Rabies is a widespread infection of warm-blooded animals. It is caused by a virus that attacks the nervous system. The virus is called the Rabies virus. Rabies has afflicted animals and humans since ancient times. The word "rabies" comes from the Latin "rabere" meaning to rage- rabid animals are often called "mad" because the symptoms of the disease in its advanced stages cause the animal to appear "mad" or in a rage.
Taxonomically, rabies viruses belong to the Rhabdoviridae family, Lyssavirus genus. The virus morphology, under electron microscopy, is a bullet-shaped enveloped cylinder. It is 100-430 nm long, and 45-100 nm in diameter. The virus consists of a phospholipid bilayer, a helicoidal RNA, and five proteins. (See structure picture below). Viral replication occurs within the cellular cytoplasm of host cells. The virus is composed of 1-2% nucleic acid, 65-75% protein (structural), 15-25% lipid, and 3% carbohydrates. The genome is linear, single stranded RNA. The total genome is about 11900 nucleotides long. The 3' terminus has no poly (A) tract. The nucleotide sequences of the 3'-terminus are inverted and complementary to similar regions on the 5' end.
After inoculation, the virus enters small nerve endings at the site of the bite. The virus slowly travels up the nerve to reach the CNS where it replicates and then travels down nerves to the salivary glands where there is further replication. The time it takes to do this depends upon the length of the nerve - a bite on the foot will have a very much longer incubation period than a bite on the surface. The incubation period may last from two weeks to six months. Very often the primary wound is healed and forgotten by the time of clinical presentation.
There are two types of rabies: furious rabies and dumb rabies. In furious rabies, when the virus reaches the CNS the patient presents with headache, fever, irritability, restlessness and anxiety. This may progress to muscle pains, salivation and vomiting. After a few days to a week the patient may experience a stage of excitement and be wracked with painful muscle spasms, triggered sometimes by swallowing of saliva or water. Hence they drool and learn to fear water (hydrophobia). The stage of excitement lasts only a few days before the patient lapses into coma and death. In animals, rabies is very similar to human rabies symptoms. Plus, in the excitement stage, the animal may bite vigorously and viciously at anything: sticks, stones, grass, other animals and humans, without provocation.
Dumb rabies starts in the same way, but instead of progressing into excitement, the subject retreats steadily and quietly down hill, with some paralysis, to death. Rabies diagnosis may easily be missed. Wild animals may be abnormally tame or appear sick.
*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.
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