The context is a region under acute water stress. The Horn of Africa experienced its warmest five-year stretch since 1981 between 2021 and 2025, with severe droughts in 2021–2022 and again in 2024 leaving soil moisture below normal across central and eastern Ethiopia. As climate pressures intensify, smallholder farmers across sub-Saharan Africa have intensified pesticide use to protect yields, introducing a growing class of emerging contaminants into already strained water resources. In eastern Ethiopia specifically, khat cultivation requires intensive management and regular pesticide application, and runoff from treated fields carries these chemicals into soil, rivers, lakes, and groundwater.

The health stakes are considerable. Long-term exposure to organophosphates has been linked to endocrine disruption, confusion, anxiety, memory loss, depression, and personality changes. Organophosphate poisoning accounts for roughly 38 percent of all emergency room poisoning admissions in Ethiopian teaching hospitals, and a retrospective study at the University of Gondar Teaching Hospital found that 90 percent of poisoning cases studied involved ingested organophosphates, with a mean patient age of just 25.5 years. Earlier work in the Ethiopian highlands detected dimethoate in surface waters at average concentrations of 0.38 micrograms per liter and in groundwater at 1.24 micrograms per liter, indicating preferential transport to aquifers during the rainy season.

To establish the scale of contamination, the team collected two-liter samples from each of the four sites, kept them refrigerated at 4 degrees Celsius, and extracted the pesticides using liquid-liquid extraction with n-hexane, achieving a pre-concentration factor of 200:1 before analysis. Khat leaf samples were dried, ground, and extracted with acetone-water solution using a similar approach. Quantification was performed by high-performance liquid chromatography with diode array detection at 210 nm, using both isocratic and gradient elution on a reversed-phase C18 column.

The results were unambiguous. Profenofos concentrations in the Haramaya, Ganda Qallu, and Adele water samples exceeded the World Health Organization’s recommended limit of 0.003 milligrams per liter, with the highest single reading at 0.14 milligrams per liter in Ganda Qallu. Dimethoate residues in Haramaya, Ganda Hayyo, and Adele samples surpassed the WHO limit of 0.02 milligrams per liter. Malathion remained below its 0.05–0.06 milligram per liter limit, but the summed pesticide load in every water sample exceeded the European Union’s guideline range of 0.001–0.003 milligrams per liter. Solubility and adsorption tests showed the pesticides dissolve readily in water and bind strongly to soil colloids, meaning they can move both in surface runoff and by leaching through soil.

For treatment, the researchers turned to the classical Fenton process, in which ferrous iron reacts with hydrogen peroxide under acidic conditions to generate hydroxyl radicals — highly reactive, non-selective oxidants that attack persistent organic molecules through hydroxylation, dealkylation, and ring-opening pathways, ultimately converting them to smaller, less hazardous intermediates and, ideally, to carbon dioxide and water. The team chose the homogeneous process deliberately: it offers direct contact between dissolved iron and peroxide, avoids the synthesis and regeneration burdens of heterogeneous catalysts, and does not depend on the light intensity and reactor geometry that solar and photo-Fenton variants require.

Batch experiments in 600-milliliter reactors explored the effects of pH, ferrous sulfate dose, hydrogen peroxide dose, and reaction time, and a response surface methodology with a central composite design was used to optimize the four variables simultaneously. The statistical model explained 99.17 percent of the variance in profenofos removal, with pH, peroxide dose, and reaction time all highly significant. The optimum emerged at pH 3.4, 0.7 grams per liter of ferrous sulfate, 5 grams per liter of hydrogen peroxide, and 42 minutes of reaction time — conditions predicted to remove 99.83 percent of profenofos. Validation runs achieved 99.21 percent removal on average, within 0.62 percent of the prediction.

The chemistry behind the optimum is well understood. Acidic pH keeps iron soluble and catalytically active, while higher pH triggers iron hydroxide precipitation and peroxide auto-decomposition. Both iron and peroxide show nonlinear behavior: beyond an optimum, excess ferrous ions and peroxide themselves scavenge hydroxyl radicals, reducing the fraction available to attack the pesticide. Strong interactions between pH and reaction time, and between iron and peroxide doses, confirmed that the reagents must be balanced rather than maximized. Thermodynamic analysis showed the process is spontaneous and endothermic, with Gibbs free energy becoming more negative as temperature rose from 298 to 328 kelvin.

