Blood and Brain Mercury Content in Infant Monkeys after Exposure to Methylmercury or Thimerosal

Thomas M. Burbacher, Ph.D.
University of Washington
R01ES003745 and P30ES007033

Background: There has been great interest and debate over the past decade regarding the use of the mercury-containing preservative thimerosal and autism. According to some published reports, autism rates around the world are rising and the rise coincides with the availability of more vaccines containing thimerosal and a lowering of the age at which vaccines are given to babies. Thimerosal, which is about 50% ethylmercury, has been used since the 1930’s in vaccines to prevent bacterial and fungal contamination, particularly in multi­dose containers. Mercury has long been known to be a neurotoxicant, but much of the scientific literature has focused on methylmercury. In 1999, an FDA review noted that with the increased number of vaccines then recommended for infants, the total amount of mercury in vaccines containing thimerosal might exceed recommended levels. To be cautious, the U.S. Public Health Service and the American Academy of Pediatrics asked doctors to minimize exposure to thimerosal­containing vaccines and manufacturers to remove thimerosal from vaccines as soon as possible.

Advance: To further understand the differences in absorption and metabolism of both forms of mercury, a team of NIEHS-supported investigators exposed 41 neonatal Macaca fasicularis, or "crab-eating" monkeys to either thimerosal or methylmercury. These monkeys are considered a very good model for human infants. Monkeys in the thimerosal group received a series of injections typical of what a human infant would receive over the first few weeks of life. Those animals in the methylmercury group were given equivalent doses of mercury through a feeding tube. Initially, absorption and distribution of total mercury appeared to be similar between the two groups. However, ethylmercury from thimerosal was cleared much faster than methylmercury. Peak blood concentrations of mercury were three times higher in the methylmercury group. Brain concentrations of total mercury were significantly lower for the thimerosal group; however, a higher percentage of the total brain mercury was in the form of inorganic mercury for the thimerosal group (37% vs. 7%).

Implications:The authors conclude that knowledge of the fate and transport of methylmercury is not a suitable surrogate for risk assessments for exposure to thimerosal. Therefore, additional research is necessary to fully characterize the biotransformation of thimerosal so that a meaningful interpretation of any developmental effects from immunization with thimerosal-containing vaccines can be determined.

Citation: TM Burbacher, Shen DD, Liberato N, Grant KS, Cernichiari E, and Clarkson T. 2005. Comparison of Blood and Brain Mercury Levels in Infant Monkeys Exposed to Methylmercury or Vaccines Containing Thimerosal Environ Health Perspect: doi:10.1289/ehp.7712. [Online 21 April 2005]