Possible pesticide link to autistic disorders

“Pregnant women who live near fields sprayed with pesticides can run more than three times the risk of having a child with autism,” the Mail Online reports.

US researchers conducted a study that examined whether living in close proximity to where four common classes of agricultural pesticides were used while pregnant was associated with a higher risk of the mother’s offspring having autism spectrum disorder (ASD), or a similar developmental disorder.

Data on pesticide use was “mapped” to the mother’s place of residence while she was pregnant.

The main findings of the study were that living near (within about 1.25km distance) to where pesticides were used at any point during pregnancy (compared to no exposure) was associated with a 60% higher risk of the child having ASD.

Despite these seemingly alarming findings, it is important to note that causation cannot be established.

It is also worth noting that this study analysed data in California – a region with a high pesticide usage, so the findings may be considered "extreme".

From what is known about ASD, it is unlikely that a single environmental factor, such as exposure to pesticides, can cause the condition. It is currently thought that the condition arises through a complex mix of genetic and environmental factors.

Where did the story come from?

The study was carried out by researchers from the University of California in the US and was funded by various grants and the University of California’s Davis Division of Graduate Studies and MIND Institute.

The study was published in Environmental Health Perspectives, a peer-reviewed open-access journal, so it is freely available to read online.

The story was picked up by the Mail Online. The headline, “Crop sprays 'raise risk of autism in unborn children'” is alarmist, as no cause and effect link has been proven.

However, the paper does provide some useful reaction quotes from independent experts. For example, the National Autistic Society is quoted as saying that “the development of autism is much more complicated than the researchers had suggested”.

What kind of research was this?

This was a type of exploratory research that used data from a wider study (the Childhood Autism Risks from Genes and Environment study or CHARGE study) and linked it with data obtained on pesticide use in California. The researchers say California is the top agricultural producing state in the US and that each year, approximately 200 million pounds of active pesticide ingredients are used throughout the state.

The CHARGE study is a population-based case-control study of over 1,600 children aged between two and five years, born in California. Cases (children with diagnosed ASD or developmental delay) are matched to controls (people without these conditions). The ongoing CHARGE study aims to look at a range of factors that may contribute to autism and developmental delay by asking parents extensive questions about environmental exposures during pregnancy.

What did the research involve?

In this latest study, the researchers aimed to investigate the association between living near to where agricultural pesticides were being used during pregnancy and the risk of ASD and developmental delay in the offspring.

They were also interested in seeing whether possible exposure to pesticides during different stages of pregnancy were associated with a higher risk.

Previous studies suggest that any type of exposure to a particular substance that occurs during the first trimester of pregnancy can have the biggest influence on subsequent development.

Parents of participants in the CHARGE study were asked to report all addresses where they lived, from three months prior to conception up to the time of delivery.

Based on previous studies, the researchers chose to investigate the following groups of pesticides:

organophosphates
carbamates
organochlorines
pyrethroids

Data on pesticides was obtained from a publicly available yearly pesticide report about pesticide use in California in areas such as parks, golf courses, cemeteries and rangeland.

Pesticide use in post-harvest treatment of agricultural commodities, in poultry and fish production, and in some livestock applications was also measured.

The researchers report excluding home and garden use, and most industrial and institutional uses of pesticides, though it is unclear from this description specifically what has been excluded.

The data includes use of these pesticides by date, square mile and the amount of chemical used.

In this latest study, mapping software was then used to determine a geographical picture for this pesticide use using radii of 1.25km, 1.5km and 1.75km around each place of residence.

Each pregnancy was then assigned an exposure profile, based on pesticide use nearby to where the mother lived and days of pregnancy on which the pesticide use occurred.

Statistical techniques were used to estimate the risk of exposure to agricultural pesticides by comparing confirmed cases of ASDs or developmental delay with a control group of children who had typical development.

Adjustments were made for some confounders (e.g. paternal education, home ownership, maternal place of birth, child's race/ethnicity, maternal pre-vitamin intake and year of birth).

What were the basic results?

The main findings of this study were:

approximately one-third of mothers lived within a 1.5km (just under one mile) radius of where one of the four classes of agricultural pesticide were used
of the pesticides evaluated, organophosphates were the most commonly used agricultural pesticide near the home during pregnancy, followed by pyrethroids

In the analyses of any exposure during pregnancy vs. no exposure:

children with autism spectrum disorder were 60% more likely to have had organophosphates applied near the home (1.25km distance; adjusted odds ratio [aOR] 1.60, 95% confidence interval [CI] 1.02 to 2.51) than mothers of children with typical development. This risk was found to be higher for exposure to organophosphates during the third trimester of pregnancy (OR 2.0, 95% CI 1.1 to 3.6)
risk for developmental delay was increased for children of mothers who lived nearby to where carbamate pesticides were being used (1.25km distance; aOR 2.48, 95% CI 1.04 to 5.91), but no specific period during pregnancy was identified as being associated with a greater risk
children of mothers living near to where pyrethroid insecticide was used just prior to conception or during the third trimester were found to be at greater risk for both ASDs and developmental delay (ORs ranged from 1.7 to 2.3)

How did the researchers interpret the results?

The researchers concluded that the children of mothers who live near agricultural areas, or who are otherwise exposed to prganophosphate, pyrethroid or carbomate pesticides during pregnancy, may be at increased risk of neurodevelopmental disorders.

Conclusion

Overall, this exploratory study provides some limited evidence of a possible link between living in close proximity to where four common classes of pesticides are used during pregnancy and their offspring having ASDs. However, it does not provide evidence of causation. The exact causes of ASDs are largely unknown, although it is thought that several complex genetic and environmental factors are involved. There may be many other factors at play that the researchers did not take into account.

There is also the possibility that there is no association at all between ASDs and pesticide use, and that these were chance findings.

Though the original sample size was fairly large, the study included only 144 children with ASDs whose mothers were exposed to pesticides at any time during pregnancy or pre-conception. When further dividing this sample of 144 children into the specific pesticide they were exposed to, and the trimester of pregnancy at which they were exposed, the numbers become smaller still. When conducting statistical analyses using small sample numbers, this increases the possibility of chance findings.

The number of children with developmental delay who had been exposed to any pesticide before birth was smaller still – only 44 children.

It is also worth noting that this study analysed data of the top agricultural state in the US: California. Because of this, more agricultural pesticides are used in this state than any other, meaning that the findings may not be generalisable to areas with different pesticide use, or to urban areas where different pesticides are used.

The authors also report some limitations to their study, including the fact that the approach used to obtain exposure to pesticides may not have included all potential sources of exposure to each of the classes of pesticides of interest. This is because not all pesticide use was captured in the publicly available report the researchers used to capture this exposure data.

In addition to this, information on the hours the mother spent in the home or elsewhere was not available, which may also contribute to errors in estimating pesticide exposure.

As stated, it is also unclear which type of industrial and institutional uses of pesticides were excluded.

The exact causes of ASDs are largely unknown, although it is thought that several complex genetic and environmental factors are involved. This study adds to the growing literature in this area.