Organophosphate pesticides are among the most heavily used chemical pest controls on Earth, accounting for roughly 40 percent of global pesticide consumption. They were widely adopted as replacements for the notoriously persistent organochlorine pesticides of the mid-twentieth century because they break down more quickly in the environment. Yet that relative impermanence has not kept them out of aquatic ecosystems. Spray drift, agricultural runoff, groundwater leaching, careless disposal of empty containers, and the washing of application equipment all provide routes by which these compounds reach lakes and rivers, where they can persist long enough to be taken up by organisms. In Ethiopia, chlorpyrifos-methyl is commonly applied in crop production, while diazinon finds its way into the environment partly through flea collars used on sheep, dogs, and cats in rural areas.

The research team, led by Tarekegn Fentie Yimer of the University of Gondar, targeted Lake Tana and Lake Hayqe because neither had ever been assessed for the ecological and human health risks of these compounds, despite intensive pesticide use by smallholder farmers and horticulture companies in their catchments. Lake Tana covers 3,150 square kilometers at an altitude of 1,786 meters, is recognized by UNESCO as a Biosphere Reserve, and supports endemic fish, bird, and plant species alongside a fishery with an estimated potential of 10,000 tons per year. Lake Hayqe, by contrast, is a closed-basin lake with a watershed of only about 77 square kilometers but an average depth of 37 meters and a maximum depth of 81 meters. Both lakes supply Nile tilapia, a staple food for surrounding communities, making them ideal natural laboratories for tracing pesticides from farm fields to dinner plates.

Between February and May 2023, the researchers collected water from 20 sites on Lake Tana and 5 sites on Lake Hayqe, along with 30 Nile tilapia from three Lake Tana fishing districts and 10 fish, five tilapia and five common carp, from Lake Hayqe. Fish muscle tissue was freeze-dried, homogenized, and extracted using the QuEChERS method, a technique whose acronym promises a process that is quick, easy, cheap, effective, rugged, and safe. All samples were then analyzed by gas chromatography coupled with triple quadrupole mass spectrometry, one of the most sensitive analytical approaches available for trace organic contaminants. The method performed well: recoveries ranged from 96.7 to 106.0 percent, well within the accepted 80 to 120 percent window, and detection limits were low enough to quantify every residue found.

Of the eighteen organophosphate pesticides screened, only a handful appeared. In water, chlorpyrifos-methyl, ethion, and malathion were detected at every site sampled, with chlorpyrifos-methyl ranging from 1.41 to 1.91 micrograms per liter. That figure is dramatic when set against the United Kingdom Environment Agency’s non-statutory maximum acceptable concentration of just 0.01 micrograms per liter for the protection of freshwater life, meaning the lakes carry concentrations more than a hundred times that benchmark. Malathion, by contrast, remained below the World Health Organization’s drinking-water guideline of 2 micrograms per liter. In fish muscle, four compounds were detected: chlorpyrifos-methyl, diazinon, ethion, and malathion, at concentrations generally between about 11 and 25 micrograms per kilogram. Notably, malathion levels in fish exceeded the WHO maximum residue limit of 2 micrograms per kilogram, while diazinon stayed comfortably within its 200 to 500 microgram limit.

The spatial patterns told a story about sources. Statistical analysis revealed significant differences among sampling sites for every detected compound, with the highest chlorpyrifos-methyl and malathion values clustering near the mouths of tributary rivers such as the Gumara, Rib, and Megech, which drain intensive agricultural areas like Fogera and Gorgora, and near Bahir Dar, the region’s largest city. Principal component analysis, a multivariate technique that condenses many correlated measurements into a few underlying axes of variation, confirmed that ethion and chlorpyrifos-methyl dominated the first component explaining nearly 60 percent of the variance in water data, while malathion distinguished the Megech site. Tilapia consistently accumulated more pesticide than common carp in Lake Hayqe, suggesting species-specific differences in uptake and metabolism.

The ecological risk assessment produced the study’s most alarming numbers. The team calculated risk quotients by dividing measured environmental concentrations by predicted no-effect concentrations derived from acute toxicity data for three representative organisms: fish, Daphnia, and algae, chosen to span primary producers, primary consumers, and higher trophic levels. For Daphnia, the risk quotients for chlorpyrifos-methyl reached 275 in Lake Tana and 268 in Lake Hayqe, values far above the threshold of 1 that signals high risk. Ethion also posed high risk to Daphnia, with quotients between roughly 19 and 22, while malathion registered a medium risk. Fish and algae fared considerably better, with low to medium risk quotients across the board. The pattern identifies Daphnia as by far the most sensitive receptor, a finding consistent with studies in Ethiopia’s Lake Hawassa and in surface waters of northwestern Bangladesh and Greece.

