run its rapid course leading to death.

Rabies is described in medical writings dating from 300 B.C., but the method of transmission or contagion was not recognized until 1804. In 1884 the French bacteriologist Louis Pasteur developed a preventive vaccine against rabies, and modifications of Pasteur's methods are still used in rabies therapy today. The Pasteur program, or variations of it, has greatly reduced the fatalities in humans from rabies. Modern treatment, following a bite by a rabid or presumed rabid animal, consists of immediate and thorough cleansing of the bite wound and injection into the wound and elsewhere of hyperimmune antirabies serum. A 14-30 day course of daily injections of rabies vaccine is then given; booster doses are given 10 days after this course and again 20 days later.

The standard vaccine contains inactivated rabies virus grown in duck eggs. It is highly effective but causes neuroparalysis in about one in 30,000 persons receiving it. In the 1970s a new vaccine was developed in France and the United States that contains virus prepared from human cells grown in the laboratory. This vaccine is safer and requires a shorter course of injections. With the widespread use of vaccine, rabies cases in the U.S. declined to fewer than five per year.

The transmission of rabies is almost invariably through the bite of an infected animal. The fact that the virus is eliminated in the saliva is of great significance, and unless saliva is introduced beneath the skin, the disease is seldom transmitted. The virus has been demonstrated in the saliva of dogs 3-8 days before the onset of symptoms. However, it has also been reported that only about 50-60% of the infected dogs shed the virus in the saliva. Rare cases of rabies have been reported where only clawing and scratching occurred, or where the skin was contaminated with saliva. The virus is most concentrated in the central nervous system and saliva, but it has also been demonstrated in various organs of the body and milk from infected animals.

In humans, the rabies virus, in addition to entering the body by the usual route through skin broken by a bite or scratch, can enter the body through intact mucous membranes, can be inhaled as an aerosol, and can be transplanted in an infected corneal graft. These four cases are the only virologically documented examples of transmission of rabies from one person to another. Vertical transmission from mother to fetus and from lactating mother to suckling young has been described in nonhuman mammals.

An engraving showing antirabies vaccination at the Pasteur Institute in Paris. Louis Pasteur (1822-1895) developed a rabies virus that was milder and had a shorter incubation period than the wild virus. A person bitten by a rabid animal would be inoculated with the Pasteur virus and rapidly develop immunity to the wild strain. The first human patient was successfully treated in 1885. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.

The incubation period in natural cases of rabies is variable. In general, the greater the quantity of virus introduced into the wound is also correlated with the length of incubation before symptoms occur. In dogs, the minimum period is ten days, the average 21-60 days, but may be as long as six months. In man, the incubation period is one to three months, with the minimum of ten days.

Rabies is caused by a number of different viruses that vary depending on geographic area and species. While the viruses are different, the disease they cause is singular in its course. The bullet-shaped virus is spread when it breaks through skin or has contact with a mucous membrane. The virus begins to reproduce itself initially in muscle cells near the place of first contact. At this point, within the first five days or so, treatment by vaccination has a high rate of success.

Once the rabies virus passes to the nervous system, immunization is no longer effective. The virus passes to the central nervous system, where it replicates itself in the system and moves to other tissues such as the heart, the lung, the liver, and the salivary glands. Symptoms appear when the virus reaches the spinal cord.

A bite from a rabid animal does not guarantee that one will get rabies; only about 50% of people who are bitten and do not receive treatment ever develop the disease. But it is best not to take any chances. If one is bitten by or has had any exposure to an animal that may have rabies, medical intervention should be sought immediately. Treatment virtually ensures that one will not come down with the disease. Any delay could diminish the treatment's effectiveness.

In humans and in animals, rabies may be manifest in one of two forms: the furious (agitated) type or the paralytic (dumb) type. Furious rabies in animals, especially in the dog, is characterized by altered behavior such as restlessness, hiding, depraved appetite, excitement, unprovoked biting, aimless wandering, excessive salivation, altered voice, pharyngeal paralysis, staggering, general paralysis, and finally death. Death usually occurs within three to four days after the onset of symptoms. The paralytic form of rabies is frequently observed in animals inoculated with fixed virus, and is occasionally observed in other animals with street virus contracted under natural conditions. Animals showing this type usually show a short period of excitement followed by uncoordination, ataxia, paralysis, dehydration, and loss of weight, followed by death.

In humans, "furious" rabies patients typically show bizarre behavior, ranging from episodes of severe agitation to periods of depression. Confusion becomes more and more extreme as the disease progresses, and the patient can become very aggressive. Hydrophobia is always seen with this type of disease, until the patient becomes comatose while showing intermittently uncontrollable inspiratory spasms. This type of rabies is also characterized by hypersalivation, from 1-1.6 qt (1-1.5 l) of saliva in 24 hours, and excessive sweating.

The paralytic form of rabies in humans is often indistinguishable from that of most viral encephalitis, except for the fact that a patient suffering from rabies remains conscious during the course of the disease. Paralysis usually begins at the extremity exposed to the bite and gradually involves other extremities finally affecting the pharyngeal and respiratory muscles.