Figure 26.19 In Most Cases, Rabies is Transmitted by the Saliva of a Biting Animal (a) Color-enhanced transmission electron micrograph of rabies virus. Notice the bullet shape. (b) Rabid raccoon caged in a Virginia animal shelter.
of viral replication in the brain, but the cells are not lysed. The virus spreads outward from the brain via the nerves to various body tissues, notably the salivary glands, eyes, and fatty tissue under the skin, as well as to the heart and other vital organs. The presence of the virus in the eyes is of some practical significance, since cases can be diagnosed before death by stained smears made from the surface of the eyes. Moreover, several cases of rabies have occurred in individuals who received corneal transplants from donors who died of an atypical form of rabies that escaped diagnosis (the cornea is the clear middle part of the eye in front of the lens).
Figure 26.20 Stained Smear of Brain Tissue from a Rabid Dog The arrow points to one of several Negri bodies within the triangular-shaped nerve cell.The Negri bodies represent the sites of rabies virus replication.
Rabies is widespread in wild animals; about 5,000 wild animal cases are generally reported each year in the United States. This represents an enormous reservoir from which infection can be transmitted to domestic animals and humans. In the United States, skunks, raccoons, and bats constitute the chief reservoir hosts. Raccoons lead the list of wildlife cases, but almost all human cases are due to contact with infected bats. The virus can remain latent in bats for long periods, and healthy looking bats can have the virus in their salivary glands.
Until the mid-twentieth century, most rabies cases in the United States resulted from dog bites, which is the primary mode of transmission in the non-industrialized world. Fortunately, since World War II, the incidence of dog rabies has dropped dramatically in the United States, and now more than 85% of reported rabies infections are in wild, as opposed to domestic, animals. The incidence of rabies in people has declined because dogs and cats have been immunized against rabies infection, in effect creating a partial barrier to the spread of the rabies virus from wild animal reservoirs to humans. Since pets vary in their tolerance to rabies vaccines depending on their age and species, several kinds of vaccines are available.
About three-quarters of dogs that develop rabies excrete rabies virus in their saliva, and about one-third of these begin excreting it 1 to 3 days before they get sick. Therefore, when a person is bitten by an unvaccinated, apparently healthy dog, the animal should be confined for 10 days to see if symptoms of rabies appear. Some dogs become irritable and hyperactive with the onset of rabies, produce excessive saliva, and attack people, animals, and inanimate objects. Perhaps more common is the "dumb" form of rabies, in which an infected dog simply stops eating, becomes inactive, and suffers paralysis of throat and leg muscles. Obviously,
682 Chapter 26 Nervous System Infections one should not be tempted to try to remove a suspected foreign body from the throat of a sick, choking, non-vaccinated dog! The reported number of rabies cases in humans now generally ranges from zero to four per year in the United States. Only about one-fourth of the cases have a history of animal bites or exposure to sick animals. It is possible that some of them contracted the infection by inhaling dust contaminated by rabies virus. The long incubation period, however, reportedly up to 6 years, and the patients' illness make the animal bite history unreliable.
A person who has been bitten by an animal should wash the wound immediately and thoroughly with soap and water and then apply an antiseptic. In people bitten by dogs having rabies virus in their saliva, the risk of developing rabies is about 30%. Louis Pasteur discovered that this risk can be lowered considerably by administering rabies vaccine as soon as possible after exposure to the virus. Presumably, the vaccine provokes a better immune response than the natural infection, inactivating free virus and killing infected cells during the long incubation period before the virus enters the nerves. Pasteur's vaccine was made from dried spinal cords of rabies-infected rabbits. Unfortunately, the rabbit nervous system tissue in the vaccine sometimes stimulated an immune response against the patient's own brain, causing allergic encephalitis. Current vaccines are essentially devoid of nervous tissue and the risk of serious side effects is very low.
If there is a reasonable possibility that the animal is rabid, the bitten individual should then receive a series of five injections of vaccine intramuscularly. Anti-rabies antibody is also injected at the wound site and intramuscularly. The anti-rabies antibody is obtained from humans who have been immunized against rabies. In the United States, about 30,000 people annually receive rabies vaccine to prevent rabies after having been bitten by suspected rabid animals.
There is no effective treatment for rabies, and only three people are known to have ever recovered from the disease. Interestingly, all of them recovered completely.
Some features of rabies are summarized in table 26.8.
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