NCCID Disease Debriefs provide Canadian public health practitioners and clinicians with up-to-date reviews of essential information on prominent infectious diseases for Canadian public health practice. While not a formal literature review, information is gathered from key sources including the Public Health Agency of Canada (PHAC), the USA Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO) and peer-reviewed literature.
This disease debrief was prepared by Shyama Nanayakkara. Questions, comments, and suggestions regarding this disease brief are most welcome and can be sent to email@example.com
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Questions Addressed in this debrief:
- What are the important characteristics of Polio?
- What is happening with current outbreaks of Polio?
- What is the current risk for Canadians from Polio?
- What measures should be taken for suspected cases of Polio contact?
What are the important characteristics of Polio?
Poliomyelitis (Polio) is a highly contagious, vaccine-preventable disease, most often seen in children under 5 years of age. Polio virus is a member of the Enterovirus genus, family Picornaviridae. Enteroviruses are transient inhabitants of the gastrointestinal tract and are stable at acidic pH. Picornaviruses are small, ether-insensitive viruses with an RNA genome. There are three poliovirus serotypes PV1, PV2, and PV3 (type 1, type 2, and type 3).
Transmission of polio is person-to-person, mainly through the fecal-oral route and less frequently via contaminated water or food. The virus multiplies in the intestine, and then invades the nervous system, sometimes causing paralysis. The incubation period is usually 7–10 days but can range from 4–35 days.
Communicability is most likely at the onset of illness, when the virus is present in greatest concentration in the throat and feces, but poliovirus may continue to be excreted in the feces for 3 to 6 weeks. In the case of some immunocompromised people, the virus may be excreted for months.
Vaccine-associated polio is possible from the oral polio vaccine (OPV), and rarely OPV may trigger a genetically altered form of the virus.
Signs, Symptoms and Severity
The majority (90%) of those infected with the polio virus experience no or mild symptoms and the disease usually goes unrecognized. In some cases, however, initial symptoms include fever, fatigue, headache, vomiting, stiffness in the neck, and pain in the limbs and will last for 2–10 days; there is complete recovery in almost all cases. After this prodromal phase, a small proportion of cases, the virus causes permanent paralysis, usually of the legs. The ratio of cases of inapparent infection to paralytic disease among susceptible individuals ranges from 100:1 to 1000:1 or more. Infection with the virus leads to viremia follows which may result in infection of central nervous system cells. Then the virus attaches and enters cells via a specific poliovirus receptor and replication of poliovirus in motor neurons of the anterior horn and brain stem, resulting in cell destruction and causing the typical clinical poliomyelitis. Depending on the site of infection and paralysis, poliomyelitis can be classified as spinal, bulbar, or spino-bulbar disease. Progression to maximum paralysis is rapid (2– 4 days); paralysis is usually associated with fever and muscle pain, and rarely progresses after the temperature has returned to normal. Spinal paralysis is typically asymmetric, more severe proximally than distally, and deep tendon reflexes are absent or diminished. Paralysis that lasts 60 days from onset is usually permanent. This can happen rapidly, within a few hours of infection. About 5-10% of cases die due to paralysis of the respiratory muscles.
Adults who contracted paralytic poliovirus as children may develop non-infectious polio syndrome 30 or more years after recovery. The post-polio syndrome is characterized by progressive muscle weakness and pain, and may include breathing and swallowing difficulties from muscle atrophy. . AFP can be caused by bacterial infections, autoimmune disorders, exposure to environmental toxins and Guillain-Barré Syndrome. Additional investigations are needed to determine the underlying cause. Sometimes no cause can be found.
Isolation of poliovirus (vaccine or wild type) from an appropriate clinical specimen; Stool samples, Viral throat swab and CSF has to be done immediately for confirmation.
Poliovirus is excreted via feces from an infected person or following vaccination intermittently for one month or more. Heavy shedding of the virus occurs just prior to the onset of paralysis and during the first two weeks after initial symptoms occur.
Two stool specimens were collected within two weeks (up to six weeks) after the onset of paralysis for viral studies and a throat swab should be submitted from all cases of acute flaccid paralysis to allow appropriate testing for poliomyelitis.
Laboratory diagnosis of poliomyelitis involves the growth and identification of polioviruses from samples using cell culture techniques. Timely collection, storage, and proper transport of samples are crucial for proper lab diagnosis of poliomyelitis.
There is no cure for paralytic polio and no specific treatment.
Depending on the severity of paralysis, physical or occupational therapy can help with arm or leg weakness and may improve long-term outcomes, particularly in the early course of illness. Healthcare providers should consider consulting neurology and infectious diseases experts to discuss possible treatments and recommend certain interventions on a case-by-case basis.
There are three poliovirus serotypes PV1, PV2, and PV3 (type 1, type 2 and type 3). Immunity to one serotype does not produce significant immunity to the other serotypes. The origins of polio eradication begin in the late 1950s. Of the 3 strains of wild poliovirus, wild poliovirus type 2 was eradicated in 1999 and no cases of wild poliovirus type 3 have been found since the last reported case in Nigeria in November 2012. Both strains have officially been certified as globally eradicated. As of 2020, wild poliovirus type 1 affects two countries: Pakistan and Afghanistan. Polio has been eradicated from most countries, however there is still a risk of polio is some parts of the world. Fewer than 175 cases were reported in 2019, compared with an estimated 350,000 cases in more than 125 endemic countries in 1988.
