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Toxicological Effects of Methylmercury
point for the Bayley scales because it is at the end of the age range for which the version of this test used in the Seychelles was standardized, leading to a substantial risk of a “ceiling effect” (i.e., too many children receiving the highest possible scores on numerous items). The next round of testing in the Seychelles will be at 8 years of age, a point in development that should be more optimal for detecting neurodevelopmental effects.
Although differences in end points assessed and age of assessment might explain the failure of the SCDS to detect the associations found in the Faroe Islands study, findings from the New Zealand study and the Seychelles pilot study suggest that the discrepancies between the Faroe Islands and the main Seychelles studies are probably not due to differences in the assessments. The New Zealand study found associations between MeHg exposure and scores on the McCarthy Scales of Children's Abilities (the primary outcome measure used in the SCDS) at about the same age of assessment as in the Seychelles study, in a study with full control for potential confounding influences. Associations with prenatal Hg exposure were even seen on the McCarthy scales and the PLS in the 217-member Seychelles pilot study at 5.5 years of age, albeit with only limited control for socioenvironmental influences.
STABLE VERSUS EPISODIC PATTERN OF EXPOSURE
The predominant source of Hg exposure in the Seychelles is daily fish consumption. Maternal fish consumption averages 12 meals per week. Hg exposure in the Faroe Islands, by contrast, is often more episodic. In the Faroe Islands, pilot-whale meals are relatively infrequent (less than once per month on the average), but whale meat has concentrations of MeHg between 10 and 20 times greater than those in many fish consumed in the Faroe Islands (Grandjean et al. 1992); thus, the whale meals might represent toxicologically more significant peak or bolus doses. Laboratory animal experiments on prenatal alcohol exposure have demonstrated that maternal ingestion of a given dose of alcohol over a short time causes greater neuronal (Bonthius and West 1990) and behavioral impairment (Goodlett et al. 1987) than that caused by gradual ingestion of the same total dose over several days. Thus, it is possible that the more episodic exposure pattern in the Faroe Islands, with