Poliomyelitis (Polio): For health professionals

Poliomyelitis (polio) infection is a highly infectious viral disease that attacks the central nervous system. Polio infections are more common in children less than 5 years of age. However, any person who is not immune to poliovirus, regardless of age, can become infected. There is no cure for polio but it can be prevented through immunization. The National Advisory Committee on Immunization (NACI) recommends routine childhood immunization against polio.

Consult the national case definition for additional information.

Agent of disease

Poliovirus is a member of the enterovirus subgroup of the Picorniviridae family. There are 3 serotypes of poliovirus: type 1, type 2 and type 3. Due to the efforts of the Global Polio Eradication Initiative (GPEI), wild-type 2 poliovirus has been eliminated from global circulation. In rare cases, polio infection can occur due to vaccine-derived poliovirus (VDPV). This is a strain of poliovirus that has genetically mutated from the strain in the oral polio vaccine (OPV), a vaccine that contains live attenuated virus. Canada does not use OPV.

Reservoir

Humans.

Spectrum of clinical illness

Most polio infections (90%) have no symptoms or very mild symptoms that can go unrecognized. In symptomatic persons, initial symptoms can include fever, fatigue, headache and vomiting. With increased disease severity, severe muscle pain and stiffness of the neck and back with or without paralysis may occur. Paralysis is generally asymmetric, dependent on the site of nerve cell damage, and commonly occurs 7 to 14 days after infection.  Weakness or paralysis still present 60 days after onset is usually permanent. One in 200 infections leads to irreversible paralysis. Although paralysis is the most visible sign of polio infection, less than 1% of cases result in paralysis. Among those paralyzed, 5% to 10% die due to respiratory failure.

Adults who contracted paralytic poliovirus during childhood may develop a non-infectious post-polio syndrome 30 to 40 years after recovery. Post-polio syndrome is characterized by a slowly progressive muscle weakness and pain and on occasion, muscle atrophy as well as breathing and swallowing difficulties.

Photos of Clinical Manifestations of Polio


Two children with polio receiving physical therapy
Source: Centers for Disease Control and Prevention

Two children with polio receiving physical therapy


Child with a severely deformed leg due to polio
Source: Centers for Disease Control and Prevention

Child with a severely deformed leg due to polio

Transmission

Transmission of poliovirus occurs predominantly through the fecal-oral route. Respiratory spread may rarely occur. The incubation period for paralytic cases of polio is generally 7 to 14 days (range, 3 to 35 days). Communicability is greatest around the onset of illness when the virus is present in high concentrations in the throat and feces. After the first week of illness, the concentration of poliovirus in the throat decreases. However, poliovirus can continue to be excreted in the feces for 3 to 6 weeks. In persons who have received oral polio vaccine (OPV), poliovirus can be present in the throat for 1 to 2 weeks following immunization and can remain in feces for several weeks. Infrequently some cases, including immunocompromised persons, poliovirus (from natural infection or OPV vaccine) can be excreted for prolonged periods of time (from greater than 6 months to a number of years).

In temperate climates, polio infection generally increases in the late summer and autumn months.

In rare cases, individuals can develop vaccine-derived polio following immunization with the OPV. The OPV contains a live, attenuated (weakened) vaccine-virus. Upon vaccination, the attenuated vaccine-virus replicates in the intestine before entering into the bloodstream to trigger a protective immune response. It is possible for the virus to become genetically altered during replication resulting in a new form of the virus. This is a vaccine-derived poliovirus which can in rare cases cause paralysis. Vaccine-associated paralytic poliomyelitis occurs in an estimated 1 in 2.7 million children receiving their first dose of oral polio vaccine.

Disease distribution (global)

Since global eradication efforts began in 1988, the annual global incidence of polio has decreased by over 99%. Countries which have stopped transmission of indigenous wild poliovirus can be intermittently affected by importations of the virus from countries where the virus is still endemic. A current list of countries with confirmed cases of wild polio is available through the Global Polio Eradication Initiative.

Risk Factors

Polio infections are more common in children less than 5 years of age. However, any person who is not immune to poliovirus, regardless of age, can become infected including those who are unimmunized or incompletely immunized.

