Methylmercury poisoning in the Iraqi suckling infant: a longitudinal study over five years

Effects of low doses of methylmercury (MeHg) exposure on definitive endoderm cell differentiation in human embryonic stem cells

Fetal development is one of the most sensitive windows to methylmercury (MeHg) toxicity. Laboratory and epidemiological studies have shown a dose-response relationship between fetal MeHg exposure and neuro performance in different life stages from infants to adults. In addition, MeHg exposure has been reported to be associated with disorders in endoderm-derived organs, such as morphological changes in liver cells and pancreatic cell dysfunctions. However, the mechanisms of the effects of MeHg on non-neuronal organs or systems, especially during the early development of endoderm-derived organs, remain unclear. Here we determined the effects of low concentrations of MeHg exposure during the differentiation of definitive endoderm (DE) cells from human embryonic stem cells (hESCs). hESCs were exposed to MeHg (0, 10, 100, and 200 nM) that covers the range of Hg concentrations typically found in human maternal blood during DE cell induction. Transcriptomic analysis showed that sub-lethal doses of MeHg exposure could alter global gene expression patterns during hESC to DE cell differentiation, leading to increased expression of endodermal genes/proteins and the over-promotion of endodermal fate, mainly through disrupting calcium homeostasis and generating ROS. Bioinformatic analysis results suggested that MeHg exerts its developmental toxicity mainly by disrupting ribosome biogenesis during early cell lineage differentiation. This disruption could lead to aberrant growth or dysfunctions of the developing endoderm-derived organs, and it may be the underlying mechanism for the observed congenital diseases later in life. Based on the results, we proposed an adverse outcome pathway for the effects of MeHg exposure during human embryonic stem cells to definitive endoderm differentiation.

Breast milk contribution to tissue mercury levels in rat pups examined by cross-fostering at birth

The developing perinatal brain is vulnerable to methylmercury (MeHg) exposure. The contribution of breast milk to tissue MeHg levels in offspring is a significant public health concern because breast milk contains a certain amount of MeHg. Here, the contribution of MeHg transferred via breast milk to the Hg levels in the tissues of pups (Wistar rats) was investigated. Mated maternal rats were fed a MeHg (2 ppm)-supplemented or a control diet during pregnancy. Following parturition, male neonates from each group were cross-fostered between exposed or control dams, and they were further raised by dams fed a MeHg-supplemented diet or a control diet during lactation. Consequently, we evaluated three pup groups, which were raised by dams exposed to MeHg during pregnancy (P pups), lactation (L pups), or pregnancy and lactation (PL pups). Total mercury (THg) concentrations in the tissues of the offspring were measured at birth (postnatal day 0 [PD0]), during lactation (PD6, PD12, and PD19), and after weaning (PD29 and PD36). Blood and brain THg levels in the P and PL pups declined dramatically during lactation, however, there were no considerable differences between the two groups at PD6 and PD12. In contrast, blood and brain THg levels in the L pups increased slightly during lactation. The increase in the THg levels in the blood and brain of L pups at PD12 were approximately 3.3% and 1.5%, respectively, compared to the corresponding THg levels in the neonates in the P and PL groups. Our results suggest that if the MeHg exposure level during pregnancy is not high enough to cause neuronal development defects in the fetus, the exposure via breast milk is not a significant concern.

Infants and mothers levels of mercury in breast milk, urine and hair, data from an artisanal and small-scale gold mining area in Kadoma / Zimbabwe

In Kadoma (Zimbabwe) gold is extracted from ore based on a simple technology using mercury. Women are exposed whilst living in an exposed area, or even more working directly with mercury. Breast fed infants are exposed via mercury contaminated milk and the general environmental mercury exposure. The aim of the study was to measure infants and mothers levels of mercury in urine and hair, as well as in breast milk. In 2006, an environmental epidemiological field study with 120 mother - child pairs was conducted. A non exposed control group (n = 42) was compared with a medium exposed group (n = 51) and a high exposed group (occupational exposure, n = 27). Urine and hair samples from mother and infants plus breast milk samples were analyzed for total mercury. 120 breast milk samples were analyzed, median (maximum) concentrations [μg Hg/l] were (i) control group < 0.50 (1.55), (ii) medium exposed group 1.10 (10.48), (iii) high exposed group 1.20 (24.80) (p < 0,001). Urine and hair results were distributed similarly for infants and mothers, with higher levels according to the exposure subgroup. All specimen results correlated very well with another, indicating there is a pathway between breast milk and infant's level of mercury. The daily uptake of mercury via breast milk was calculated. The reference dose of 0.3°[μg Hg/kg BW/d] was used to compare the burden of the infants. No infant from the control group, but 17.6% of the medium and 18.5% of the high exposed group were above the reference dose. Mercury in breast milk is generally higher in artisanal and small scale gold mining areas. Breast fed infants were burdened with toxic mercury via breast milk and via the general environment, both are important public health issues.

Early Developmental Low-Dose Methylmercury Exposure Alters Learning and Memory in Periadolescent but Not Young Adult Rats

Few studies have assessed the effects of developmental methylmercury (MeHg) exposure on learning and memory at different ages. The possibility of the amelioration or worsening of the effects has not been sufficiently investigated. This study aimed to assess whether low-dose MeHg exposure in utero and during suckling induces differential disturbances in learning and memory of periadolescent and young adult rats. Four experimental groups of pregnant Sprague-Dawley rats were orally exposed to MeHg or vehicle from gestational day 5 to weaning: (1) control (vehicle), (2) 250 μg/kg/day MeHg, (3) 500 μg/kg/day MeHg, and (4) vehicle, and treated on the test day with MK-801 (0.15 mg/kg i.p.), an antagonist of the N-methyl D-aspartate receptor. The effects were evaluated in male offspring through the open field test, object recognition test, Morris water maze, and conditioned taste aversion. For each test and stage assessed, different groups of animals were used. MeHg exposure, in a dose-dependent manner, disrupted exploratory behaviour, recognition memory, spatial learning, and acquisition of aversive memories in periadolescent rats, but alterations were not observed in littermates tested in young adulthood. These results suggest that developmental low-dose exposure to MeHg induces age-dependent detrimental effects. The relevance of decreasing exposure to MeHg in humans remains to be determined.

Assessing pre/post-weaning neurobehavioral development for perinatal exposure to low doses of methylmercury

Fetuses and neonates are known to be high-risk groups for Methylmercury (MeHg) exposure. MeHg can be transferred to the fetus through the placenta and to newborn offspring through breast milk. The aim of the present study was to investigate the neurotoxic effects of low doses of MeHg (1 and 5μg/mL in drinking water) administration, from gestational day 1 to postnatal day (PND) 21, on the neurobehavioral development of rats. The results showed that the no-observed-effect level of MeHg is somewhere in the range of 1-4μg/mL. Neurobehavioral development analysis revealed a delayed appearance of cliff drop and negative geotaxis reflexes in the 5μg/mL MeHg exposure group. Developmental exposure to MeHg affected locomotor activity functions for the females, but not for the males, implying that the female pups were more vulnerable than the male pups. All pups exposed to 5μg/mL of MeHg showed a significant deficit in motor coordination in the rotarod test compared with controls, and the highest accumulated concentrations of Hg were found in the cerebellum, followed by the hippocampus and cerebral cortex, indicating that the cerebellum is a possible target for MeHg toxicity. We demonstrated adverse effects of developmental exposure to MeHg associated with tissue concentrations very close to the current human body burden of this persistent and bioaccumulative compound.