History of Eradication
Over the last decade, the Global Polio Eradication Initiative (GPEI) made steady progress on the path to eradication. In 1994, the International Commission for the Certification of Poliomyelitis Eradication declared the Americas to be polio-free. By 2002, three WHO Regions (the Americas, Western Pacific and European Regions) had been certified polio-free. Wild poliovirus types 2 and 3 (WPV2 and WPV3) were declared eradicated in 2015 and 2019, respectively; the World Health Organization (WHO) South-East Asia Region was declared free of poliovirus in 2014, and most recently, the WHO African Region was certified free of wild poliovirus (WPV) in August 2020. However, the final steps towards eradication have proven the most difficult.
In 1988, the World Health Assembly adopted a resolution for the worldwide eradication of polio, marking the launch of the GPEI, supported by national governments, WHO, Rotary International, the US Centers for Disease Control and Prevention (CDC), UNICEF, and later joined by the Bill & Melinda Gates Foundation and Gavi, the Vaccine Alliance. Wild poliovirus cases have decreased from 99% since 1988. An estimated 350 000 cases in more than 125 endemic countries then to 175reported cases in 2019. Canada was certified by the World Health Organization as being free of wild poliovirus in 1994. Cases of paralytic polio reported in Canada since then have been associated with the use of the OPV vaccine and importation of the wild poliovirus.
In 2020, COVID-19 disrupted routine immunizations, resulting in more than 80 million children at increased risk of vaccine-preventable diseases, including polio. The thirty-second Polio IHR Emergency Committee (EC) unanimously agreed that the risk of international spread of poliovirus remains a Public Health Emergency, with the possibility of poliovirus outbreaks tripling from 2019 to 2020.
Prevention and Control
As there is no cure for polio, it can only be prevented by immunization. Almost all children (>99%) who have been immunized with all recommended doses of inactivated polio vaccine will be protected from poliovirus. Good hand washing hygiene, using soap and water is essential; poliovirus is not killed by alcohol-based hand sanitizers.
There are two types of vaccines: Inactivated poliovirus vaccine (IPV) is given as an injection in the leg or arm; and Oral poliovirus vaccine (OPV) which is a live attenuated vaccine. An estimated 1.5 million childhood deaths have been prevented through the systematic administration of vitamin A (to also prevent Vit A deficiency) during polio immunization. Since 1988, GPEI efforts have decreased wild poliovirus cases by over 99%.
However, attenuated polioviruses in OPV can undergo genetic changes during replication, and in communities with low vaccination coverage, can result in vaccine-derived polioviruses (VDPVs) that can cause paralytic polio same as the disease caused by WPVs. It will be necessary to stop oral polio vaccine (OPV) use globally to achieve eradication, because the attenuated viruses in the vaccine may rarely cause polio. The World Health Assembly endorsed the phased withdrawal of OPV and introduction of inactivated poliovirus vaccine (IPV) into childhood routine immunization schedules. In 2016, the type 2 oral polio-containing vaccine was withdrawn from all routine immunization programmes worldwide, replaced by the trivalent oral polio vaccine (tOPV) containing attenuated poliovirus vaccine serotypes 1, 2 and 3 with bivalent oral polio vaccine (bOPV) containing only types 1 and 3.
OPV was withdrawn from use in Canada in 1995/6 due to the fact that most cases of paralytic polio were attributed to OPV.
Switching from tOPV to bOPV is not without risks. The primary risk is the re-emergence of outbreaks involving type 2 circulating vaccine-derived polioviruses. The updated GPEI Polio Eradication Strategy 2022-2026 added, with the recent risk of emerging cVDPV2 type 2, novel oral poliovirus vaccine (nOPV2). The vaccine is a next-generation version of mOPV2, and considered genetically more stable and less likely to be associated with vaccine-derived poliovirus.
Poliomyelitis vaccines used in Canada contain three types of wild poliovirus and are available as trivalent IPV or in combination vaccines.
The Canadian Immunization Guide recommends that children receive four doses of polio vaccine between the ages of 2 and 18 months (23 months at the outside), with a booster to be received before school entry (4 to 6 years of age).
Rapid investigation of suspected poliomyelitis cases is critical for identifying possible wild poliovirus transmission. Early detection permits timely implementation of controls to limit the spread of imported wild poliovirus or cVDPVs and maintain the eradication of wild poliovirus.
To ensure there are no new cases of polio, an active Surveillance is carried out to investigate all Acute Flaccid Paralipsis (AFP) cases which is a notifiable disease. AFP is the acute onset of paralysis in one or more limbs and may occur for different reasons including neurological conditions. This active surveillance to find all AFP cases is used to rule out the presence of polio. It provides ongoing evidence that the countries which are eradicated polio remain polio-free.
