4. Discussion
Large numbers of pathogens such as viruses, bacteria and other microorganisms can be introduced to water sources due to inefficiency or lack of protection of these sources. This makes the water quality unsuitable for consumption and causes outbreaks of waterborne diseases [
24]. Isolated in groundwater, river water, sewage water and drinking water and causative of severe gastroenteritis in children, Rotavirus is responsible for approximately 24 million patients seeking hospitals, 2.4 million hospitalizations, 114 million episodes of gastroenteritis and per year 450,000 deaths of children under 5 years, documented majority in low-income countries [
25].
In tropical regions, Rotavirus prevalence can occur throughout the year, but has seasonal peaks during the dry period or cold months [
26]. In the MidWest region of Brazil, where the State of Goiás is located, the highest prevalence of rotavirus circulation occurs in the coldest months or in the dry period between May and September [
27]. The RV-A found in the study samples were found in water samples both in the dry (54,7%) and rainy periods (20%), but there was a higher prevalence in samples from the dry period. The results brought from the study also demonstrated a concordant relationship with the seasonality normally found by Rotavirus in the State of Goiás, where a strong statistical association between the dry season and the presence of Rotavirus (association test (χ²) resulted in a p-value < 0.001 and the Exact Test Fisher's test with p-value < 0.001).
These results are in line with what was found in the study by Stobnicka-Kupiec&Górny [
28] where the prevalence of positive rotavirus samples found in winter (dry period) was higher than those found in summer (rainy period) (73.3% vs. 26.7%), with there also being an statistics association between the prevalence and the period found (x2: p = 0.028; Fisher's Exact test: p = 0.033). Different from what was found in our study, in a study in the State of Minas Gerais, where there is also a higher incidence of rotavirus during the dry period, the RVA were found in water samples collected on both dry (54.1 %=13/24) and rainy period (70.8 %=17/24) but no significant association between the viral detection and the period in which the study was realized were found(p=0.371)[
21].
Outbreaks of infection not reported in the study areas, the elimination of feces with high concentrations of the virus (which can reach 10
8 to 10
11 particles/gram of feces) by infected patients, in addition to characteristics specific to the virus, may explain its greater detection of RV in the samples [
9,
10,
16,
20]. RVs survive diverse conditions and are capable of surviving for long periods, contributing to their prevalence in the environment [
28].
In the dry months (winter) the UV radiation is less intense and the temperatures are lower, which will lead to greater stability of Rotaviruses in the water [
29]. UV irradiation penetrates cellular structures, damages genetic material and interferes with cell reproduction. The viral capsid, nucleic acids and enzymes responsible for virus replication are damaged when present at high temperatures, preventing the adsorption of the virus to its host [
9]. In tropical regions, RVs associated with the incidence of diarrheal disease decrease by 10% for every 1ºC increase in temperature [
30].
Contrary to expectations, enteric viruses are the most likely microorganisms to contaminate groundwater. In rural areas, not all homes are connected to the sewage collection network, but rather to septic tanks, from where sewage can drain into the ground and migrate to deeper layers, reaching groundwater [
31,
32,
33]. Due to their extremely small size – between 25 and 100 nm – it allows them to easily infiltrate and pass through soil pores, which are capable of retaining bacteria and larger protozoa, reaching these sources [
31]. These are sources that, as they do not undergo treatment, are extremely vulnerable to contamination [
34]. For Fongaro et al. [
34] in groundwater source the prevalence of RV was higher in the rainy season than in the dry season (80% vs. 60%), with genomic copies varying from 4.5×10
4 to 4.5×10
8 gc/L for the rainy season and 1.2×10
3 to 1.5×10
5 gc/L in the dry season. In groundwater, the presence of RVA was positively correlated in dry and rainy periods (P<0.05, r2 = 0.92).
Shallow wells was the only groundwater source where Rotavirus contamination showed a higher percentage of positive samples in the dry season, with a strong statistical association (p-value < 0.001). Shallow wells are more vulnerable to viral contamination as the depth of the wells are generally smaller and may not have lining or protection factors, thus pipe failures and/or septic tank overflows, in addition to the presence of animals nearby and surface contaminants that can be taken to the wells through precipitation, increasing the contamination of this source by viruses [
35,
36]. Protection against UV radiação, low microbial activity and lower temperatures are reasons why viruses survive longer in groundwater [
9]. It has been shown that Rotaviruses were able to survive up to 7 months when stored in the dark [
20].
