Poliomyelitis

Polio

Read the Ninth Report of the Independent Monitoring Board of the Global Polio Eradication Initiative (May 2014).

Clinical Description

Identification

A viral infection recognized by acute onset of flaccid paralysis. Infection occurs in the GI tract with spread to regional lymph nodes and, in a minority of cases, to the CNS. Flaccid paralysis occurs in less than 1% of infections; over 90% are unapparent or result in non-specific fever. Aseptic meningitis occurs in about 1% of infections. Fever, malaise, headache, nausea and vomiting are recognized in 10% of infections. If disease progresses to major illness, severe muscle pain and stiffness of the neck and back with flaccid paralysis may occur. Paralysis of poliomyelitis is usually asymmetric, with fever present at onset. Maximum extent of paralysis is usually reached within 3–4 days. The site of paralysis depends on the location of nerve cell destruction in the spinal cord or brain stem; legs are affected more often than the arms. Paralysis of the respiration and/or swallowing muscles can be life-threatening. Some improvement in paralysis may occur during convalescence, but paralysis still present after 60 days is likely to be permanent. Infrequently, recurrence of muscle weakness following recovery may occur many years after the original infection has resolved (“postpolio syndrome”); this is not believed to be related to persistence of the virus itself. With progress made towards global eradication, poliomyelitis must now be distinguished from other paralytic conditions by isolation of virus from stool. Other enteroviruses (notably types 70 and 71), echoviruses and coxsackieviruses have been reported to cause an illness simulating paralytic poliomyelitis.

The most frequent cause of acute flaccid paralysis (AFP) that must be distinguished from poliomyelitis is Guillain-Barré syndrome (GBS). Paralysis in GBS is typically symmetrical, and may progress for periods as long as 10 days. Fever, headache, nausea, vomiting and pleocytosis characteristic of poliomyelitis are usually absent in GBS; high protein and low cell counts in CSF and sensory changes are seen in the majority of GBS cases. Acute motor axonal neuropathy (“China paralytic syndrome”) is an important cause of AFP in northern China, and is probably present elsewhere; it is seasonally epidemic and closely resembles poliomyelitis. Fever and CSF pleocytosis are usually absent, but paralysis may persist for several months. Other causes of AFP include transverse myelitis, traumatic neuritis, infectious and toxic neuropathies, tick paralysis, myasthenia gravis, porphyria, botulism, insecticide poisoning, polymyositis, trichinosis, and periodic paralysis.

Infectious Agents

Poliovirus (genus Enterovirus) types 1, 2 and 3; all types cause paralysis. Wild poliovirus type 1 is isolated from paralytic cases most often, and type 3 less so. Circulating wild type 2 poliovirus has not been detected since October 1999. Type 1 most frequently causes epidemics. Paralytic polio cases have also been reported, rarely, due to outbreaks caused by circulating vaccine-derived polioviruses (cVDPVs) types 1, 2 and 3. Most vaccine-associated cases are due to type 2 or 3 Sabin-like polioviruses.


Diagnosis

Differential diagnosis of acute non-paralytic poliomyelitis includes other forms of acute nonbacterial meningitis, purulent meningitis, brain abscess, tuberculous meningitis, leptospirosis, lymphocytic choriomeningitis, infectious mononucleosis, the encephalitides, neurosyphilis and toxic encephalopathies.

Definitive laboratory diagnosis requires isolation of the wild poliovirus from stool samples, CSF or oropharyngeal secretions. Specialized laboratories can differentiate “wild” from vaccine virus strains. Rises in antibody levels (4-fold or greater) are less helpful in the diagnosis of wild poliomyelitis infection, since type-specific neutralizing antibodies may already be present when paralysis develops, and significant titer rises may not be demonstrable in paired sera. Antibody response following immunization mimics the response after infection with wild type viruses, and the widespread use of live polio vaccines makes interpretation of antibody levels difficult, except to rule out polio in cases where no antibody has developed in immunocompetent children.


