In the current study, the overall prevalence of NoV was 8.9%. The magnitude was increased among under 5 children (12.5%) and the elder people (33.3%). This substantial prevalence indicated that NoV is an important cause of AGE in ANRS, Northwest Ethiopia. Our finding is in agreement with the previous studies conducted in African countries (34, 35), and China (36). When the present finding was stratified by age, it was also consistent with the previous study conducted among outpatient under-5 children in Ethiopia (19). In contrast to our finding, a lower prevalence was reported in Iraq, 6% (37), and Saudi Arabia, 3.6% (38). The difference might be due to the use of a small sample size, sociodemographic variation, and study period.
Our finding was lower than a study done in Congo, 27.4% (34), Burkina Faso, 20% (39), China, 38% (40), a review done in Africa, 20.2% (18), and in the world, 20% (12). One important source of variation might be associated with the non-pharmaceutical interventions seated for COVID-19 management and control during the data collection period. These practices can reduce the potential person-to-person transmission of NoV (22, 41). In support of this, a rapid decrease in the incidence of viral gastroenteritis was reported during the COVID-19 pandemic (41–43). The other source of variation might be the study design with large sample sizes in 5 to 10 years of surveillance and outbreak investigations (44). The study in Congo (34) and Burkina Faso (39) for example, only considered hospitalized individuals for acute diarrhea (within 48 hours) that might increase the incidence of NoV as reported elsewhere (45). A raw fish consumption culture in China might also increase the prevalence of NoV (46).
Norovirus-GII was found predominantly, 82.6% (95% CI: 68.6, 92.2%) among all ages. More specifically, the prevalence was 80% in under-5 children. This finding is consistent with the previous report in Ethiopia, 98.3% (19), Cameron, 97.4% (35), Botswana, 93.9% (47), China, 95.67% (40), USA, 89% (48). These confirm an increased circulation of NoV-GII since its emergency globally. The mean Ct-value for NoV-GII was 29.8 lower than 33 for NoV-GI. The Ct-value confirmed that most of the positive samples, especially NoV-GII had an increased viral load which supports the predominance of NoV-GII.
The present study demonstrated that no human SaV was detected. This is in agreement with the previous report from China (49). A lower prevalence of SaV was also reported in another report from China, 0.6% (50), and Thailand (0.9%) (51). In contrast to this study, a higher or non-zero prevalence of SaV was reported (19, 30). The source of this variation might be either the lower prevalence of the virus during the study period in the region, reduced viral load or the predominance of other viral, bacterial and parasitic associated gastroenteritis as seen elsewhere (52, 53).
In the previous study conducted among under-5 children, the NoV positivity rate was not significantly different between the two sampling sites (19). However, the present study found a significant difference between sampled sites. A possible explanation might be the study season where our study was conducted between May and November (inclusive of the Ethiopian summer season and school entrance) compared to the previous study conducted from November to April (the winter season). Different studies reported the seasonality of this virus (54, 55). The prevalence of NoV was higher in Debre Tabor. One important reason might be low access to potable drinking water in this specific area. In line with this, the probability of accessing potable or safe drinking water in the Debre Tabor area was lower (32.34%), compared to the Debre Markos site (this study). The cold weather condition in the Debre Tabor area might also favor the proliferation of NoV (51). Similarly, individuals living around Bahir Dar were more infected with NoV which might be due to the predominance of elder people in the sampled population and the larger sample size allocated for this specific area (34.3%).
In the present study, the positivity rate of NoV infection was significantly higher in under-5 children and elder people compared to the adult study participants with increased severity of the disease (longer duration of diarrhea, increased frequency of diarrhea, and vomiting). This finding is in agreement with some studies (33, 44, 56). The possible explanation for the increased prevalence of NoV among these extreme age groups might be due under developed and or reduced activity of the immune system in these age groups (57). It might also be due to reduced self-care and increased environmental contact (22). Although NoV infection was not significantly associated with a history of food consumption and toilet utilization, it was higher among individuals with a previous history of AGE, family contact history with a related case of AGE, and those with poor hand-washing practice. This implies that NoV is highly contagious, increased probability of transmission between family members and associated with prolonged viral shedding (25). Our finding is in agreement with different studies (29, 58).
One important limitation of this study is that due to the existing war and instability during the data collection period in the Northern region of Ethiopia, we were unable to include the North-East Amhara region which might affect the scope of our study. The other is that due to a lack of primers and probes, we only detected NoV-GI and NoV-GII, and were unable to detect GIV-NoV, leading to the possibility of underestimating the detection rate of NoV. The other is that due to the scarcity of sequencing, we were unable to report the different genotypes and strains of NoV which makes it difficult to report exactly the dynamicity or evolution of the virus. Despite these, this study provides comprehensive updated data on the prevalence, genogroup distributions, and associated factors of NoV by including all ages in a multicenter approach with a wide geographic coverage.