Concepts in Wastewater Disease Monitoring Part 1: How do we find cases with no symptoms?
How the campaign to eradicate polio gave rise to wastewater-based epidemiology — and why finding silent, asymptomatic transmission is the whole point.
In 1959, public health officials set a radical goal: to eradicate a disease from the world. Vaccines made such bold goals possible and the first target was smallpox. The coordinated global campaign was wildly successful, tallying the last case in 1977 and declaring victory in 1980. With this success notched, they turned their sights to polio. Before vaccination, polio killed or paralyzed over 500,000 people, mainly children, every year. Vaccines developed by Jonas Salk and Albert Sabin had dramatically changed the landscape of global polio. When the Global Polio Eradication Initiative launched in 1988, 60 countries were already polio-free thanks to vaccination campaigns.
As eradication efforts made the disease rarer, a new problem emerged — how to find communities where the virus is still circulating? For most pathogens, there is some rate of silent transmission, where cases with no symptoms are contagious. Polio takes this to an extreme. For every 500 infected people, 375 people will be asymptomatic. Another 124 will have mild symptoms, mostly diarrhea for which people rarely seek medical care. Of the 500 infected individuals, only 1 will have the acute flaccid paralysis we associate with polio. It makes sense for clinical surveillance to focus on the serious, life-threatening cases, but people focused on eradication knew the other 499 infected people were continuing to spread the virus. To eradicate the virus, they had to know where these pockets of silent transmission were happening.
It’s surprising to many people that polio is caused by an enterovirus that infects the gut and is diagnosed by stool testing. To find the pockets of silent transmission, the polio eradication team needed to test stool from as many asymptomatic people as possible. Conveniently, wastewater systems already collected stool from communities every day. By regularly testing wastewater, public health officials could quickly and effectively identify communities where poliovirus was present, even if no one was sick. These communities were then prioritized for vaccinations, breaking the chain of silent transmission.
It’s been a longer process than hoped, but this combined surveillance and vaccination approach is working. In 2025, only two countries still have wild-type poliovirus. And it opened up the idea of wastewater as a health information source, creating the field of wastewater-based epidemiology.