In September 2005, an unvaccinated
7-month-old infant with severe combined
immunodeficiency in an Amish community
without vaccine coverage for polio was
diagnosed with poliovirus by stool
sample. The type 1 poliovirus was
identified as a vaccine-derived
poliovirus from oral poliovirus vaccine
(OPV). Three other children in the
household had the same virus recovered
from stool samples, although the
children were not ill. Vaccine-derived
poliovirus occurs via replication in an
immunocompromised patient or via
circulating virus in communities with
low vaccine coverage; therefore, the
risk for transmission to other
low-vaccination communities and for an
outbreak in the United States is
heightened.[9]
Incidence
Prevaccination cases of poliovirus
reached a peak in 1952 with more than
21,000 cases of paralytic disease.[10]
In the United States, the last known
case of wild virus polio infection
occurred in 1979.[10,11]
For the year 2005, the number of
countries with endemic polio decreased
from 7 to 4 compared with 2002 and from
125 compared with 1988.[12]
Eight countries that were polio-free
were reinfected, although transmission
has been reduced in all except Somalia.
These 8 countries were Angola,
Bangladesh, Chad, Ethiopia, Indonesia,
Nepal, Somalia, and Yemen. Polio remains
endemic in 3 Asian countries --
Afghanistan, India, and Pakistan.
Routine vaccination coverage for infants
is estimated at 80% worldwide varying
from 69% in Africa to 94% in Europe.
Worldwide there were 1948
virus-confirmed cases of polio in 2005,
with none reported in America, Europe,
or the Western Pacific.[13]
One risk for infection is contact
with low-vaccinated communities.[9]
There is a risk for vaccine-associated
paralytic poliomyelitis (VAPP) seen with
the live attenuated poliovirus vaccine.
The risk is estimated at 1 in 2.4
million does of OPV, with first-dose
risk of 1 in 750,000.[11,14]
The last case of VAPP in the United
States was in 1999.[10]
Pathogenesis
Poliovirus is an RNA picornavirus and
enterovirus.[10] A highly
contagious virus, it is spread from
person to person by the fecal-to-oral
route by contact with stool,
contaminated sewage or water, oral
secretions, or fomites.[9]
After exposure, the virus replicates in
the oropharynx and intestine with a
resultant viremia that can infect the
central nervous system. There are 3
serotypes -- 1, 2 and 3 -- with most
infections caused by type 1, type 3, and
type 2 in that order. The incubation
period is 6-20 days (range, 3-35), and
infectivity lasts 4-6 weeks. More than
90% of household contacts of an
individual with wild poliovirus
infection become infected. Humans are
the only reservoir, and infection
results in lifelong immunity.[10,14]
Long-term carriage can occur in
immunodeficient individuals, but is
otherwise rare.[14]
Poliovirus peaks in the summer months
for temperate climates with no seasonal
variation in tropical zones.[10]
Clinical Manifestations
The majority, up to 95%, of
poliovirus infections are asymptomatic.
Some infections, 4% to 8%, present as a
mild febrile illness that may take the
form of an upper respiratory tract
infection, a gastrointestinal illness,
or a flulike illness. Finally, 1% to 2%
present as a mild prodromal illness
followed by aseptic meningitis. Less
than 1% or 1 in 200 infections result in
acute flaccid paralysis, with a 2- to
3-day prodromal illness followed in 1-10
days by an asymmetric paralysis.[9-11,14]
The virus has an affinity for anterior
horn cells and the brainstem, resulting
in spinal (79% of cases), bulbar (2%),
or a combination disease (19%).[10,14]
Maximum paralysis occurs in 2-4 days
with fever and muscle pain. Most
patients recover muscle function.
Persistent weakness or paralysis 12
months post infection is usually
permanent.[10]
The case-fatality rate for paralytic
illness is 2% to 5% for children, 15% to
30% for adults, and 25% to 75% for
bulbar disease.[10] Postpolio
syndrome, consisting of muscle pain and
progressive or new weakness or
paralysis, occurs in 25% to 40% of
individuals infected in childhood and
presents 30-40 years after initial
infection, and is related to wild
poliovirus infections.[14]
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