Environmental contaminants and children's health: Cause for concern, time for action

The present paper provides an outline of the developmental and behavioural characteristics that make children, especially the fetus and young child, more vulnerable to contaminants than adults. The major categories of contaminants are briefly described. The evidence for their possible effects on neurobehavioural development; immune, endocrine and respiratory systems; childhood cancer based on research studies with animals; children exposed to catastrophic 'accidents' involving overdose exposures; and pregnant women and children from communities with high 'background' levels of contamination who participated in studies is reviewed. While the data are worrisome, especially for children living in northern and certain urban communities, much remains to be learned about possible subtle effects and the potential for long term effects of the current background contamination experienced by the majority of Canadian children before its significance to their health can be fully evaluated. The present regulatory processes, which are based on risk assessment, are so cumbersome and costly that the great majority of chemicals in use have not been fully evaluated, and the ingenuity of new chemical production continually exceeds the capacity to test the new chemicals. Moreover, despite past insistence on scientific proof of adverse effects and safety, unanticipated effects have occurred that will threaten the sustainability of human life unless more effective control measures are taken to limit the release of toxic substances and persistent chemicals into the environment. Therefore, the shortcomings of risk assessment are discussed, and the precautionary principle, which is used in some countries and is proposed for use internationally as an alternative measure that may offer improved control for the future, is outlined. Finally, opportunities for physician action are suggested.

Systematic review of birth cohort studies in South East Asia and Eastern Mediterranean regions

BACKGROUND

Few longitudinal studies of children have taken place in the developing world, despite child mortality being concentrated there. This review summarises the methodologies and main outcomes of longitudinal studies of pre-school children (0 to 59 months) in the World Health Organization's South East Asia (SEA) and Eastern Mediterranean (EM) Regions.

METHODS

A systematic search of literature using pre-defined criteria revealed 7863 papers. After application of quality criteria, 120 studies were selected for analysis.

RESULTS

The search revealed 83 studies in the SEA region and 37 in the EM region, of which 92 were community-based and 8 facility-based. Objectives were diverse but topics included growth (n = 49 studies), mortality (n = 28), nutrition (n = 24), and infectious diseases (n = 33). Only 12 studies focused on non-communicable diseases. Duration ranged from 7 to 384 months. Measurements included anthropometric (n = 56 studies), socioeconomic (n = 50) and biological sampling (n = 25), but only one study was DNA-based.

CONCLUSION

Biobanks have emerged as the most successful approach to generating knowledge about disease causes and mechanisms. Little of this is possible to undertake in the in SEA or EM regions, however. Further longitudinal studies of young children with DNA sampling should be set up to better understand determinants of diseases in low-income countries.

Involvement of oxidative stress-mediated ERK1/2 and p38 activation regulated mitochondria-dependent apoptotic signals in methylmercury-induced neuronal cell injury

Methylmercury (MeHg) is well-known for causing irreversible damage in the central nervous system as well as a risk factor for inducing neuronal degeneration. However, the molecular mechanisms of MeHg-induced neurotoxicity remain unclear. Here, we investigated the effects and possible mechanisms of MeHg in the mouse cerebrum (in vivo) and in cultured Neuro-2a cells (in vitro). In vivo study showed that the levels of LPO in the plasma and cerebral cortex significantly increased after administration of MeHg (50μg/kg/day) for 7 consecutive weeks. MeHg could also decrease glutathione level and increase the expressions of caspase-3, -7, and -9, accompanied by Bcl-2 down-regulation and up-regulation of Bax, Bak, and p53. Moreover, treatment of Neuro-2a cells with MeHg significantly reduced cell viability, increased oxidative stress damage, and induced several features of mitochondria-dependent apoptotic signals, including increased sub-G1 hypodiploids, mitochondrial dysfunctions, and the activation of PARP, and caspase cascades. These MeHg-induced apoptotic-related signals could be remarkably reversed by antioxidant NAC. MeHg also increased the phosphorylation of ERK1/2 and p38, but not JNK. Pharmacological inhibitors NAC, PD98059, and SB203580 attenuated MeHg-induced cytotoxicity, ERK1/2 and p38 activation, MMP loss, and caspase-3 activation in Neuro-2a cells. Taken together, these results suggest that the signals of ROS-mediated ERK1/2 and p38 activation regulated mitochondria-dependent apoptotic pathways that are involved in MeHg-induced neurotoxicity.

Is dental amalgam safe for humans? The opinion of the scientific committee of the European Commission

It was claimed by the Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR)) in a report to the EU-Commission that "....no risks of adverse systemic effects exist and the current use of dental amalgam does not pose a risk of systemic disease..." [1, available from: http://ec.europa.eu/health/ph_risk/committees/04_scenihr/docs/scenihr_o_016.pdf].SCENIHR disregarded the toxicology of mercury and did not include most important scientific studies in their review. But the real scientific data show that:(a) Dental amalgam is by far the main source of human total mercury body burden. This is proven by autopsy studies which found 2-12 times more mercury in body tissues of individuals with dental amalgam. Autopsy studies are the most valuable and most important studies for examining the amalgam-caused mercury body burden.(b) These autopsy studies have shown consistently that many individuals with amalgam have toxic levels of mercury in their brains or kidneys.(c) There is no correlation between mercury levels in blood or urine, and the levels in body tissues or the severity of clinical symptoms. SCENIHR only relied on levels in urine or blood.(d) The half-life of mercury in the brain can last from several years to decades, thus mercury accumulates over time of amalgam exposure in body tissues to toxic levels. However, SCENIHR state that the half-life of mercury in the body is only "20-90 days".(e) Mercury vapor is about ten times more toxic than lead on human neurons and with synergistic toxicity to other metals.(f) Most studies cited by SCENIHR which conclude that amalgam fillings are safe have severe methodical flaws.

Neurotoxicological effects of low-dose methylmercury and mercuric chloride in developing offspring mice

Mercury is a well-known toxic metal and potently induces severe neurotoxicological effects, especially in infants and children. The purpose of this study was to explore the underlying mechanisms of neurotoxic effects of mercurial compounds on the different stages of developing mice. Low-doses (the probability of human exposure in mercury-contaminated areas) of methylmercury (MeHg) (M, 0.02mg/kg/day) and mercury chloride (HgCl(2)) (H, 0.5mg/kg/day) were administered to mice of the following groups: (1) treatment with distilled water for 7 consecutive weeks after weaning (control-vehicle (CV)); exposure to mercurial compounds at different stages; (2) for 7 consecutive weeks after weaning (control-MeHg (CM) and control-HgCl(2) (CH)); (3) only during perinatal and weaning stages (MeHg-vehicle (MV) and HgCl-vehicle (HV)); and (4) in all experimental stages (MeHg-MeHg (MM) and HgCl(2)-HgCl(2) (HH)). Results revealed the neurobehavioral defects (increased locomotor activities, motor equilibrium impairment, and auditory dysfunction) that correlated with increasing Hg accumulation in CM and CH groups. However, it revealed a decrease and an increase in locomotor activities in MV and HV groups, respectively; these became more severe in MM and HH groups than in MV and HV groups. Motor equilibrium performance in MV and HV groups remained normal, while that in MM and HH groups was decreased. The most severe auditory defects (altered auditory brainstem response, ABR test) found in MM and HH groups than those in the respective CM and CH, MV and HV, including absolute wave III delays and interwave I-III latencies, which suggested that the irreversible auditory dysfunction caused by mercurial compounds. Furthermore, the alteration of lipid peroxidation (LPO), Na(+)/K(+)-ATPase activities, and nitric oxide (NO(x)) in the brain tissues contributed to the observed neurobehavioral dysfunction and hearing impairment. These findings provide evidence that fetuses were much more susceptible to the effects of mercurial compounds with regard to inducing severely neurotoxicological injuries as that found in human beings. The signaling of ROS/Na(+)-K(+)-ATPase/NO(x) plays a crucial role in the underlying mechanism for mercurial compound-induced toxic effects in offspring.