The Public Health Agency of Canada and the Canadian Pediatric Society collaborate on surveillance of acute flaccid paralysis (AFP) cases in children younger than 15 years. Cases reported since 1996 (27 to 64 AFP cases per year) in children under 15 were not attributed to wild or vaccine-derived poliovirus.
According to the Public Health Agency of Canada:
Data on cases of AFP are collected from the following surveillance programs, managed by the Canadian Paediatric Society:
- Canadian Paediatric Surveillance Program (CPSP): A surveillance network that gathers data from over 2,500 peadiatricians and peadiatric subspecialists each month to monitor rare diseases and outcomes in Canadian Children.
- IMPACT (Immunization Monitoring Program, ACTive): A paediatric hospital-based active surveillance network for adverse events following immunization, vaccine failures and selected infectious diseases that are, or will be, vaccine-preventable.
AFP surveillance results are published annually in the CPSP Results publications and are also reported to the Polio Global Eradication Initiative at the Pan-American Health Organization (PAHO) on a weekly basis.
CTV News reports that the Public Health Agency of Canada will test wastewater for polio virus in several “key high-risk communities” in Canada (updated news report August 15, 2022).
What is happening with current outbreaks of Polio?
Since the last Thirty-second EC meeting in February 2022, Pakistan has reported ten WPV1 cases from North Waziristan and two WPV1 positive environmental samples from the neighbouring district. With the ongoing WPV1 circulation in South KP, the risks to the rest of Pakistan have escalated. In April 2022, the WHO confirmed that seven vaccine-derived poliovirus type 3 cases were found in children in Jerusalem, Israel; six were asymptomatic.
A multi-country response to the WPV1 outbreak is continuing with retrospective case searching, surveillance, strengthening and improving essential immunization are all ongoing. The EC noted, “that while administrative coverage was high, problems with population data made these coverage estimates unreliable. Monitoring coverage by Lot Quality Assurance Sampling (LQAS) showed far lower coverage, and the committee noted that countries that have long been polio-free needed assistance from GPEI partners in the implementation of supplementary immunization activities (SIAs).”
While surveillance is continuing, the Global Polio Laboratory Network (GPLN) in London has confirmed the isolation of type 2 vaccine-derived poliovirus (VDPV2) from environmental samples, but no associated cases of paralysis have been detected. According to GPLN “Initially, vaccine-like type 2 poliovirus (SL2) had been isolated from samples taken from the same site between February and May 2022. Genetic analysis suggests that the new VDPV2 and previous SL2 isolates have a common origin, still to be identified, but the technical definition and criteria for ‘circulation’ of VDPV2 are not met at this time.”
Malawi reported a case of wild poliovirus type1 (WPV1) in January 2022. Sequencing analysis confirmed that the WPV1 isolate was genetically linked to a sequence detected in Pakistan in 2020. In May 2022 the health authorities in Mozambique declared an outbreak of wild poliovirus type 1 after confirming that a child in the country’s north-eastern Tete province, the second case in southern Africa this year, following an outbreak in Malawi in mid-February.
In June 2022, the CDC announced it is coordinating with New York State health authorities concerning a confirmed case of polio type 2 VDPV. Public health experts are working to understand how and where the individual was infected. The patient, age 20, was hospitalized in June and had recently travelled to Poland and Hungary. Authorities are working to provide protective measures, such as vaccination services to prevent the spread of polio to under- and unvaccinated individuals.
The Polio Eradication Strategy 2022–2026 has two primary goals: 1) to permanently interrupt all poliovirus transmission in endemic countries and; 2) to stop cVDPV transmission and prevent outbreaks in non-endemic countries. The updated GPEI Polio Eradication Strategy 2022-2026 includes expanded use of the type 2 novel oral poliovirus vaccine (nOPV2) to avoid new emergences of cVDPV2 during outbreak responses.
It is considered essential that all countries, those with a high volume of travel and contact with polio-affected countries and areas, strengthen surveillance to rapidly detect any new virus importation and to facilitate a rapid response. Countries, territories, and areas are encouraged to maintain high routine immunization coverage at all levels of health authority to protect children from polio and to minimize the consequences of any new virus being introduced.
What is the current risk for Canadians from Polio?
Canada has been polio-free since 1994. However, the virus still poses a threat to anyone who remains unvaccinated. It is possible to spread polio through global travel networks and wastewater systems. The Public Health Agency of Canada (PHAC) together with the Canadian Paediatric Society, conducts enhanced surveillance of AFP in children less than 15 years of age to ensure Canada is free of polio.
As there is still a risk of polio in some areas of the world, the PHAC recommends that travellers get vaccinated against polio when going to countries where there is a risk of ongoing transmission.
The immunization program in Canada for children is recommended polio vaccination 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.
It is also recommended to have primary immunization for non-immune adults and a booster for adults travelling to epidemic or endemic areas or for those with other exposure risks.
What measures should be taken for suspected cases of Polio contact?
PHAC advises contacting your doctor if you get flu-like symptoms after travelling to a country where polio is still present.
The latest news says the PHAC will begin testing wastewater for the polio virus in several key high-risk communities following reports it was detected in sewage in the U.S. and the United Kingdom.