A comparative study of the toxicity of mercury dichloride and methylmercury, assayed by the Frog Embryo Teratogenesis Assay--Xenopus (FETAX)

Toxicogenomic signatures associated with methylmercury induced developmental toxicity in the zebrafish embryos

Methylmercury (MeHg) is a toxicant with adverse effects on embryogenesis from fish to man. The developmental outcomes of MeHg are well understood, but molecular understanding of toxicity is rather limited. We performed here a genome-wide transcriptional analyses of 6, 30, and 50 μg/L MeHg exposed zebrafish embryos from 4 to 72 h post-fertilization (hpf) using RNA-sequencing and microarray, and conducted a systematical comparison of MeHg-induced transcriptomic responses reported in this and our previous studies. We observed MeHg significantly to disrupt expression of 1050, 1931, and 2996 genes, respectively including gene ontologies in terms of visual and sensory perception, phototransduction, ferroptosis, and GABAergic synapse. Significantly altered genes were associated with ontology categorized into metabolism, such as fatty acid, amino acid, and glutathione metabolism across all experiments. Expression of genes involved in Wnt, Shh, and Notch signaling pathways previously demonstrated to be crucial for development was changed at varying levels dependent on exposure concentrations and durations. Our findings show MeHg significantly to affect expression of genes associated with tissue and/or organs developmental processing including eye, lateral line, fins, and brain, especially in embryos exposed to 6 μg/L, which did not cause obviously toxic effects on zebrafish embryos. We obtain 21 genes being significantly altered by MeHg in a concentration and stage independent manner, and might be served as signatures for developmental toxicity and/or teratogenic effects.

Evaluation of the developmental toxicity of solvents, metals, drugs, and industrial chemicals using a freshwater snail (Biomphalaria glabrata) assay

A freshwater snail assay was employed to assess the embryotoxicity of solvents including acetone, methanol, ethanol, isopropanol, dimethyl-sulfoxide, glycerin, metals/metalloids including mercuric chloride (HgCl₂), cadmium chloride (CdCl₂,), antimony salts Sb⁺³ and Sb⁺⁵, drugs including colchicine, hydroxyurea, cyclophosphamide, an industrial chemical sodium azide (SA), an anionic surfactant dodecyl sodium sulfate-(DSS), H₂O₂ and sodium chloride (NaCl). The assay consists of exposing Biomphalaria glabrata egg masses (EM) to the substances for 96-hr and following up embryo/snail development for lethality, abnormal morphology (teratogenicity), and day of hatching up to day 10 or 14 after spawning. Based upon concentration-response relationships, LC_50%s (embryolethality), EC_50%s (teratogenicity) and IC_50%s (hatching retardation) and 95%CIs were determined for tested chemicals. The LOECs indicated that HgCl₂ (37 nM) and CdCl₂ (140 nM) are potent embryotoxic agents in snails. Teratogenic indices (TI = LC₅₀/EC₅₀) for almost all tested chemicals were lower than or close to unity suggesting that these compounds were not teratogenic in this assay. The snail assay may be adequately performed in a cost-effective standardized protocol which enables testing a number of environmental chemicals over a broad concentration range. The snail assay needs to undergo further validation to be recognized for an internationally harmonized hazard identification in ecotoxicity risk assessment.

Evaluating Multiple Biochemical Markers in Xenopus laevis Tadpoles Exposed to the Pesticides Thiacloprid and Trifloxystrobin in Single and Mixed Forms

Pesticide exposure is thought to be one of the common reasons for the decline in amphibian populations, a phenomenon that is a major threat to global biodiversity. Although the single effects of pesticides on amphibians have been well studied, the effects of mixtures are not well known. The present study aimed to evaluate the acute toxicity of the insecticide thiacloprid and the fungicide trifloxystrobin on early developmental stages of Xenopus laevis using various biochemical markers (glutathione S-transferase, glutathione reductase, acetylcholinesterase, carboxylesterase, glutathione peroxidase, catalase, alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, Na⁺ K⁺ -adenosine triphosphatase [ATPase], Ca²⁺ -ATPase, Mg²⁺ -ATPase, and total ATPase). The median lethal concentrations (LC50s) of thiacloprid and trifloxystrobin were determined to be 3.41 and 0.09 mg a.i. L^-1 , respectively. Tadpoles were exposed to the LC50, LC50/2, LC50/10, LC50/20, LC50/50, and LC50/100 of these pesticides. Both pesticides significantly affected (inhibited/activated) the biomarkers even at low concentrations. The pesticides showed a synergistic effect when applied as a mixture and altered the biomarkers more than when applied individually. In conclusion, we can assume that tadpoles are threatened by these pesticides even at environmentally relevant concentrations. Our findings provide important data to guide management of the ecotoxicological effects of these pesticides on nontarget amphibians.  Environ Toxicol Chem 2021;40:2846-2860. © 2021 SETAC.

