08
May
Neonicotinoid Exposure Threatens Fish Health, Highlighting Cascading Effects to Humans as Consumers
(Beyond Pesticides, May 8, 2025) A study, published in Environmental Pollutants and Bioavailability, assesses the impacts on Nile tilapia (Oreochromis niloticus) with subacute and chronic exposure to thiamethoxam, a neonicotinoid insecticide, and finds genotoxicity, oxidative stress, and changes in tissue structure, among other threats to organ function and overall fish health. “The study focused on biochemical markers, genetic damage, pesticide residue levels in fish flesh, and histopathological changes in fish exposed to different concentrations of thiamethoxam,†the authors state. The threats do not end there; human health is also at risk from the consumption of these contaminated fish.
“Unfortunately, neonicotinoids, rapidly washed into surface water from agricultural areas, pose a significant threat to environmental water quality and can harm non-target species, particularly aquatic organisms,†the researchers state. The accumulation of these chemicals leads to “ultimately harming both aquatic ecosystems and human health,†they say.
In particular, the study highlights that prolonged exposure to high doses of thiamethoxam can cause “significant negative effects on fish health,†the authors note. They continue: “This exposure led to increased levels of urea and ALT [alanine aminotransferase] in the blood, indicating potential damage to the kidneys and liver. Additionally, thiamethoxam caused oxidative stress, as evidenced by elevated levels of MDA [malondialdehyde, a product of lipid peroxidation] and upregulated TLR2 [toll-like receptor 2] gene expression.â€
Residues of thiamethoxam in Nile tilapia muscle correlate with increasing exposure and elevated urea levels. Even lower thiamethoxam concentrations are associated with subsequent kidney dysfunction. “Thiamethoxam also negatively impacted the fish’s antioxidant defense system,†the researchers say. “It significantly reduced the expression of key antioxidant genes like ghrelin and catalase [CAT],†they note, which “could be a consequence of thiamethoxam-induced gastroenteritis†and indicative of damage to gut cells.
Lastly, the authors share, “Histological examination revealed severe degenerative changes in the gills and liver tissues of the fish exposed to thiamethoxam, with the severity of the damage increasing with the concentration of the pesticide.†These results are concerning, as chemicals such as thiamethoxam are able to bioaccumulate and biomagnify within the food chain, leading to impacts on various organisms, including humans.
The study, carried out in Egypt, utilizes 200 Nile tilapia within one control group and four test groups exposed to multiple concentrations of thiamethoxam for varying lengths of time. “Prior to the main study, preliminary trials were conducted to establish non-lethal thiamethoxam concentrations, which were subsequently used as sublethal doses in toxicity assessments,†the researchers say.
In analyzing biochemical markers, this study determines pesticide toxicity. “Specifically, this study examined the impact of thiamethoxam on serum liver and kidney biochemical functions, oxidative stress markers (SOD [superoxide dismutase], MDA, and TAC [total antioxidant capacity]), gene expression (TLR2, ghrelin, and catalase), tissue residue levels, and histopathological changes in the gills and liver of fish,†the authors explain.
The results show a reduction in both SOD and TAC biomarkers. “Collectively, these findings provide compelling evidence that TMX [sublethal exposure to thiamethoxam] exposure induces significant oxidative stress in Nile tilapia, characterized by diminished antioxidant enzyme activity, increased lipid peroxidation, and compromised overall antioxidant capacity,†the researchers note. Additional results, including the significantly elevated creatinine levels, also indicate potential renal dysfunction, as well as impacts on hepatic gene expression “associated with growth (ghrelin), immunity (TLR2), and oxidative stress (CAT).â€
“The analysis revealed significant, concentration-dependent alterations in the expression of these genes across all treatment groups compared to the control,†the authors say. They continue: “Notably, ghrelin expression demonstrated a significant downregulation… indicative of potential growth inhibition. Conversely, TLR2 expression exhibited a substantial upregulation…suggesting a pronounced activation of the immune response. Furthermore, CAT gene expression displayed a significant downregulation… signifying heightened oxidative stress.†These results highlight thiamethoxam’s ability to disrupt physiological pathways in fish species by compromising growth, immune function, and the management of oxidative stress.
Lastly, the fish exposed to different concentrations of thiamethoxam all show a range of impacts and damage to their gills. From inflammation and hemorrhaging to exacerbated gill lesions, even the lower concentrations yielded concerning results. As the concentrations increase, more severe hepatic pathologies are induced. The researchers note necrosis (cell death) within tissues and congestion of hepatic blood vessels at the higher exposure levels.
“These findings demonstrate a clear concentration-dependent increase in histopathological damage to the gills and liver of Nile tilapia following [thiamethoxam] exposure, indicative of significant toxicological effects,†the authors write. “The histopathological findings support the biochemical results, showing direct damage to gill and liver tissues caused by thiamethoxam.â€
Previous research confirms these results, within Nile tilapia and similar species, to both thiamethoxam and other neonicotinoids. Important study results include:
- “Research indicates that sublethal exposure to thiamethoxam (TMX) significantly disrupts the physiology of Labeo rohita over 120 hours” and also “documented notable elevations in urea and creatinine, suggesting impaired kidney function, along with increased alanine aminotransferase (ALT) and aspartate aminotransferase (AST), indicators of liver damage. Additionally, the study found a rise in malondialdehyde (MDA) concentrations, a marker of lipid peroxidation, demonstrating that TMX induces significant oxidative stress in the fish.â€
- Another study “further demonstrated that TMX administration exacerbates oxidative stress through the inhibition of crucial antioxidant enzyme activities. Specifically, they observed statistically significant reductions in catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione (GSH) enzyme activities, coupled with an increase in MDA levels.â€
- The sublethal effects of thiamethoxam in a study of Oreochromis niloticus reveal “dose-dependent increases in serum ALT, AST, ALP, BUN, urea, uric acid, creatinine, and cortisol, alongside decreased albumin after a 7-day exposure. Kidney tissue exhibited significantly reduced glutathione (GSH) and heat shock protein 70 (HSP70) levels, while thiobarbituric acid reactive substances (TBARS) surged, indicating heightened oxidative stress.â€
- “TMX significantly weakened the immune response in fish, as evidenced by reduced serum lysozyme activity, respiratory burst activity, and phagocytosis, alongside a notable increase in pro-inflammatory interleukins (IL-1β and IL-6).†Nile tilapia exposed to thiamethoxam also “has led to liver damage, including congestion of central veins, diffuse hydropic degeneration of hepatocytes, activation in melanomacrophage centers (MMCs), and vacuolation of hepatocytes.†(See study here.)
- A study of Clarias batrachus exposed to sublethal doses of thiamethoxam reports a significant increase in serum protein, which can be an indicator of various physiological changes, including immune responses, nutritional status, and chronic stress.
- A significant increase in ALT and AST enzyme levels is observed in Nile tilapia exposed to imidacloprid.
- Two studies (see here and here) “reported elevated creatinine levels in fish exposed to thiamethoxam and imidacloprid pesticides.â€
- A study “reported increased urea levels in Labeo rohita exposed to thiamethoxam at concentrations ranging from 0 to 2.0 mg/L over 120 hours.â€
- “[E]levated urea levels may be associated with gill dysfunction, as observed histopathologically in Nile tilapia, and increased creatinine may result from muscle damage or glomerular disorders, renal tissue breakdown, or decreased kidney filtration.†(See studies here and here.)
- Rainbow trout exposed to thiamethoxam “showed decreased SOD, CAT, and glutathione peroxidase (GPx) levels, along with increased MDA and myeloperoxidase (MPO) levels.†(See here.)
- Another study “reported decreased antioxidant enzyme activities (SOD, GPx, CAT, GST) and reduced glutathione (GSH) levels in fish exposed for 96 hours to thiamethoxam.â€
- Cultivated fish in thiamethoxam-treated paddies show evidence of weakened digestive systems. (See study here.)
- “[H]istopathological changes, such as muscle damage, degeneration, necrosis, edema [tissue swelling], and inflammation, observed in fish exposed to acetamiprid and imidacloprid, are likely caused by oxidative stress.†(See study here.)
- Another study “observed similar gill tissue damage in Nile tilapia exposed to imidacloprid, including epithelial lifting, fusion of secondary lamellae, dilated blood vessels, and increased blood flow.â€
- “[S]evere gill damage in Nile tilapia exposed to acetamiprid and thiamethoxam, including swelling, increased cell proliferation, bleeding, and fusion of the secondary lamellae [projections in gills]†is noted in various studies. (See here and here.)
- Imidacloprid “exhibited cell damage, cytoplasmic vacuolations, and loss of cell structure in Nile tilapia liver.†(See study here.)
The above findings highlight the vast array of effects on fish health from neonicotinoid exposure that consequently affect human health when contaminated fish are consumed. As a result of their study, the researchers conclude, “These results underscore the toxic impact of thiamethoxam on aquatic organisms, emphasizing the need for stringent monitoring and regulatory measures to protect aquatic ecosystems.â€
Regulatory processes fail to safeguard human health and the environment. (See extensive coverage here.) As a solution, Beyond Pesticides advocates eliminating petrochemical pesticide and fertilizer use because of their threat to all organisms and their ecosystems. Instead, Beyond Pesticides urges the adoption of organic land management in order to create a world free of toxic chemicals.
Learn more about the health and environmental benefits of organic (see here and here) and stay informed with the Daily News Blog, which offers information on the hazards of pesticides, pesticide regulation and policy, pesticide alternatives, and cutting-edge science. Sign up now to get our Action of the Week and Weekly News Updates delivered right to your inbox in order to take action and stay engaged.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Mansour, A. et al. (2025) Exploring the toxicity of thiamethoxam on aquatic ecosystems: an overarching assessment of biochemical parameters, genotoxicity, fish flesh residues and histological alterations in Nile tilapia, Environmental Pollutants and Bioavailability. Available at: https://www.tandfonline.com/doi/full/10.1080/26395940.2025.2493050.
