Despite there being multiple active Covid-19 cases in Yellowknife, daily wastewater surveillance results have identified “no detection” of the virus on several occasions since May 15.
Sewage testing has been used in larger NWT communities for months as a form of safety net, checking samples of wastewater for traces of the virus that causes Covid-19. The system is credited with unearthing a cluster of cases involving Tu Nedhé-Wiilideh MLA Steve Norn in April.
However, the same system that triggered an appeal for people to get tested when just a few cases were suspected now appears to sometimes find no trace of the coronavirus, even when a number of cases are known to be active.
The territorial government says it is “learning that a lack of a positive signal does not necessarily mean there is no Covid-19 in the community.”
“In regions with few or no cases, wastewater signals appear to follow a pattern characterized by rapid increases in signals followed by equally rapid signal declines to baseline,” said Dawn Ostrem, a spokesperson for the NWT government’s Covid-19 Secretariat.
Wastewater surveillance in Yellowknife has followed this same pattern, with the signal increasing sharply in early May when the NJ MacPherson cluster was first identified before returning to trace levels.
Ostrem said this mirrors what health workers are seeing clinically – that most cases have recovered and the remaining cases are recovering.
But why the signal spikes dramatically and then drops so speedily, even as Covid-19 infections linger, remains a puzzle.
Officials say they have a few theories.
Those include “the potential for some spring run-off to further explain some of the differences we are seeing,” said Ostrem, “although changes in flow have been relatively minimal.”
Water collected by Yellowknife’s street sewer drains runs through the same treatment plant as sewage that is collected for Covid-19 testing, possibly watering down the sample during the spring melt.
“The fact that most cases in Yellowknife during this event are among children is quite unique,” Ostrem continued. “This might also further explain some of the differences we are seeing.”
Ostrem added the science of wastewater surveillance to identify Covid-19 remains new, and the NWT government is working with researchers, labs, wastewater utilities, and public health authorities across Canada to learn more.
While there has been no recent detection of Yellowknife’s three remaining active cases in wastewater, the signal may still “bounce in and out of detection levels,” she said.
“We’ve seen previously it takes some time for the signal to wash out of the system.”