Mercury exposure and children's health

Acute or chronic mercury exposure can cause adverse effects during any period of development. Mercury is a highly toxic element; there is no known safe level of exposure. Ideally, neither children nor adults should have any mercury in their bodies because it provides no physiological benefit. Prenatal and postnatal mercury exposures occur frequently in many different ways. Pediatricians, nurses, and other health care providers should understand the scope of mercury exposures and health problems among children and be prepared to handle mercury exposures in medical practice. Prevention is the key to reducing mercury poisoning. Mercury exists in different chemical forms: elemental (or metallic), inorganic, and organic (methylmercury and ethyl mercury). Mercury exposure can cause acute and chronic intoxication at low levels of exposure. Mercury is neuro-, nephro-, and immunotoxic. The development of the child in utero and early in life is at particular risk. Mercury is ubiquitous and persistent. Mercury is a global pollutant, bio-accumulating, mainly through the aquatic food chain, resulting in a serious health hazard for children. This article provides an extensive review of mercury exposure and children's health.

A prospective study of thimerosal-containing Rho(D)-immune globulin administration as a risk factor for autistic disorders

BACKGROUND

This study evaluated the relationship between prenatal mercury exposure from thimerosal (49.55% mercury by weight)-containing Rho(D)-immune globulins (TCRs) and autism spectrum disorders (ASDs).

METHODS

The Institutional Review Board of the Institute for Chronic Illnesses approved the present study. A total of 53 consecutive non-Jewish Caucasian patients with ASDs (Diagnostic and statistical manual of mental disorders, fourth ed. - DSM IV) born between 1987 and 2001 who presented to the Genetic Centers of America for outpatient genetic/developmental evaluations were prospectively collected from June 1, 2005 through March 31, 2006. Imaging and laboratory testing were conducted on each patient to rule out other causal factors for their ASDs. As race-matched controls, the frequency of Rh negativity was determined from 926 non-Jewish Caucasian pregnant women who had presented for outpatient prenatal genetics care to the Genetic Centers of America between 1980 and 1989.

RESULTS

Children with ASDs (28.30%) were significantly more likely (odds ratio 2.35, 95% confidence interval 1.17-4.52, p < 0.01) to have Rh-negative mothers than controls (14.36%). Each ASD patient's mother was determined to have been administered a TCR during her pregnancy.

CONCLUSION

The results provide insights into the potential role prenatal mercury exposure may play in some children with ASDs.

Postnatal exposure to methyl mercury from fish consumption: a review and new data from the Seychelles Child Development Study

BACKGROUND

Fish is an important source of nutrition worldwide. Fish contain both the neurotoxin methyl mercury (MeHg) and nutrients important for brain development. The developing brain appears to be most sensitive to MeHg toxicity and mothers who consume fish during pregnancy expose their fetus prenatally. Although brain development is most dramatic during fetal life, it continues for years postnatally and additional exposure can occur when a mother breast feeds or the child consumes fish. This raises the possibility that MeHg might influence brain development after birth and thus adversely affect children's developmental outcomes. We reviewed postnatal MeHg exposure and the associations that have been published to determine the issues associated with it and then carried out a series of analyses involving alternative metrics of postnatal MeHg exposure in the Seychelles Child Development Study (SCDS) Main Cohort.

METHODS

The SCDS is a prospective longitudinal evaluation of prenatal MeHg exposure from fish consumption. The Main Cohort includes 779 subjects on whom recent postnatal exposure data were collected at the 6-, 19-, 29-, 66-, and 107-month evaluations. We examined the association of recent postnatal MeHg exposure with multiple 66- and 107-month outcomes and then used three types of alternative postnatal exposure metrics to examine their association with the children's intelligence quotient (IQ) at 107 months of age.

RESULTS

Recent postnatal exposure at 107 months of age was adversely associated with four endpoints, three in females only. One alternative postnatal metric was beneficially associated with 9-year IQ in males only.

CONCLUSIONS

We found several associations between postnatal MeHg biomarkers and children's developmental endpoints. However, as has been the case with prenatal MeHg exposure in the SCDS Main Cohort study, no consistent pattern of associations emerged to support a causal relationship.

Epidemiologic evidence of relationships between reproductive and child health outcomes and environmental chemical contaminants

This review summarizes the level of epidemiologic evidence for relationships between prenatal and/or early life exposure to environmental chemical contaminants and fetal, child, and adult health. Discussion focuses on fetal loss, intrauterine growth restriction, preterm birth, birth defects, respiratory and other childhood diseases, neuropsychological deficits, premature or delayed sexual maturation, and certain adult cancers linked to fetal or childhood exposures. Environmental exposures considered here include chemical toxicants in air, water, soil/house dust and foods (including human breast milk), and consumer products. Reports reviewed here included original epidemiologic studies (with at least basic descriptions of methods and results), literature reviews, expert group reports, meta-analyses, and pooled analyses. Levels of evidence for causal relationships were categorized as sufficient, limited, or inadequate according to predefined criteria. There was sufficient epidemiological evidence for causal relationships between several adverse pregnancy or child health outcomes and prenatal or childhood exposure to environmental chemical contaminants. These included prenatal high-level methylmercury (CH(3)Hg) exposure (delayed developmental milestones and cognitive, motor, auditory, and visual deficits), high-level prenatal exposure to polychlorinated biphenyls (PCBs), polychlorinated dibenzofurans (PCDFs), and related toxicants (neonatal tooth abnormalities, cognitive and motor deficits), maternal active smoking (delayed conception, preterm birth, fetal growth deficit [FGD] and sudden infant death syndrome [SIDS]) and prenatal environmental tobacco smoke (ETS) exposure (preterm birth), low-level childhood lead exposure (cognitive deficits and renal tubular damage), high-level childhood CH(3)Hg exposure (visual deficits), high-level childhood exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (chloracne), childhood ETS exposure (SIDS, new-onset asthma, increased asthma severity, lung and middle ear infections, and adult breast and lung cancer), childhood exposure to biomass smoke (lung infections), and childhood exposure to outdoor air pollutants (increased asthma severity). Evidence for some proven relationships came from investigation of relatively small numbers of children with high-dose prenatal or early childhood exposures, e.g., CH(3)Hg poisoning episodes in Japan and Iraq. In contrast, consensus on a causal relationship between incident asthma and ETS exposure came only recently after many studies and prolonged debate. There were many relationships supported by limited epidemiologic evidence, ranging from several studies with fairly consistent findings and evidence of dose-response relationships to those where 20 or more studies provided inconsistent or otherwise less than convincing evidence of an association. The latter included childhood cancer and parental or childhood exposures to pesticides. In most cases, relationships supported by inadequate epidemiologic evidence reflect scarcity of evidence as opposed to strong evidence of no effect. This summary points to three main needs: (1) Where relationships between child health and environmental exposures are supported by sufficient evidence of causal relationships, there is a need for (a) policies and programs to minimize population exposures and (b) population-based biomonitoring to track exposure levels, i.e., through ongoing or periodic surveys with measurements of contaminant levels in blood, urine and other samples. (2) For relationships supported by limited evidence, there is a need for targeted research and policy options ranging from ongoing evaluation of evidence to proactive actions. (3) There is a great need for population-based, multidisciplinary and collaborative research on the many relationships supported by inadequate evidence, as these represent major knowledge gaps. Expert groups faced with evaluating epidemiologic evidence of potential causal relationships repeatedly encounter problems in summarizing the available data. A major driver for undertaking such summaries is the need to compensate for the limited sample sizes of individual epidemiologic studies. Sample size limitations are major obstacles to exploration of prenatal, paternal, and childhood exposures during specific time windows, exposure intensity, exposure-exposure or exposure-gene interactions, and relatively rare health outcomes such as childhood cancer. Such research needs call for investments in research infrastructure, including human resources and methods development (standardized protocols, biomarker research, validated exposure metrics, reference analytic laboratories). These are needed to generate research findings that can be compared and subjected to pooled analyses aimed at knowledge synthesis.

