A study of autism rates in California suggests that cases continue to rise despite the removal of the mercury containing vaccine from childhood vaccinations.
Researchers from the California state Department of Public Health looked at recorded cases of autism between 1995 and 2007. They found that autism rates rose steadily over this 12 year period.
Some researchers and many parents believed that the mecury containing preservative thimerosal, which was routinely added to childhood vaccinations, was playing a role in the increasing number of cases of the condition. As pressure from parents, advocacy groups, and some doctors mounted, thimerosal stopped being added to childhood vaccines in 2001.
The researchers and many commentators say that the study indicates that mercury in vaccinations cannot be a factor in the development of autism. Since autism is typically diagnosed beyond the age of two, they say that autism rates should have dropped between 2004-2007 but this doesn't seem to be the case.
For the study, which is published in the January issue of the journal Archives of General Psychiatry, California public health officials calculated the autism rate by analyzing a database of state-funded centres that care for people with autism spectrum disorders.
The analysis revealed that the prevalence of autism in the state in children aged 3 to 12 increased throughout the study period. Over a 10 year period from 1993 to 2003 the rate of children born who went on to develop autism by age 3 had increased from 0.3 in 1000 to 1.3 in 1000, a more than fourfold increase. The researchers report that this pattern is repeated at different ages.
"These time trends are inconsistent with the hypothesis that thimerosal exposure is a primary cause of autism in California," the researchers concluded.
While many doctors and organizations are holding the study up as definitive proof that mercury and vaccinations are not involved in autism and the rise in cases, many unsolved mysteries remain.
For example, the symptoms of mercury poisoning are remarkably similar to those exhibited by children with autism. A number of studies have also shown that important enzymes involved in detoxification, such as glutathione, are abnormally low in autistic children. Researchers involved in these studies have concluded that this leaves autistic children less able to detoxify environmental toxins such as mercury.
Geraldine Dawson, the chief science officer for the advocacy group Autism Speaks, told the AP that the California research was "a very important study," and said all possible causes -- genetic and environmental -- need to be explored aggressively.
"The bulk of the evidence thus far suggests that mercury is not involved, but I think parents still have many questions," said Dawson. "I think until parents are satisfied, we need to continue to examine the question."
Source: Archives of General Psychiatry: http://archpsyc.ama-assn.org