Rabies is a disease of the central nervous system that affects mammalian hosts and is generally transmitted through the saliva of an infected animal. According the Center for Disease Control, most cases of rabies in the U.S. occur in wildlife populations (92%) but rabies has also been reported in domestic dogs, cats, ungulates and humans. In developing countries the majority of rabies incidents occur in dogs that are allowed to run free.
Etiology and Natural History
Rabies is a virus with a distinct bullet shape in the family Rhabdoviridae. The rabies genus, Lyssavirus, includes viruses such as Lagos bat, Mokola virus, Duvenhage virus, European bat virus 1 and 2, and Australian bat virus.
Transmission
The rabies virus replicates inside warm-blooded hosts including ungulates, rodents, canines, felines and even hominids. Rabies infection may potentially be transmitted by aerosol, contact with mucous membranes, ingestion of infected tissues, and corneal transplants (Rosatte 1987). Most often the rabies virus is spread through the saliva of an infected animal.
The period of time during which the animal can transmit the disease varies with the particular strain of virus that the animal is infected with as well as what species the host is. All animals are able to transmit the disease ("shed virus") while they are symptomatic, dogs and cats usually for three to seven days, except for in Ethiopian dogs, which may shed virus for up to 14 days prior to the onset of symptoms. Bats, skunks, and ferrets may excrete the virus for up to 12, 8 and 10 days, respectively. Rabid horses will not transmit the disease for more than 10 days prior to showing symptoms and rabid cows not at all (Manitoba Public Health Communicable Disease Control Unit).
Pathogenesis
When the virus enters the body it travels through the central nervous system infecting the brain and salivary glands of its host. All mammals are susceptible to the rabies virus, but only a few are considered reservoirs for the disease, including raccoons, skunks, foxes and coyotes (CDC).
Once the rabies virus enters the body of the host it may incubate for up to several years, but more commonly the virus will incubate for 3-12 weeks. During the incubation period, the infected animal will show no symptoms of the disease and it cannot be easily detected. The virus travels through the central nervous system and infects the brain of the animal, causing behavioral changes and clinical symptoms. Once an animal becomes symptomatic the disease is nearly always fatal.
Symptoms and Diagnosis
Symptoms of the rabies virus include feelings of discomfort or uneasiness, headache, fever, spasms of swallowing muscles, delirium and convulsions (Manitoba Public Health Communicable Disease Control Unit). Hydrophobia often occurs as a result of the pain associated with swallowing, and acute irritation can be brought on by noise, wind, light and other stimuli.
A post-mortem direct fluorescent antibody test (dFA) on the brain tissue of a suspected carrier is the most effective way to diagnose rabies in animals. This test reveals whether the animal has the rabies antigen in the nervous tissue (CDC). Diagnoses in humans is more difficult and requires extensive testing before a diagnosis of rabies can be made.
In the United States, options for prevention and control of the disease have expanded to include prevention in wildlife populations.
Non-linked References
Rosatte, Richard C. 1987. Advances in Rabies Research and Control: Applications for the Wildlife Profession. Wildlife Society Bulletin 15:504-511
Krista McDermid - Krista has a Master's Degree in Wildlife Ecology, and works at an environmental consulting company. She has always been passionate about ...
