Rabies is an acute and progressive nervous system infection caused by viruses from the Lyssavirus genus, or group.
Rabies is of great public health concern because it can be transmitted to humans by the bite of infected animals and once clinical signs occur is nearly 100% fatal. With the advent of vaccines for domestic animals in the 1960s, wildlife became the primary source of rabies exposure for domestic animals and humans. Since then cases of human rabies in the United States have declined. However, with shrinking wildlife habitat and more opportunities to encounter wildlife, avoiding wild animals that are behaving abnormally is very important.
All mammals can potentially become infected with the rabies virus. In North America, the major wildlife reservoirs for this disease are raccoons, coyotes, skunks, bats, and foxes. Unvaccinated domestic or feral cats and dogs are also at risk of acquiring and transmitting rabies.
Rabies is not known to infect birds or reptiles.
Rabies is currently found on every continent except Antarctica and was likely introduced to North America from Europe around the 1700s. The United Kingdom and Japan have successfully eradicated rabies. Some other island nations are also free of the disease.
There are multiple strains of the virus associated with specific wildlife species. The Eastern raccoon strain is most common in the eastern United States. Skunk rabies is more common in the central United States and California, while fox rabies is found in Texas, Arizona, and Alaska. Although raccoon rabies is very common in New England, this strain of the virus has only been reported in three cases of human infection. Most human cases in the United States and Canada are caused by bat-strain rabies. Bats are a particular concern because they are active at night and their teeth are so small that people often do not realize that they have been bitten.
Rabies is transmissible to all mammals and is almost always transmitted through the saliva via the bite of an infected animal. However, it can be transmitted by non-bite exposures, which include infected saliva entering an open wound or intact mucous membranes, like the eyes or the mouth. Rabies transmission has been reported in rare cases via inhalation of infected particles in a bat cave or a laboratory, and transplantation of infected tissue.
The rabies virus travels slowly via the peripheral nerves from the site of the bite wound to the spinal cord and then ascends into the brain. Once in the brain, the virus replicates on a very large scale and then spreads to the salivary glands and other tissues. If the exposure site is farther from the brain (i.e. toward the legs or feet), it will take longer for the virus to travel to the brain, thus giving the host a longer incubation period. Depending on the species and the location of the exposure site on the body, incubation periods can range between a few days to 7 years, but typically are reported as 6 months (The World Organization for Animal Health).
Once the rabies virus has reached the brain, the infected host will begin to exhibit marked behavioral changes (encephalitis), which may be subtle such as gait abnormalities or voice changes, but most commonly they are readily seen as abnormal. Rabies will eventually lead to coma and death within 1 to 10 days of the onset of clinical signs. Clinical signs can vary widely and resemble many other neurological conditions, so a diagnosis cannot be reached based on clinical signs alone. However, generally speaking, abnormal behavior is the most common sign.
Currently, the only way to confirm a rabies infection is to examine the brain of a suspected infected animal. Rabies suspects must be euthanized and submitted for laboratory testing. The brain must be intact and should be refrigerated (not frozen) before testing. The brain tissue is tested using a fluorescent antibody test (FAT), which detects antibodies against the rabies virus. A new test, the DRIT, is showing promise as another accurate test that can be performed outside of a laboratory.
The local health department should be contacted immediately in the event of a possible exposure. Basic post-exposure treatment of cleaning the wound with soap and water, vaccination and immunoglobulin treatment has a high success rate if administered quickly after exposure to rabies and before the onset of clinical signs. More information about medical treatment of rabies exposure in humans can be found on the Centers for Disease Control and Prevention website. If treatment is not administered prior to the appearance of clinical signs, there is no generally accepted further treatment for rabies. However, 1-3 individuals are thought to have survived in recent years with induced coma and supportive treatment, though with permanent neurologic impairment.
Rabies vaccinations for domestic animals, wildlife and people are available to help prevent rabies and control spread in reservoir populations. Prevention through vaccination has been very successful in domestic animals. People with greater risk of exposure such as veterinarians, wildlife biologists, wildlife rehabilitators, and animal handlers should receive pre-exposure vaccinations. Hunters and trappers should remember to wear gloves whenever handling carcasses.
Vaccination of free-ranging wildlife species is expensive but may be implemented to attempt to control rabies in reservoir populations. Oral Rabies Vaccine (ORV) programs have been employed throughout the country to deliver oral rabies vaccines within fishmeal bait to wildlife. These programs have successfully eliminated fox rabies in several countries in Europe, curtailed the virus significantly in Ontario and eliminated coyote rabies in southern Texas. Since 1997, USDA Wildlife Services has been working with local, state, and federal parties in the eastern US to develop and monitor ORV programs to establish a vaccination zone from Maine to Alabama to prevent the westward and northern spread of raccoon rabies. Every year, millions of baited vaccines are distributed throughout the eastern United States.
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