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Reservoir
Humans are the only known reservoir of poliovirus. The virus has been
found in human feces, sewage and bodies of water such as rivers, lakes,
and streams that receives water that has come in contact with raw sewage.
Transmission
Person-to-person spread of poliovirus via the fecal-oral route is the
most important route of transmission, although the oral-oral route may
account for some cases.
Temporal
pattern
Poliovirus infection typically peaks in the summer months in temperate
climates. There is no seasonal pattern in tropical climates.
Communicability
Poliovirus is highly infectious. Cases are most infectious from 7 to
10 days before and after the onset of symptoms, but poliovirus may be present
in the stool from 3 to 6 weeks.
United
States
In the immediate pre-vaccine era, 13,000 to 20,000 paralytic cases
were reported annually. Following IPV introduction in 1955, the incidence
dramatically decreased. The decline continued following OPV introduction
in 1961. In 1960, a total of 2,525 paralytic cases were reported, compared
with 61 in 1965. The last cases of paralytic poliomyelitis caused by endemic
transmission of wild virus in the United States were in 1979. The Western
Hemisphere was declared free of indigenous poliomyelitis in 1994. From
1980 through 1999, a total of 152 confirmed cases of paralytic poliomyelitis
were reported. Six cases were due to imported poliovirus. 144 (95%) cases
were vaccine-associated. In order to eliminate vaccine-associated paralytic
poliomyelitis from the United States, ACIP recommended in 2000 that IPV
be used exclusively in the United States.
Last
documented cases of indigenous transmission of wild poliovirus in the United
States
1.1970,
on the Texas-Mexico border, 22 cases of polio occurred, all in children
4 years of age or less.
2.1972,
in a Christian Science school in Connecticut, eight cases of paralytic
poliomyelitis and three of non-paralytic occurred in persons from 7 to
18 years of age.
3.1979
ten paralytic and five non-paralytic cases of poliomyelitis occurred among
the Amish in Pennsylvania, Missouri, Iowa and Wisconsin.
Post-Polio
Syndrome
After an interval of about 30-40 years, 25-40% of people who contracted
paralytic poliomyelitis in childhood experience muscle pain and weakness
or develop paralysis. This disease is referred to as post-polio syndrome
(PPS). These symptoms often worsen after exercise. Some patients also have
trouble breathing or swallowing and suffer from muscle twitches. While
not life threatening, PPS can severely limit a patient's lifestyle and
mobility. Increasing length of time since acute poliovirus infection and
presence of permanent residual impairment after recovery from the acute
illness enhance the risk of post-polio syndrome. The pathogenesis of post-polio
syndrome is thought to involve the failure of oversized motor units created
during the recovery process of paralytic poliomyelitis. Unfortunately,
the exact cause of PPS is unknown. Rest, pain relievers, physical therapy,
and assistive devices such as canes and wheelchairs help patients control
their symptoms. Post-polio syndrome is not an infectious process, and persons
experiencing the syndrome do not shed poliovirus. There is no known cure.