15.21A: Rabies
- Examine the causes and symptoms associated with infection by the rabies virus
Rabies is a viral disease that causes acute encephalitis (inflammation of the brain) in warm-blooded animals. Rabies literally means “madness” in Latin. The disease is zoonotic and can be transmitted from one species to another, commonly by a bite from an infected animal. In humans, rabies is almost invariably fatal if postexposure prophylaxis is not administered prior to the onset of severe symptoms.
The rabies virus infects the central nervous system, travelling from the peripheral nerves to the brain. In humans, the incubation period between infection and the first sign of symptoms is typically two to 12 weeks, although periods as short as four days and longer than six years have been documented. Incubation period depends on the quantity of virus introduced and the distance it must travel to reach the central nervous system. Once there, symptoms begin to show and the infection is virtually untreatable. Early-stage symptoms include malaise, headache and fever, violent movements, uncontrolled excitement, depression, confusion, agitation, anxiety, and hydrophobia. Late stage symptoms extend to paranoia, terror, mania, and hallucinations progressing into delirium. Once symptoms have presented, survival is rare. Death almost invariably occurs within two to 10 days. Treatment with human rabies immunoglobulin (HRIG) and rabies vaccine is highly successful if administered before the onset of symptoms.
Rabies causes about 55,000 human deaths annually worldwide, with 95% of human deaths occurring in Asia and Africa. Roughly 97% of human rabies cases result from dog bites. In the U.S., animal control and vaccination programs have effectively eliminated domestic dogs as reservoirs of rabies. In several countries, including Australia and Japan, rabies carried by terrestrial animals has been eliminated entirely. While rabies was once eradicated in the United Kingdom, infected bats have recently been found in Scotland. In the U.S., the widespread vaccination of domestic dogs and cats and the development of effective human vaccines and immunoglobulin treatments has dropped the number of recorded human deaths from 100 or more annually in the early 20 th century, to one to two per year (mostly caused by bat bites). Modern cell-based vaccines are similar to flu shots in terms of pain and side effects. The old nerve-tissue-based vaccinations that require multiple painful injections into the abdomen with a large needle are cheap, but are being phased out and replaced by affordable World Health Organization intradermal vaccination regimens.
Rabies may be diagnosed by PCR or viral culture of brain samples after death, or from skin samples taken before. Diagnosis can be made from saliva, urine, and cerebrospinal fluid samples with less accuracy. Cheaper rabies diagnosis will become possible for low-income settings using basic light microscopy techniques.
Key Points
- The rabies virus infects the central nervous system, travelling from the peripheral nerves to the brain.
- Symptoms include hydrophobia, paranoia, terror, mania, hallucinations, and delirium.
- Once symptoms have presented, survival is rare, but treatment administered before the onset of symptoms is highly successful.
Key Terms
- hydrophobia : An aversion to water, as a symptom of rabies.
- zoonotic : of or relating to zoonosis, the transmission of an infectious disease between species.