15 May 2008
Could insecticides in pet shampoos trigger autism spectrum disorders? That’s the suggestion of one of the first large-scale population-based studies to look how environmental factors and their interactions with genes contribute to the condition.
Mothers of children with an autism spectrum disorder (ASD) were twice as likely to have reported using pet shampoos containing a class of insecticide called pyrethrins as those of healthy children, according to survey results presented Thursday at the International Meeting for Autism Research in London. The risk was greatest if the shampoo was used during the second trimester of pregnancy.
Meanwhile, another study suggests that exposure to organophosphate insecticides double the risk of developmental disorders, including autism. Organophosphates have previously been linked to Gulf War syndrome.
While many chemicals have previously been blamed for triggering autism, there have been very few rigorous studies designed to investigate the link.
To remedy this, Irva Hertz-Picciotto and her colleagues at the University of California in Davis, US, studied 333 children with ASD and 198 healthy children, and their families.
They collected blood and urine samples, as well as conducting in-depth questionnaires on medical history and any possible exposure to medications, household products or metals that could have occurred around the time of conception, during pregnancy, or after birth. They also collected information on lifestyle, and whether the children were breast-fed, for example.
Previous studies in insects and rodents have suggested that pyrethrins could damage the blood-brain barrier during early life, and cause neuronal damage. They may also interfere with the transmission of signals along nerve fibres.
“Autism is associated with an imbalance between excitatory and inhibitory neurotransmitters within the brain, and one could hypothesise that children with an imbalance in this system may be more sensitive to the effects of pyrethrins,” says Isaac Pessah at UC Davis, who was also involved in the study.
‘No single answer’
However, Hertz-Picciotto and other experts caution that pyrethrins were unlikely to be the only cause of autism, which is increasingly recognised as being caused by a complex interplay of both genetic and environmental factors.
“This is a candidate, but there is unlikely to be a single answer to what causes autism,” says Tony Charman of University College London, UK, and chairman of IMFAR.
“The findings now need to be replicated and more studies need to be done to understand how this toxin affects development,” he says.
Neither are pyrethrins likely to be the only chemicals that increase the risk of ASD.
For example, in a separate study, Brenda Eskenazi and her colleagues at the University of California in Berkeley, measured levels of metabolites from organophosphates in urine samples taken from 400 women during pregnancy, and their children. Such metabolites were associated with a two-fold risk of developmental disorders in the children.
Pessah says that chemicals like pyrethrins and organophosphates are likely to increase the risk of autism in children who already have an underlying genetic susceptibility.
“ASD is a spectrum of disorders, some of which are very mild, and chemical or environmental factors may shift that profile so that children who would otherwise not receive a diagnosis of ASD will receive one.”
Geri Dawson, chief scientific officer at research charity Autism Speaks, agrees.
“Probably in the majority of cases autism is a complex disorder involving multiple genes and multiple environmental factors,” she says. “However, at least if we can identify the environmental factors, there is something we can do about them.”
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