What do we know about COVID-19 and sewage?
A version of this story was first published by COVID-19 Waterblog. Read the original.
There has been quite some talk about SARS-CoV-2 shedding in faeces and what that might mean for the water industry. Here, Susan Petterson provides a snapshot of the current data.
As I see it, there are two aspects to this conversation: the first is a concern that sewage may contain infectious SARS-CoV-2 viruses; and the second relates to the more theoretical potential of using SARS-CoV-2 RNA concentration in sewage as a public health surveillance tool.
1. Is sewage contaminated with infectious SARS-CoV-2 viruses?
While COVID-19 is primarily a respiratory illness, the possibility of faecal-oral transmission was raised quite early (Yeo et al. 2020). From the information we have to date, it appears as though many people infected will excrete SARS-CoV-2 RNA in their faeces. A snapshot of reported presence in stool samples includes:
- Six studies reported from China: 9 out of 17 patients were positive (Pan et al. 2020); 39 out of 73 patients positive (Xiao et al. 2020); 8 out of 10 children positive (Xu et al. 2020); 44 out of 153 faecal samples positive (Wang et al. 2020); 12 out of 22 patients positive (Chen et al. 2020); 41 of 74 patients positive (Wu et al. 2020)
- In Singapore, 4 out of 8 patients were positive (Young et al. 2020)
- The first reported case in the United States tested positive on day 7 (Holshue et al. 2020)
- In Germany, 8 out of 9 patients were positive (Woelfel et al. 2020)
- In France, 2 out of 5 patients were positive (Lescure et al. 2020)
However, most importantly as highlighted in the World Health Organisation technical brief, there is limited indication of infectious viruses in faeces, let alone survival to sewage effluent.
This is comforting, and yet it made me wonder: how many studies have tried to culture SARS-CoV-2 from faecal samples?
Of the studies listed above, to my knowledge only two tried to culture the virus: Wang et al. (2020) reported successfully culturing 2 out of 4 samples, identifying the ‘live’ virus by electron microscopy. Woelfel et al. (2020) attempted to culture 13 samples taken between days six to twelve from four patients without success.
There is a need for more information on the success and failure of culture of SARS CoV-2 in faecal samples.
2. Enumeration of SARS-CoV-2 in sewage to support public health surveillance.
As highlighted in a previous blog post , many in the water industry, myself included, see the tremendous potential to use enumeration of RNA from sewage to support public health surveillance.
If we are to do this well, we need to understand how SARS-CoV-2 is shed with the faeces of infected individuals in order to model a link between the number of infections in the community with estimated numbers in wastewater. From the studies cited above we can deduce:
- Not all infected people are positive for SARS-CoV-2 in their faeces. Any predictions to the community infection rate would need to correct for this.
- The duration of shedding varies between individuals. Of the 8 children who were persistently positive in stool samples in China, the duration varied from 5 to 28 days with a mean of 21 (Xu et al. 2020); and of the 41 who tested positive by Wu et al. (2020), the duration of positive samples varied from 1-39 days with a mean of 14 (Wu et al. 2020)
- The magnitude of shedding varies between individuals and over the course of infection in any one person. Viral loads reported by Pan et al., 2020 ranged from 550 copies per ml to 1.21 × 105 copies per mL; and Lescure et al. (2020) reported one patient with 6.2 – 6.8 Log10 copies per g; and the other patient 7.4 – 8.1 Log10 copies per g. The most complete data so far appears to be from Woelfel et al. (2020) where the viral load in stool samples was plotted over the course of the infection for 8 patients and reached close to 108 RNA copies per gram at its’ peak in one patient
It is clear than any attempt to model RNA concentration in sewage from shedding data will not be simple or straight forward, and will need to give appropriate consideration to the variability and uncertainty associated with these excretion patterns.
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