May 20, 2008 (London, United Kingdom) — Compared with mothers of typically developing children, mothers of children with autism spectrum disorders were twice as likely to report that they had shampooed their pets with pyrethrin-containing antiflea/antitick shampoos around the time of their pregnancy, in a Californian case-control study that looked at household pesticide use.
The findings were presented by lead study author Irva Hertz-Picciotto, PhD, from the University of California, Davis, at the 7th Annual International Meeting for Autism Research.
When participants in the study, the Childhood Autism Risks and the Environment (CHARGE) trial, were questioned about their prenatal, gestational, and postnatal use of pesticides, the researchers found that products containing pyrethrin — pet shampoos and certain sprays for controlling flies, ants, and cockroaches — were associated with an increased risk for autism spectrum disorders.
These are initial findings and need to be confirmed in other population studies, Dr. Hertz-Picciotto told Medscape Psychiatry, adding: “The bottom line here is [that pyrethrin] is something that really deserves further study.”
It is important to remember that autism is multifactorial, she stressed, explaining that “generally speaking, probably most cases of autism arise from multiple genetic as well as multiple environmental factors.”
Outdoors, pyrethrin has a very short half-life, but indoors it lingers for a long time — for example, in pet hairs — so that people continue to be exposed, she noted. Concerned consumers can seek out more natural, nontoxic alternatives (such as boric acid for cockroaches).
Pesticides are designed to attack the central nervous system of lower species such as insects and rodents, Dr. Hertz-Picciotto explained.
A recent study suggests that there is a link between maternal exposure to commercially applied organochlorine pesticides and subsequent risk of autism in the child, she said. Eric M. Roberts, MD, from the California Department of Public Health, in Richmond, and colleagues reported that pregnant mothers who lived in the California Central Valley close to fields where organochlorine pesticides were being applied had an increased risk of giving birth to a child with autistic spectrum disorder (Roberts EM et al. Environ Health Perspect. 2007;115:1482-1489).
The current study aimed to examine the relationship between use of household pesticides during pregnancy and subsequent autism in the child.
The researchers looked at data from the CHARGE study of 2- to 5-year-olds living in California — 333 children with autism spectrum disorder and 198 typically developing children. The children’s diagnosis of autism was confirmed using the Autism Diagnostic Observation Schedule (ADOS) and the Autism Diagnostic Interview (ADI).
In a 90- to 100-minute telephone interview, the children’s mothers were asked about their use of household pest-control products (such as insecticide sprays, ant poisons, pet shampoos, and weed-control products) during the exposure period (which was defined as 3 months prior to conception until the child was 1 year old).
After adjustment for socioeconomic variables, compared with mothers of typically developing children, mothers of children with autistic spectrum disorders were twice as likely to report having shampooed their pets with antiflea/antitick shampoos during the exposure period (odds ratio [OR], 2.0; 95% CI, 1.2 – 3.6). The adjusted odds ratio for this association was strongest for the second trimester (OR, 2.6; 95% CI, 1.3 – 6.0).
The researchers determined that the active ingredient in these pet shampoos was pyrethrin, which came into use about 20 years ago to replace organophosphates, said Dr. Hertz-Picciotto.
When the researchers went back and analyzed the data according to products containing pyrethrin (which included some sprays for controlling flies, ants, and cockroaches, as well as pet shampoos) instead of by product type, the association between pyrethrin and autism spectrum disorder remained.
Was Recall Bias a Factor?
Because these data were collected retrospectively, it is possible that the mothers of typically developing children tended to forget about their use of pesticides around the house, unlike the mothers of the children with autistic spectrum disorders, who may have been more attuned to thinking about this. However, since the researchers did not find an association between autism and products not containing pyrethrin, recall bias was not likely to be a strong factor.
What Does This Mean?
Pyrethrins have largely replaced organophosphates for flea control, the group writes. In insects, pyrethrins affect the nervous system and sodium channels, which results in repeated firing of neurons and death. In rodent studies, pyrethrin exposure when the fetal brain is developing was found to compromise the blood-brain barrier.
Although pyrethrin-containing pesticides have been tested for safety, animal findings and the current study findings raise concerns about the long-term neurodevelopmental effects from prenatal, gestational, and early postnatal exposure to pyrethrin-based products.
The group is planning to submit this work for publication in the near future.
In another presentation at the meeting, Brenda Eskenazi, PhD, from the University of California, Berkeley, reported that in a study of low-income Mexican farmworker families in California, organophosphates were associated with pervasive developmental disorder (or autism spectrum disorder) in children.
The study participants, from the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) birth cohort study, included over 400 children as well as their mothers. Urine samples from pregnant mothers and from their offspring were analyzed to detect organophosphate metabolites. The presence of these agents was linked with increased odds of the mothers reporting that their children had pervasive developmental disorder at age 2 years.