[Usefulness of umbilical cord mercury concentrations as biomarkers of fetal exposure to methylmercury]

Prenatal exposure to low-level methylmercury alters the child's fine motor skills at the age of 18 months

OBJECTIVES

To compare motor, cognitive and language characteristics in children aged 18 months who were prenatally exposed to low-level methyl-mercury (MeHg), and to analyze the eventual differences in these characteristics in relation to cord blood THg concentration.

PATIENTS AND METHODS

The total number of 205 child-mother pairs was included in the study, and total cord blood mercury was measured in 198 of them. Out of the 198 already measured samples, 47 of them have also been tested for methyl-mercury in cord blood. Data regarding the 47 samples of MeHg levels has been used for calculating the correlation between cord blood THg and cord blood MeHg. MeHg and THg showed a significant correlation (r=0.95, p<0.05). One month after the delivery, mothers were asked to complete the questionnaire regarding socioeconomic factors, breastfeeding of their infants, and dietary habits during pregnancy. Neurodevelopmental assessment of motor, cognitive and language skills were conducted on 168 children using The Bayley Scales of Infant and Toddler Development, Third Edition (BSID-III). Regarding the cord blood THg concentration, 135 children were divided in 4 quartile groups. Their neurodevelopmental characteristics have been compared.

RESULTS

The cord blood THg concentration median and inter-quartile range was 2.98ng/g (1.41-5.61ng/g). There was a negative correlation between cord blood THg concentration and fine motor skills (rho=-0.22, p=0.01). It is evident that children grouped in 2nd ,3rd and 4th quartile had statistically significant lower fine motor skills assessment related to those grouped in 1st quartile (2nd quartile -1.24, p=0.03; 3rd quartile -1.28, p=0.03; 4th quartile -1.45, p=0.01). The differences in fine motor skills assessments between children in 2nd and 3rd and 3rd and 4th quartile were not statistically significant.

CONCLUSION

Intrauterine exposure to low-level THg (MeHg) is associated with alterations in fine motor skills at the age of 18 months.

Umbilical cord blood mercury levels in China

Mercury (Hg) is a well-known neurotoxicant. Hg exposure at high levels can harm individuals of all ages. Even low level exposure to Hg can damage the brain of fetuses and young children, and affect their central nervous system and cognitive development. The aims of our study were to measure total Hg levels in infant umbilical cord blood and to investigate the risk factors associated with total Hg cord blood levels in various cities in China. Our goal was to provide clues for the prevention of Hg exposure in utero. The results indicated that the average cord blood mercury levels (CBMLs) were (1.81 +/- 1.93) microg/L, which were lower than those found in most previous studies. The concentrations also differed according to geographic region. The CBMLs were not only associated with family economic and living conditions, but also with diet in pregnant women, especially the intake of marine fish, shellfish, poultry, formula milk and fruits.

Recent evidence from epidemiological studies on methylmercury toxicity

More than fifty years have passed since the outbreak of Minamata disease, and large-scale methylmercury poisoning due to industrial effluents or methylmercury-containing fungicide intoxication has scarcely happened in developed countries. On the other hand, widespread environmental mercury contamination has occurred in gold and mercury mining areas of developing countries. In this article, we provided an overview of recent studies addressing human health effects of methylmercury, which we searched using the PubMed of the US National Library of Medicine. The following suggestions were obtained for low-level methylmercury exposure: (1) In recent years, the proportion of human studies addressing methylmercury has tended to decrease. (2) Prenatal exposure to methylmercury through fish intake, even at low levels, adversely affects child development after adjusting for polychlorinated biphenyls and maternal fish intake during pregnancy, whereas maternal seafood intake has some benefits. (3) Long-term methylmercury exposure through consumption of fish such as bigeye tuna and swordfish may pose a potential risk of cardiac events involving sympathovagal imbalance. (4) In measuring methylmercury levels in preserved umbilical cord collected from inhabitants born in Minamata areas between 1945 and 1989, the elevated concentrations (≥1 mg/g) were observed mainly in inhabitants born between 1947 and 1968, and the peak coincided with the peak of acetaldehyde production in Minamata. (5) Since some developing countries appear to be in similar situations to Japan in the past, attention should be directed toward early recognition of a risky agent and precautions should be taken against it.

Long-term exposure to methylmercury and its effects on hypertension in Minamata

Recent studies suggest potential adverse effects of methylmercury exposure on cardiovascular disease, although the evidence of association with hypertension is still inconsistent. Therefore, we evaluated the effects of methylmercury exposure on hypertension in Minamata. We used data derived from the 1971 population-based survey in Minamata and neighboring communities. We also utilized data on hair mercury content of the participants (derived from a 1960 investigation). We adopted two exposure indices (residential area and hair mercury content) and two hypertension outcomes (past history of hypertension and hypertension defined by measurements in the examination). Then, we estimated the adjusted prevalence odds ratio (POR) and its confidence interval (CI) of both hypertension outcomes in relation to residential area and hair mercury content. In the Minamata area (high exposure area), 87% (833) of the eligible population (aged > or =10 years) participated in the 1971 investigations. In the Goshonoura area (middle exposure area) and the Ariake area (low exposure area), 93% (1450) and 77% (755), respectively, of the eligible population participated. Compared with subjects in the Ariake area, the subjects in the Minamata area manifested hypertension more frequently, and PORs observed for two hypertension outcomes were 1.6 (95% CI: 1.2-2.1) and 1.4 (95% CI: 1.1-1.9), respectively. Furthermore, dose-response trends with hair mercury content were observed for both hypertension outcomes. The present finding supports the causal relationship between methylmercury exposure and hypertension.

Assessment of intrauterine methylmercury exposure affecting child development: messages from the newborn

In evaluating the health effects in the offspring born to the mothers exposed to methylmercury (MeHg) during pregnancy, biomarkers obtained from maternal blood and hair, and umbilical cord have been employed for the exposure assessment. However, which biomarker is employed may affect the study conclusion. In this sense, umbilical cord mercury concentration appears to be a direct exposure biomarker of the fetus. We present an overview of the studies addressing umbilical cord and mercury and scrutinize the usefulness of umbilical cord samples for intrauterine exposure assessment of MeHg. The mean total mercury concentration in cord blood ranged from 0.5 to 35.6 microg/L among the studies reviewed. In Japanese populations, MeHg concentrations in dry cord tissue did not exceed 0.4 microg/g. Also, dry cord tissue would be preferable to wet tissue because the definition of wet weight of the umbilical cord is ambiguous. The cord blood-to-maternal blood ratio of mercury concentrations differed considerably among mother-child pairs despite the significant positive correlation between the cord and maternal mercury concentrations. Moreover, the increased cord mercury concentration was associated with some neurobehavioral and neurophysiological deficits in child. Therefore, mercury concentrations in cord blood and tissue can be accepted as more useful and valid biomarkers of fetal exposure to MeHg, for assessing a dose-response relationship in children at the developmental stage, than maternal mercury concentrations at parturition. Because umbilical cord blood has been used for cord-blood stem cell transplants, dry umbilical cord tissue should be effectively utilized for the advancement of medical sciences.