The human health picture was far more reassuring. Using standard exposure assumptions, a fish consumption rate of 30 grams per day and an adult body weight of 60 kilograms, the researchers estimated daily intakes and hazard quotients for each pesticide. Every hazard quotient came in well below 1, and the combined hazard index was approximately 0.06 for both lakes, indicating a negligible non-carcinogenic risk from eating fish under the conditions assessed. Diazinon contributed the largest share of the hazard index, yet even at maximum detected concentrations the total remained an order of magnitude below the level of concern. The authors caution, however, that these estimates rest on generalized consumption and body weight assumptions that may not reflect every local community, and that the assessment addressed only non-cancer endpoints.

The study’s authors are candid about its limitations. Composite sampling, while analytically practical, blurred spatial resolution, particularly in Lake Hayqe where all water samples were merged into a single composite. Sampling occurred during a single season, so rainfall-driven pulses of pesticide runoff may have gone unmeasured, and the smaller sample size from Lake Hayqe limits direct comparison between the lakes. Still, the baseline established here carries a clear message for policymakers: pesticide governance in Ethiopia remains underdeveloped, many generic pesticides circulate without full registration, and farmers often handle chemicals without precautions. The researchers call for multi-season monitoring, larger and non-composited samples, and site-specific exposure data, alongside improved management practices to stem the flow of organophosphates into lakes that sustain fisheries, tourism, and hydropower. Protecting the plankton at the base of these ecosystems, the data suggest, may be the most urgent task of all.

Subject of Research: Ecological and human health risk assessment of organophosphate pesticide residues in water and fish from two Ethiopian highland lakes

Article Title: Ecological and human health risk assessments of organophosphate pesticide residues in water and fish from two highland lakes in Ethiopia

Article References: Yimer, T. F., Kassa, Y., Mulugeta, M., Akele, M. L., Alemu, A. K., Yenealem, D., Amare, A., Moges, Z., Birhan, T. A., & Tibebe, D. (2026). Ecological and human health risk assessments of organophosphate pesticide residues in water and fish from two highland lakes in Ethiopia. Heliyon, 12(15), Article e45551. https://doi.org/10.1016/j.heliyon.2026.e45551

Image Credits: AI Generated

DOI: 10.1016/j.heliyon.2026.e45551

Keywords: organophosphate pesticides, Lake Tana, Lake Hayqe, Ethiopia, chlorpyrifos-methyl, Daphnia, ecological risk assessment, bioconcentration, Nile tilapia, water contamination, fish consumption, risk quotient

The study was led by Peter M. Chilipweli and Elias C. Nyanza of the Catholic University of Health and Allied Sciences in Mwanza, together with Aiwerasia Vera Ngowi of Muhimbili University of Health and Allied Sciences and Karim Manji, also of Muhimbili. Their team recruited 337 mother–child pairs from reproductive and child health clinics across four districts in the Iringa and Njombe regions. Of these, 220 came from districts classified in advance as pesticide-exposed — Kilolo and Makambako, both centers of intensive horticulture — while 117 came from comparison districts, Njombe Town and Iringa Municipal, where agricultural chemical use is expected to be lower.

The central measurement was erythrocyte acetylcholinesterase activity, assessed during the first trimester of pregnancy. Acetylcholinesterase is the enzyme that breaks down acetylcholine, the neurotransmitter released at nerve synapses, and organophosphate insecticides work precisely by inhibiting this enzyme, causing overstimulation of nerve pathways. Because red blood cell acetylcholinesterase mirrors the enzyme found at nerve endings, its activity in maternal blood serves as a widely used proxy for the body’s internal dose of organophosphate exposure. The researchers classified each mother as having either low inhibition or high or moderate inhibition of the enzyme. Overall, 38.3 percent of the mothers showed moderate or high inhibition, a strikingly high burden of biological exposure in a general population of pregnant women rather than a group of occupationally selected workers.

Alongside the blood measurements, the team administered a structured pesticide-use inventory, classifying the products farmers reported using according to the international IRAC, FRAC and HRAC classification systems for insecticides, fungicides and herbicides. The picture that emerged was one of heavy reliance on organophosphates, particularly chlorpyrifos and profenofos, alongside pyrethroids and neonicotinoids. Notably, 24.3 percent of the reported use involved pesticide mixtures, including combinations of neonicotinoids with organophosphates — a pattern of co-formulation and co-application that toxicologists increasingly worry about, since different chemical classes can interact in ways that single-compound risk assessments do not capture.

When the infants reached approximately six months of age, the researchers assessed their development using the Malawi Developmental Assessment Tool, an instrument designed and validated for African settings that evaluates four domains: gross motor function, fine motor function, language and hearing, and social behavior. The overall picture was reassuring at the population level. Most infants — 88.4 percent — showed normal composite development, and across the individual domains, between 94.7 and 97.3 percent scored within normal ranges. Yet beneath that generally healthy surface, the statistical analysis revealed a consistent signal tied to prenatal exposure.

