Revised:Thimerosal, Methylmercury React Differently in the Brains of Infants

Newswise — Researchers have uncovered greater detail about differences in how thimerosal—a preservative used in vaccines since the 1930s—and methylmercury are distributed in and eliminated from the brain and body, as reported in a study published online today by the peer-reviewed journal Environmental Health Perspectives. Among other effects, researchers found that brain concentrations of total mercury following thimerosal exposure were nearly threefold lower than those following methylmercury exposure.

These findings are important because they demonstrate that methylmercury is not a suitable reference for determining risk from exposure to thimerosal. The current debate over a potential link between thimerosal in vaccines and autism has led many families to question whether the risk of developing the disorder is greater than the benefit of vaccination.

Thimerosal breaks down in the body to ethylmercury and thiosalicylate. Because few health effects data exist for ethylmercury, methylmercury guidelines have been used to predict the toxicokinetics and neurodevelopmental effects of ethylmercury exposure. An earlier study calculated that children receive 187.5 micrograms of ethylmercury from thimerosal-containing vaccines given over the first 14 weeks of life, which can exceed EPA guidelines for methylmercury exposure during pregnancy.

In the present study, researchers exposed 41 infant Macaca fascicularis, or crab-eating monkeys, to thimerosal and methylmercury. These monkeys are among the best proxies for infant humans. Infants assigned to the thimerosal group received the typical schedule of injected vaccines for human infants, while infants assigned to the methylmercury group were exposed through a feeding tube.

Absorption and initial distribution of total mercury proved to be similar for both thimerosal and methylmercury. However, injected thimerosal reacted differently from methylmercury in that it cleared from the infant much more quickly. Also the peak blood mercury concentration in the methylmercury group rose to a level three times higher than the thimerosal infants after the fourth dose. Brain concentrations of total mercury were significantly lower for the thimerosal group compared to the methylmercury group.

These results suggest that ethylmercury is dealkylated much more extensively than methylmercury. While dealkylation is thought to be a detoxification mechanism that helps protect the central nervous system, previous work by Burbacher and his group has shown that inorganic mercury can affect certain types of cells in the brain such as the microglia. Recent reports have indicated abnormal microglia in the brains of children with autism.

According to the researchers, more research is needed to accurately predict how immunization with thimerosal-containing vaccines may affect children. "Knowledge of the biotransformation of thimerosal . . . is urgently needed to afford a meaningful interpretation of the potential developmental effects of immunization with thimerosal-containing vaccines in newborns and infants," the study authors write. "This information is critical if we are to respond to public concerns regarding the safety of childhood immunizations."

Dr. Jim Burkhart, science editor for EHP, says, "This study emphasizes that thimerosal and methylmercury behave differently in the body. Given that we routinely inject thimerosal into millions of infants, the study authors' call for more in-depth research on the subject is the right way to go."

The lead author of the study was Thomas M. Burbacher of the University of Washington. Other authors included Danny D. Shen, Noelle Liberato, Kimberly S. Grant, Elsa Cernichiari, and Thomas Clarkson. The study was funded by the National Institute of Environmental Health Sciences, the National Institute of Allergy and Infectious Diseases, the National Institute of Child Health and Human Development, the National Center for Research Resources, and the University of Rochester. The article is available free of charge at http://ehp.niehs.nih.gov/docs/2005/7712/abstract.html, and will be published in an upcoming print edition of the journal.

EHP is published by the National Institute of Environmental Health Sciences, part of the U.S. Department of Health and Human Services. EHP is an Open Access journal. More information is available online at http://www.ehponline.org/.