Cerebellum cholinergic muscarinic receptor (subtype-2 and -3) and cytoarchitecture after developmental exposure to methylmercury: an immunohistochemical study in rat

The developing central nervous system (CNS) is a target of the environmental toxicant methylmercury (MeHg), and the cerebellum seems the most susceptible tissue in response to this neurotoxicant. The cholinergic system is essential for brain development, acting as a modulator of neuronal proliferation, migration and differentiation processes; its muscarinic receptors (MRs) play pivotal roles in regulating important basic physiologic functions. By immunohistochemistry, we investigated the effects of perinatal (GD7-PD21) MeHg (0.5 mg/kg bw/day in drinking water) administration on cerebellum of mature (PD36) and immature (PD21) rats, evaluating the: (i) M2- and M3-MR expression; (ii) presence of gliosis; (iii) cytoarchitecture alterations. Regarding to M2-MRs, we showed that: at PD21, MeHg-treated animals did not display any differences compared to controls, while, at PD36 there was a significant increase of M2-immunopositive Bergmann cells in the molecular layer (ML), suggesting a MeHg-related cytotoxic effect. Similarly to M2-MRs, at PD21 the M3-MRs were not affected by MeHg, while, at PD36 a lacking immunoreactivity of the granular layer (IGL) was observed after MeHg treatment. In MeHg-treated rats, at both developmental points, we showed reactive gliosis, e.g. a significant increase in Bergmann glia of the ML and astrocytes of the IGL, identified by their expression of glial fibrillar acidic protein. No MeHg-related effects on Purkinje cells were detected neither at weaning nor at puberty. These findings suggest: (i) a delayed MeHg exposure-related effect on M2- and M3-MRs, (ii) an overt MeHg-related cytotoxic effect on cerebellar oligodendroglia, e.g. reactive gliosis, (iii) a selective vulnerability of granule cells and Purkinje neurons to MeHg, with the latter that remain unharmed.

Searching for novel biomarkers of centrally and peripehrally-acting neurotoxicants, using surface-enhanced laser desorption/ionisation-time-of-flight mass spectrometry (SELDI-TOF MS)

The neurotoxicity of chemicals to humans is difficult to monitor as there are no suitable methods of detecting early neuronal dysfunction. Here, a proof of principle study was designed to assess the potential of identifying protein biomarkers in accessible biofluids for this purpose. Groups of rats were treated with a range of doses of the model neurotoxicants, acrylamide (0, 2, 10, 50mg/kg) and methylmercury (0, 0.2, 1, 5mg/kg) for up to 3 weeks and samples of serum, urine, and cerebral spinal fluid analysed by surface-enhanced laser desorption/ionisation-time-of-flight mass spectrometry. There was no neuropathology up to the highest dose tested. Protein profiles were obtained from all samples and changes in the levels of many proteins were detected in both serum and urine, although not cerebral spinal fluid. In serum, the combination of three protein ion levels with m/z values of 4968, 9402 and 12,948 was able to correctly classify the treatment groups thus: 88% control, 100% acrylamide, 92% methylmercury. In urine, three protein ions with m/z values of 4944, 12,966 and 21,992 classified correctly the groups: 67% control, 94% acrylamide, 97% methylmercury. Similar classifications using other serum and urinary protein ions were also possible. This indicates the potential of serum and urine protein biomarkers for the assessment of sub-clinical neurotoxicity.

Time of perinatal immunization, thimerosal exposure and neurodevelopment at 6 months in breastfed infants

AIM

Brazilian newborns immunized with hepatitis-B (thimerosal containing vaccine, TCV) receive the first dose within 24 h if delivered in public hospitals, but at a later time if born in private hospitals. We compared neurodevelopment (ND) in infants born in a state hospital (immunized within 24 h) and in privately run hospitals (immunized 2-4 days postnatally).

METHODS

We used the Gesell Developmental Schedules in 82 healthy exclusively breastfed infants at 6 months to assess motor skills, language development, comprehension capacity and social skills.

RESULTS

Compared to the group immunized 2-4 days after hospital discharge, the group immunized within 24 h showed no significant difference in ND delays. Despite the variation in gestational age (range 36-42 weeks) and TCV-ethylmercury (EtHg) dose (5.7-11.3 microg Hg/kg b.w.) at birth, time of exposure to TCV showed no significant association with ND. Gesell Developmental Score was not significantly correlated with total parenteral EtHg/unit of body mass neither with the relative increase in hair-Hg (as an additional challenge to prenatal Hg exposure).

CONCLUSION

In breastfed infants, differences in early exposure to TCV-EtHg cannot portend clinical neurodevelopment delays at 6 months. We speculate that breastfeeding remains a significant strategy to improve central nervous system protection of infants facing early exposure challenges.

A case series of children with apparent mercury toxic encephalopathies manifesting with clinical symptoms of regressive autistic disorders

Impairments in social relatedness and communication, repetitive behaviors, and stereotypic abnormal movement patterns characterize autism spectrum disorders (ASDs). It is clear that while genetic factors are important to the pathogenesis of ASDs, mercury exposure can induce immune, sensory, neurological, motor, and behavioral dysfunctions similar to traits defining or associated with ASDs. The Institutional Review Board of the Institute for Chronic Illnesses (Office for Human Research Protections, U.S. Department of Health and Human Services, IRB number IRB00005375) approved the present study. A case series of nine patients who presented to the Genetic Centers of America for a genetic/developmental evaluation are discussed. Eight of nine patients (one patient was found to have an ASD due to Rett's syndrome) (a) had regressive ASDs; (b) had elevated levels of androgens; (c) excreted significant amounts of mercury post chelation challenge; (d) had biochemical evidence of decreased function in their glutathione pathways; (e) had no known significant mercury exposure except from Thimerosal-containing vaccines/Rho(D)-immune globulin preparations; and (f) had alternate causes for their regressive ASDs ruled out. There was a significant dose-response relationship between the severity of the regressive ASDs observed and the total mercury dose children received from Thimerosal-containing vaccines/Rho (D)-immune globulin preparations. Based upon differential diagnoses, 8 of 9 patients examined were exposed to significant mercury from Thimerosal-containing biologic/vaccine preparations during their fetal/infant developmental periods, and subsequently, between 12 and 24 mo of age, these previously normally developing children suffered mercury toxic encephalopathies that manifested with clinical symptoms consistent with regressive ASDs. Evidence for mercury intoxication should be considered in the differential diagnosis as contributing to some regressive ASDs.

Prenatal methylmercury exposure affects spatial vision in adult monkeys

Decades of research have demonstrated that exposure to methylmercury (MeHg), a ubiquitous environmental pollutant, can have both early and long-term neurobehavioral consequences in exposed offspring. The present study assessed visual functioning in adult macaque monkeys (Macaca fascicularis) exposed in utero to 0, 50, 70, or 90 microg/kg/day of MeHg hydroxide. Twenty-one full-term, normal birth weight offspring (9 controls, 12 exposed) were tested at approximately 11-14.5 years of age on a visual contrast sensitivity task. A forced-choice tracking procedure was utilized with spatial frequencies of 1, 4, 10, and 20 cycles per degree of visual angle. On each test session, a single spatial frequency was presented across five levels of contrast, each differing by 3 dB. Methylmercury-exposed monkeys exhibited reduced contrast sensitivity thresholds, particularly at the higher spatial frequencies. The degree of visual impairment was not related to MeHg body burden or clearance and almost half of the exposed animals were unimpaired. The results from this study demonstrate that chronic in utero MeHg exposure, at subclinical levels, is associated with permanent adverse effects on spatial vision in adult monkeys.