Lead, Mercury and Cadmium in Fish and Shellfish from the Indian Ocean and Red Sea (African Countries): Public Health Challenges

The main aim of this review was to assess the incidence of Pb, Hg and Cd in seafood from African countries on the Indian and the Red Sea coasts and the level of their monitoring and control, where the direct consumption of seafood without quality control are frequently due to the poverty in many African countries. Some seafood from African Indian and the Red Sea coasts such as mollusks and fishes have presented Cd, Pb and Hg concentrations higher than permitted limit by FAOUN/EU regulations, indicating a possible threat to public health. Thus, the operationalization of the heavy metals (HM) monitoring and control is strongly recommended since these countries have laboratories with minimal conditions for HM analysis.

Multidirectional Changes in Parameters related to Sulfur Metabolism in Frog Tissues exposed to Heavy Metal-related Stress

The investigations showed changes of the cystathionine γ-lyase (CTH), 3-mercaptopyruvate sulfurtransferase (MPST) and rhodanese (TST) activity and gene expression in the brain, heart, liver, kidney, skeletal muscles and testes in frogs Pelophylax ridibundus, Xenopus laevis and Xenopus tropicalis in response to Pb²⁺, Hg²⁺ and Cd²⁺ stress. The results were analyzed jointly with changes in the expression of selected antioxidant enzymes (cytoplasmic and mitochondrial superoxide dismutase, glutathione peroxidase, catalase and thioredoxin reducatase) and with the level of malondialdehyde (a product of lipid peroxidation). The obtained results allowed for confirming the role of sulfurtransferases in the antioxidant protection of tissues exposed to heavy metal ions. Our results revealed different transcriptional responses of the investigated tissues to each of the examined heavy metals. The CTH, MPST and TST genes might be regarded as heavy metal stress-responsive. The CTH gene expression up-regulation was confirmed in the liver (Pb²⁺, Hg²⁺, Cd²⁺) and skeletal muscle (Hg²⁺), MPST in the brain (Pb²⁺, Hg²⁺), kidney (Pb²⁺, Cd²⁺), skeletal muscle (Pb²⁺, Hg²⁺,Cd²⁺) and TST in the brain (Pb²⁺) and kidney (Pb²⁺, Hg²⁺, Cd²⁺). Lead, mercury and cadmium toxicity was demonstrated to affect the glutathione (GSH) and cysteine levels, the concentration ratio of reduced to oxidized glutathione ([GSH]/[GSSG]) and the level of sulfane sulfur-containing compounds, which in case of enhanced reactive oxygen species generation can reveal their antioxidative properties. The present report is the first to widely describe the role of the sulfane sulfur/H₂S generating enzymes and the cysteine/glutathione system in Pb²⁺, Hg²⁺ and Cd²⁺ stress in various frog tissues, and to explore the mechanisms mediating heavy metal-related stress.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Visualization of mercury(ii) accumulation in vivo using bioluminescence imaging with a highly selective probe

Mercury is a highly toxic environmental pollutant that negatively affects human health. Thus, an in vivo method for noninvasive imaging of mercury(ii) and visualization of its accumulation within living systems would be advantageous. Herein, we describe a reaction-based bioluminescent probe for detection of mercury(ii) in vitro and accumulation in vivo. The application of this probe would help to shed light on the intricate contributions of mercury(ii) to various physiological and pathological processes.

