Is wastewater the answer for tracking all disease outbreaks?
Covid-19 tested the concept. Other pathogens are now being studied too.
Early in the Covid-19 pandemic, we could not track the spread of the virus in our communities because we did not have enough tests. Later in the pandemic, we had the tests, but people stopped using them—or started using ones (like at-home rapid tests) that don’t always get reported to officials.
Wastewater may hold the key to detecting outbreaks for many communicable diseases. By now, most of you will have heard of this idea: checking sewage for the genetic material of SARS-CoV-2, the nasty coronavirus that causes Covid-19. The hope is that we can follow the virus’s rise and falls regardless of whether testing in a community is low or high.
The problem is that a neat correlation between detected levels of SARS-CoV-2 RNA and case counts has not been precisely worked out. Even if a good correlation can be worked out, the ratios may change with new variants and other factors. Still, qualitatively, it’s pretty clear that when SARS-CoV-2 wastewater levels go up, reported Covid-19 case counts follow suit. That knowledge has been powerful. The CDC now has a national wastewater surveillance system online that anyone can browse.
Now, new research published in the New England Journal of Medicine found that the same approach can be taken for tracking MPOX (the disease formerly known as monkeypox). The same research group which published this report has previously looked at influenza and RSV (in addition to Covid-19), and with similarly encouraging results. Meanwhile, New York State is tracking poliovirus using wastewater. The possibilities are endless.
The pattern that is emerging is that wastewater levels often increase before cases are identified in communities. That means that epidemiologists can use wastewater as a warning system to keep tabs on many potential pathogens, and ramp up testing in at-risk patients whenever warranted.
At the moment, the qualitative insights are more reliable than the quantitative ones. Using Covid as example, when wastewater levels go up, documented cases are likely to rise as well (provided clinicians test people). But linking certain levels of genetic material in the wastewater to exact numbers of new cases per capita? That remains an inexact science for Covid and other pathogens too.
I suspect that we’ll get better at the quantitative game in time, but that things will always remain at least somewhat inexact for biological reasons. For example, it’s unknown whether certain viral variants are more or less likely to show up in wastewater early or late in an infection, or whether vaccination status or antiviral use modifies output levels. There’s still a lot to learn.
For now though, it’s promising that for a growing number of diseases we would like to track, we can now turn to wastewater for surveillance, getting a big-picture assessment of whether an outbreak of importance is about to erupt in a community.
Harnessing that information could stop an outbreak in its tracks, and save many lives.