The polioviruses are three related enteroviruses : Types 1, 2 and
3. All three types cause paralysis. Type 1 causes paralysis most
frequently. The type 2 virus has not been detected worldwide since
October 1999. Type 3 virus is now isolated very infrequently in
India.
Immunity
Protective immunity against poliovirus infection develops by immunization
or natural infection. Immunity to one poliovirus type does not
protect against other poliovirus types. Immunity following natural
infection or by live oral polio vaccine (OPV) is believed to be
lifelong. Infants born to mothers with high antibody levels against
poliovirus are protected for the first weeks of life.
Occurrence
Poliomyelitis occurred worldwide, but is not limited to Asia and
African continents. During 2003 the disease was endemic in 6 countries
India, Nigeria, Niger, Pakistan, Afghanistan and Egypt. However
importations from these endemic countries have been reported recently
in other polio free countries and states. The disease is seasonal,
with cases starting to increase sharply in June with peaks during
July through September. Cases continue to be high in areas with
low immunization coverage and poor sanitation.
Reservoir
Poliovirus infects only human beings and there is no animal reservoir.
Clinical spectrum
Inapparent (sub clinical ) infection. This occurs approximately
in 95 per cent of poliovirus infection.
Abortive polio or minor illness : Occurs in approximately 4 to 8
per cent of the infections.
Non paralytic polio : Occurs in approximately 1 per cent of all
infections.
Paralytic polio : Occurs in less than one per cent of infections.
