SAN FRANCISCO, Oct. 20 (UPI) -- Researchers analyzing new study results have concluded the government at least partly botched its job when it was assessing the danger posed by the mercury-based preservative thimerosal that once was widely used in children's vaccines.
The story begins in the fall of 1971 with a mission of mercy to famine-ravaged Iraq. In October of that year, the country received 90,000 metric tons of wheat seed, which, intended for planting as crops, had been treated with methylmercury, a fungicide similar to yet, as new research suggests, significantly different from the ethylmercury found in thimerosal.
Rather than being put in the ground in the rural communities that acquired it, much of the seed instead was ground and baked as bread. Of the estimated 50,000 consumers of the treated wheat, more than 6,000 were hospitalized for mercury poisoning, 450 died and many pregnant women gave birth to children with mental retardation, seizures, impaired vision or hearing and other birth defects.
The U.S. Environmental Protection Agency studied the amounts of mercury in samples of the mothers' blood and hair and the occurrence of disability in their babies along with other toxicological information to calculate a "safe" level of exposure to the toxin.
For added protection, the agency then further divided that number by a factor of 10 to come up with limits considered safe for children receiving vaccines preserved with thimerosal.
The use of these data to formulate guidelines for the U.S. childhood immunization program was predicated on two critical assumptions: that the effects of methylmercury and ethylmercury on the human body are the same and that these do not differ between fetuses still in the womb and babies already born. Neither, it turns out, is true, scientists say.
Numerous studies have shown a greater susceptibility to poisons of the developing central nervous system of the fetus than that of the newborn, researchers say.
Putting aside the differences between the two types of mercury, that would mean the EPA erred on the side of caution, at least when it comes to shots given children, not the ones administered to pregnant women -- who to this day are given thimerosal-containing flu vaccines.
As far as the two types of mercury go, other research paints a picture of beasts of a different color, and it's not yet clear which one has a darker disposition toward causing neurological harm in babies, doctors say.
Because of all the uncertainty, the net effect on a youngster of the small amounts of toxin once commonly found in immunizations remains highly disputed.
No one denies that mercury in large doses can wreak developmental havoc and that exposure to the neurotoxin can affect maturing brains, whether in the womb or cradle. But there's emphatic disagreement among scientists on just how much is too much and exactly what damage the child suffers as a result.
The non-profit Environmental Working Group recently challenged a long-held view of the womb as a shield against toxins. The analysis of 10 samples of umbilical cord blood from infants born in U.S. hospitals in August and September 2004 detected 287 of the 413 chemicals for which the researchers tested.
Of these, 180 have been associated with cancer in humans or animals, 217 with irregularities in the brain and nervous system and 208 with birth defects or abnormal development in animals, the environmentalists said.
The EPA itself estimates each year even before they're born, more than 300,000 infants may be exposed to enough mercury to increase their risk of learning disabilities.
That grim conclusion is based on a 1999-2000 analysis that showed some 8 percent of women in the childbearing years of 16 to 49 had higher than recommended concentrations of the neurotoxin in their blood. Other studies showed the rate dropping to 3.9 percent in 2001-2002.
In early 2006, however, interim results released from an ongoing survey detected mercury levels exceeding EPA safety guidelines in as many as one in five women in that age group.
The results raise concern "because mercury exposure in the womb can cause neurological damage and other health problems in children," said an accompanying statement by the environmental groups Sierra Club and Greenpeace.
The organizations tested hair samples from more than 6,600 Americans of all ages from all 50 states in what they called the largest such project ever conducted. In nearly all of the cases, the mercury source was a fish-rich diet.
The health consequences of such eating habits are under intense debate.
Studies from the Faroe Islands, an archipelago of 18 isles in the North Sea between Iceland and Norway, have reported a possible connection between subtle cognitive deficits, such as performance on attention, language and memory tests, and methylmercury levels previously thought to be safe.
But critics double fault the research, which involved a pilot-whale-eating populace. For one, the multitude of pollutants contaminating the meat muddies the waters, both literally and figuratively, making it difficult to discern the effects attributable to mercury, they say. For another, whatever the consequences, they likely would not apply to American children, few of whom munch on Moby Dick sandwiches, they point out.
Less disputed, though still controversial, are the results of a long-running survey, now in its 16th year, of 779 youngsters born in 1989 or 1990 in the Seychelles Islands, a tiny Indian Ocean nation off the coast of Africa, whose waters are purer and menu selections more akin to those gracing U.S. dinner tables.
Other than methylmercury, there are no known contaminants in the tuna, swordfish and other ocean-dwelling delicacies favored by the locals, say the investigators from the University of Rochester Medical Center in New York.
Thus far, they say they have observed no evidence of harm in any of the children, who are now reaching their 16th birthday, from their mothers wolfing down an average of 12 fish meals a week during pregnancy -- about 10 times the amount of seafood most Americans find palatable.
The horrifying fallout of consuming mercury-polluted fare on the fetus caught the world off guard in the 1950s during a poisoning outbreak in Minamata and Niigata, Japan.
Some pregnant women who feasted on noxious seafood gave birth to babies with severe developmental disabilities, although they themselves suffered no ill effects, raising questions of extra fetal sensitivity to the neurotoxin.
The levels of food contamination with mercury from industrial pollution -- 50 parts per million -- have not been duplicated since. Some 20 years later, Iraq's tragedy spawned of tainted bread led to studies that suggested there might be adverse effects from exposures as low as 10 to 20 ppm.
The Rochester researches report fish typically consumed today around the world usually contains less than 1 ppm, and rarely more than 4 or 5 ppm.
Funded by the National Institute of Environmental Health Sciences, the scientists will continue their study at least until 2010 to see the Seychelles children through their teens, a time when, recent animal studies indicate, the mercury effects on learning, memory and behavior might start showing up.
More dust of controversy was kicked up by another provocative study, this one coming out of Texas. The survey of the state's nearly 1,200 school districts noted those with the highest levels of mercury spewed by fossil-fuel-burning power plants also have the greatest rates of special-education students and autism diagnoses. Suggestive as it may be, no proof of a connection has been established.
The disputed significance of these findings aside, it's important to note they apply to the type of mercury not found in vaccines.
Because the research cupboard for ethylmercury's effects is virtually bare, government and public health officials have been basing their safety standards on the more studied and better understood fallout of methylmercury.
While such a crossover may seem justified, it is not scientifically validated, scientists say. In fact, they say, evidence is mounting for striking differences in the way the two compounds are distributed, metabolized and excreted.
"Methylmercury is not same as (the mercury) in thimerosal; it's surprising how different they are," said Thomas Burbacher, associate professor of environmental health, researcher at the Center on Human Development and Disability, director of the Infant Primate Research Laboratory at the University of Washington School of Public Health and Community Medicine in Seattle and lead investigator on the first study to directly compare the blood and brain levels of the two chemicals in infant primates.
"It's been used (in EPA guidelines) because there are no data on ethylmercury, but there are on methylmercury, which has been studied since 1950," he added. "So you use the information that's available -- assuming it's relevant."
From their experiments with 41 newborn crab-eating monkeys, the investigators concluded that it's not.
"The current study indicates that (methylmercury) is not a suitable reference for risk assessment from exposure to thimerosal," they wrote.
(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: Mercury effects on the body
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