Ped Med: Mercury differences raise concern

By LIDIA WASOWICZ, UPI Senior Science Writer  |  Nov. 30, 2006 at 12:57 PM
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SAN FRANCISCO, Oct. 25 (UPI) -- Recent research shows the type of mercury the government relied on as a reference in setting safety standards for children's vaccines differs significantly from the type that was actually present in the shots -- and still remains in some of them.

Thomas Burbacher's study -- one of the first to look beyond blood and into the brain to determine vaccine-related mercury distribution -- confirmed, overturned and added to previous findings on how ethylmercury and methylmercury compare and contrast.

Ethylmercury is found in thimerosal, a preservative once commonly used in children's shots which detractors hold responsible for a rise in diagnoses of autism and other neurodevelopmental problems in America's young.

Because much more research has been conducted on the effects on the human body of methylmercury -- the kind found in power plant pollution -- than of ethylmercury, the former was used as a measuring stick when the government formulated its safety limits for thimerosal amounts in vaccines.

Thimerosal has been phased out of most shots administered to children, with the exception of some booster and flu immunizations recommended for pregnant women and babies as young as six months.

In the first study to directly compare the blood and brain levels of the two chemicals in infant primates, Burbacher and his team at the Center on Human Development and Disability and the Infant Primate Research Laboratory at the University of Washington School of Public Health and Community Medicine in Seattle came to some startling conclusions.

Their study showed the ethylmercury in shots is eliminated about three times faster from the bloodstream than is the methylmercury in fish, as previous research had suggested.

Burbacher's study nixed the widespread assumption that of the pair, ethylmercury -- long viewed as the lesser of the two evils -- has a more difficult time getting to the brain. Instead, it noted that it's just as easy, and maybe even easier, for the thimerosal toxin to make the trip.

The study also came up with some new and potentially troubling findings, which showed that in the body, thimerosal breaks down to ethylmercury, which rapidly crosses the blood-brain barrier, where it converts to so-called "inorganic" mercury that stays put in the sensitive organ for at least a year and likely much longer.

However, the research did not go beyond tracing the path of mercury through the body.

While critical, the results tell nothing of the actual effects of the compounds on human growth and development, Burbacher emphasized.

That both defenders and detractors of a thimerosal-autism link saw the findings as a boost for their respective points of view reflects the complexity of the question, the meagerness of our understanding of the complexity of the issue and the eagerness of each side to gather support for its position.

Confounding the public further, news reports of the results showed a similarly confusing duality of interpretation. Consider the following two examples:

Story 1:

"The (National Institutes of Health)-funded study ... found that thimerosal, best known for its use as an ethylmercury-based preservative in infant vaccines and pregnancy shots, is actually more toxic to the brain than methylmercury."

Story 2:

"The mercury contained in some vaccines is processed differently in the body and is possibly less toxic to children than mercury found in pollution and fish."

The lead investigator's take: "Neither side is right."

The ongoing research stands at the very beginning of a long and winding road toward determining what, if any, effects thimerosal in the amounts found in vaccines has on a baby's maturing system, Burbacher said in a telephone interview.

"Thus far, what we've tried to do is provide very basic information on what happens to mercury when you inject it in a vaccine because that information is not known," he explained. "It's surprising we'd be (vaccinating children) all these years without knowing (what the mercury does)."

His preliminary results showed nearly three times less total mercury in newborn monkeys' brains after they got a thimerosal-containing shot than after they ingested a comparable oral dose of methylmercury.

But, the researchers also noted ethylmercury's fast breakdown left double the dose of the enduring inorganic form trapped in the brains of the thimerosal-injected group.

"Having less total mercury in your brain is good news, but having this inorganic mercury twice as high in your brain is concerning," Burbacher said.

In general, the nervous system shows sensitivity to all forms of mercury, which, at high levels, can permanently damage the brain, kidneys and developing fetus, resulting in irritability, shyness, tremors and problems with vision, hearing, attention, language and memory, among other effects.

However, the mechanisms by which organic mercury is converted to the inorganic form in the brain are unknown, and there is no consensus on the type and extent of damage that results.

"Previous studies indicate inorganic mercury may be toxic, but there are no studies in a developing brain, so we don't know," Burbacher stressed.

Research he's conducted with adult monkeys suggests the inorganic form can affect certain brain cells, including microglia, the immune system scavengers that gobble up dead matter and other debris -- the same cells that appear damaged in children with autism.

"I'm not looking to see if monkeys have autism but whether monkeys who got a dose of thimerosal have changes in the brain and, if so, the next question will be is the change functionally important, does it affect learning or memory," Burbacher said.

It will take years, perhaps decades, to fit all the pieces to the puzzle, he said.

In the meantime, "you can tell parents they need to be careful in what they read in the paper," Burbacher advised. "I'm a strong proponent of vaccination. I wouldn't like this data to be used (as a reason for) children not (getting) their vaccination."

Among investigations delving deeper into thimerosal's effects, the National Institute of Allergy and Infectious Diseases is sponsoring research of a large number of infants, including premature and low-birth-weight babies, in Buenos Aires, where mercury-containing vaccines are still administered to children.

(Note: In this multi-part installment, based on dozens of reports, conferences and interviews, Ped Med is keeping an eye on autism, taking a backward glance at its history and surrounding controversies, facing facts revealed by research and looking forward to treatment enhancements and expansions.)

Next: Case rests on circumstantial evidence


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