Concern over insect repellent

Wednesday August 5 2009

“Insect repellents used by millions of holidaymakers each year are potentially toxic,” claims the Daily Mail, which says that scientists now say an ingredient of the sprays could cause fits in children and should not be used by pregnant women. The chemical DEET, found in many mosquito sprays, has been shown to be toxic to nerves in animals and insect studies. The researchers say their results highlight potential implications for human health.

In this research scientists found that DEET blocked the enzyme cholinesterase in rats. The enzyme is essential for transmitting messages from the brain to the muscles, and chemicals that interfere with it can cause excessive salivation and eye-watering in low doses. This can be followed by muscle spasms and, ultimately, death.

The harmful effect on insects is not surprising since the chemical is used to repel them; however, DEET had previously been thought solely to affect insects’ sense of smell. This discovery of the effect it has on an important enzyme in the nervous system (which is also found in humans) warrants further investigation.

Where did the story come from?

This research was carried out by Dr Vincent Corbel and colleagues from the Institut de Recherche pour le Développement in Montpellier, France and other institutes in Benin and Slovenia. The study was financially supported by the French National Research Agency and published in the journal BMC Biology.

What kind of scientific study was this?

This was a laboratory-based study using toxicological, biochemical and electrophysiological techniques to investigate whether the chemical DEET inhibits cholinesterase activity in the nervous systems of insects and mammals.

DEET, otherwise known as diethyl toluamide, is an insect repellent chemical used in a large proportion of repellent sprays. The authors of this study estimate that each year around 200 million people use insect repellents containing DEET.

Despite being the standard against which other repellents are often tested, how DEET works is poorly understood. Its toxic effects are known to reduce insects’ sense of smell, which is thought to prevent them from detecting the scent of humans. Exactly how DEET affects the insects’ sense of smell and nervous systems has not previously been investigated.

The researchers were particularly interested in the effect DEET had on acetylcholinesterase, an enzyme found in the spaces between nerves. Its function is to break down the neurotransmitter acetylcholine, which plays an important role in transmitting nerve signals around the body. The enzyme’s breakdown of acetylcholine allows the nerve to return to its resting state after it has been active.

The researchers looked at the death rates of selected insects, including the dengue fever mosquito, when exposed to a range of DEET-treated fly papers. The doses used were similar to those that could be applied to human skin. They then looked at the effects of the chemical on the abdominal nerves of dissected cockroaches and in the diaphragm nerves of mice. The techniques they used measured the voltages in the synapse, the gap between nerves, after a single burst of activity created by stimulating the nerve further up.

The researchers also looked at the interaction between DEET and the common insecticide known as carbamate, which also blocks the acetylcholinesterase enzyme. They did this in the test tube at a molecular level using a device called a spectrometer, which is able to assess how DEET binds to the acetylcholinesterase enzyme.

What were the results of the study?

The researchers showed that DEET does not simply modify the behaviour of insects, but also directly inhibits enzyme activity, both in insect and mammal nerves.

They also showed that DEET interacts and strengthens the toxicity of carbamates, a class of insecticides also known to block acetylcholinesterase.

What interpretations did the researchers draw from these results?

The researchers conclude that their findings “question the safety of DEET, particularly in combination with other chemicals”, and “highlight the importance of a multidisciplinary approach to the
development of safer insect repellents for use in public health”.

What does the NHS Knowledge Service make of this study?

This laboratory study has raised important questions regarding the mechanisms behind the action of DEET, a common ingredient of many insect repellent products. Further research is required to answer these questions effectively.

Although it is not surprising that an insect repellent is toxic to insects, the newly discovered mechanism of the repellent is something that will need evaluation. Particularly important will be studies that investigate the ideal dose that can repel insects without running the risk of serious toxic effects in humans. These results must also be weighed up in relation to the risk of diseases carried by mosquitoes in tropical countries, particularly malaria. The potential harms of restricting DEET use must be considered in order to reach a balanced view on this issue.

This study appears to have been well conducted, and it would seem wise for individuals to avoid the use of this repellent if pregnant as a precautionary measure, as it is unknown whether the chemical might cross the placenta and affect an unborn child. There has been newspaper speculation that products containing DEET might trigger fits in children, but this was not shown through this research and this remains a theory based on extrapolation of this science from an effect seen in animal nerves.

Analysis by Bazian
Edited by NHS Choices