hands separately, fast cadence (i.e., as fast as possible); and (3) both hands, fast cadence (Beuter et al. 1999b). Seven dimensions of performance were measured: duration, range, maximum slope, similarity in shape, smoothness, sharpness, and coherence. Significant group difference were found on most end points, and the results of nested analyses provided additional evidence that group differences in Hg concentrations were probably contributory.

Neurotoxic Effects in Adult Animals

Experimental studies of the effects of MeHg exposure on adult animals have reported neurological effects similar to those reported for adult humans. Studies using monkeys, rodents, and cats have reported effects consistent with adult MD (Harada 1995). Some of those studies are summarized in Table 5-12. Neurotoxic signs reported reflect the regional specificity of the neuropathological effects observed in adult subjects. Signs of ataxia, constriction of the visual field, and sensory disturbances are commonly associated with pathological lesions in the calcarine cortices, dorsal root ganglia, and cerebellum (Chang 1980).

A study of macaque and squirrel monkeys has reported ataxia, tremor, and constriction of visual fields in animals with blood Hg concentrations between 1 and 2 ppm (Evans et al. 1977). The latency for the onset of symptoms in that study was 135 to 140 days.

Constriction of the visual field was reported in macaques following variable dosing schedules that produced blood MeHg concentrations from 1.5 to 3 ppm. The onset of visual-field disturbances preceded overt signs of toxicity (Merigan et al. 1983).

Ataxia, tremor, and apparent blindness was reported in adult female macaques exposed orally to doses of MeHg hydroxide at 70 µg/kg per day and above (Burbacher et al. 1988). The durations to onset of symptoms ranged from 177 days to 392 days.

In rodents, several studies have reported severe neurological effects, such as ataxia, paralysis, spasms, and hindlimb crossing in adult rats and mice, from exposure to MeHg (see Table 5-12). In general, the onset of symptoms is dependent on the dose and duration of exposure. In rats, overt signs of neurotoxicity were reported at doses ranging from 0.8 mg/kg per day for 6 weeks (Chang and Hartmann 1972) to 10 mg/kg

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