Chronic effects of mercury on Bufo gargarizans larvae: Thyroid disruption, liver damage, oxidative stress and lipid metabolism disorder

Mercury is severely detrimental to organisms and is ubiquitous in both terrestrial and aquatic ecosystems. In the present study, we examined the effects of chronic mercury (Hg) exposure on metamorphosis, body size, thyroid microstructures, liver microstructural and ultrastructural features, and transcript levels of genes associated with lipid metabolism, oxidative stress and thyroid hormones signaling pathways of Chinese toad (Bufo gargarizans) tadpoles. Tadpoles were exposed to mercury concentrations at 0, 6, 12, 18, 24 and 30 µg/L from Gosner stage 26-42 of metamorphic climax. The present results showed that high dose mercury (24 and 30 µg/L) decelerated metamorphosis rate and inhibited body size of B. gargarizans larvae. Histological examinations have clearly exhibited that high mercury concentrations caused thyroid gland and liver damages. Moreover, degeneration and disintegration of hepatocytes, mitochondrial vacuolation, and endoplasmic reticulum breakdown were visible in the ultrastructure of liver after high dose mercury treatment. Furthermore, the larvae exposed to high dose mercury demonstrated a significant decrease in type II iodothyronine deiodinase (Dio2) and thyroid hormone receptor α and β (TRα and TRβ) mRNA levels. Transcript level of superoxide dismutase (SOD) and heat shock protein (HSP) were significantly up regulated in larvae exposed to high dose mercury, while transcript level of phospholipid hydroperoxide glutathione peroxidase (PHGPx) was significantly down regulated. Moreover, exposure to high dose mercury significantly down regulated mRNA expression of carnitine palmitoyltransferase (CPT), sterol carrier protein (SCP), acyl-CoA oxidase (ACOX) and peroxisome proliferator-activated receptor α (PPAPα), but significantly up regulated mRNA expression of fatty acid elongase (FAE), fatty acid synthetase (FAS) and Acetyl CoA Carboxylase (ACC). Therefore, we conclude that high dose mercury induced thyroid function disruption, liver oxidative stress and lipid metabolism disorder by damaging thyroid and liver cell structures and altering the expression levels of relevant genes.

Determination of metals and pharmaceutical compounds released in hospital wastewater from Toluca, Mexico, and evaluation of their toxic impact

Due to the activities inherent to medical care units, the hospital effluent released contains diverse contaminants such as tensoactives, disinfectants, metals, pharmaceutical products and chemical reagents, which are potentially toxic to the environment since they receive no treatment or are not effectively removed by such treatment before entering the drain. They are incorporated into municipal wastewater, eventually entering water bodies where they can have harmful effects on organisms and can result in ecological damage. To determine the toxicological risk induced by this type of eflluents, eight metals and 11 pharmaceuticals were quantified, in effluent from a hospital. Developmental effects, teratogenesis and oxidative stress induction were evaluated in two bioindicator species: Xenopus laevis and Lithobates catesbeianus. FETAX (frog embryo teratogenesis assay-Xenopus) was used to obtain the median lethal concentration (LC₅₀), effective concentration inducing 50% malformation (EC₅₀), teratogenic index (TI), minimum concentration to inhibit growth (MCIG), and the types of malformation induced. Twenty oocytes in midblastula transition were exposed to six concentrations of effluent (0.1, 0.3, 0.5, 0.7, 0.9, 1%) and negative and positive (6-aminonicotinamide) controls. After 96 h of exposure, diverse biomarkers of oxidative damage were evaluated: hydroperoxide content, lipid peroxidation, protein carbonyl content, and the antioxidant enzymes superoxide dismutase and catalase. TI was 3.8 in X. laevis and 4.0 in L. catesbeianus, both exceed the value in the FETAX protocol (1.2), indicating that this effluent is teratogenic to both species. Growth inhibition was induced as well as diverse malformation including microcephaly, cardiac and facial edema, eye malformations, and notochord, tail, fin and gut damage. Significant differences relative to the control group were observed in both species with all biomarkers. This hospital effluent contains contaminants which represents a toxic risk, since these substances are teratogenic to the bioindicators used. The mechanism of damage induction may be associated with oxidative stress.

