Delayed neurotoxicity in monkeys exposed developmentally to methylmercury

Five monkeys (Macaca fascicularis) were dosed from birth to 6.5-7.0 years of age with 50 micrograms/kg/day of mercury as methylmercuric chloride. Blood mercury levels were 0.6-0.9 ppm throughout most of the period of dosing. Blood mercury levels declined rapidly after cessation of dosing to levels below 0.01 ppm. When these monkeys were approximately 13 years old, some individuals displayed clumsiness during routine exercise. This observation was examined by cage-side observation and independent clinical neurological assessment by two staff veterinarians. Fine motor performance was assessed by timing retrieval of raisins from recessed grids. Treated monkeys took longer to retrieve raisins than age-matched controls. Clinical neurological assessment revealed apparent insensitivity to touch and pin-prick in mercury-exposed monkeys, and exposed monkeys were clumsier and slower in the exercise cage. These results indicate that overt signs of toxicity can be manifested long after exposure to methylmercury has ceased.