Polio Virus

Source:  Polio Virus    Tag:  about 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.