Polio Virus

Polio is caused by a Polio Virus!!

Polio Virus belongs to the Picornaviridae (Picornaviruses) family. Its diameter is about 22 to 30 nm and it is composed of single-stranded RNA of the (+) type, contained within an icosahedral capsid. No lipid containing envelope surrounds the nucleocapsid.

Two main characteristics of polio viruses are their affinity for nervous tissue and their narrow host range. Besides humans, Most strains of polio viruses infects only monkeys and chimpanzees.

There are three immunological types of polio viruses. Type 1 is the common epidemic type, Type 2 is associated with endemic infections, and Type 3 is an occasional cause of epidemics.

Polio Viruses are transmitted in nose and throat discharges and in the feces of the infected individuals. It enters into the body through respiratory route or oral-intestinal route.

Polio Virus multiply in the oropharynx or intestinal mucosa, pass to the lymphatic system and then it eventually reaches the bloodstream. In most instances the disease is sub-clinical. In 4-6% of cases, non paralytic poliomyelitis occurs, this is characterized by fever and stiffness or pain in the back and neck muscles. In about 0.1% of cases, paralytic poliomyelitis occurs.

This blood borne virus infects the central nervous system and causes inflammation of gray matter of the spinal cord, especially the anterior horn, in which the cell bodies of motor neurons are located. When these cells are damaged, motor responses to the affected parts are weakened or destroyed.

Paralytic poliomyelitis can result in death. The seriousness of the disease is also emphasized by the crippling effects on the survivors. One well known example is of Franklin.D.Roosevelt, who acquired paralytic poliomyelitis as an adult and could walk only with the greatest difficulty for the rest of his life.


VACCINES

Two types of vaccines are available for immunization against poliomyelitis.

• SABIN vaccine.
• SALK vaccine.

The SABIN vaccine is an easily administered oral vaccine that is nearly 100% effective. It consists of live attenuated strains of three immunological types. These strains infect the intestinal tract but, unlike virulent strains, they do not cause paralytic disease. Rather they produce a long lasting immunity by stimulating the formation of secretory IgA antibodies in the intestine and the serum antibodies in the bloodstream. The secretory antibodies can neutralize the infectivity of virulent strains that may be subsequently encountered, thus these antibodies can prevent primary intestinal infection.

The SALK vaccine is administered in a series of three intramuscular injections and is 70% - 90% effective. It consists of formalin inactivated strains. This vaccine stimulates the production of serum antibodies but not intestinal secretory IgA antibodies. Although the serum antibodies cannot prevent intestinal infection by a virulent polio virus, they can prevent polio virus in the bloodstream from reaching the spinal cord and causing paralysis.

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.