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Exposure to pyrethroid insecticides linked to behaviour problems

Even low environmental doses met by public may be associated with abnormal behaviour

Louise Prime

Thursday, 02 March 2017

Exposure to pyrethroid insecticides is associated with childhood behavioural difficulties, research has revealed. Authors of the study,* published online today in Occupational & Environmental Medicine, said that although they hadn’t established a causal relationship, they found the link even at the low environmental doses encountered by the general public.

Pyrethroid insecticides are commonly used synthetic chemicals found in some mosquito repellents as well as treatments for head lice, scabies and fleas. They are considered a safer alternative to organophosphates but, because they work by damaging nerves, a group of researchers from INSERM-IRSET in France decided to investigate potential impact of environmental exposure to pyrethroid insecticides on child neurodevelopment. They enrolled 3,421 pregnant women from Brittany, France between 2002 and 2006; and randomly selected 428 of these mothers when the children turned six, of whom 287 (67%) agreed to join the study.

The research team measured levels of five pyrethroid metabolites in urine samples collected from mothers at 6-19 gestational weeks and from the children at six years old. On home visits, they assessed the children’s behaviour at the age of six using the Strengths and Difficulties Questionnaire (SDQ). They considered three subscales of the SDQ – prosocial behaviour (altruism), internalising disorders (inability to share problems and ask for help) and externalising disorders (defiant and disruptive behaviours). The mothers also completed a detailed questionnaire on socioeconomic factors, lifestyle, their child’s behaviour, and various environmental exposures.

The study authors reported that three metabolites (trans-DCCA, cis-DBCA, and cis-DCCA) occurred the most frequently in the urine samples of both the mothers (100%, 68%, and 65% respectively) and their children (96.5%, 85%, and just under 65% respectively).

After taking account of potential confounding factors they found that a higher antenatal level of cis-DCCA in the mothers’ urine was associated with a higher risk of internalising behaviours in their six year olds. They also found that increasing levels of another metabolite (3-PBA) in the children’s urine samples were associated with an increased risk of externalising behaviours; for the intermediate and highest category of metabolite levels, the odd ratios were almost tripled (OR 2.93) and doubled (OR 1.91), respectively, for abnormal or borderline social behaviour. High levels of trans-DCCA in children’s urine were associated with a lowered risk of externalising behaviours.

The researchers suggested that their findings might be explained by pyrethroids altering neurochemical signalling in the brain. They pointed out that their observational study could not show causal association; and also that the speed with which pyrethroid metabolites are cleared from the body makes it difficult to assess accurately pyrethroid exposure from urine samples.

But they concluded: “The current study suggests that exposure to certain pyrethroids at the low environmental doses encountered by the general public may be associated with behavioural disorders in children.”

* Viel J-F, Rouget F, Warembourg C, et al. Behavioural disorders in 6-year-old children and pyrethroid insecticide exposure: the PELAGIE mother-child cohort. Occup Environ Med, published online first: 01 March 2017. DOI: 10.1136/oemed-2016-104035.

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