There is a diary published earlier today about autism and a newly published study that links certain pesticides with autism. The diarist also asked if Monsanto still wanted to fund anti-vaxxers in the title of the diary. I'm going to say right now that this diary has nothing to do with vaccines. What got me was the possibility that environmental factors could impact autism incidence and prevalence rates. I have friends, family and students affected by autism. I decided to chase this rabbit down the hole.
It didn't take long for me to find the original study from the UC Davis web site. It's published in a peer reviewed journal called Environmental Health Perspectives (EHP). I skimmed through it and found it to be interesting. It's a step beyond an earlier study done in 2007 that looked at pesticides and neurodevelpmental delays. Then, I thought about the chemicals I have lying around the house and all the studies I'm aware of that link pesticides with negative health outcomes and genotoxicity.
The idea that pesticides are toxic to adults, children and pets; plus cause a lot of unintended consequences (pdf) is as old as DDT. The comment thread on the diary was full of, lets call it, skepticism. It was skeptical of the idea that pesticides could be associated with increased autism incidence rates. The bibliography of the UC Davis study shows this study is only building on the idea that pesticides are associated with multiple neurological and behavioral issues. This study was only another brick in the wall. Being skeptical is one thing, but it became clear to me that no one was looking at the number of peer reviewed studies already published with findings that build a strong case to suspect pesticides as an environmental factor influencing autism.
With Autism incidence rates rising, it's no wonder our government is trying to get a bead on the problem. What I was surprised to find out is how long and how diverse autism research is and that there is much more we need to learn about autism.
What NIH says about autism
What causes autism?
Scientists aren’t certain about what causes ASD, but it’s likely that both genetics and environment play a role. Researchers have identified a number of genes associated with the disorder. Studies of people with ASD have found irregularities in several regions of the brain. Other studies suggest that people with ASD have abnormal levels of serotonin or other neurotransmitters in the brain. These abnormalities suggest that ASD could result from the disruption of normal brain development early in fetal development caused by defects in genes that control brain growth and that regulate how brain cells communicate with each other, possibly due to the influence of environmental factors on gene function. While these findings are intriguing, they are preliminary and require further study. The theory that parental practices are responsible for ASD has long been disproved.
The preponderance of meta analysis and study reviews is that environmental factors interact with genetics in exposures that need better definitions of dosage, duration and pathways.
Searching for Studies
I found links through the U.S. National Library of Medicine® (NLM)'s PubMed and MedLine. I found links at a private search site, Jstor. I even found them with Google's scholarly article search.
All of these search methods lead to peer reviewed studies. Some give better abstracts than others, but if you put in the time, you can find out the current thinking on a link between environmental factors including pesticides and autism.
Abstracts and Full Articles of Peer Reviewed Publications
Genetic links to autism has been studied for several decades. I found multiple studies linking environmental factors such as pesticide exposure to autism, birth defects and neural behavioral disorders with data going back to 1990. Other studies linking advanced parental age to autism has been around for some time. Genetic damage and chemical exposure is also found. No one study is likely to identify a single recipe for autism. ASD is likely to be a matrix of variables of chemicals, neurotransmitters, dosages, time frames, intensity, genetics that interact in multiple ways that lead to autism. For now, all we're going to see is risk factors. Pesticides might not be definitive for autism, but they are associated with a whole host of negative health outcomes and worth avoiding.
Here's the list of what I found
This 1995 study of British twins revisited a previous study and then, added to it. It showed a significant genetic associative component of autism.
This 1997 study (pdf) compared 25 families with two autistic siblings with 30 families with Down Syndrome Probands and found higher rates of social and communication deficits and stereotyped behaviors were in the relatives in the families with multiple-incidence autism. Twenty-five vs. thirty is too small a cohort to use for statistical significance, but it can suggest a path for further study. In this case the suggestion was "to delineate the boundaries of the broader autism phenotype and that this broader phenotype should be included in some future genetic analyses of this disorder."
This full article was published in 2000 by Environmental Health Perspectives. It called for more study because the then current findings in autism and in related disorders pointed to the possibility of a gene-environment interaction causation.