Epidemiology

Mode of Transmission

Primarily person-to-person spread, principally through the fecal-oral route; virus is detectable more easily and for a longer period in feces than in throat secretions. Where sanitation levels are high, pharyngeal spread may be relatively more important. In rare instances, milk, foodstuffs and other materials contaminated with feces have been incriminated as vehicles. No reliable evidence of spread by insects exists.

Incubation Period

Commonly 7–14 days for paralytic cases; reported range of 3 to possibly 35 days.

Period of Communicability

Not precisely defined, but transmission is possible as long as the virus is excreted. Poliovirus is demonstrable in throat secretions as early as 36 hours, and in feces 72 hours, after exposure to infection in both clinical and inapparent cases. Virus typically persists in the throat for approximately 1 week, and in feces for 3–6 weeks. Cases are most infectious during the days before and after onset of symptoms.

Reservoir

Humans, most frequently people with inapparent infections, especially children. No long-term carriers of wild type poliovirus have been detected.

Susceptibility

Susceptibility to infection is universal; paralysis occurs in only about 1% of infections. Residual paralysis is observed in 0.1% to 1% of cases, depending primarily on the virulence of the strain. The rate of paralysis among infected non-immune adults is higher than that among non-immunized infants and young children. Type-specific immunity, apparently of lifelong duration, follows both clinically recognizable and inapparent infections. Second attacks are rare and result from infection with a poliovirus of a different type. Infants born of immune mothers have transient passive immunity.

Intramuscular injections, trauma or surgery during the incubation period or prodromal illness may provoke paralysis in the affected extremity. Tonsillectomy increases the risk of bulbar involvement. Excessive muscular activity in the prodromal period may predispose to paralysis.

Occurrence

Historically, poliomyelitis occurred worldwide sporadically and as epidemics, with an increase during the late summer and autumn in temperate countries. In tropical countries, a less pronounced seasonal peak occurred in the hot and rainy season. With improved immunization and the global initiative to eradicate poliomyelitis, by the end of 2007 polioviruses were limited to only four countries that had not succeeded in interrupting transmission (Afghanistan, India, Nigeria and Pakistan). The greatest risks of polio are now in south Asia (70% of cases in 2007) and in West/Central Africa (30% of cases in 2007). Poliomyelitis remains primarily a disease of infants and young children. In the four countries that have not yet succeeded in interrupting transmission, 80%–90% of cases are in children aged less than three, and virtually all cases are in those under five. Clusters of susceptible persons—including groups that refuse immunization, minority populations, migrants and other unregistered children, nomads, refugees and the urban poor—are at high risk.

Although wild poliovirus transmission has ceased in most countries, importation remains a threat. A large outbreak of poliomyelitis occurred in 1992–1993 in the Netherlands among members of a religious group that refuse immunization, and virus was also found among members of a related religious group in Canada, although no cases occurred. Between 2003 and 2007, imported wild poliovirus caused paralytic cases in 27 countries, primarily in Africa, Asia and the Middle East. Until recent changes in immunization policy, and with the exception of rare imported cases, the few cases of poliomyelitis recognized in industrialized countries were caused by vaccine virus strains. About half of vaccine-associated paralytic poliomyelitis (VAPP) cases occurred among adult contacts of vaccinees.

Since 2000, 10 polio outbreaks due to cVDPVs have been reported from 9 countries, all of which continue to use OPV for routine immunization. These outbreaks have been associated with areas of low OPV coverage, and the cases were clinically indistinguishable from polio caused by wild poliovirus.
 


Prevention and Control

1)    Preventive measures:

a)    Educate the public on the advantages of immunization in early childhood.

b)    Both a trivalent live, attenuated oral poliovirus vaccine (OPV) and an injectable, inactivated poliovirus vaccine (IPV) are commercially available for routine immunization. Since 2005, monovalent oral poliovirus vaccines (mOPV) types 1 and 3 have been developed and licensed for use in mass campaigns to provide higher, type-specific seroconversion rates in areas where one or both of these serotypes are circulating.

