What causes autism?
This is the question that researchers have attempted to answer for years. As the prevalence of autism (ASD) increases with each passing year (U.S. autism cases grew by 72% from 2007 to 2013), researchers, like those at CMSRI, are anxious to pinpoint the exact cause of this neurological disorder.
Researchers are getting closer, but as scientists C A Shaw, S Sheth, D Li and L Tomljenovic point out in their “Etiology of autism spectrum disorders: Genes, environment, or both?” paper, published in OA Autism, genetic causes have dominated the focus of ASD etiology research, but not to the field’s advantage. Many believe that individuals on the spectrum have a genetic predisposition for the disorder, and some research has shown that ASD can be inherited. However, not all cases of ASD are explained by genetics, which multiple twin studies have helped reveal. These studies found that 55% of ASD cases between twins were the result of environmental factors, while 37% were the result of genetics. This suggests that an environmental stimulus, which could even act as a genetic trigger, may often be at play.
Aside from twin studies, there’s an abundance of scientific evidence to suggest ASD is the result of environmental influences. One of the most obvious is the commonalities between ASD and other neurological diseases like Alzheimer’s, ALS, and Parkinson’s disease, all of which share many of the symptoms of ASD, and all of which have largely been determined to be the product of environmental influences, rather than genetic mutations. Scientists have actually been able to replicate the symptoms of ALS in outbred mice with no gene mutations without altering their genetic makeup.
Individuals with ASD also often have abnormal neural connectivity, and as Shaw et. al point out, there is a growing body of research that suggests this abnormality is the result of immune signaling interfering with the development of circuitry, and in turn, causing symptoms of ASD. These neural connections have also been shown to maintain neuroinflammation, which suggests immune-related pathways in the brain have been altered. The presence of autoimmune manifestations like immunoglobins and CNS autoantibodies in ASD individuals, manifestations not found in neurotypical brains, have also been found in those with Alzheimer’s and Parkinson’s disease. Multiple findings suggest that an ASD individual’s abnormal connectivity is the result of what’s known as immune insults, or damage to the immune system, shortly after birth.
This connection between immune insults soon after birth and an abnormal central nervous system has already been substantiated by research, though not specifically in the context of autism; for example, it’s been shown that peri- and post-natal immune insults cause abnormalities in behavior and cognitive responses, deficiencies in social and sensorimotor abilities, and heightened anxiety. All of these abnormalities appear in individuals with ASD, and all are caused by disruptive environmental influences.
So the question becomes, what environmental factors are responsible for the dramatic increase in ASD? Though we may not have a precise answer, it has been found that xenobiotics like mercury, aluminum, and lead can cause neuroinflammation and immune dysfunction, which produce each of the observed abnormalities. Exposure to these xenobiotics can come from various sources, but the only source that’s almost universally exposed to pregnant women and children in the United States is vaccines. Many flu vaccines today include the mercury compound Thimerosal, which was also present in many childhood vaccines before being banned in 2001. Aluminum, another toxic xenobiotic, is still used as an adjuvant in a large number of pediatric and adult vaccines.
The potential role vaccines might play in ASD development is a topic that merits more full-bodied analysis and discussion; however, the rapid increase in cases of autism since 1980, in conjunction with compelling scientific evidence, does strongly suggest one thing: environmental factors should be more closely analyzed as potential causes of autism.