per day given every 3 days for 15 days (Leyshon and Morgan 1991). Studies in mice reported that doses from 0.6 mg/kg per day for approximately 2 years (Mitsumori et al. 1990) to 1.9 mg/kg per day for 24 weeks (MacDonald and Harbison 1977) caused paralysis. A study by Jacobs et al. (1977) using New Zealand rabbits reported ataxia and decreased muscle tone following a dose of 30 mg/kg per day for 7 days. Two studies using cats reported ataxia and impaired hopping after long-term exposure at approximately 0.05 mg/kg per day (Khera et al. 1974; Charbonneau et al. 1976).

A few studies using rodents have reported less severe symptoms, such as altered sleep cycles, hindlimb weakness, or increased brainstem-auditory-response thresholds following exposure to MeHg. Altered sleep cycles in rats were reported by Arito and Takahashi (1991) following 2 days of exposure at 4 mg/kg per day. Hindlimb weakness in mice was reported by Berthoud et al. (1976) following exposure at 1 mg/kg per day for 60 days. Wassick and Yonovitz (1985) reported increased brainstem-auditory thresholds in mice following 17 days of exposure at 4 mg/kg per day or 6 days of exposure at 8 mg/kg per day.

In summary, reports from animal models of adult MD have provided supportive evidence for the neurological signs reported in humans. These studies have also provided detailed descriptions of the associated neuropathological effects from high-dose MeHg exposures (Chang 1979, 1990). Studies using adult animal models of chronic low-dose MeHg effects have been sparse, most likely because of the focus on neurodevelopmental effects following in utero or early postnatal MeHg exposure.

CONCLUSIONS

  • MeHg is highly toxic. The data reviewed in this chapter indicate that the adverse effects of MeHg exposure can be expressed in multiple organ systems throughout the lifespan.

  • Studies in humans on the carcinogenic effects of MeHg are inconclusive. Renal tumors have been seen in male mice but only at or above the MTD of MeHg.

  • The effect of MeHg on the human immune system is poorly understood. However, studies in vitro and in animals suggest that



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