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February 10, 2012

Rabies (cont.)

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What is the history of rabies, and what is the cause of rabies?

Rabies has been noted in history since about 2000 BC when owners of rabid dogs were cautioned against getting bitten. The word rabies is derived from the Latin word rabies, which meant "madness or rage." For many centuries, it was common practice to kill any animal or human that showed symptoms of the disease. Rabies has been a problem in many countries, even into the 21st century as China developed a "one-dog" rule for people in Beijing to better control an increasing rate of dog rabies in 2006. Although the U.S. has had no people infected by dogs in years, there is an increasing incidence in wild animals, especially in the southern states since the 1970s.

In the 15th century, Italian physician Girolamo Fracastoro studied rabies and concluded that it was a communicable disease transmitted to people by direct contact with saliva from infected animals. He termed the disease "rabies." In 1895, after studying the data presented by Fracastoro centuries before, Louis Pasteur was able to produce a vaccine against the disease-causing viruses without actually discovering or isolating the viruses

The virus that causes rabies is Lyssavirus (Lyssa is the Greek goddess of madness, rage, and frenzy) rabies, a cylindrical or bullet-shaped virus that is enveloped and contains a negative-sensed RNA that makes up its genetic material.

Picture of the longitudinal view of the viral structure.

Picture of cross-sectional view of the viral structure.
Figure 2: CDC diagrams of the bullet-shaped rabies virus. The top image is the longitudinal view, and the bottom image is the cross-sectional view of the viral structure. SOURCE: CDC

The virus is termed a neurotropic virus because it proliferates in nerve tissues, especially the brain tissues of humans and animals. It proceeds from the bite or entrance wound along nerves to eventually infect the brain. The treatment use of human immune globulin and vaccine is designed to interrupt and kill the virus before it makes the journey to the brain. The virus enters nerve cells, takes over the cells systems, and develops virus replicating sites (termed Negri bodies that can be seen microscopically inside cells) that produce new viruses to continue viral spread to other body sites like the brain and, most important for spreading the viruses to other animals, the salivary glands. The rabies life cycle is relatively simple; the virus is transmitted to a wild animal by bites or saliva, the virus replicates in the bitten animal that, in turn, bites another animal and the cycle is completed. Humans are incidentally infected and rarely transmit the virus so humans play almost no role in the rabies life cycle.

How is rabies transmitted?

Almost every person who gets rabies has the virus transmitted to them by the bite of an infected animal. The transmission of rabies from human to human is rare; the highest number recorded happened when corneal transplants that were unknown before transplant to contain rabies virus caused rabies in eight patients. Aerosols of infected saliva or bat guano may also transmit rabies. In developing countries, the majority of people are infected by rabid dogs. Rabid dogs are often aggressive and sometimes are drooling, but in other cases, they act very withdrawn.

Picture of a rabid dog drooling saliva onto teeth and lower lip.
Figure 3: Rabid dog drooling saliva onto teeth and lower lip. SOURCE: CDC


In underdeveloped and developed countries, there are a number of animals that can transmit rabies to humans. Bats, foxes, coyotes, skunks, raccoons, cats, wolves, opossums, and other animals are some of the prevalent wild or feral animals that may become infected. In Mexico, cows have become infected from bat bites and can transmit the virus to humans via a bite or saliva if the human skin is abraded or cut. Currently, in the U.S., bats and raccoons cause the most bites that may lead to rabies. Readers should note that drooling saliva is not always present in wild or domestic animals with rabies. Any animal (domestic or wild) that acts strangely (for example, unusually aggressive or docile, unusual gait, has seizure-like activity) should be suspected of having rabies.



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