Motor function following developmental exposure to PCBS and/or MEHG

Recent studies raise concern for combined exposure to polychlorinated biphenyls (PCBs) and methylmercury (MeHg), two environmental contaminants that are found in fish and seafood. Past accidental poisonings in humans show that exposure to high levels of either contaminant is associated with motor impairments, including alterations in cerebellar functions such as balance and coordination. Epidemiological studies of lower level exposures suggest some neuromotor impairment in exposed children, but the majority of these studies have focused on cognitive endpoints rather than examining a full-range of motor function. In particular, the cerebellum could be a sensitive target for combined PCB and MeHg toxicity. MeHg exposure during development damages the cerebellum along with cortical areas, and PCBs may also cause cerebellar damage via thyroid hormone disruption during development. In addition, in vitro studies report interactive effects of PCBs and MeHg on ryanodine-sensitive calcium signaling. Ryanodine receptors are found especially within the cerebellum, and alterations in calcium signaling within the cerebellum could impair long-term depression and subsequent motor learning. This article reviews the motor impairments reported in humans and laboratory animals following exposure to PCBs and/or MeHg during development. There is need for a better understanding of the interactive effects of PCBs and MeHg, especially in regard to motor function.

Effects of breast feeding on neuropsychological development in a community with methylmercury exposure from seafood

Breastfeeding has been associated with an advantage to infant neurobehavioral development, possibly in part due to essential nutrients in breast milk. However, breast milk may be contaminated by environmental neurotoxicants, such as methylmercury. In the Faroe Islands, where maternal consumption of pilot whale may cause transfer of marine toxicants into breast milk, a cohort of 1022 consecutive singleton births was generated during 1986-87. Methylmercury exposure was assessed from mercury concentrations in cord blood and in the hair of the child at age 12 months, and the duration of breastfeeding was recorded. At approximately 7 years of age, 917 (90%) of the children underwent detailed neurobehavioral examination. After adjustment for confounders, breastfeeding was associated with only marginally better neuropsychological performance on most tests. These associations were robust even after adjustment for cord-blood and hair mercury concentration at age 1 year. Thus, in this cohort of children with a relatively high prenatal toxicant exposure and potential exposure to neurotoxicants through breast milk, breastfeeding was associated with less benefits on neurobehavioral development than previously published studies though not associated with a deficit in neuropsychological performance at age 7. Although the advantage may be less, Faroese women can still safely breastfeed their children.

Biological markers for metal toxicity

Exposure assessment is often considered the weakest link in risk assessment. It is important for investigators to continue to utilize the full potential of biomarkers for chemicals whose exposure is of global concern. This review is concerned with the biomarkers of metal toxicity, as the overall exposure to metals encountered occupationally or in the environment would continue causing indirect, delayed effects therefore ecoepidemiology, using designed molecular probes and noninvasive diagnostics will be the leading component for future management of environmental health. An attempt is made here at appraising the need for the development of more biomarkers for use in environmental epidemiology and health risk assessment.

Dose-dependent effects of methylmercury administered during neonatal brain spurt in rats

Rapid brain growth occurs primarily during the third trimester in humans, whereas in rats it occurs after parturition. Therefore, we hypothesized that the effects of methylmercury (MeHg) on the postnatal developing rat nervous system may help in understanding the neurotoxicity on the human fetal brain when the brain is most vulnerable. In the present experiment, the dose-response effects of MeHg treatment during the postnatal developing phase in rats were studied. Male Wistar rats were orally administered 0, 1, 3, and 5 mg/kg/day methylmercury chloride (MMC), respectively, on postnatal day 1 and for 30 consecutive days. The body weight decline began from day 25 and typical symptoms, such as hind-limb crossing and ataxia, were observed in rats treated with 5 mg/kg/day MMC. The weight loss and typical symptoms were not observed in rats treated with 1 and 3 mg/kg/day. Mercury (Hg) concentrations in the brain were 2.6, 4.5, and 9.6 microg/g in the rats treated with 1, 3, and 5 mg/kg/day, respectively, on the day after the final MMC treatment. At 5 to 6 weeks of age, dose-dependent deficits of motor coordination in the rotarod test and learning disability in the passive avoidance response test were observed. Histopathological examination of a proportion of the MeHg-treated rats revealed widespread neuronal degeneration manifested by neuron loss and astrocytosis in the cerebral cortex, striatum, and cerebellum, where severity of the lesions seemed to increase in proportion to the administered dose of MMC. These findings using neonatal rats will be useful for better understanding of the effects of MeHg in the developing human brain during gestation.

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.

Declining risk of methylmercury exposure to infants during lactation

Methylmercury (MeHg) can be transferred to infants through milk, in addition to passage through the placenta during intrauterine life. The higher MeHg accumulation and susceptibility to toxicity in the fetus than in the mother during the gestation period is well known. However, the contribution of Hg exposure through breast milk to the MeHg concentration in infants is not clear. Our objective in this study was to examine the changes in MeHg levels in infants who were reared on breast milk to evaluate the risks between fetal and breast-feeding periods based on Hg concentrations in red blood cells (RBCs-Hg). RBCs-Hg and plasma-Hg concentrations (Plasma-Hg) in seven pairs of maternal and infant blood samples were compared at birth and 3 months after parturition. RBCs to Plasma-Hg ratio was approximately 8:1 for mothers at parturition and after 3 months, suggesting that their Hg source is MeHg from consumed fish as is general in Japanese. In all seven cases, RBCs-Hg in the umbilical cords were higher than those in the mothers at parturition. The geometric mean of RBCs-Hg in umbilical cords (10.6ng/g) was about 1.4 times higher than that in the mothers (7.1ng/g). There was a strong correlation in RBCs-Hg in mothers and umbilical cords. However, all the infants showed declines in Hg concentrations throughout the breast-feeding period. The geometric mean RBCs-Hg at 3 months of age was 5.8ng/g, accounting for 54% of that in the umbilical cords. Consequently, maternal RBCs-Hg surpassed that of infants at 3 months, opposite to the situation at parturition. The decline in infant RBCs-Hg during the breast-feeding period can be explained by the low Hg transfer through breast milk and the rapid growth of infants after birth. The geometric mean Milk-Hg was low (0.21ng/g), around 20% of that in maternal Plasma-Hg. The average body weight of infants at 3 months increased to about 1.9 times of that at birth. Thus, offspring are subjected to MeHg exposure through both the gestation and the breast-feeding periods; the risk is especially high during gestation but may decrease during breast-feeding.

Evaluation of changes in methylmercury accumulation in the developing rat brain and its effects: a study with consecutive and moderate dose exposure throughout gestation and lactation periods

Methylmercury (MeHg) can be transferred to the fetus through the placenta and to newborn offspring through breast milk. The higher mercury (Hg) accumulation and susceptibility to toxicity in the fetus than in the mother during the gestation period is well known. However, the contribution of MeHg exposure through breast milk to the brain Hg concentration in offspring is not clear. The purposes of this study were to evaluate the changes in Hg concentration in the brain of offspring and its effects on the developing rat brain, based on consecutive and moderate doses of MeHg throughout gestation and lactation. Adult female rats were given a diet containing 5 ppm Hg (as MeHg) for 8 weeks. The administration level was thought not to cause adverse effects in adult rats. The rats were then mated and subsequently given the same diet throughout gestation and after parturition. The newborn offspring were placed with the mothers until postnatal day 30. The offspring were exposed to MeHg throughout their intrauterine life through the placenta, and during the postnatal developing phase via contaminated milk. Furthermore, they were given the same diet containing MeHg for 2 months following weaning. On the day of parturition, the concentration of Hg in the brains of newborns was 1.4 times higher than that in the mothers. During the suckling period the concentration in the brain of the offspring rapidly declined to 1/5 of that at birth, suggesting that MeHg transport by milk was limited while the brain and body volumes increased rapidly. The concentration increased gradually again after the offspring started the contaminated diet. In behavioral tests performed at 5 and 6 weeks of age, MeHg-exposed rats showed a significant deficit in motor coordination in the rotarod test and a learning disability in the passive avoidance response test, compared with controls. Histopathologically, focal cerebellar dysplasia, including the heterotopic location of Purkinje cells and granule cells, was observed. These abnormalities may be induced by the effect of highly accumulated MeHg in the brain during the gestation period. Thus, although offspring are subjected to consecutive and moderate dose MeHg exposure throughout both the gestation and suckling periods, the risk is especially high during gestation but may decrease during lactation.

