For the last couple of months, Kendal/Crosslands has been testing its sewage to see if there is any indication of Covid-19. The process is explained in this post, and the question of whether a single, isolated case could be detected is discussed in this post. Up until recently, we really had no confirmation showing the usefulness of the testing process for us. The reports on each of our weekly samples came back with “no virus detected”.
The process finally showed its promise in late July. On July 23, the administration learned that an asymptomatic Crosslands resident (who had gone to the hospital for treatment for a different condition) had tested positive for Covid-19 at the hospital. The resident had stayed briefly in Firbank (the Crosslands medical facility) before being transferred to the hospital.
Given the possibility that the resident had already had the virus before leaving, Firbank staff and residents, as well as others who might have had contact with the sick resident were tested. Those tests have turned up one positive but asymptomatic staffer (who is now quarantined at home) and no positive residents.
Our first positive sewage test. Meanwhile, our wastewater testing had provided a hint of what was coming. A weekly sewage sample had been taken on July 15, and it was positive for Covid-19—the first positive result since sampling began in mid-May. The level of virus that was detected suggested that a small number of people—possibly as few as a single person—were infected as of the time the sample was taken.
Based on that positive result, additional samples were then taken during the week of July 21 of wastewater from each of the three communities (Kendal, Crosslands, and Cartmel) that feed the treatment plant, in an effort to pin down the source of the virus. No virus was detectable in any of those samples.
What did this episode tell us? First, we now know that wastewater testing is a valuable monitoring tool. It picked up the presence of a local infection in a sample taken July 15—eight days before it was detected in the normal course of business. (We didn’t know about the sewage test result immediately, though, because of the multi-day turnaround process with the Florida testing lab.)
It’s not clear at this point whether the positive sample was due to just the one resident case or perhaps to the combination of that case plus the asymptomatic staffer. The amount of virus shed per day varies with the course of the infection, and the level of shedding may possibly vary with the severity of the illness, so there will always be some uncertainty around the number of cases associated with a positive sample.
It’s unclear exactly why none of the samples taken the week of July 21 from any of the three campuses came back positive for virus. When the samples were taken, the Crosslands resident had already left the campus. Perhaps the asymptomatic staffer (who was still on campus) was not shedding enough virus, or did not contribute enough waste, to be detected. In any case, the negative result was reassuring, and the process showed how the location of an outbreak could be narrowed through additional testing.
To me, this episode shows that wastewater testing has proven itself to be a useful tool for catching Covid-19 cases early. Even with weekly testing intervals and a several-day turnaround for analysis, it provided our earliest indication of a Covid-19 case on one of our campuses.
There are two areas where the process might have the potential for improvement. First, I think a case could be made for more frequent sampling than once a week, so that we would not have to wait so long to know that we had virus in our wastewater. Second, perhaps it will be possible to find a local lab to do the testing, so the turnaround for that could be reduced as well (so far, our analysis has been done by labs in Massachusetts and Florida).