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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