Neurobehavioral toxicity of methylmercury and PCBs Effects-profiles and sensitive populations

A large and growing body of literature is available on the neurotoxicity of methylmercury and PCBs as expressed in the behavior of both humans and laboratory animals. Methylmercury and PCBs will be compared with PCBs with attention directed at overlaps and distinctions in their profiles of neurotoxicity. It is possible with methylmercury and, to a lesser extent, with PCBs to characterize the sensory, motor, and cognitive consequences of exposure. Methylmercury is emerging as a life-span developmental neurotoxicant: adverse effects of exposure have been identified in development and during aging in human populations as well as in laboratory animals. Less is known about the PCBs on this count. While the mechanisms of neurotoxicity are not understood for either class of compounds, emerging clues are pointing to the possibility of overlap in some mechanisms of neurotoxicity.

Placental to fetal transfer of mercury and fetotoxicity

Mercury vapor is known penetrate the placental barrier more easily than inorganic mercury. A relative amount of mercury accumulates in the fetus after exposure of pregnant animals to mercury vapor. Mercury concentration in fetal organs is much lower than that in maternal organs except the liver, and fetal liver shows significantly higher mercury concentrations than maternal liver. In fetal liver, a substantial portion of mercury is bound to metallothionein (MT), which plays an important role as a reservoir of mercury during the prenatal period. The mercury retained in fetal liver is redistributed to other organs, such as the brain and kidney, with diminishing MT levels during postnatal development. Consequently, an increase in mercury concentration in the brain and kidney of the neonate is observed. In studies on animal offspring in utero exposed to mercury vapor, behavioral changes, such as radial arm maze, morris maze and lever-press durations, are observed when the levels of mercury vapor exceed the threshold limit value (TLV).

Perinatal and lifetime exposure to methylmercury in the mouse: blood and brain concentrations of mercury to 26 months of age

Chronic, low-level exposures to environmental toxicants, because they often begin prenatally and then persist throughout the individual's lifetime, pose challenging issues to risk assessment. Exposure to low levels of methylmercury through the diet, based largely on consumption of fish and sea mammals, follows this pattern. Early development is considered to be a period of heightened vulnerability during which even low-level exposures may produce undetected, "silent", damage that is revealed only under conditions that challenge the functional capacities of the individual. Aging, with its diminished functional capacities and compensatory reserves provides such a challenge, but, to explore this possibility, requires basic information about blood and brain levels under conditions of chronic lifetime exposure. The current research was undertaken to provide such information. One hundred female B6C3F1/HSD mice were assigned to one of three dose groups, 0, 1, or 3 ppm methylmercury chloride administered in a 5 nM sodium carbonate drinking solution. They were bred with male CBA/J HSD mice to produce the trihybrid offspring B6C3F1/ HSD x CBA/J HSD. Dosing of the females began 4 weeks prior to breeding and continued for the two methylmercury-exposed groups throughout breeding and gestation. The methylmercury-treated litters were split into two subgroups, one exposed throughout its lifetime (set at 26 months) to the original dose, the other exposed through postnatal day 13 (PND 13). Brain and blood concentrations were assayed by cold-vapor atomic absorption. Samples were obtained on PND 4 and 21, and then at the end of months 14 and 26. On PND 4, brain and blood levels closely reflected maternal dosing. In all groups, concentrations fell sharply from PND 4 to 21, but to a greater extent in the perinatal groups. Blood levels in the 1 ppm lifetime group remained unchanged between months 14 and 26, but brain levels rose modestly. In the 3 ppm lifetime group, both brain and blood levels rose significantly between months 14 and 26, suggesting an interaction between dose and age.

Exposure analysis of five fish-consuming populations for overexposure to methylmercury

Mercury, in the form of methylmercury, is found in a myriad of fish species consumed by recreational and subsistence fishers around the world. Many agencies have attempted to address the issue of mercury consumption, resulting at times in the placement of advisories on waterways used for fishing. In this study, consumption rates of three Native American populations and two recreational fishing populations consuming freshwater or saltwater fish species were examined. These consumption rates were combined with fish contamination data to assess the level of exposure to methylmercury and to determine if any of these populations exceed a derived tolerable daily intake (TDI) for methylmercury (0.035 to 0.08 microg/kg/day). The TDI is unlikely to result in adverse health effects and is based on scientific studies investigating sensitive endpoints in children of mothers who consume fish over prolonged periods of time. Results from the exposure analysis indicate that many within the Native American populations exceed the TDI. This occurs even though the mercury concentrations in certain fish species are comparable to concentrations found in fish from open waters where "background" levels are assumed. Recreational anglers consuming freshwater species have exposure levels below the TDI as do nearly all anglers consuming saltwater species. Similar populations or populations having comparable consumption patterns of fish with equal or higher mercury levels in other areas will also exceed the TDI level for mercury. The public health implications of this exposure analysis are discussed.

Determination of a site-specific reference dose for methylmercury for fish-eating populations

Environmental risk-management decisions in the U.S. involving potential exposures to methylmercury currently use a reference dose (RfD) developed by the U.S. Environmental Protection Agency (USEPA). This RfD is based on retrospective studies of an acute poisoning incident in Iraq in which grain contaminated with a methylmercury fungicide was inadvertently used in the baking of bread. The exposures, which were relatively high but lasted only a few months, were associated with neurological effects in both adults (primarily paresthesia) and infants (late walking, late talking, etc.). It is generally believed that the developing fetus represents a particularly sensitive subpopulation for the neurological effects of methylmercury. The USEPA derived an RfD of 0.1 microg/kg/day based on benchmark dose (BMD) modeling of the combined neurological endpoints reported for children exposed in utero. This RfD included an uncertainty factor of 10 to consider human pharmacokinetic variability and database limitations (lack of data on multigeneration effects or possible long-term sequelae of perinatal exposure). Alcoa signed an Administrative Order of Consent for the conduct of a remedial investigation/feasibility study (RI/FS) at their Point Comfort Operations and the adjacent Lavaca Bay in Texas to address the effects of historical discharges of mercury-containing wastewater. In cooperation with the Texas Natural Resource Conservation Commission and USEPA Region VI, Alcoa conducted a baseline risk assessment to assess potential risk to human health and the environment. As a part of this assessment. Alcoa pursued the development of a site-specific RfD for methylmercury to specifically address the potential human health effects associated with the ingestion of contaminated finfish and shellfish from Lavaca Bay. Application of the published USEPA RfD to this site is problematic; while the study underlying the RfD represented acute exposure to relatively high concentrations of methylmercury, the exposures of concern for the Point Comfort site are from the chronic consumption of relatively low concentrations of methylmercury in fish. Since the publication of the USEPA RfD, several analyses of chronic exposure to methylmercury in fish-eating populations have been reported. The purpose of the analysis reported here was to evaluate the possibility of deriving an RfD for methylmercury, specifically for the case of fish ingestion, on the basis of these new studies. In order to better support the risk-management decisions associated with developing a remediation approach for the site in question, the analysis was designed to provide information on the distribution of acceptable ingestion rates across a population, which could reasonably be expected to be consistent with the results of the epidemiological studies of other fish-eating populations. Based on a review of the available literature on the effects of methylmercury, a study conducted with a population in the Seychelles Islands was selected as the critical study for this analysis. The exposures to methylmercury in this population result from chronic, multigenerational ingestion of contaminated fish. This prospective study was carefully conducted and analyzed, included a large cohort of mother-infant pairs, and was relatively free of confounding factors. The results of this study are essentially negative, and a no-observed-adverse-effect level (NOAEL) derived from the estimated exposures has recently been used by the Agency for Toxic Substances and Disease Registry (ATSDR) as the basis for a chronic oral minimal risk level (MRL) for methylmercury. In spite of the fact that no statistically significant effects were observed in this study, the data as reported are suitable for dose-response analysis using the BMD method. Evaluation of the BMD method used in this analysis, as well as in the current USEPA RfD, has demonstrated that the resulting 95% lower bound on the 10% benchmark dose (BMDL) represents a conservative estimate of the traditional NOAEL, and that it is superior to the use of "average" or "grouped" exposure estimates when dose-response information is available, as is the case for the Seychelles study. A more recent study in the Faroe Islands, which did report statistically significant associations between methylmercury exposure and neurological effects, could not be used for dose-response modeling due to inadequate reporting of the data and confounding from co-exposure to polychlorinated biphenyls (PCBs). BMD modeling over the wide range of neurological endpoints reported in the Seychelles study yielded a lowest BMDL for methylmercury in maternal hair of 21 ppm. This BMDL was then converted to an expected distribution of daily ingestion rates across a population using Monte Carlo analysis with a physiologically based pharmacokinetic (PBPK) model to evaluate the impact of interindividual variability. The resulting distribution of ingestion rates at the BMDL had a geometric mean of 1.60 microg/kg/day with a geometric standard deviation of 1.33; the 1st, 5th, and 10th percentiles of the distribution were 0.86, 1.04, and 1.15 microg/kg/day. In place of the use of an uncertainty factor of 3 for pharmacokinetic variability, as is done in the current RfD, one of these lower percentiles of the daily ingestion rate distribution provides a scientifically based, conservative basis for taking into consideration the impact of pharmacokinetic variability across the population. On the other hand, it was felt that an uncertainty factor of 3 for database limitations should be used in the current analysis. Although there can be high confidence in the benchmark-estimated NOAEL of 21 ppm in the Seychelles study, some results in the New Zealand and Faroe Islands studies could be construed to suggest the possibility of effects at maternal hair concentrations below 10 ppm. In addition, while concerns regarding the possibility of chronic sequelae are not supported by the available data, neither can they be absolutely ruled out. The use of an uncertainty factor of 3 is equivalent to using a NOAEL of 7 ppm in maternal hair, which provides additional protection against the possibility that effects could occur at lower concentrations in some populations. Based on the analysis described above, the distribution of acceptable daily ingestion rates (RfDs) recommended to serve as the basis for site-specific risk-management decisions at Alcoa's Point Comfort Operations ranges from approximately 0.3 to 1.1 microg/kg/day, with a population median (50th percentile) of 0.5 microg/kg/day. By analogy with USEPA guidelines for the use of percentiles in applications of distributions in exposure assessments, the 10th percentile provides a reasonably conservative measure. On this basis, a site-specific RfD of 0.4 microg/kg/day is recommended.

