Detection of thermotolerant coliforms and SARS-CoV-2 RNA in sewage and recreational waters in the Ecuadorian coast: A call for improving water quality regulation

Abstract

Wastewater surveillance represents an alternative approach to regulating contamination and the early detection of infectious agents and outbreaks of diseases of public health importance. This study evaluated domestic wastewater effects on recreational waters in estuarine and seawater bodies in Guayas and Santa Elena provinces in Ecuador, South America. Fecal indicator bacteria (thermotolerant coliforms) served as key indicators for evaluation. Physical, chemical, and microbiological quality markers following the Ecuadorian environmental quality standard and the discharge of effluents to the water resource were analyzed. Samples were collected from 44 coastal sites and 2 oxidation lagoons during the dry and rainy seasons of 2020 and 2021, respectively. SARS-CoV-2 RNA was detected in samples with higher E. coli concentrations using reverse transcription quantitative PCR to detect the genes N and ORF1ab. All samples analyzed for SARS-CoV-2 showed Ct ˂ 40 for at least one gene. Four samples showed at least 20 genome copies of gene N per reaction. These were at an artisanal fishing port, an estuarine area (Palmar), a recreational bay, and an oxidation lagoon. A moderate correlation was found between SARS-CoV-2 RNA, thermotolerant coliform and E. coli (p-value ≤ 0.0037), and a strong and positive correlation between thermotolerant coliform and E. coli. (p-value ≤ 0.00001), highlighting the utility of these established parameters as a proxy of the virus. Significant differences were found in the concentrations of thermotolerant coliforms between seasons (p-value = 0.016) and sites (p-value = 0.005). The highest levels of coliforms were found in the dry season (63000 MPN/100 mL) in Anconcito and during the rainy season (14000 MPN/100 mL) at Esterillo in Playas County. It is recommended that the decentralized autonomous governments of the surveyed provinces in Ecuador implement urgent corrective actions and establish medium-term mechanisms to minimize a potential contamination route. Additional parameters must be included in the monitoring, such as Enterococcus and intestinal parasites, due to their public health implications. In the oxidation lagoons, maintenance actions must be carried out, including the dissolution of sediments, an increase in water retention times, and in situ treatment of the sludge, to improve the system's performance.

Fig 1. Sampling sites locations and population density of studied sites.

(A) Sample site location within Guayas and Santa Elena Provinces in Ecuador. (B) Oxidation lagoons of Playas (OL-A) situated in Guayas province. (C) Oxidation lagoons of Punta Carnero (OL-B) situated in Santa Elena province. (D) The sites studied and its population density.

Fig 2

Physico-chemical and biological quality parameters assessed in seawater of beaches during dry (blue) and rainy (red) seasons. TC: Thermotolerant coliforms; DO (mg/L): Dissolved oxygen; COD (mg/L): Chemical oxygen demand; BOD (mg/L): Biological oxygen demand; temp (Celsius): Temperature; Sal (Practical salinity units): Salinity; tens (mg/L): Tensoactives; O & F (mg/L): Oils and fats; TDS (mg/L): Total dissolved solids; Cl (mg/L): Chlorine; NH₄ (mg/L): Ammonium.

Fig 3

Spatial distribution of E. coli (left) and thermotolerant coliforms (right) in the seawater and wastewater treatment systems during 2020–2021. The concentration of thermotolerant coliforms in OL-A and OL-B are light green in dry season and dark green in the rainy season, Thermotolerant coliforms on beaches are yellow in the dry season and orange in the rainy season.

Fig 4. Non-metric multidimensional scaling ordination, derived from Euclidean distance constructed using square root transformed total chemical, physical and microbiological parameters at 42 sampling sites during dry season (2020) and rainy season (2021).

The upper plot has used for all variables and seasons and is coded for season, while the lower plot was restricted to Thermotolerant Coliforms for a seasons.