After treatment, profenofos in the Haramaya, Ganda Hayyo, and Adele samples fell to 0.00084, 0.0006, and 0.0012 milligrams per liter respectively — all below the WHO limit — corresponding to removal efficiencies of 97.6 to 98.9 percent. Dimethoate removals ranged from roughly 98 to 99 percent, and overall organophosphate removal reached approximately 98.5 to 99 percent across the treated sites. Chemical oxygen demand also dropped substantially, from 128 to 32 milligrams per liter at Haramaya and from 256 to 96 milligrams per liter at Adele, bringing those samples within permissible ranges, while Ganda Qallu and Ganda Hayyo achieved complete COD removal at the optimal pH.

An economic assessment using SuperPro Designer simulation, based on a 10-cubic-meter-per-day treatment capacity operating 300 days per year, estimated a total capital investment of about 78,450 US dollars, an annual operating cost of roughly 42,680 dollars, and a treated-water cost of approximately 0.56 dollars per cubic meter. Chemicals dominated the cost structure at about 42 percent of total expenditure, followed by power at 18 percent. With a payback period of 2.7 years, a net present value of about 135,800 dollars over ten years, and an internal rate of return near 28.4 percent, the authors argue the process is financially viable and well suited to decentralized deployment in regions where advanced infrastructure is limited. They caution that pesticide disappearance does not guarantee complete mineralization, and that further work on transformation products and large-scale operation is needed — but for a region where khat farming, drought, and pesticide-laced runoff converge, a cheap bottle of iron salt and peroxide may prove to be a powerful line of defense.

Subject of Research: Fenton oxidation removal of organophosphate pesticides from contaminated water and wastewater in eastern Ethiopia

Article Title: Organophosphate pesticides in water/wastewaters in eastern Ethiopia: identification and abatement study using fenton oxidation process

Article References: Jabesa, A., Gindaba, G. T., Abo, L. D., Hamda, A. S., Edae, I. T., & Areti, H. A. (2026). Organophosphate pesticides in water/wastewaters in eastern Ethiopia: identification and abatement study using fenton oxidation process. Environmental Challenges, 25, Article 101684. https://doi.org/10.1016/j.envc.2026.101684

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101684

Keywords: organophosphate pesticides, Fenton oxidation, water treatment, Ethiopia, profenofos, dimethoate, malathion, advanced oxidation processes, response surface methodology, khat cultivation, wastewater remediation, hydroxyl radicals

The research team collected Aedes aegypti mosquitoes from two contrasting environments: the densely urbanized metropolitan area of Kingston and the parish of St Andrew, referred to in the study as KSA, and the rural community of Lances Bay in the parish of Hanover. They also collected Aedes albopictus, the Asian tiger mosquito, from the urban KSA area to assess whether a second potential vector species shared the same vulnerability profile. Field-derived mosquitoes were then subjected to Centers for Disease Control and Prevention bottle bioassays, a standardized technique in which mosquitoes are exposed to a diagnostic dose of insecticide inside coated glass bottles and mortality is recorded over time. The CDC threshold for declaring a population susceptible requires mortality at or above 98 percent at the diagnostic time point, and by that measure every population tested failed, often spectacularly.

The numbers reveal the depth of the problem. At the 30-minute diagnostic time point, urban Aedes aegypti from KSA showed zero percent mortality to permethrin, meaning not a single mosquito was killed by the pyrethroid. Mortality to deltamethrin, another pyrethroid, stood at just 22.0 percent with a standard deviation of 2.7 percent, while malathion, an organophosphate, fared somewhat better at 70.5 percent with a standard deviation of 9.8 percent, still far below the susceptibility threshold. The rural Hanover population was, if anything, in worse condition: zero percent mortality to both permethrin and malathion, and only 8.1 percent mortality to deltamethrin. Aedes albopictus from KSA also demonstrated resistance to deltamethrin, with mortality below the CDC threshold, confirming that resistance is not confined to a single species.

What makes these results particularly concerning for public health officials is the multi-class nature of the resistance. Pyrethroids and organophosphates act through fundamentally different mechanisms: pyrethroids target voltage-gated sodium channels in insect neurons, while malathion inhibits acetylcholinesterase, an enzyme essential for nerve impulse termination. When a mosquito population simultaneously withstands compounds from both classes, control programs lose their two most important chemical tools at once. In Jamaica, where dengue outbreaks recur and ultra-low-volume spraying with pyrethroids is a mainstay of emergency response, the practical implications are immediate. Spraying campaigns that would have knocked down susceptible populations may now leave the majority of vector mosquitoes unharmed, while giving communities a false sense of protection.

To understand the molecular machinery behind this resistance, the researchers turned to transcriptomics, the study of gene expression patterns across the entire genome. They exposed Aedes aegypti collected from KSA to two compounds: malathion, the organophosphate already in use, and pyriproxyfen, an insect growth regulator belonging to a class of chemicals used to disrupt mosquito development. Using RNA sequencing, the team compared gene expression in exposed mosquitoes against untreated field mosquitoes and against the Rockefeller strain, a long-established insecticide-susceptible laboratory reference line. Principal component analysis and hierarchical clustering of the sequencing samples confirmed that the biological replicates grouped consistently by treatment, lending statistical confidence to the downstream comparisons.