Prevention and control

Until worldwide eradication of polio has been achieved, routine vaccination to prevent poliomyelitis should continue.

There are 2 main types of polio vaccine:

  • inactivated polio virus vaccines (IPV) which contains only dead virus and are given by injection
  • oral poliovirus vaccines (OPV) which contain live attenuated virus which are administered orally

IPV was introduced in Canada in 1955 and OPV in 1962. Vaccine programs in Canada switched from OPV to IPV in 1995/1996. OPV is no longer recommended or available in Canada because most cases of paralytic polio from 1980 to 1995 were associated with OPV vaccine. OPV vaccine continues to be widely used internationally.

Poliomyelitis vaccines used in Canada contain three types of wild poliovirus and are available as trivalent inactivated polio vaccines (IPV) or in combination vaccines.

  • Routine immunization of children is recommended at 2, 4, and 18 months of age with a booster dose at 4-6 years of age. It is also acceptable to give an additional dose of IPV at 6 months of age for convenience of administration in combination with DTap and Hib.
  • Similar to vaccination of children, vaccination of adults is recommended to prevent the introduction and circulation of polio.
  • Primary immunization for non-immune adults and a booster is recommended for adults travelling to epidemic or endemic areas or for those with other exposure risks (Immunization of Travellers).

For specific recommendations about polio-containing vaccines please refer to the Canadian Immunization Guide as well as NACI statements on poliomyelitis).

Epidemiology of Polio in Canada

The incidence of polio in Canada was dramatically reduced by the introduction of immunization programs in the 1950s. The last indigenous case of wild poliovirus in Canada was in 1977. In 1994, Canada was certified as being free of wild poliovirus by the World Health Organization. Cases of paralytic polio in Canada reported since that time have been associated with importations of wild poliovirus and the use of OPV vaccine. Vaccine programs switched from OPV vaccine to inactivated poliomyelitis vaccines (IPV) exclusively in 1995/1996.

Until polio eradication has been achieved globally, there remains a small risk of importation of polio into Canada. To ensure that Canada remains polio-free, the Public Health Agency of Canada, in conjunction with the Canadian Paediatric Society, conducts surveillance of cases of acute flaccid paralysis (AFP) in children less than 15 years of age. Since 1996, between 27 and 64 AFP cases in children less than 15 years of age have been reported each year, none attributed to wild or vaccine-derived poliovirus.

Figure 1: Poliomyelitis, paralytic - reported cases, Canada, 1950-2012

Figure 1
Text Equivalent - Figure 1

The image consists of two line graphs showing the reported number of cases of poliomyelitis in Canada over time in years. There is one main graph and a smaller graph embedded within the main graph at the top right corner.

In the main graph, the x axis represents time in years between 1950 and 2012 and the y axis represents the number of cases starting with 0 at the bottom to 6000 at the top. The years in which Canada introduced the inactivated poliovirus vaccine and the oral poliovirus vaccine are depicted with arrows as 1955 and 1961 respectively. The line inside the main graph represents the number of cases, which start from 911 in 1950, sharply increasing to a peak of 5384 by 1953. The number of reported cases declined sharply to 185 by 1957. The number increased to another peak of a1887 in the year 1959. We notice another sharp decline in cases to 19 in 1964.

The smaller graph inserted within the main graph shows the number of poliomyelitis cases reported between the years 1965 and 2012. The x axis depicts the time in years, and the y axis the number of reported cases starting from 0 at the bottom to 7 at the top. The line representing the case reports indicates that only 3 cases were reported in 1965, declining to 0 cases in 1968. However, the number increased to 6 by 1971, declined to 2 by 1972, rose to 4 by 1973 and, fell once again to 1 by 1976. The last case of wild poliovirus that originated in Canada (indigenous case) occurred in 1977. Another peak at 6 cases is seen by 1978 due to an importation of the virus. By 1980, however, the reported number of poliomyelitis cases declined to zero. Between 1983 and 1995, case reports fluctuate between 0 and 3 cases all related to either importations or vaccine associated paralytic poliomyelitis. No cases of paralytic poliomyelitis have been reported since then.

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