The team, led by A. O. Olalemi with colleagues O. O. Olaniyi, D. O. Olugbade and O. F. Olowoporoku, sampled the dam biweekly over a six-month period that spanned both wet and dry seasons. Crucially, they did not restrict their sampling to the water surface. Samples were collected both just below the surface and near the dam bed, a distinction that matters because microbial distributions in reservoirs are rarely uniform. Pathogens and fecal indicator organisms can settle into sediments, where they may persist longer than in the open water column, and resuspension during storms or disturbance can return them to the water that people draw for domestic use. Capturing both zones gives a more realistic estimate of the microbial population that users of the dam are actually exposed to.

The numbers reported for fecal indicator bacteria are striking. Escherichia coli, the standard benchmark organism for fecal pollution in water, was detected at concentrations ranging from 3.0 to 4.3 log10 colony-forming units per 100 milliliters. In practical terms, that means between roughly 1,000 and 20,000 viable E. coli cells in each 100-milliliter sample. Proteus species ranged from 3.6 to 4.5 log10 cfu/100 ml, and Shigella, one of the classic causes of bacillary dysentery, was present at 3.3 to 4.1 log10 cfu/100 ml. International guidelines for drinking water quality set the acceptable level of E. coli in treated drinking water at zero per 100 milliliters, so the measured concentrations represent contamination that is orders of magnitude above what would be considered safe for direct consumption.

Beyond the cultured indicator organisms, the researchers used molecular methods to characterize the broader bacterial community in the dam. Sequencing revealed that the water was dominated by two phyla: Proteobacteria, which accounted for 57.63 percent of the community, and Bacteroidota, which made up 26.63 percent. This composition is typical of freshwater systems influenced by organic matter and fecal inputs, since both phyla contain large numbers of organisms associated with the animal and human gut as well as with decaying organic material in aquatic environments. The dominance of these two groups is consistent with the picture painted by the culture data of a reservoir under continuous contamination pressure.

Drilling down further, the community analysis showed that at the order level Pseudomonadales was the most abundant group at 40.43 percent, followed by Cytophagales at 28.29 percent. At the family level, Pseudomonadaceae dominated with 39.17 percent of sequences, with Spirosomaceae close behind at 28.29 percent. The most prevalent individual species identified were Pseudomonas sp003428805, accounting for 27.61 percent of the community, and Arcicella rosea at 19.09 percent. Pseudomonas species are ubiquitous in soil and water and are not themselves typically fecal indicators, but their dominance alongside high fecal indicator counts suggests a reservoir rich in organic nutrients, conditions that allow both environmental organisms and gut-derived pathogens to survive and sometimes multiply outside their hosts.

Perhaps the most concerning findings concern the protozoan parasites. Cryptosporidium parvum was detected in 83.33 percent of the water samples, and Giardia lamblia was found in 50 percent of them. Both are single-celled parasites that cause prolonged diarrheal illness, and both are globally recognized as major causes of waterborne disease outbreaks. Cryptosporidium in particular is notorious in water treatment circles because its oocysts are highly resistant to chlorine disinfection, the mainstay of municipal water treatment in many places. An organism present in more than four out of five samples from a source used for domestic purposes represents a persistent and widespread exposure hazard, not an occasional anomaly.

To translate these measurements into estimates of actual health risk, the team applied a dose-response approach, the core of quantitative microbial risk assessment. This framework uses experimentally derived relationships between the number of pathogen cells or cysts ingested and the probability of infection, combined with estimates of how much water people consume and how contaminated that water is. The calculated risks of infection from the pathogens detected in the dam all exceeded 10 to the power of minus 4 per exposure, a benchmark often used in public health as the maximum acceptable annual risk of infection from waterborne pathogens. In other words, the estimated infection probabilities were above one in ten thousand, the level at which intervention is generally considered necessary.

The risk estimates also revealed a clear hierarchy among the pathogens. When ranked by infection risk, the order was Klebsiella, lowest, followed by Giardia lamblia, then Salmonella, then Cryptosporidium parvum, and finally Shigella, which carried the highest estimated risk. This ordering reflects both the concentrations at which each organism was present and the differing infectivity of each pathogen; Shigella requires very few cells to establish an infection, which makes even moderate concentrations in drinking water a serious concern. The finding that protozoan parasites ranked above most of the bacteria underscores the particular threat posed by Cryptosporidium and Giardia in untreated surface waters.

The authors interpret the consistently high bacterial and protozoan loads across the sampling period as evidence of continuous contamination of the dam rather than isolated pollution events. Multipurpose dams like Owena typically serve a range of functions, including domestic water supply, irrigation, fishing and livestock watering, and such multiple uses, combined with runoff from surrounding settlements and farmland during the wet season, create numerous pathways for fecal material to enter the reservoir. The fact that elevated counts persisted through both wet and dry periods suggests that contamination sources are not merely seasonal runoff but ongoing inputs, whether from human activity around the dam, animal access to the water, or the survival and regrowth of organisms within the reservoir itself and its sediments.

The practical message of the study is unambiguous: water drawn from the Owena dam must be adequately treated before it is consumed. For households and water managers in the region, that means treatment capable of addressing both bacteria and chlorine-resistant protozoa, such as filtration, boiling, or appropriately designed multi-barrier treatment systems. More broadly, the study illustrates the value of pairing traditional culture-based monitoring with molecular community profiling and formal risk assessment. Culture methods reveal viable indicator organisms and known pathogens, sequencing exposes the full ecological structure of the microbial community, and dose-response modeling converts both into estimates of human health risk that decision-makers can act on. As pressure on freshwater resources grows across sub-Saharan Africa, studies of this kind provide an evidence base for protecting the millions of people who rely on surface waters like the Owena dam for their daily needs, and they make clear that in the absence of treatment, the microbial hazards in such waters are neither marginal nor hypothetical.

Subject of Research: Microbial contamination and quantitative infection risk assessment of water from the Owena multipurpose dam in Nigeria

Article Title: Microbial risk assessment of enteric bacteria and protozoa in Owena multipurpose dam in Igbara-Oke, Nigeria

Article References: Olalemi, A. O., Olaniyi, O. O., Olugbade, D. O., & Olowoporoku, O. F. (2026). Microbial risk assessment of enteric bacteria and protozoa in Owena multipurpose dam in Igbara-Oke, Nigeria. PLOS Water, 5(7), e0000595. https://doi.org/10.1371/journal.pwat.0000595

Image Credits: AI Generated

DOI: 10.1371/journal.pwat.0000595

Keywords: water quality, microbial risk assessment, E. coli, Shigella, Cryptosporidium parvum, Giardia lamblia, faecal indicator bacteria, Nigeria, Owena dam, PLOS Water, quantitative microbial risk assessment, waterborne disease