The malathion results were striking. Exposure to the organophosphate triggered strong overexpression of genes associated with detoxification and with cuticle structure. Detoxification genes in mosquitoes typically include the three great enzyme families of insecticide metabolism: cytochrome P450 monooxygenases, glutathione S-transferases, and carboxylesterases, all of which can chemically modify or sequester toxic compounds before they reach their molecular targets. Cuticle-associated genes matter because the insect exoskeleton is the first barrier an insecticide must cross; a thicker or chemically altered cuticle can slow penetration and give internal detoxification systems time to neutralize the incoming dose. The coordinated upregulation of both gene categories suggests that Jamaican mosquitoes are mounting a layered defense, combining reduced uptake with enhanced metabolic breakdown.

Pyriproxyfen produced a different and more nuanced transcriptional signature. Rather than a concentrated detoxification response, the insect growth regulator elicited a moderate but broad response spanning metabolic and endocrine pathways. This pattern is biologically coherent: pyriproxyfen mimics juvenile hormone, a master regulator of insect metamorphosis, so exposure would be expected to perturb the expression of genes involved in hormone signaling and development alongside general metabolic machinery. The authors interpret these findings as evidence that transcriptional responses to insecticide exposure occur across multiple gene families rather than within a single pathway, a realization with important consequences for how resistance is monitored and predicted.

The transcriptomic dataset also included a comparison between the Rockefeller susceptible strain and untreated field-derived mosquitoes, capturing the baseline genetic differences that distinguish resistant Jamaican populations from their susceptible counterparts even before any laboratory exposure. Supplementary tables accompanying the paper document the significantly differentially expressed genes in each comparison, complete with log2 fold-change values, adjusted P-values, and normalized expression levels, providing a public resource for other researchers studying Caribbean vector populations. The study was supported by a Research and Publication Grant from the University of the West Indies, and the team included entomologists from the CDC’s Entomology Branch within the Division of Parasitic Diseases and Malaria, underscoring the international collaboration behind the work.

For the Caribbean region, the study arrives at a critical moment. Aedes aegypti is the principal urban vector of dengue virus, which causes periodic explosive outbreaks across the islands, as well as chikungunya and Zika, both of which swept through the Americas in the last decade. Resistance in Aedes aegypti has previously been documented in Jamaican urban and rural settings, but the susceptibility of other potential vector species and the transcriptional responses to insecticide exposure had remained poorly characterized. By showing that Aedes albopictus shares deltamethrin resistance with its more famous cousin, and by mapping the gene families involved, the study fills two significant gaps in regional surveillance knowledge. It also demonstrates that resistance profiles can differ dramatically between locations separated by only a few hundred kilometers, with the rural Hanover population showing even lower mortality than the urban KSA population for two of the three insecticides tested.

The authors conclude that insecticide resistance in Jamaican Aedes populations is widespread, heterogeneous, and dynamic, and they argue that these findings support locally informed resistance surveillance and integrated vector management strategies that go beyond reliance on pyrethroids. Integrated vector management, endorsed by the World Health Organization, combines chemical, biological, and environmental approaches, including source reduction through the elimination of standing water, larval control with agents less prone to resistance, and community engagement, reserving insecticides for targeted use where susceptibility still exists. The alternative, continuing to spray compounds to which local mosquitoes no longer respond, wastes scarce public health resources and accelerates the selection of even hardier mosquito populations. As dengue risk grows across a warming Caribbean, the Jamaican data serve as a warning that the chemical arsenal against Aedes mosquitoes is eroding, and that molecular surveillance of the kind performed in this study will be essential to staying ahead of the next outbreak.

Subject of Research: Insecticide resistance and resistance-associated gene expression in Jamaican Aedes mosquito populations

Article Title: Phenotypic insecticide resistance in Jamaican Aedes spp. and resistance-associated transcripts in Aedes aegypti

Article References: Tai, D., Irvine, W., Francis, F., Mackenzie-Impoinvil, L., Buchanan, L., Reimer, L., Lenhart, A., Roye, M., & Francis, S. (2026). Phenotypic insecticide resistance in Jamaican Aedes spp. and resistance-associated transcripts in Aedes aegypti. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07666-w

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07666-w

Keywords: Aedes aegypti, Aedes albopictus, insecticide resistance, Jamaica, pyrethroids, malathion, pyriproxyfen, transcriptomics, CDC bottle bioassay, dengue, vector control, Caribbean