In this study a molecular characterization of three gastroenteric viruses, RVA, NoV and HAstV was performed using a WBE approach. In comparison to surveillance based on clinical diagnostic testing, monitoring enteric viruses in a WBE approach present a major advantage of comprehensive population based surveillance of circulating virus by capturing persons who did not have diagnostic testing performed during gastroenteritis as well as including patients with asymptomatic infection (Lin et al 2021). The objective of this work was to obtain a snapshot of the circulating strains in the population of three Uruguayan cities throughout a year (2017–2018). Analyzing wastewater samples provides an estimate of the epidemiology of viruses infecting the populations served by the sanitation network and enables the observation of gastroenteritis outbreaks caused by the studied viruses. Unfortunately, no epidemiological information from clinical samples was available in our country in order to compare the presence of these viruses in wastewater with gastroenteritis outbreaks in the population.
The prevalence of RVA was higher when compared with NoV and HAstV in all three cities studied. Moreover, RVA was detected throughout the year, indicating its continuous circulation in these areas which discharge their wastewater into the Uruguay River. These findings are consistent with previous studies from southern Brazil based on clinical samples, where RVA was also detected year-round (Gutierrez et al., 2020). The ability of rotaviruses to remain infectious in harsh environments and wastewater treatment process may contribute to their prevalence as one of the most common enteric viruses (Tavakoli et al., 2020). However, despite these findings, previous studies from Brazil, Uruguay and Iran reported a marked RVA seasonality during colder months in both environmental and clinical samples (Atabakhsh et al., 2019; Bortagaray et al., 2019; Gutierrez et al., 2020).
It is worth noting that currently in Uruguay, the RVA vaccination is not mandatory and the vaccine is not provided for free, which suggest that only a small portion of the population is likely to be vaccinated. The benefits of a mandatory vaccination are evident in Brazil, where a decline in gastroenteritis cases and hospitalization caused by RVA have been observed after 13 years of vaccination (Gutierrez et al., 2020).
Considering the distribution of NoV GI and GII throughout the study period of this study in the three cities, no clear seasonality pattern was observed. However, previous studies suggested that NoV has greater incidence and persistence in cold environments and its spread may be affected by environmental factors such as relative humidity and latitude, as well as demographic characteristics. In our study, although NoV GI and GII were detected in both cold and warm seasons, the virus was more frequently distributed during the warm season. This contrast with numerous clinical studies where NoV VAGE cases and outbreaks tend to occur during winter or cooler months (Eftim et al. 2017; Shamkhali Chenar & Deng 2017; Farkas et al. 2018; van Beek et al. 2018; Huang et al 2022). Additionally, marked seasonality of NoV has been observed in China where the majority of NoV disease peaks occur from October to March during the cold season (Na Wei et al., 2021). In our study NoV and HAstV were also detected in all the seasons throughout the year, without any significant seasonality. In the literature there is no information about Astrovirus seasonality in the environment. It is important to highlight that this is a one year period study and further studies for longer periods would be necessary to define the seasonality of these viruses in this region.
Regarding the molecular characterization of RVA, we found strains that clustered with genotypes P8 and G3. These genotypes have been previously reported in other studies, with G3P8 being the most prevalent in southern Brazil and corresponding to an equine-like genotype (Gutierrez et al., 2020). However, in our study, we did not observed a high genetic diversity in RVA strains, since only one VP4 and one VP7 genotype were detected. This limited diversity could be due to the small sample size of our study, which included only 36 samples or maybe because of the predominance of this two genotypes. It has been observed that both the P8 and G3 genotype continue to circulate in our country (Lopez-Tort et al., 2015).
Our study found a higher frequency of NoV GII compared to GI, which is in agreement with previous studies based on clinical samples reporting a higher frequency of gastroenteritis outbreaks caused by GII than GI (Matthews et al. 2012; Parikh et al. 2020).
Regarding the molecular characterization of NoV GI, we observed a high genetic variability detecting genotypes GI.2, GI.5, GI.6, GI.3 and GI.7. Specifically, we detected GI.5 in Fray Bentos during January and February 2018, which suggests the occurrence of at least one outbreak in this city during that period. Similarly, we observed the circulation of GI.7 in Salto during October to December 2017. In contrast, the detection of NoV GII revealed the presence of genotypes GII.2, GII.5, GII.6, and GII.4, with the predominance of GII.4, variants New Orleans 2009 and Sydney 2012, which is consistent with a study conducted in Brazil (Hernandez et al. 2018). Several studies, including Huang and coworkers (2021) reported the detection of NoV GI.2, GI.5, and GI.3, with the more recent detection of genotype GI.6 at a high frequency since 2014. In our study, we also observed the presence of genotypes GI.2, GI.5, GI.3, and GI.6, but with the additional detection of genotype GI.7. For NoV GII, the frequently detected genotypes include GII.4, GII.17, GII.2, GII.3 and GII.13, while in our study we detected the presence of genotypes GII.2, GII.5, GII.6 and GII.4, with the latter two being detected in both studies. Since 2012 the GII.4 Sydney 2012 variant has been the most commonly detected strain in wastewater and has been associated with VAGE outbreaks worldwide. These findings are in agreement with recent molecular epidemiological analyses of NoV over the past five years (Zhou et al. 2020; Cannon et al. 2021; Utsumi et al. 2021; Huang et al 2022).
Comparing our study with a previous one performed by our team in 2011–2012 in our country, it is observed that NoV GI.8, GI.6, GI.5 and GI.7 were previously circulating but they were not detected in the present study. NoV GI.3 continued circulating and GI.7 and GI.2 currently circulate but were absent in 2011–2012. Regarding NoV GII, GII.3, GII.13, GII.17 and GII.1 no longer circulate compared to the previously work. GII.4, GII.2 and GII.6 continue to circulate while other genotypes were not detected (Lizasoain et al 2015; Victoria et al 2016).
In addition, co-circulation of different NoV genotypes within a defined human population during the same period provides opportunities for genetic recombination between strains (Na Wei et al., 2021). Our study detected the presence of both NoV GI and GII strains in the same sample collected in February 2018 and August 2017 in Fray Bentos, as well as in May 2017 in Salto, suggesting the co-circulation of these two genotypes in the same city at the same time.
By analyzing the HAstV stains, we observed only the presence of MLB1 genotype. In a previous study performed in Uruguay it was also observed the presence of MLB1 (Lizasoain et al., 2015). On the other hand, in a study with wastewater in Japan, it was identified MLB2, VA1, and VA2 genotypes (Hata et al 2015). Moreover, in the United States, only types 1 and 2 MLB-AstVs were identified from the wastewater samples collected, whereas all known genotypes of MLB-AstVs (types 1–3) were found from samples collected in Nepal (Hata et al., 2018). These findings suggests the widely distribution of until recently called emergent HAstV.