Epidemiology of rabies
Worldwide, canine rabies causes- 55,000 human deaths each year, most in Asia and Africa. In North America, bats, raccoons, skunks, and foxes carry endemic rabies. Bats cause the most human cases in the United States; rabies can be transmitted by minor or unrecognized bat bites.
Causes of rabies
Rabies is caused by a virus. Although there is only one type of virus that causes the disease, there are about five other similar viruses that have been found in various parts of Africa. Two of these viruses are reported to cause a similar infection in the human brain.
Rabies is found almost everywhere in the world, although in most places it is confined to the wild animal population, and only very rarely does it actually spread to human beings. Transmission of the disease occurs when an infected animal bites another animal or a human. The rabid animal will eventually die of the disease, so biting another animal ensures the survival of the rabies virus.
The virus attacks the nervous system and alters the behavior or some animals once they are infected. So that they become uncharacteristically aggressive and attack whatever comes near them – humans or other animals alike. This characteristic is important to the survival of the virus, and is brought about in part by hydrophobia, the major symptom of rabies, which is a thirst for water accompanied by terror and spasms at the sight of water. By keeping the pool of affected animals small and isolated, the disease can be made self limiting.
Once the infecting virus has made its way through the skin, it may act immediately or it may lie dormant for some time. The incubation period has been known is some cases to extend to years before symptoms appear.
Generally, though, the incubation period ranges from 20 to 90 days. After the initial infection through the skin, the virus enters the nerves of its victim and works its way up the nerves through the nerve cells until it reaches the brain. Once it has become established in the brain the symptoms of the disease break out.
That the virus has to make this journey through the nerves explains why the incubation period seems to be longer in those who are bitten on the foot compared with those who are bitten on the hand or the face; the farther the virus has to go from the site of entry to reach the brain, the longer, it takes for the symptoms of rabies to appear.
Symptoms and signs of rabies
Rabies starts out as a nonspecific, flulike illness. Symptoms usually begin within 20 to 90 days after exposure to the virus, but can occur within only a few days or up to one year or more after this exposure. Fever, headache, and malaise are common symptoms. Pain, numbness, and tingling may occur at the injury site. As the illness progresses, the person develops changes in behavior, hallucinations, and agitation. Difficulty in swallowing develops from neurological dysfunction, leading to fear of water. Excessive salivation, convulsions, cardiac arrhythmias, respiratory failure, and death usually follow. In almost all cases, once classic symptoms appear death results.
Pathogenesis if rabies
The incubation period varies from 2 weeks to > 1 year (mean, 1-3 months). During most of this period, rabies virus is present at or close to the site of the bite. Virus replicates at the inoculation site and spreads to peripheral nerves and then to the central nervous system (CNS), dispersing rapidly throughout the gray matter. Establishment of CNS infection is followed by centrifugal spread along peripheral nerves to other tissues, including salivary glands— hence the excretion of virus in the saliva of rabid animals. The most characteristic pathologic CNS finding is the Negri bodyâ€”and eosinophilic cytoplasmic inclusion that is found within neurons and is composed of rabies virus particles in an amorphous matrix.
Other methods of transmission of rabies
A bite is not the only way that the disease can be transmitted. It is possible for the virus to enter the body through a cut or scratch in the skin, so even a lick from an infected animal can transmit the disease. There is even a documented case of someone’s is becoming infected with the virus by inhaling droplets of virus carrying liquid. This has happened in laboratory accidents, and in even more bizarre circumstances, for instance as a result of breathing in a cave atmosphere contaminated with the urine of infected bats.
Treatment of rabies
There is no effective treatment once symptoms appear. Wounds inflicted by a potentially rabid animal should be washed with soap and water. Immediate medical attention should be sought. The geographical location of the incident, the animal involved and its vaccination status, and whether or not provocation was involved is important. Capture of the animal and testing of postmortem tissue, or, in the case of a domesticated animal that has documented up to date rabies vaccine status, confinement and observation for a specified period of time are needed. In the latter case, the animal is watched for any signs of rabies.
For potential exposures to rabies, immune globulin and five doses of rabies vaccine are administered. This is referred to as post exposure prophylaxis. Immune globulin provides antibody to the virus and is administered around the site of the wound. The vaccine induces the body to produce antibody to rabies virus, a situation that takes some time. The vaccine is administered in the upper arm and is given over a period of 4 weeks.
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