The characteristics of tubular wells of having smaller opening diameter, wall coverings made of PVC plastic material and being greater in depth, ensuring greater isolation and durability, did not ensure that that the water samples collected from this source were free from RV contamination [
37]. With regard to the construction aspect of the wells, Allen et al. [
38] obtained a positive association when relating the probability of detecting viruses in wells that do not have casing, that is, wells that are in direct contact with the aquifer (open well interval). Tube wells with longer open intervals will be better able to capture agreater flow of groundwater, thus increasing the likelihood of including a virus source gradient in the flow. In addition to problems with the structure and sealing of the well, sources of contamination that are at a distance of up to 10 meters, problems with the sealing of the well pump, and latrines close to the well, which, due to their lack of plumbing, facilitate the percolation of excreted in the soil, are some of the risk factors that contribute to viral contamination in tube wells [
39]. For the study by Verheyen et al [
40], the fact of having latrines within a 50-meter radius of the sample collection sites would already be a risk factor for viral contamination of wells.
Another groundwater source that also had a strong statistical association with the presence of RV were water springs (p-value = 0.02). The positive samples for adenovirus and rotavirus, in springs, found by Gonella et al. [
41], are explained by the presence of animals in its surroundings, which may be transmitting these agents, as it is located in an ecological park. Springs that are located in lower regions may be more subject to contamination than in higher regions; human density and agricultural activities are also factors that can lead to contamination from this source [
37].
The discharge of sewage into surface water sources is an important source of fecal pathogens, as bodies of water that receive untreated sewage or are subject to inefficient treatment often constitute public water supply sources, making them a risk to those who uses them [
42,
43]. In rural areas, most of the final disposal of sewage can occur directly on the ground or in streams, rivers and lakes, contaminating bodies of water and increasing the risk of waterborne diseases [
44,
45].
Collections carried out by Bortagaray et al. [
30], in two rivers, Santa Lucia and Uruguay, obtained a statistically significant association with the presence of rotavirus in the dry period (winter) (Uruguay river: p=0.0014, Santa Lucia river: p=0.0008). In the warmer months (rainy period), the presence of rotavirus was not found in any of the rivers surveyed. Similar to what was found by Bortagaray, the present study did not find RV in surface water samples during the rainy season (warm months) and in addition to finding the presence of RV in the dry period (colder months), it also found a strong statistical association between the viruses and dry period (p = 0.001).
Temperature, presence of microbiota and desiccation are factors that effectively contribute to the destruction and decomposition of viruses and microbial pathogens on surface water, in addition, when exposed to solar radiation (UV light), the viruses are readily inactivated [
46,
47]. It is known that these factors are capable of denaturing proteins, causing damage to the nucleic acid, or dissociation of the capsid. The presence of native aquatic microorganisms, such as bacteria, would lead to bacterial production of proteolytic enzymes, which works as a mechanism for the antiviral activity of bacteria [
8,
48,
49,
50]. These factors may justify the absence of RV in surface samples during the rainy season, as the temperature is higher during this period.
Considered the gold standard, the Polymerase Chain Reaction (PCR) and quantitative PCR (qPCR) molecular detection technique is emerging very quickly as a method for detecting viruses in environmental samples. Compared to cell culture, the main advantages when using PCR for viral diagnosis include high sensitivity and specificityand for detecting viruses that can be cultivated or not, and in addition to releasingrapid results, in the order of hours. The main disadvantage of PCR methods is that unlike the cell culture method, which are capable of detecting infectious viruses that causes infection in the cultured cell or demaged non-infectious viral particles, the PCR method can potentially detect both infectious viruses and non-infectious viruses [
9,
28,
51,
52].
Humic and fulvic acids, heavy metals, nucleases and polyphenols are organic and inorganic components, that can causedegradation of nucleic acids, impairing the extraction stage or affecting the polymerase and reverse transcriptase, preventing the amplification and quantification of the genetic material present in the samples [
53]. They are easily found in the environment [
53]. Therefore, when in the viral concentration of the samples step occurs, interferents and inhibitors of viral detection may be concentrated together with the samples [
53].
In addition to epidemiological issues, the difference in detection between the two periods studied may be due to: with an increase in water volume, rainwater may have increased the concentration of inhibitors in the samples. In this study, the method for concentrating samples using negatively charged membranes is based on the structure of the virus, thus, the detection of free genetic material (RNA) will be reduced [
54]. The two RNA/DNA extraction kits used in this study are not validated for environmental samples (specifically water), according to the manufacturer's instructions they are ideal for whole blood, plasma, serum and other biological fluids free of cells other than animal/plant tissue. Furthermore, there is a difference in the detection kits using Sybr Green® (with different protocols and reagents), different thermocyclers used, freezing/thawing cycles of samples degrading viral RNA or a bad pairing of the oligonucleotides used and when collecting samples for the dry period, the samples got through the cDNA synthesis stage, unlike in the rainy season, which only carried out the RT-q-PCR stage, which may be factors that affected the findings in both study periods [
53].