A shot at explanation
A study tests the mercury-autism link
Back in 1994, medical geneticist Judith Miles and her colleagues at Mizzou began treating a few patients with autism. Now, the Thompson Center for Autism and Neurodevelopmental Disorders has treated around 900 children. During that time span, one question has persisted: Why?
It’s a question on the lips of parents who have watched their children go from happy and social to withdrawn and even mute — sometimes over years, and sometimes virtually overnight. It’s also a question on the minds of the Thompson Center’s doctors, who don’t just treat autism. They seek answers by researching its causes.
In a study published in the American Journal of Medical Genetics, Miles and fellow researcher Nicole Takahashi have added to the evidence ruling out one potential cause: thimerosal. Thimerosal is a preservative that was used in immunizations before 2002, and it contains mercury.
Parents and researchers have theorized that mercury exposure could cause autism. Diagnoses of autism spectrum disorders (see sidebar) in children have exploded in the past 20 years or so (data in Missouri showed a 300 percent increase from 1988 to 1995 alone, for example). During that same time period, the number of childhood immunizations also has increased.
For the autism community, this raised a question: Is a medical miracle designed to save lives and prevent disease actually causing more harm than good?
The answer, according to Miles’ study and others, is a qualified no.
Judith Miles and her peers at the Thompson Center treat patients with autism and perform research on the disorder, from causes to new treatments. Photo by Martha Routier.
Genetics and environment
“Most of the research suggests that autism is a genetically controlled disorder,” says Miles, the William S. Thompson Endowed Chair for Autism. “But with these huge increases in diagnoses, people start to ask, ‘Is there something in the environment that causes this?’ ”
Research has shown a connection between mercury exposure and fetal development, and fetal brains are especially susceptible to toxins. The idea of Miles’ research was that if small doses of thimerosal caused autism in young children, it would be even more toxic to developing fetuses.
Miles looked at data on mothers of children with autism treated at Mizzou. She was looking specifically for how many of those mothers had been given RH immune globulin (RhIg) during pregnancy. RhIg is routinely given to mothers who are Rh negative, a condition that can cause a mother’s body to produce antibodies that harm a fetus. RhIg controls that condition and prevents the resulting damage. However, like immunizations, RhIg before 2002 contained thimerosal.
Over about three years, Miles compared data from 214 mothers of children with autism with data from comparison groups with no autism connection. If fetal thimerosal exposure indeed causes autism, then the percentage of thimerosal-exposed mothers of children with autism should have been higher than in other groups. But the results showed that it was not higher. It was statistically the same.
Miles’ study is one piece of research among many, though. Other studies have shown no association between thimerosal in vaccines and autism. Likewise, data shows a continued increase in autism diagnoses, even after thimerosal was removed from immunizations in the U.S. after 2002, Miles says.
“We can’t unequivocally say that small doses of mercury in thimerosal can’t cause autism,” she says, “but there have been lots and lots of studies, and there hasn’t been any consistent data that shows that it does. This is just one piece of the puzzle, but it’s a hot issue.”
Why it still matters
Out of precaution, several medical and governmental agencies called for the removal of thimerosal from immunizations by 2001, so American children immunized from 2002 to the present have not been exposed to mercury.
That’s not true in the developing world, where the much cheaper immunizations that contain thimerosal are still in use. “The United States is a wealthy country, so we can do it,” Miles says. “But if you’re looking at a third-world country, where they’re deciding whether they’re going to have children dying of diseases because they can't afford immunizations, they’ve got to make decisions based on data.”
Miles also remembers the days when kids wouldn't go swimming out of fear of getting polio, or when residents at hospitals saw all kinds of diseases that are virtually eradicated today because of immunizations. Eliminating fear is important to her.
“Hopefully, this will just reassure parents, because immunizations have changed the face of childhood,” Miles says.
Beyond that, sorting what does and doesn’t cause autism remains crucial to researchers — and to distraught parents who need answers.
“What I’ve always told people is that if we don’t know more next year than we know this year,” Miles says, “then we’re not doing our jobs.”