Methylmercury exposure during early Xenopus laevis development affects cell proliferation and death but not neural progenitor specification

Methylmercury (MeHg) is a widespread environmental toxin that preferentially and adversely affects developing organisms. To investigate the impact of MeHg toxicity on the formation of the vertebrate nervous system at physiologically relevant concentrations, we designed a graded phenotype scale for evaluating Xenopus laevis embryos exposed to MeHg in solution. Embryos displayed a range of abnormalities in response to MeHg, particularly in brain development, which is influenced by both MeHg concentration and the number of embryos per ml of exposure solution. A TC50 of ~50μg/l and LC50 of ~100μg/l were found when maintaining embryos at a density of one per ml, and both increased with increasing embryo density. In situ hybridization and microarray analysis showed no significant change in expression of early neural patterning genes including sox2, en2, or delta; however a noticeable decrease was observed in the terminal neural differentiation genes GAD and xGAT, but not xVGlut. PCNA, a marker for proliferating cells, was negatively correlated with MeHg dose, with a significant reduction in cell number in the forebrain and spinal cord of exposed embryos by tadpole stages. Conversely, the number of apoptotic cells in neural regions detected by a TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay was significantly increased. These results provide evidence that disruption of embryonic neural development by MeHg may not be directly due to a loss of neural progenitor specification and gene transcription, but to a more general decrease in cell proliferation and increase in cell death throughout the developing nervous system.

Bioaccumulation and maternal transfer of mercury and selenium in amphibians

Amphibian population declines have been documented worldwide and environmental contaminants are believed to contribute to some declines. Maternal transfer of bioaccumulated contaminants to offspring may be an important and overlooked mechanism of impaired reproductive success that affects amphibian populations. Mercury (Hg) is of particular concern due to its ubiquity in the environment, known toxicity to other wildlife, and complex relationships with other elements, such as selenium (Se). The objectives of the present study were to describe the relationships between total Hg (THg), methlymercury (MMHg), and Se in three amphibian species (Plethodon cinereus, Eurycea bislineata cirrigera, and Bufo americanus) along a Hg-polluted river and floodplain, and to determine if B. americanus maternally transfers Hg and Se to its eggs in a tissue residue-dependent manner. Total Hg and MMHg concentrations in all species spanned two orders of magnitude between the reference and contaminated areas, while Se concentrations were generally low in all species at both sites. Strong positive relationships between THg and MMHg in tissues of all species were observed throughout. Both Hg and Se were maternally transferred from females to eggs in B. americanus, but the percentage of the females' Hg body burden transferred to eggs was low compared with Se. In addition, Hg concentrations appeared to positively influence the amount of Se transferred from female to eggs. The present study is the first to confirm a correlation between Hg concentrations in female carcass and eggs in amphibians and among the first to describe co-transference of Se and Hg in an anamniotic vertebrate. The results suggest future work is needed to determine whether maternal transfer of Hg has transgenerational implications for amphibian progeny.

Developmental selenomethionine and methylmercury exposures affect zebrafish learning

Methylmercury (MeHg) is a ubiquitous environmental pollutant and has been shown to affect learning in vertebrates following relatively low exposures. Zebrafish were used to model long-term learning deficits after developmental MeHg exposure. Selenomethionine (SeMet) co-exposure was used to evaluate its role in neuroprotection. Embryos were exposed from 2 to 24h post fertilization to (1) MeHg without SeMet, (2) SeMet without MeHg and (3) in combination of MeHg and SeMet. In case (1), the levels of MeHg were 0.00, 0.01, 0.03, 0.06, 0.10, and 0.30 microM. In case (2), the levels of SeMet were 0.00. 0.03, 0.06, 0.10, and 0.30 microM. In case (3), co-exposure levels of (MeHg, SeMet) were (0.03, 0.03), (0.03, 0.06), (0.03, 0.10), (0.03, 0.30), (0.10, 0.03), (0.10, 0.06), (0.10, 0.10), and (0.10, 0.30) microM. Learning functions were tested in individual adults, 4 months after developmental exposure using a spatial alternation paradigm with food delivery on alternating sides of the aquarium. Low levels of MeHg (<0.1 microM) exposure delayed learning in treated fish; fish exposed to higher MeHg levels were unable to learn the task; SeMet co-exposure did not prevent this deficit. These data are consistent with findings in laboratory rodents. The dorsal and lateral telencephalon are the primary brain regions in fish involved in spatial learning and memory. Adult telencephalon cell body density decreased significantly at all MeHg exposures >0.01 microM MeHg. SeMet co-exposure ameliorated but did not prevent changes in telencephalon cell body density. In summary, MeHg affected both learning and brain structure, but SeMet only partially reversed the latter.