Using parsimonious multivariable Firth penalized logistic regression — a technique chosen because it produces more stable estimates when outcomes are imbalanced and sample sizes are modest — the researchers found that high maternal acetylcholinesterase inhibition, indicating high pesticide exposure, was associated with roughly triple the odds of mild or delayed composite neurodevelopment in the infants, with an adjusted odds ratio of 2.94 and a 95 percent confidence interval of 1.02 to 8.33, reaching statistical significance at p equal to 0.045. The confidence interval was wide, reflecting the modest number of delayed cases, but the association aligned with a substantial body of evidence from other parts of the world.

Perhaps the most intriguing finding was a significant interaction between pesticide exposure and the sex of the child, with an adjusted odds ratio of 4.30 and a 95 percent confidence interval of 1.05 to 17.56, at p equal to 0.042. This suggests that the relationship between maternal pesticide exposure and infant neurodevelopment may differ for boys and girls, echoing a recurring theme in the environmental health literature, where male fetuses have often appeared more vulnerable to neurotoxic insults, although the direction and magnitude of sex differences vary across studies. The authors themselves are careful to note that this particular estimate was imprecise and requires confirmation in larger, adequately powered, sex-stratified analyses before firm conclusions can be drawn.

The study also identified protective social factors. Mothers who had completed secondary or tertiary education had lower odds of having an infant with mild or delayed composite development, with an adjusted odds ratio of 0.47 and a confidence interval of 0.22 to 1.00, just crossing the threshold of significance at p equal to 0.050. Increasing maternal age was associated with lower odds of language and hearing delay, with an adjusted odds ratio of 0.86 per year of age and a confidence interval of 0.74 to 0.99 at p equal to 0.042. These findings underscore that developmental risk in such communities is shaped by a web of influences — schooling, maternal experience, household environment — that interact with chemical exposures rather than operating in isolation.

The broader context gives the findings weight. Agricultural intensification in Tanzania’s Southern Highlands has driven a marked increase in reliance on organophosphate pesticides, and evidence from cohorts such as the CHAMACOS study of farmworker families in California has linked prenatal organophosphate exposure to poorer cognitive, motor and behavioral outcomes in childhood. What has been missing is data from low- and middle-income countries, and especially from sub-Saharan African smallholder communities, where women often work in the fields themselves, handle concentrates without adequate personal protective equipment, and face exposure through spraying, mixing, re-entering treated plots and laundering contaminated clothing. This study provides a rare, early-window measurement in exactly that population, and it does so using an objective biomarker rather than self-reported exposure alone.

The authors emphasize that the cross-sectional design, the single exposure measurement in the first trimester, and the wide confidence intervals all counsel caution, and they call for longitudinal studies incorporating repeated, pesticide-specific exposure measurements, longer neurodevelopmental follow-up and properly powered sex-stratified analyses. Still, the practical implications are immediate. The team recommends strengthening safer pesticide-use practices in horticultural communities, providing antenatal counseling on occupational exposure, and building developmental surveillance into routine reproductive and child health services so that infants at elevated risk are identified early. The work was funded by Tanzania’s Ministry of Education, Science and Technology through the Higher Education for Economic Transformation Project, and approved by the Catholic University of Health and Allied Sciences and Bugando Medical Centre Research Ethics and Review Committee. For the farming families of Iringa and Njombe, the message is that the first weeks of pregnancy may be a critical window in which pesticide exposure leaves measurable traces in an infant’s developing nervous system — and that protecting mothers in those early months is an investment in the next generation’s minds.

Subject of Research: Prenatal organophosphate pesticide exposure and infant neurodevelopment in Tanzanian smallholder farming communities

Article Title: First-trimester maternal erythrocyte acetylcholinesterase activity and neurodevelopmental outcomes at 6 months among children of smallholder horticultural growers in the Southern Highlands of Tanzania

Article References: Chilipweli, P. M., Nyanza, E. C., Ngowi, A. V., & Manji, K. (2026). First-trimester maternal erythrocyte acetylcholinesterase activity and neurodevelopmental outcomes at 6 months among children of smallholder horticultural growers in the Southern Highlands of Tanzania. BMC Pediatrics. https://doi.org/10.1186/s12887-026-07698-2

Image Credits: AI Generated

DOI: 10.1186/s12887-026-07698-2

Keywords: acetylcholinesterase, organophosphate pesticides, chlorpyrifos, neurodevelopment, Malawi Developmental Assessment Tool, prenatal exposure, smallholder horticulture, Tanzania, infant health, occupational exposure, child sex differences, public health