Polio (poliomyelitis): Wastewater surveillance
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- Current situation
- Risk and prevention
- Forms of poliovirus
- Wastewater testing
- If poliovirus is detected
On December 23, 2022, the Public Health Agency of Canada detected positive signs of vaccine-derived poliovirus in 2 wastewater samples collected in August 2022, from Montreal, Quebec. We're working with public health authorities in Quebec to investigate.
Poliovirus reproduces in an infected person, who sheds or excretes it in their stool (feces). It's then flushed into the sewage system (wastewater), where it can be detected by laboratory testing.
To help monitor for poliovirus, the National Microbiology Laboratory (NML) now tests wastewater for signs of poliovirus in key high-risk areas with low polio vaccination rates. If found, it can mean that someone in the community is shedding 1 of the 3 forms of poliovirus:
- wild poliovirus
- Sabin (oral vaccine-strains)
- vaccine-derived poliovirus
Risk and prevention
Canada has high levels of vaccine coverage, so our overall risk is low. However, as long as polio exists anywhere in the world, it poses a risk to unvaccinated people.
Immunization is the best way to prevent polio. Contact your healthcare provider or local public health unit to ensure your vaccinations are up to date.
Forms of poliovirus
Wild poliovirus occurs naturally in the environment. It causes severe disease, such as paralysis, in less than 1% of infections. Wild poliovirus has been almost completely wiped out by vaccination.
Sabin (oral vaccine-strains) is the weakened strain of poliovirus in the oral polio vaccine (OPV), which is still used in some countries. When someone is vaccinated with it, the live weakened virus triggers an immune response to protect them against paralytic disease.
Vaccine-derived poliovirus is a rare strain of poliovirus that has evolved from the OPV strain and mutated. After someone is vaccinated with OPV, they shed the vaccine virus in their stool, and it can spread to unvaccinated people. If the virus passes from person to person over a long period of time (about 12 to 18 months), it can change and take on a form that can cause paralytic disease, just like the wild poliovirus. This can happen in communities with low vaccination coverage. Canada doesn't use OPV, but it's still used in some parts of the world. It can pose a risk to unvaccinated people but is not a concern in Canadian communities with high vaccination rates.
Wastewater testing includes a technique called genome sequencing, which can identify the type of poliovirus detected. This work is technically challenging, but it provides a detailed view of the virus. It allows experts to compare cases or wastewater detections with those circulating locally and internationally. Through these comparisons, we can learn about the chain of transmission of the virus. This information, along with clinical surveillance, helps public health experts create a plan for managing the virus.
Testing wastewater for poliovirus is a developing science and can be affected by:
- the amount of poliovirus circulating
- contamination from nearby industrial facilities
- extreme rainfall or snow events that cause flooding
If poliovirus is detected
If poliovirus is detected in wastewater, more laboratory testing is done to:
- identify the type of poliovirus
- link cases
- study chains of transmission, and
- compare its genetic sequence to wastewater detections and clinical cases elsewhere
Poliovirus in wastewater usually comes from people who were exposed to poliovirus through contact with someone shedding the virus.
In Canada, a wild poliovirus detection in wastewater likely means that someone travelled to an area where polio is common or with a current outbreak. It is not a cause for concern in areas with high vaccination rates.
A Sabin (oral vaccine-strains) poliovirus detection in wastewater is not a cause for concern in areas with high vaccination rates. It likely came from someone who was recently vaccinated with OPV in another country.
A vaccine-derived poliovirus detection in wastewater doesn't necessarily mean there is local spread of polio. It may have come from:
- a traveller who was infected in an area with circulating vaccine-derived polioviruses
- an immunocompromised person who was vaccinated with OPV, or exposed to Sabin strains of the poliovirus
In immunocompromised people, the OPV strain can mutate over a long period of time into vaccine-derived poliovirus. Additional laboratory testing will help determine the source and whether there is local transmission.
If polio is confirmed in wastewater, it's immediately reported to local, provincial, territorial and international public health authorities as required.
A public health investigation of the wastewater catchment site and neighbouring sites is conducted to determine:
- possible origin
- vaccination coverage of the local population
- presence of high-risk groups, such as children under 5 or people who are not fully vaccinated
Local or regional health authorities lead public health investigations, with help from provincial, territorial and federal authorities.
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