Selenomethionine reduces visual deficits due to developmental methylmercury exposures

Developmental exposures to methylmercury (MeHg) have life-long behavioral effects. Many micronutrients, including selenium, are involved in cellular defenses against oxidative stress and may reduce the severity of MeHg-induced deficits. Zebrafish embryos (<4 h post fertilization, hpf) were exposed to combinations of 0.0-0.30 microM MeHg and/or selenomethionine (SeMet) until 24 hpf then placed in clean medium. Fish were tested as adults under low light conditions ( approximately 60 microW/m(2)) for visual responses to a rotating black bar. Dose-dependent responses to MeHg exposure were evident (ANOVA, P<0.001) as evidenced by reduced responsiveness, whereas SeMet did not induce deficits except at 0.3 microM. Ratios of SeMet:MeHg of 1:1 or 1:3 resulted in responses that were indistinguishable from controls (ANOVA, P<0.001). No gross histopathologies were observed (H&E stain) in the retina or optic tectum at any MeHg concentration. Whole-cell, voltage-gated, depolarization-elicited outward K(+) currents of bipolar cells in intact retina of slices adult zebrafish were recorded and outward K(+) current amplitude was larger in bipolar cells of MeHg-treated fish. This was due to the intense response of cells expressing the delayed rectifying I(K) current; cells expressing the transient I(A) current displayed a slight trend for smaller amplitude among MeHg-treated fish. Developmental co-exposure to SeMet reduced but did not eliminate the increase in the MeHg-induced I(K) response, however, I(A) responses increased significantly over MeHg-treated fish to match control levels. Electrophysiological deficits parallel behavioral patterns in MeHg-treated fish, i.e., initial reactions to the rotating bar were followed by periods of inactivity and then a resumption of responses.

Comparative evaluation of the toxicity and genotoxicity of cadmium in amphibian larvae (Xenopus laevis and Pleurodeles waltl) using the comet assay and the micronucleus test

The toxic and genotoxic potential of Cadmium (CdCl(2)) were evaluated by the micronucleus test (MNT) and comet assay (CA) using amphibian larvae (Xenopus laevis and Pleurodeles waltl). Acute toxicity results showed that Cd is toxic to Xenopus larvae exposed from 2 to 50 mg/L and to Pleurodeles from 5 to 50 mg/L, depending on the nature of the water (reconstituted water containing mineral salts or mineral water MW (Volvic)). The MNT results obtained in MW showed that Cd (2 mg/L) is genotoxic to Xenopus, whereas it was not genotoxic to Pleurodeles at all concentrations tested. The CA established that the genotoxicity of Cd to Xenopus and Pleurodeles larvae depends on the concentration, the exposure times, and the comet parameters (Tail DNA, ETM, OTM, and TL). The CA and MNT results were compared for their ability to detect genotoxic effects, considering the concentrations of Cd applied and the exposure time. The CA showed Cd to be genotoxic from the first day of exposure. In amphibians, the CA appears to be a sensitive and suitable method for detecting genotoxicity such as that caused by Cd.

Molecular cloning and expression of alpha2,8-sialyltransferase (ST8Sia I, GD3 Synthase) in Xenopus

GD3, a minor ganglioside in most normal tissues, is involved in important biological events and its expression could increase in pathological conditions. Organism integrity requires a tight balance between the anabolic and catabolic processes, thus it is important to control the intracellular expression of those "key" enzymes, which act at the "branching point" of ganglioside metabolism; one of these is the GD3-synthase (ST8Sia I). In this paper, we report the sequences of two ST8Sia I mRNAs found in Xenopus laevis and their genomic organization; the canonical form resulted constituted of 5 exons and 4 introns, while the "short" mRNA lacks of the exon 2. The expression of the two ST8Sia I mRNAs during embryo development and their tissue distribution in adult animals showed the single or simultaneous presence of the two forms. Experiments of in vitro expression and evaluation of enzymatic activity of the two hypothetical proteins turned out to be ST8Sia I. In the end, considering the growing interest toward the specie Xenopus tropicalis, due to its diploid genome that render it more suitable for genetic studies, we also cloned X. tropicalis ST8Sia I.

