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Wednesday, February 20, 2013

Polio and Poliomyelitis

Structure and Characteristics of Polio virus:

   1.            Non-enveloped, RNA virus
   2.            Replicates in cytoplasm
   3.            Single, stranded, Icosahedral nucleocapsid
   4.            Acid and bile stable but no long term carrier state
   5.            Three serologic (antigenic) types  based on different antigenic determinants on the outer capsid protein
   6.            Release upon death of the cell
A child suffering from Poliomyelitis

Disease caused by Polio virus:

Disease cause by polio virus is poliomyelitis

Pathogenesis of Poliomyelitis:

          Transmission: faeco-oral route (by ingestion of contaminated food and drink)
          Host: Human
          Incubation period: 7-14 days
          Mechanism of pathogenesis:
                           1.            Polio virus enters into the body by faeco-oral route
                           2.            Replicate in the lymphoid tissue of the orophraynx and SI (Peyer’s patches)
                           3.            Spreads through blood stream to the CNS and also spreads retrograde along nerve axons
                           4.            Then replicates in the motor neurons located in the ant. Horn of the spinal cord
                           5.            Death of nerve cells
                           6.            Result: Paralysis of the muscle innervated by those neurons. Also affects the brain stem, leading to bulbar poliomyelitis (with respiratory paralysis) but rarely damage the cerebral cortex.


Prevention/ Prophylaxis / Vaccination against Polio virus:

1. Live attenuated (sabin/oral vaccine/ opv)

Vaccination against Polio Virus
         
Dose
Time
Time
1st
At birth
4th-14th weeks after birth
5th-9months of birth
2nd
6 weeks after birth
3rd
10 weeks after birth

2. Killed (salk/inactivated/IPV) vaccine:

·       At first 3 doses are given at intervals of 1-2 months
·       4th dose 6-12 months after that


Advantages of OPV (Oral Polio Vaccine):

   1.            Cheaper, easy to administer
   2.            Produce local gut immunity by provoking the production of intestinal IgA
   3.            Manufacture is easy
   4.            Longer and strong immunity given

Disadvantages of OPV  (Oral Polio Vaccine):

   1.            It may revert back to virulent form
   2.            Can also spread from vaccine to controls
   3.            Cannot be given to pregnant women, immunocompromised and aged person
   4.            Presence of other virus may interfere with gut colonization
   5.            Multiple doses are necessary to establish immunity
   6.            Must be kept in refrigerator to prevent heat-inactivation

Advantages of IPV  (Inactivated Polio Vaccine):

   1.            Dose not revert back to virulent form
   2.            No reduction of potency due to interference
   3.            Preservation is easy
   4.            Less chance of contamination
   5.            Can be given to pregnant women, immunocompromised and aged person

Disadvantage of IPV (Inactivated Polio Vaccine):

   1.            No local immunity
   2.            Patient compliance is not good
   3.            Large dose is required
   4.            Expensive, shorter immunity
   5.            More difficult to manufacture

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