Rabies or hydrophobia is a viral neuroinvasive disease that causes acute encephalitis (inflammation of the brain) in warm-blooded animals. It is zoonotic (i.e. transmitted by animals), most commonly by a bite from an infected animal, but occasionally by other forms of contact. It is fatal if left untreated. In some countries it is a significant killer of livestock.
The rabies virus makes its way to the brain by following the peripheral nerves. The incubation period of the disease depends on how far the virus must travel to reach the central nervous system, usually taking a few months. Once the infection reaches the central nervous system and symptoms begin to show, the untreated infection is usually fatal within days.
In the beginning stages of rabies, the symptoms are malaise, headache, and fever, while in later stages it includes acute pain, violent movements, uncontrolled excitements, depressions, and the inability to swallow water (hence the name hydrophobia). In the final stages, the patient begins to have periods of mania and lethargy, and coma. Death generally occurs due to respiratory insufficiency.
The term is derived from the Latin rabies, "madness". This, in turn, may have come from the Sanskrit rabhas, "to do violence". The Greeks derived the word "lyssa", which is derived from "lud" or "violent", this terminology is used in the name of the genus of rabies lyssavirus.
The rabies virus is the type species of the Lyssavirus genus, which encompasses other similar viruses. Lyssaviruses have helical symmetry, with a length of about 180 nm and a cross-sectional diameter of about 75 nm. These viruses are enveloped and have a single stranded RNA genome with negative-sense. The genetic information is packaged as a ribonucleoprotein complex in which RNA is tightly bound by the viral nucleoprotein. The RNA genome of the virus encodes five genes whose order is highly conserved. These genes are nucleoprotein (N), phosphoprotein (P), matrix protein (M), glycoprotein (G) and the viral RNA polymerase (L).
From the point of entry, the virus travels quickly along the neural pathways into the central nervous system (CNS), and then further into other organs. The salivary glands receive high concentrations of the virus thus allowing further transmission.
Any warm-blooded animal, including humans, may become infected with the rabies virus and develop symptoms. Indeed the virus has even been adapted to grow in cells of poikilothermic vertebrates though natural transmission has only been documented among mammals. Most animals can be infected by the virus and can transmit the disease to humans. Infected bats, monkeys, raccoons, foxes, skunks, cattle, wolves, dogs, mongoose (normally yellow mongoose) or cats provide the greatest risk to humans. Rabies may also spread through exposure to infected domestic farm animals, groundhogs, weasels, bears and other wild carnivores. Rodents (mice, squirrels etc) are seldom infected.
The virus is usually present in the nerves and saliva of a symptomatic rabid animal. The route of infection is usually, but not necessarily, by a bite. In many cases the infected animal is exceptionally aggressive, may attack without provocation, and exhibits otherwise uncharacteristic behavior.
Transmission between humans is extremely rare. A few cases have been recorded through transplant surgery.
After a typical human infection by bite, the virus enters the peripheral nervous system. It then travels along the nerves towards the central nervous system. During this phase, the virus cannot be easily detected within the host, and vaccination may still confer cell-mediated immunity to prevent symptomatic rabies. Once the virus reaches the brain, it rapidly causes encephalitis. This is called the “prodromal” phase, and is the beginning of the symptoms. Once it reaches this point and symptoms show, there is no treatment, and death is certain. Rabies may also inflame the spinal cord producing myelitis.
Rabies in animals:
Rabies is infectious to mammals. Three stages of rabies are recognized in dogs and other animals. The first stage is a one- to three-day period characterized by behavioral changes and is known as the prodromal stage. The second stage is the excitative stage, which lasts three to four days. It is this stage that is often known as furious rabies due to the tendency of the affected dog to be hyperreactive to external stimuli and bite at anything near. The third stage is the paralytic stage and is caused by damage to motor neurons. Incoordination is seen due to rear limb paralysis and drooling and difficulty swallowing is caused by paralysis of facial and throat muscles. Death is usually caused by respiratory arrest.
As recently as 2004, a new symptom of rabies has been observed in foxes. Probably at the beginning of the prodromal stage, foxes, who are extremely cautious by nature, seem to lose this instinct. Foxes will come into settlements, approach people, and generally behave as if tame. How long such "euphoria" lasts is not known. But even in this state such animals are extremely dangerous, as their saliva and excretions still contain the virus and they remain very unpredictable.