Twenty-seven years studying the human neurotoxicity of methylmercury exposure

Research at the University of Rochester (U of R) has been focused on mercury for nearly half a century. Initially studies focused on dosimetry, especially the accuracy of measuring exposure, and experimental work with animal models. Clinical studies in human populations started when the U of R mercury group was asked to assist with dosimetry in the Iraq epidemic of 1971-1972. Initial clinical studies described the effects of methylmercury (MeHg) poisoning on adults and children. A dose-response curve for prenatal exposure was determined and it suggested that relatively low exposures might be harmful to the fetus. Since most human exposure to MeHg is dietary from fish consumption, these theoretical dangers had far-reaching implications. After Iraq, the Rochester team pursued exposure from fish consumption in both adults and children. Populations with high fish consumption were identified in Samoa and Peru for studying adults and in Peru and the Seychelles islands for studying children. The possible health threat to the fetus from maternal fish consumption quickly became the focus of research efforts. This paper reviews the Rochester experience in studying human exposure to MeHg from fish consumption.

Mechanisms of injury in the central nervous system

Neurotoxicants with similar structural features or common mechanisms of chemical action frequently produce widely divergent neuropathologic outcomes. Methylmercury (MeHg) produces marked cerebellar dysmorphogenesis during critical periods of development. The pathologic picture is characterized by complete architectural disruption of neuronal elements within the cerebellum. MeHg binds strongly to protein and soluble sulphydryl groups. Binding to microtubular -SH groups results in catastrophic depolymerization of immature tyrosinated microtubules. However, more mature acetylated microtubules are resistant to MeHg-induced depolymerization. In contrast to MeHg, the structurally similar organotin trimethyltin (TMT) elicits specific apoptotic destruction of pyramidal neurons in the CA3 region of the hippocampus and in other limbic structures. Expression of the phylogenetically conserved protein stannin is required for development of TMT-induced lesions. Inhibition of expression using antisense oligonucleotides against stannin protects neurons from the effects of TMT, suggesting that this protein is required for expression of neurotoxicity. However, expression of stannin alone is insufficient for induction of apoptotic pathways in neuronal populations. The aromatic nitrocompound 1,3-dinitrobenzene (DNB) has 2 independent nitro groups that can redox cycle in the presence of molecular oxygen. Despite its ability to deplete neural glutathione stores, DNB produces edematous gliovascular lesions in the brain stem of rats. Glial cells are susceptible despite high concentrations of reduced glutathione compared with neuronal somata in the central nervous system (CNS). The severity of lesions produced by DNB is modulated by the activity of neurons in the affected pathways. The inherent discrepancy between susceptibility of neuronal and glial cell populations is likely mediated by differential control of the mitochondrial permeability transition in astrocytes and neurons. Lessons learned in the mechanistic investigation of neurotoxicants suggest caution in the evaluation and interpretation of structure-activity relationships, eg, TMT, MeHg, and DNB all induce oxidative stress, whereas TMT and triethyltin produce neuronal damage and myelin edema, respectively. The precise CNS molecular targets of cell-specific lipophilic neurotoxicants remain to be determined.

Kinetics of methylmercury and inorganic mercury in lactating and nonlactating mice

The elimination of mercury was followed for 9 days (Days 10-19 of lactation) in milk and/or 21 days in blood and plasma of lactating and nonlactating mice administered a single iv injection of either 203Hg-labeled methylmercuric chloride or 203Hg-labeled mercuric chloride (0.5 mg Hg/kg body wt). Demethylation of methylmercury to inorganic mercury was taken into consideration by analyzing the data with a combined pharmacokinetic model based on the assumption of constant blood plasma ratios for methylmercury and inorganic mercury. A three-compartment model fitted the blood and plasma concentrations vs time profiles for both compounds. Plasma clearance and volume of distribution at steady state for methylmercury were 95. 3 ml/h/kg and 18,500 ml/kg, respectively, in lactating mice, and significantly higher than in nonlactating mice with values of 47.1 ml/h/kg and 9400 ml/kg, respectively. The terminal half-lives of methylmercury in plasma were similar, 170 h in lactating and 158 h in nonlactating mice. No differences were observed between the pharmacokinetic parameters in lactating and nonlactating mice administered inorganic mercury. The lactational transfer of mercury was more efficient following administration of inorganic mercury than after administration of methylmercury, with a five times higher peak concentration in milk, higher milk:plasma concentration ratios, and 8% of the administered dose excreted in milk compared with 4% for methylmercury. Mercury concentrations in milk following an iv dose of inorganic mercury decreased with a terminal half-life of 107 h, whereas after administration of methylmercury, the concentration of total mercury in milk remained at an almost constant level during the whole period of investigation. There was a nonlinear relationship between mercury in milk and plasma following inorganic mercury administration. It is suggested that inorganic mercury enters the mammary gland by a carrier-mediated transport system, which is saturated at high plasma levels of inorganic mercury. The present study shows that physiological changes during lactation alter the pharmacokinetics for methylmercury in mice but not for inorganic mercury.