i)    OPV simulates natural infection by inducing both circulating antibody and resistance to infection of the pharynx and intestine, and also immunizes some susceptible contacts through secondary spread. In developing countries, lower rates of seroconversion and reduced vaccine efficacy for OPV have been reported; this can be overcome by administration of numerous extra doses in immunization programs and/or supplemental campaigns. Breastfeeding does not cause a significant reduction in the protection provided by OPV. WHO recommends the use of OPV alone for immunization programs in developing countries because of the capacity to induce mucosal immunity, low cost, ease of administration, and superior capacity to provide population immunity through community spread.

ii)    IPV, like OPV, provides excellent individual protection by inducing circulating antibody that blocks the spread of virus to the CNS, and also protects against pharyngeal infection, but does not induce intestinal immunity comparable to OPV. Many middle-income countries and most industrialized countries have switched to IPV alone for routine immunization, because wild type polioviruses have been eliminated, ongoing global eradication efforts have reduced the risk of importations, and the risk of paralysis from OPV in these countries is considered greater than that from wild poliovirus.

ii)    A few individuals with underlying primary immune deficiency disorders have been identified who chronically excreted an OPV-derived poliovirus, and the significance of these chronic excreters as a risk to polio eradication is under review. No secondary cases have been associated with or attributed to long-term excreters of vaccine-derived polioviruses.

iv)   More troublesome are outbreaks of poliomyelitis caused by circulating vaccine-derived polioviruses (cVDPVs). These are capable of spreading through populations and becoming manifest in non-vaccinated or incompletely vaccinated individuals. The extent of this problem is being evaluated, with on average one such outbreak being detected each year, arising primarily in areas of low OPV coverage
.

c)    Recommendations for routine and supplementary immunization:

i)    In developing countries, WHO recommends 4 doses of OPV at 6, 10 and 14 weeks of age, with an additional dose at birth or at the measles contact (usually 9 months of age), depending on the endemicity and/or risk of polio in the country. In endemic countries, WHO recommends the use of national supplemental immunization campaigns administering two or more doses of OPV one month apart to all children under five regardless of prior immunization status. These campaigns should be conducted during the cool, dry season to achieve maximum effect. On the attainment of a high level of control in a country, targeted house-to-house mop-up immunization campaigns in high-risk areas are recommended to interrupt the final chains of transmission.

ii)    Where polio is still endemic or at high risk of importation and spread, WHO recommends the use of OPV for all infants, including those who may be infected with HIV in whom it has been shown to be safe. Diarrhea is not a contraindication to OPV. In industrialized countries, contraindications to OPV frequently include congenital immunodeficiency (B-lymphocyte deficiency, thymic dysplasia), current immunosuppressive treatment, disease states associated with immunosuppression (e.g. lymphoma, leukemia, and generalized malignancy), and the presence of immunodeficient individuals in the households of potential vaccine recipients. IPV should be used in such people. OPV causes paralytic poliomyelitis in vaccine recipients or their healthy contacts at a rate of approximately one in every 2.5 million doses administered, or 1 in 800 000 first vaccinations. In Romania, multiple injections of antibiotics at the time of vaccination were associated with an increased risk of vaccine-associated poliomyelitis (VAPP).

iii)    With progress towards eradication, the risk profile of paralytic poliomyelitis is changing, particularly in industrialized and high/middle income countries. Many of these have decided that the risks of paralytic poliomyelitis due to adverse events associated with continued use of OPV in routine immunization are greater than those due to the handling or circulation of wild poliovirus, and have adopted one of two approaches to prevent or minimize immunization-related adverse events:

(1) Replacement of OPV by inactivated poliovirus vaccine (IPV) for routine immunization.


(2) Introduction of mixed OPV/IPV use—for example, effective January 2000, all children in the USA were to receive 4 doses of IPV at ages 2, 4 and 6–18 months and 4–6 years. In these countries, OPV is now reserved for special circumstances, such as mass campaigns to control possible outbreaks.

d)    Immunization of adults: Routine immunization for adults is not considered necessary. Primary immunization is advised for previously non-immunized adults traveling to endemic countries, members of communities or population groups in which poliovirus disease is present, laboratory workers handling specimens containing poliovirus, and health care workers who may be exposed to patients excreting wild type polioviruses. In most industrialized countries, IPV is recommended for adult primary immunization, e.g. 2 doses at a 1–2 month interval and a third dose 6–12 months later. Those having previously completed a course of immunization and currently at increased risk of exposure are often given an additional dose of IPV. A single, lifetime booster is recommended for previously immunized adults traveling to polio-infected areas.

