Polio Facts

                                                                               Illustration of the Polio virus from www.cdc.gov

•  Polio is caused by the smallest viruses that infect humans called enteroviruses that belong to the Picornaviridae family and Humans are the only known reservoir.  (Kew, 2005). 

•  Polioviruses are stable viruses even in an acid environment and are able to survive for long periods of time in water and sewage (Melnick, 1989).     

•  Polio is a highly contagious disease.  In endemic areas, many children are infected within the first year of life.   The largest percentage of infections does not cause any symptoms or may cause only a mild illness.  However, in about 1% of cases, Acute Flaccid Paralysis (AFP) occurs which can lead to lifelong paralysis and lameness. (www.cdc.gov) 

• Transmission peaks in the summer and fall and decreases during winter hence it exhibits seasonality however these seasonal differences are less pronounced in tropical areas (www.cdc.gov). 

• It is transmitted via the fecal-oral route and the oral-oral route. 

• Major risk factors for transmission include the following (Kew, 2005):

1.       Poor sanitation and hygiene

2.       Tropical and subtropical conditions

3.       Large birth cohorts

4.       High population densities

      According to the Center for Disease Control:

 “The virus enters through the mouth, and primary multiplication of the virus occurs at the site of implantation in the pharynx and gastrointestinal tract.  The virus is usually present in the throat and in the stool before the onset of illness.  One week after the onset there is fewer viruses in the throat, but virus continues to be excreted in the stool for several weeks.  The virus invades local lymphoid tissue, enters the bloodstream, and then may infect cells of the central nervous system.  Replication of poliovirus in motor neurons of the anterior horn and brain stem results in cell destruction and causes the typical manifestations of poliomyelitis.   Many persons who develop AFP recover completely and in most muscle function returns to some degree however, weakness or paralysis still present 12 months after the acute onset is usually permanent” (www.cdc.gov).      

Vaccines:

•  Currently, there are two vaccines available: the inactivated poliovirus vaccine or IPV and the oral polio vaccine or OPV.  IPV is also known as the Salk vaccine- named for the man who developed the vaccine in the 1950.

•  IPV is made from inactivated or killed polioviruses (all three types are used) and requires that it be given by injection (www.cdc.gov). 

•  OPV, also known as the Sabin vaccine- named for the man who developed the vaccine, contains live viruses that  have been attenuated so that they are no longer neurovirulent for humans.  In its trivalent form, it contains all three types of polioviruses.

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