This 2002 study showed a correlation between fathers who used pesticides and children with birth defects. This study is used as a proof of concept that pesticides (neurotoxins) and genetics interact and effect offspring.
In another 2002 proof of concept study pesticides and tobacco smoking both were determined associated with significant amounts of genotoxicity.
This 2006 Israeli study found advanced paternal age is an associated (correlated) link to autism.
This 2006 study (pdf) looked at of 254 NYC minority infants born in 1997 and found that "high in utero exposures to organophosphates were associated with significantly lower birth weight and shorter length. Those children who were exposed to the highest levels of chlorpyrifos in utero showed impaired mental and motor development at 36 months." The researchers then recruited 25 Hispanic pregnant women in rural Southern Texas and found the presence of a variety of pesticides in the indoor air of homes of pregnant women in South Texas and documented their reports of frequent pesticide application. This study is significant in that it shows that airborne pesticides in the urban New York apartments was higher than in rural South Texas homes, which suggests pesticide levels need to be measured both in the home and on the job in addition to accounting for agrarian exposure.
This 2007 case analysis of children born between 1996-1998, can't prove a causation either, but it found a correlation between specific types of pesticides and ASD (Autism Spectrum Disorder).
This 2007 study looked at Organophosphate (OP levels by following 531 mothers through their pregnancy to the children turning 2 years of age. 372 children were followed throughout the study. They found a correlation between pesticide levels and mental development and pervasive developmental problems with the children at 24 months of age. This study did not specify autism, but a broader category of neurological developmental issues that includes ASD.
This 2008 study also links (associates, correlates) advanced parental age with ASD.
This 2010 Study (pdf) showed a correlation between pesticide exposure and neurodevelopmental delays in Ecuador. Again, not autism, but a related issue - developmental delay.
This 2010 meta analysis reviewed the link between advanced paternal age and ASD and found a significant correlation that further strengthens the idea of genetic damage due to age is linked to autism.
This 2011 study looked at the months of conception for 6,604,975 children born from 1990 to 2002 and found a correlation of children conceived during December, January and February having a higher chance of autism which suggests environmental factors influence ASD.
This 2012 article surveyed multiple studies and determined that pesticides were a strong candidate for a link to autism. Their caveat: "Although we have described several possible avenues by which pesticide exposure may influence autism, the dearth of studies on large occupational and pregnancy cohorts with adequate exposure assessment impedes our understanding of a) whether pesticides are consistently associated with autism risk, and b) if so, which pesticide compounds and which components of those compounds might actually contribute to autism risk."
This 2012 study looked at chlorpyrifos (CPF), an organophosphate insecticide and found High-exposure children also showed frontal and parietal cortical thinning, and an inverse dose–response relationship between CPF and cortical thickness. This study reports significant associations of prenatal exposure to a widely used environmental neurotoxicant, at standard use levels, with structural changes in the developing human brain. We have other studies that show people with ASD have differences in brain development.
This 2013 meta-analysis looked at the "accumulating evidence shows that exposure to neurotoxic compounds is related to ASD" trying to find a commonality. Some studies did find a commonality, but there were exceptions leading their conclusion to be regarding neurotoxins: "Other factors also might be related, possibly altering the mechanisms at work, such as time and length of exposure as well as dose of the compound. Future research should focus on identifying the pathway through which these factors interact with exposure to neurotoxic compounds making use of human studies."
This 2014 study compared the brains of deceased children with and without autism and found distinct differences within brain tissue, which further bolsters the concept that autism starts before birth. "Building a baby's brain during pregnancy involves creating a cortex that contains six layers," says researcher Eric Courchesne, PhD. He continues: "We discovered focal patches of disrupted development of these cortical layers in the majority of children with autism. This defect indicates that the crucial early developmental step of creating six distinct layers with specific types of brain cells - something that begins in prenatal life - had been disrupted."
Tue Jun 24, 2014 at 10:05 AM PT: Thanks for the Rescue. I thought I thanked you earlier. Sorry for the delay. I got caught up in a statistics problem for work....the numbers aren't adding up.