Assessment of Lead Ecotoxicity in Water using the Amphibian Larvae (Xenopus laevis) and Preliminary Study of its Immobilization in Meat and Bone Meal Combustion Residues

Lead (Pb) is a major chemical pollutant of the environment. It has been associated with human activities for the last 6000 years. Quite rightly, it remains a public health concern today. The present investigation evaluates the toxic potential of Pb in larvae of the toad Xenopus laevis after 12 days exposure in lab conditions. Acute toxicity, genotoxicity and Pb bioaccumulation were analyzed. The genotoxic effects were analyzed in the circulating blood from the levels of micronucleus induction according to the French standard micronucleus assay (AFNOR 2000 Association française de normalization. Norme NFT 90-325. Qualité de l'Eau. Evaluation de la génotoxicité au moyen de larves d'amphibien (Xenopus laevis, Pleurodeles waltl)). Lead bioaccumulation was analyzed in the liver of larvae at the end of exposure. Moreover, the toxic potential of lead, in aquatic media, was investigated in the presence of meat and bone meal combustion residues (MBMCR) known to be rich in phosphates and a potential immobiliser of lead. Previously, acute toxicity and genotoxicity of MBMCR alone were evaluated using Xenopus larvae. The results obtained in the present study demonstrated: (i) that lead is acutely toxic and genotoxic to amphibian larvae from 1 mg Pb/l and its bioaccumulation is significant in the liver of larvae from the lowest concentration of exposure (1 microg Pb/l), (ii) MBMCR were not acutely toxic nor genotoxic in Xenopus larvae, (iii) lead in presence of MBMCR induced inhibition or reduction of the toxic and genotoxic potential of lead in water at concentrations that do not exceed the capacity of MBMCR of Pb-binding (iv) Pb accumulation in larvae exposed to lead with MBMCR in water was lower than Pb-accumulation in larvae exposed to lead alone except at the concentration of 0.01 mg Pb/l suggesting complex mechanisms of MBMCR interaction in organisms. The results confirm the high toxicity and genotoxicity of lead in the aquatic compartment and suggest the potential utility of MBMCR for use in remediation.

Influence of metal ions on gene expression of BALB 3T3 fibroblasts

It is widely recognized that metal compounds may modify gene expression. In this context, we have searched for genes whose expression may be affected by cadmium and platinum ions within the context of a cell culture system. Cadmium is well known for its carcinogenic potential while platinum is destined to become more and more interesting because of its increasing use in the automotive industries. By applying differential display to cultures of mouse fibroblast, we have identified two transcripts (acute lymphoblastic leukemia-1, All-1, and a novel gene named metal-responsive gene, MERE-1) that were responsive to platinum and cadmium ions. Moreover, further experiments with a panel of metal compounds have shown that MERE-1 was strongly induced also by La(NO3)2 and Cr(NO3)3, and to a lesser extent, by Na2CrO4 and (NH4)2TeCl6.

Environmental exposure to mercury and its toxicopathologic implications for public health

Mercury is a toxic and hazardous metal that occurs naturally in the earth's crust. Natural phenomena such as erosion and volcanic eruptions, and anthropogenic activities like metal smelting and industrial production and use may lead to substantial contamination of the environment with mercury. Through consumption of mercury in food, the populations of many areas, particularly in the developing world, have been confronted with catastrophic outbreaks of mercury-induced diseases and mortality. Countries such as Japan, Iraq, Ghana, the Seychelles, and the Faroe Islands have faced such epidemics, which have unraveled the insidious and debilitating nature of mercury poisoning. Its creeping neurotoxicity is highly devastating, particularly in the central and peripheral nervous systems of children. Central nervous system defects and erethism as well as arrythmias, cardiomyopathies, and kidney damage have been associated with mercury exposure. Necrotizing bronchitis and pneumonitis arising from inhalation of mercury vapor can result in respiratory failure. Mercury is also considered a potent immunostimulant and -suppressant, depending on exposure dose and individual susceptibility, producing a number of pathologic sequelae including lymphoproliferation, hypergammaglobulinemia, and total systemic hyper- and hyporeactivities. In this review we discuss the sources of mercury and the potential for human exposure; its biogeochemical cycling in the environment; its systemic, immunotoxic, genotoxic/carcinogenic, and teratogenic health effects; and the dietary influences on its toxicity; as well as the important considerations in risk assessment and management of mercury poisoning.