http://ehp.niehs.nih.gov/press/042105.html

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April 21, 2005 (revised April 27, 2005)

Thimerosal, Methylmercury React Differently in the

Brains of Infants, Study Says

Report in Environmental Health Perspectives finds

methylmercury not a suitable reference for risk

assessment

[RESEARCH TRIANGLE PARK, NC] 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, producing

higher levels of inorganic mercury in the brain. 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 M. Burbacher

of the University of Washington. Other authors

included Danny D. Shen, Noelle Liberato, S.

Grant, Elsa Cernichiari, and son. 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/.

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