Wastewater Surveillance

When you feel chilly and achy, you take your temperature with a thermometer. Wastewater surveillance is a bit like sticking that thermometer, not under one person's arm, but deep into a whole city's sewer pipes.

With some diseases, the germ that causes them comes out not only in coughs but also mixed into stool, urine, or saliva. The virus that causes polio is one of those. On the other hand, for diseases that do not leave the body this way, this method cannot be used. People whose symptoms are mild and who never think of going to a clinic, and asymptomatic carriers who show no symptoms at all, still use the toilet every day and leave traces of the pathogen in the water.

If people only get tested by going to a clinic on their own, it is hard to capture the true picture of a whole city. Many untested people drop out of the statistics. But in any home connected to a sewer, the traces of untested people are carried along in the water too. So looking at wastewater lets us estimate the share that the statistics missed.

Think about what happens when you catch an infectious disease. After a germ enters your body, it takes time for symptoms to show up. Then you drag yourself to a clinic to be tested, and it takes a few more days before the result is recorded with public health authorities.

But the moment you flush the toilet, the genetic material of the germ reaches the wastewater treatment plant through the pipes in just a few hours. Before a patient even develops a fever and knocks on a clinic door, traces of the pathogen are already gathering in the water.

Thanks to this, health authorities can spot a rising signal of an outbreak several days earlier than clinic records would show. Just as you prepare for a typhoon by watching waves roll in from far out at sea, you gain precious time to secure hospital beds and prepare treatments before patients flood out of one area.

However fast wastewater surveillance is, it cannot solve every public health problem by itself. It can tell you that a virus turned up in the water at a treatment plant, but it cannot point to exactly which person in which home released it. If you narrow down where the samples are taken, you can shrink the range to a neighborhood or a building, but figuring out who inside it takes testing people separately. Directly finding individuals, like contact tracing, which retraces the movements of one confirmed patient, is not possible.

That is why public health runs clinical surveillance, which examines patients who visit a hospital, and environmental surveillance, which examines wastewater, together as two branches of one surveillance system. Eradication, the complete elimination of a disease such as polio, is not the name of a method but the final state at the end of that road. To reach it, clinical surveillance that searches thoroughly for patients with paralyzed legs and environmental surveillance that combs through sewer water move forward in step.

The sharpest point of surveillance goes beyond simply counting a disease that is spreading. Proving that the disease is not around us at all is also at the heart of surveillance. Wastewater surveillance checks the whole city for hidden carriers, and so becomes a solid support for proving that a disease is gone.