Cross-fostering study of methyl mercury retention, demethylation and excretion in the neonatal hamster

The cross-fostering technique was used in order to compare methyl mercury (MeHg) metabolism in hamsters following prenatal (in utero) and neonatal (lactational) exposure. Pregnant Syrian golden hamsters were administered radiolabeled MeHg on day 12 of gestation. The offspring was nursed by foster mothers unexposed to MeHg, while the pups from the unexposed animals were nursed by the MeHg-administered animals. Under these conditions, each pup in the litter received a dose of MeHg in utero corresponding to 0.9% of the maternal dose. The average amount of mercury found in the pups exposed via milk corresponded to 4.5% of the total body burden of the foster dam at the onset of lactation. This was about half the amount received by the pups exposed in utero. The total body burden of mercury, and the amount of mercury in the liver, brain and kidney of the pups exposed in utero began to decrease at seven days of age. The rate of decrease differed among the tissues and was lowest in the kidney. The amount of mercury in pups exposed via milk reached a peak level when the pups were 10-15 days old. The total body burden of mercury showed a slow decrease while the liver, brain and kidney levels decreased rapidly. In both groups of animals, up to 80% of the total body burden of mercury was found in the pelt. These data show that milk may be a significant exposure route for mercury and that neonatal hamsters are unable to demethylate MeHg and excrete mercury in urine and faeces.

Breast-feeding exposure of infants to cadmium, lead, and mercury: a public health viewpoint

The purpose of this report is to provide an overview of the public health implications of exposure via breast milk to cadmium, lead, and mercury for nursing infants and to provide health-based guidance. Daily intakes were calculated and compared with guidance values used for public health assessments at hazardous waste sites. Cadmium, lead, and mercury under normal conditions are found in breast milk at concentration ranges of < 1 microgram/L, 2-5 micrograms/L, and 1.4-1.7 micrograms/L, respectively. Women exposed environmentally or occupationally can have higher levels in their breast milk. Concentrations of about 5 micrograms/L (cadmium), 20 micrograms/L (lead), and 3.5 micrograms/L (mercury) appear to be adequate screening levels. Many factors affect both the distribution of cadmium, lead, and mercury in breast milk and the health consequences to an infant. It is not clear what additional impact low-level exposure via breast milk may have on an infant born with a body burden to one of these metals. There is sufficient evidence to make the case that contaminated breast milk is a source of potential risk to infants in certain populations. Prevention strategies that include behavior modification and proper nutrition should be communicated to women at risk. Identification and elimination of exposure pathways and a critical analysis of the benefits of breast feeding versus heavy metal exposure are needed on a site-specific or individual basis. Research is required to better understand the impact of low-level exposure to heavy metals via breast milk. Breastfeeding should be encouraged under most circumstances.

Lactational exposure to methylmercury in the hamster

Syrian Golden hamster dams were administered 203Hg-labelled methyl mercury (MeHg; 1.6 mumol/kg) 1 day after parturition and milk was collected twice during the 1st week. The excretion of 203Hg in milk and the uptake, retention and tissue distribution of 203Hg in the pups was studied using gamma counting. The fraction of inorganic Hg in milk and in the kidneys of the pups was determined following separation of inorganic Hg and MeHg by ion exchange chromatography. The concentration of 203Hg in milk on the 1st day after MeHg administration was 0.12 nmol/g. 203Hg was mainly (80-90%) excreted as MeHg during the first 6 days of lactation. The whole body and tissue concentration of 203Hg in the pups increased for 10-15 days and decreased thereafter. The content of 203Hg in the pelt and the fraction of inorganic Hg in the kidney increased throughout the study period (4 weeks). The excretion of MeHg in milk corresponded to at least 5% of the dose administered to the dam. Our study demonstrates that breast milk may be a significant source of MeHg exposure during the critical neonatal period.

Milk transfer and tissue uptake of mercury in suckling offspring after exposure of lactating maternal guinea pigs to inorganic or methylmercury

Maternal guinea pigs were injected with mercuric chloride (HgCl2; 1 mg Hg/kg body weight) or methylmercury (MeHg; 1 mg Hg/kg) 12 h after parturition, and exposure of the offspring to mercury (Hg) via breast milk were studied on days 3, 5 and 10 postpartum. Milk Hg concentrations were lower than maternal plasma Hg concentrations regardless of the form of Hg given to the dams. Milk Hg was higher in HgCl2-treated dams than in MeHg-treated dams. In MeHg-treated dams, MeHg was separately determined. While the ratio of MeHg to T-Hg decreased in the dams' plasma, it did not in the milk. There was a strong correlation between milk and plasma T-Hg concentrations in HgCl2 treated dams. In the milk of MeHg-treated dams, the plasma MeHg concentrations correlated better than did the plasma T-Hg concentrations. In the offspring, regardless of the chemical forms of Hg given to the dams, the highest Hg concentrations were found in the kidney, followed by the liver and the brain. Brain Hg concentrations were, however, significantly higher in the offspring of MeHg-treated dams than in those of HgCl2-treated dams. In addition, Hg levels in the major organs of the offspring of HgCl2-treated dams peaked on day 5 postpartum, while those of MeHg-treated dams did not show a significant decrease up to day 10 postpartum. These facts indicate that the two chemical forms of Hg were transferred to the offspring via the breast milk and were distributed differently, depending on the chemical form, to the offspring's tissues.

Exposure to mercury via breast milk in suckling offspring of maternal guinea pigs exposed to mercury vapor after parturition

Exposure to an exogenous chemical via milk causes superimposition on the body burden that might have already existed in the neonatal organism due to in utero exposure. Such exposure via breast milk is avoidable, although transplacental exposure cannot be avoided at present. In this paper, mercury exposure via breast milk was examined in an animal model. Maternal guinea pigs were shortly exposed to mercury vapor at the mean concentrations of 6-10 mg/m3 immediately after parturition. Newborn guinea pigs were then nursed by the natural mothers. Mercury concentrations in breast milk and tissue samples were determined on d 3, 5, and 10 postpartum. Mercury concentrations in breast milk were slightly lower than plasma mercury concentrations of the maternal guinea pigs over the observation period. However, the decrease in the mercury concentration in breast milk with time was slower than that in maternal plasma. Mercury concentrations in major organs of the neonates were highest in kidney, followed by liver and lung. In brain and whole blood, mercury concentrations were slightly elevated compared with the nonexposed control. Organ distribution of mercury in the suckling neonates indicated that they were exposed via breast milk to inorganic, not elemental, mercury.

Review of the health effects of methylmercury

The study critically reviews recent data relating to the health effects of methylmercury in man and the attendant dose-response relationships. New data obtained from animal studies, including pre-and postnatal exposure, are also examined. The consumption of fish and fish produce represents the major source of methylmercury exposure in the general population. Reported mercury concentrations in fish throughout the world are examined, particularly in the Mediterranean Sea. Here there is limited knowledge of methylmercury intake in critically exposed populations such as fishermen, employees of the fish industries and their families. The measurement of mercury in hair is now regarded as the most useful indicator of exposure but more experimental data are still required to increase the value of this index. The threshold levels of methylmercury in blood, hair and for dietary intake, as estimated by the World Health Organization, have been largely endorsed. However, new information from Japan and Canada suggests the existence of a latency period for some effects, so that the frequency or probability of their occurrence is inversely related to the duration of exposure. Incorporation of such findings would therefore lead to the designation of lower threshold values than are presently recognized. Pregnant women and the fetus have been identified as groups that are at special risk. The fetal blood mercury level is up to twice that of the mother and the sensitivity of both mother and fetus may be higher than in non-pregnant adults. This should be taken into account when assigning protective threshold concentrations.

The many faces of methylmercury poisoning

Methylmercury (MM) is a very potent neurotoxic agent. Its role in polluting the environment is well documented. A vast amount of study over the past several decades has finally provided insight into many aspects of its effect. Exposure to MM may be through ingestion of poisoned fish or inadvertent misuse of grain treated with the poison as a fungicide. Major epidemics have occurred in Japan (Fetal Minamata disease), Iraq, Pakistan, Guatemala, and Ghana. Sporadic incidences have occurred in the United States and Canada. There is no effective antidote to counteract the effect of MM on the central nervous system, although the information documented should provide hope for more effective therapy in acute cases.