Poliomyelitis (polio) is an infection caused by a virus called poliovirus that enters the body through the mouth mainly through food or water that is contaminated with feces as a result of poor personal hygiene, poor sanitation or poor control of sewage. Polio infections are more common in children less than five years of age; however, any person who is not immune to poliovirus, regardless of age, can become infected. Most people have no symptoms when they are infected. However, in some cases (approximately 1% of cases), polio can infect and damage nerve cells in the spinal cord resulting in muscles that are weak or paralyzed. The nerve damage is permanent and can result in death if it is the muscles that facilitate breathing that are affected. Because less than 1% of polio infections ever result in paralysis, the most visible sign of polio, poliovirus can spread widely before an outbreak is detected. There is no cure for polio but it can be prevented through immunization.
Polio infection occurs as a result of circulating wild poliovirus. There are three serotypes of poliovirus: type 1, type 2 and type 3. Type 1 is more virulent and associated with more epidemics compared to the other types. Only types 1 and 3 wild polioviruses continue to circulate globally as wild-type 2 poliovirus has been eliminated. In rare cases, polio infection can occur due to vaccine-derived poliovirus (VDPV) which are strains of poliovirus that have genetically mutated from the strain in the oral polio vaccine (OPV), a vaccine that contains live attenuated virus. Canada does not use OPV.
The majority (90% to 95%) of polio infections are asymptomatic. In symptomatic persons, initial symptoms occur on average 7 to 14 days after infection and 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 and dependent on the site of nerve cell damage. Weakness or paralysis still present 60 days after onset is usually permanent. Although paralysis is the most visible sign of polio infection, less than 1% of cases result in paralysis. Typically, 1 in 100 - 200 infections leads to paralysis, usually in the legs; however, this may range from 1 in 50 to 1 in 1,000 infections. The case fatality ratio for paralytic polio ranges from 2% to 5% among children and from 15% to 30% for adults.
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.
The incubation period for polio is generally 7 to 14 days with a range of 3 to 35 days. Communicability is greatest just prior and post onset of illness, when the virus is present in high concentrations in the throat and excreted in 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 from 3 to 6 weeks. In persons who have received the oral polio vaccine, poliovirus can be present in the throat for 1 to 2 weeks following immunization and can remain in feces for several weeks. In rare cases, including immunocompromised patients, poliovirus can be excreted for prolonged periods of time, for example, greater than 6 months.
The spread of wild poliovirus occurs from person to person through either indirect or direct contact. The virus is spread predominantly indirectly from the feces by contamination of water, food or hands (commonly referred to as the fecal-oral route). In settings or situations of poor hygiene and sanitation the virus can spread rapidly in the environment (e.g. contaminated community drinking water). Direct spread through the respiratory route (coughing, sneezing and the inhalation of droplets) requires close contact with the infected person and occurs less frequently.
In rare cases, individuals can develop vaccine-derived polio following immunization with the oral polio vaccine. The oral polio vaccine contains a live, attenuated (weakened) vaccine-virus. Upon vaccination, the weakened vaccine-virus replicates in the intestine before entering into the bloodstream to trigger a protective immune response. In some cases, the virus can 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 (VAPP) occurs in an estimated 1 in 2.7 million children receiving their first dose of oral polio vaccine.
In 1988, the World Health Assembly established the Global Polio Eradication Initiative with the goal of eliminating polio worldwide. This initiative has become the largest international public health effort to date and is currently lead by the World Health Organization (WHO), the US Centers for Disease Control and Prevention, Rotary International and UNICEF.
Since its inception in 1988, many advances have been made in the eradication of polio. In 1994, the WHO Region of the Americas was certified polio-free followed by the WHO Western Pacific Region in 2000 and the WHO European Region in June 2002. Furthermore, the number of polio cases has decreased by over 99% and the number of countries in which polio is endemic decreased from 125 to 3. The remaining endemic countries include Afghanistan, Nigeria and Pakistan.
Until worldwide eradication of polio has been achieved, routine vaccination should continue. Poliomyelitis can be prevented by vaccination.
For further information about polio immunization, please refer to the most recent version of the Canadian Immunization Guide.
There are two main types of polio vaccine - inactivated polio virus vaccines (IPV) which contains only dead virus and is given by injection; and oral poliovirus vaccine (OPV) which contains live weakened virus which is administered by drops into the mouth. IPV was introduced in Canada in 1955 and OPV in 1962. Vaccine programs in Canada switched from OPV to IPV in 1995/1996.
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. More recent cases of paralytic polio in Canada have been associated with importations of wild poliovirus and the use of OPV vaccine. Between 1980 and 1995, 11 cases of paralytic polio associated with OPV vaccine use were reported in Canada. Vaccine programs switched from OPV vaccine to inactivated poliomyelitis vaccines (IPV) in 1995/1996. As recently as 2007 and 2009, Canada has seen isolated cases of polio or asymptomatic importations of vaccine associated poliovirus related to immunization of individuals with OPV vaccine in other countries.
Polio has been nationally notifiable since 1924 and is included in the Canadian National Notifiable Diseases Surveillance System (CNDSS)
In addition, to ensure Canada maintains its polio-free status in accordance with World Health Organization, 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. AFP is an outcome of poliovirus infection that involves the acute onset of paralysis in one or more limbs. This syndromic surveillance program was established in 1996 and collects data through both the Canadian Paediatric Surveillance Program (CPSP) and the Canadian Immunization Program ACTive (IMPACT).
The last case of indigenous wild paralytic poliomyelitis in Canada occurred in 1977.
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. AFP surveillance results are published annually in the CPSP Results publications and are also reported to PAHO on a quarterly basis.