Norovirus causes 677 million annual cases of acute gastroenteritis globally, resulting in more than 210,000 deaths and encumbering the global economy ~$60 billion per year. Norovirus is highly infectious and induces a rapid onset of symptoms lasting between 2-4 days.
Here we investigated the effects of social distancing and other infection control measures on norovirus levels in wastewater within the community. We also conducted molecular surveillance of circulating norovirus in Sydney and Melbourne during 2018-20 using both clinical and wastewater samples in order to identify prevalent strains, novel recombinant viruses and those with pandemic potential.
Nucleic acid was extracted from both clinical and wastewater samples and norovirus RNA amplified by RT-PCR and probe-based qRT-PCR. Amplicons from clinical samples were Sanger sequenced and data used for phylogenetic analysis. Amplicons from wastewater samples were sequenced on the Illumina MiSeq platform to determine strain diversity and prevalence.
Comparison of norovirus levels in monthly wastewater samples between 2019 and 2020 revealed an average overall decrease of nearly 80% pre (1.2 x 106 genome copies/L sewage) and post lockdown (2.6 x 105 genome copies/L sewage), which demonstrates the success of the implementation of social distancing restrictions. In 2018-20, human genogroup II (GII) noroviruses caused 91.3% of total clinical cases/outbreaks (n=548/600), whilst the remaining were caused by genogroup I (GI) noroviruses (8.7%, n=42/600). GII recombinant strains were the most prevalent in clinical samples and included GII.4 Sydney 2012[P16] (23.3%, n=140/600), followed by GII.2[P16] (19.5%, n=117/600) and childhood virus, GII.3[P12] (18.3%, n=110/600). The genetic diversity of noroviruses detected in wastewater largely reflected that of clinical samples.
Wastewater analysis provided independent insight into the true levels of circulating noroviruses in the community. A more comprehensive picture of the molecular epidemiological trends of norovirus in a population is obtained when used in conjunction with clinical analyses.