Some countries have established special immunization requirements for travelers from polio-endemic and re-infected countries, and travelers should check immunization requirements prior to departure.

2)    Control of patient, contacts and the immediate environment:

a)    Report to local health authority: Obligatory case report of paralytic cases as a Disease under surveillance by WHO, Class 1. In countries undertaking poliomyelitis eradication and/or certification, each case of acute flaccid paralysis (AFP), including Guillain-Barré syndrome, in children aged under 15 years must be reported and fully investigated. Non-paralytic cases are also reported to the local health authority, Class 2. (See Annex: Reporting of Communicable Diseases)

b)    Isolation: Enteric precautions in the hospital for wild virus disease; of little value under home conditions because many household contacts are infected before poliomyelitis has been diagnosed.

c)    Concurrent disinfection: Throat discharges, feces and articles soiled therewith. In communities with modern and adequate sewage disposal systems, feces and urine can be discharged directly into sewers without preliminary disinfection. Terminal cleaning.

d)    Quarantine: Of no community value.

e)    Protection of contacts: Immunization of familial and other close contacts is recommended but may not contribute to immediate control; the virus has often infected susceptible close contacts by the time the initial case is recognized.

f)    Investigation of contacts and source of infection: Occurrence of a single case of poliomyelitis due to wild poliovirus in a country that has interrupted transmission is a public health emergency prompting immediate investigation and planning for a large-scale response. A thorough search for additional cases of AFP in the area around the case assures early detection, facilitates control, and permits appropriate treatment of unrecognized and unreported cases.

g)    Specific treatment: None; attention during acute illness to complications of paralysis requires expert knowledge and equipment, especially for patients in need of respiratory assistance. Physical therapy is used to attain maximum function after paralytic poliomyelitis and can prevent many deformities that are late manifestations of the illness.

3)    Epidemic measures:

In any country that has previously interrupted transmission of wild poliovirus, a single case of poliomyelitis must now be considered a public health emergency, requiring an extensive supplementary immunization response over a large geographic area. Responses should be initiated within 4 weeks of confirmation of the index case, and should consist of a minimum of 3 mass immunization rounds spaced 4–6 weeks apart (with at least 2 rounds after the last detected case), using the appropriate type-specific monovalent OPV, covering a minimum of 2–5 million children, and achieving at least 95% coverage in each administrative area.

4)    Disaster implications:

Overcrowding of non-immune groups and collapse of the sanitary infrastructure pose an epidemic threat.

5)    International measures:

a)    Poliomyelitis is a Disease under surveillance by WHO and targeted for eradication. Since 2007, countries party to the International Health Regulations (2005) are required to inform WHO immediately of individual cases of paralytic polio due to wild poliovirus, and to report details and extent of virus transmission. Countries should also report wild poliovirus isolated from other sources (e.g. environmental sampling), and polio cases due to circulating vaccine-derived poliovirus. Planning of a large-scale immunization response must begin immediately and be completed within 72 hours, and if appropriate, be coordinated with bordering countries. Primary isolation of the virus is best accomplished in a designated Global Polio Eradication Laboratory. Once a wild poliovirus is isolated, molecular epidemiology can help trace the source. At-risk countries should submit weekly reports on cases of poliomyelitis, AFP cases and AFP surveillance performance to their respective WHO offices until the world has been certified polio-free.

b)    International travelers visiting polio-infected areas should be adequately immunized. For more information see the WHO publication International travel and health.

c)    WHO Collaborating Centres provide support as required. More information can be found at: http://www.who.int/collaboratingcentres/database/en/

d)    Further information can be found at: http://www.who.int/gpv/
 

Source: Heymann (Ed.). (2008). Control of Communicable Diseases Manual, 19th edition. Washington, DC: American Public Health Association.
 


Available Vaccines

WHO-Prequalified Poliomyelitis Vaccines

 

Common Disease Taxonomy: