Adverse effects of sodium fluoride exposure on the development, thyroid morphology and disease resistance in rainbow trout (Oncorhynchus mykiss) embryos and larvae.

Endocrine disruptors (EDs) are substances that pose a threat to both human and animal health by interacting with the endocrine system. They are widely present in the environment due to their extensive use in agricultural, industrial, and domestic applications. These substances can manipulate the endocrine system and lead to developmental and reproductive abnormalities in animals and humans (Casals-Casas & Desvergne, 2011). In addition to disrupting endocrine functions, EDs can also affect the holobiont, development, and the immune system (IS), raising concerns at scientific, societal and regulatory levels.

Historically, research has first focused on EDs that interfere with sex steroid hormone metabolism and reproduction. Only recently, the disruption of the thyroid hormone system (THS) has gained significant attention due to the crucial role of the thyroid cascade in various physiological processes across vertebrates (Dang et al., 2021). The THS is involved in several functions in physiology, metabolism and sex determination (Yuan et al., 2021), as well as multiple developmental processes such as metamorphosis, craniofacial, cardiac (Yamakawa et al., 2021), brain and eye development, as well as swim bladder inflation (Baumann et al., 2019; Gölz et al., 2022). It also plays a key role in regulating the immune system (IS), making it a potential target for EDs (Hasani & Baumann, 2025). Indeed, thyroid hormones (THs) mediate crosstalk between endocrine and immune systems, exerting immunomodulatory effects, while the immune system also contributes to the TH regulation (Montesinos & Pellizas, 2019). THs, including triiodothyronine (T3) and thyroxine (T4), influence a broad range of cellular and humoral immune functions such as phagocytosis, oxidative burst, chemotaxis and cytokine synthesis, as well as differentiation and maturation of macrophages and lymphocytes (Wenzek et al., 2022).

While THS-IS interactions are well documented in mammals (Klein, 2006; Wenzek et al, 2022), they remain poorly explored in fish. Several studies suggest a toxicologically relevant link between THS and IS disruption (Birgersson et al., 2021; Quesada-Garcia et al., 2014; Quesada-Garcia et al., 2016), reporting adverse effects of THS-disrupting chemicals (THSDCs) on the IS, including increased susceptibility to pathogens. As observed with estrogenic EDs and sexual differentiation, early development may represent a critical window during which THS disruptions can impact IS maturation. However, only a few studies have demonstrated persistent immune dysfunction later in life or across generations (Kernen et al., 2020).

Additionally, there is growing evidence that environmental exposure can induce long-lasting effects on individuals and even on unexposed descendants via epigenetic mechanisms, thereby affecting population viability, fitness, and evolutionary trajectories (Pierron et al., 2021). Since epigenetic marks play a central role in differentiation, cellular memory and immunity, epigenetics must be considered to fully understand the complexity of thyroid hormone system-disrupting chemicals (THSDC) effects (Ballestar et al., 2020). Disruptions of the endocrine and immune systems may also lead to alterations in morphological development and behavior in exposed fish, affecting stress responses, exploratory abilities, and social interactions – traits essential for fitness and survival in natural populations (Le Du-Carrée et al., 2021b).

Among EDs, sodium fluoride (NaF) is a chemical compound frequently detected in the environment. It is used in the manufacture of various pesticides and cosmetics, and is also present in bottled waters. In 2021, ANSES classified NaF among the 16 priority substances in the SNPE2 (stratégie nationale sur les perturbateurs endocriniens 2) framework due to its suspected effects on development and reproduction in humans and animals (Chai et al., 2016; Jianjie et al., 2016). Under Article 57 of the REACH regulation, EDs are considered substances of very high concern (SVHC), which classifies them according to criteria such as persistence (P), bioaccumulation (B), toxicity (T), or very high persistence and bioaccumulation (vPvB).

Although NaF is generally found in low concentrations, values ranging from 1.5 to 20 mg F^–/L have been reported in polluted waters (WHO, 2004; Di Paola et al., 2022; Alarcon-Herrera et al., 2013; Staub et al., 2025; Shaji et al., 2024). The World Health Organization recommends an acceptable range of 0.7 to 1.0 mg/L for drinking water (WHO, 2004), whereas the European Council Directive 98/83/EC sets a maximum of 1.5 mg/L.

NaF is known to inhibit phosphorylation-related activities, which are crucial for numerous cellular functions and may affect immune signalling pathways. It is also suspected to disrupt the THS (Jianjie et al., 2016; Lu et al., 2022) and influence IS development through its effects on the liver (Wang et al., 2022). Fluoride (F^–), a monoatomic anion ubiquitous in water, soil, air, plants, and animals, can induce genotoxic, neurotoxic, and oxidative damage (Dey Bhowmik & Chattopadhyay, 2019). Fluoride enters the body via drinking water, food, cosmetics and medicines (Kabir et al., 2019) and is easily absorbed by the gastrointestinal tract (Jha et al., 2011). Although widely promoted for its benefits in dental health, the use and toxicity of fluoride remain controversial, raising persistent public health concerns (Guth et al., 2021; Petersen & Lennon, 2004; Vieira, 2021). Demonstrated effects include impacts on teeth, bones, kidneys, liver, pancreas, as well as the nervous, cardiovascular, and reproductive systems, with particular concern regarding parental and embryonic exposure (Wei et al., 2019; Wei et al., 2024). Generally, the effects of fluoride on human health and the environment are chronic and long-term (Wei et al., 2024).

This project aims to investigate the impact of NaF exposure on the overall health of rainbow trout (Oncorhynchus mykiss) with a special focus on the THS and the IS. Early life-stages were exposed to five concentrations ranging from 0.5 mg/L of F^– (the lower acceptable limit for drinking water) to 15 mg/L of F^– (high environmental concentration) for 15 or 23 days. We examined development, behavior, thyroid follicles and eye development, as well as resistance to Infectious Hematopoietic Necrosis Virus (IHNV), a pathogenic virus of major importance in salmonid farming.