Rabies

Rabies is a disease caused by Rabies Virus transmitted through a bite of an infected animal.

Rabies Virus is the most important member of the Rhabdovirus family. It is a parasite of domestic as well as wild mammals. It is transmitted to humans through the bite of dogs, cats, bats, and skunks.

Rabies is essentially an overwhelming encephalomyelitis. In humans the incubation period from the time of infection is usually about 3-8 weeks. The development of symptoms and the length of the incubation period depend largely on the severity and location of the bite.

Symptoms in humans includes severe headache and high fever with alternating stages of excitement and depression.

Patients have difficulty in swallowing, and slight stimuli incite muscular spasms in the throat and chest. Death usually follows paralysis. The mortality rate is nearly one hundred percent.

If a person has been bitten by a rabid animal, the long incubation period for rabies allows time for measures to be taken to prevent the virus from reaching to the central nervous system. These measures include a combination of PASSIVE IMMUNIZATION and ACTIVE IMMUNIZATION.

PASSIVE IMMUNIZATION :-  

                                                      Administration of immune human or horse globulin, which provides an immediate source of antibodies against the virus. But it lasts for only about 14 days.

ACTIVE IMMUNIZATION :- 

                                                    Administration of a rabies vaccine, to stimulate a long-lasting production of antibodies by a patient.

In 1885, Pasteur produced an effective rabies vaccine by using brain tissue from rabbits in which the rabies virus had been propagated. By drying the infected nerve tissue for increasing periods of time, the infectivity of the virus could be progressively diminished to the point where the preparation could be used to immunize persons who had been bitten by rabid animals. In 1919, Semple modified the Pasteur vaccine by adding phenol to completely inactivate the virus.

The only difficulty with the Pasteur and Semple vaccines is that they sometimes cause an allergic encephalitis in the recipient, because of the presence of nerve tissue in the vaccines.

In 1949, a different type of vaccine was developed. It contained a live attenuated flurry strain of rabies virus, which could be grown in embryonated duck eggs. This vaccine has been widely used. However, it has relatively low immunogenicity, and although it lacks large amounts of nerve tissue antigens, it still occasionally cause allergic encephalitis. 

In 1980, an inactivated vaccine prepared from virus propagated in cultures of diploid human cells devoid of nerve tissue antigens was licensed for use. This new vaccine appears to be both safe and highly immunogenic.