Paracelsus' poison

No, pesticides are not causing microcephaly in Brazil

The explosive expansion of the Zika virus in the Americas has only been exceeded by the rise of conspiracy theories blaming everything from vaccines to genetically modified mosquitoes. While not a conspiracy theory, recently the internet and news media have been running with a report that claims the birth defects attributed to the Zika virus is in fact due to pesticides.

Short summary: this claim is not plausible.

Just to remind you, the Zika virus is a generally mild, mosquito-borne virus that has recently arrived in the Americas and is spreading rapidly. The virus has been linked to the rise of microcephaly, where babies are born with very small heads, in Brazil and its rapid spread is the focus of international concern.

However, a recent report from the group “Physicians in the Crop-Sprayed Villages” has claimed that the outbreak of microcephaly is due to a pesticide used to control the mosquitoes that carry Zika virus (and Dengue fever).

How does pyriproxyfen work?

The pesticide in question is pyriproxyfen, a replacement for the organophosphate pesticides originally used for mosquito control that the mosquitoes are becoming resistant to. Pyriproxyfen acts by interfering with the hormonal control of the insect growth cycle. Most insects do not hatch as small adults and get bigger, but go thorough successive stages of larvae, to pupa, then to adult.

Pyriproxyfen mimics one of the key hormones in this process, juvenile hormone, killing insects at very low concentrations. This hormone control system does not exist in organisms with backbones, such as humans, and pyriproxyfen has very low toxicity in mammals as a result.

How toxic is pyriproxyfen?

This isn’t taken on faith though, pyriproxyfen has been extensively tested in animals for both acute and chronic toxicity. We have also learnt from the DDT story, and test extensively for reproductive and birth defects in animals exposed to any chemicals that are brought onto the market.

Based on animal studies, an adult human would need to eat a teaspoon full of the raw pesticide to reach the threshold levels for toxicity seen in long-term feeding experiments in animals. Regulatory levels are of course set much lower than this (0.1 milligram per kilogram body weight).

At the regulatory levels that are permitted in drinking water, a 60 kilogram person would need to drink 200 litres of water to reach the toxic threshold[1]. In terms of how much is actually present in water reservoirs that have been sprayed with pyriproxyfen to control mosquito larvae, a person would have to drink well over 1,000 litres of water a day, every day, to achieve the threshold toxicity levels seen in animals.

Does pyriproxyfen cause brain defects in animals?

The effect of pyriproxyfen on reproduction and foetal abnormalities is well studied in animals. In a variety of animal species large quantities of pyriproxyfen are required produce either small body weight or some changes in the skeleton. Importantly, pyriproxyfen does not cause the defects seen during the recent Zika outbreak. Neither brain abnormalities nor effects on neurodevelopment have been seen in animals exposed to pyriproxyfen during foetal development.

We have quite a lot of experience with pyriproxyfen, it is used extensively world-wide. It has been trialled for controlling Salt Marsh mosquitoes in Australia, and is present in many of the flea control preparations we use on our pets. Despite animals ingesting 100 milligrams of pyriproxyfen in tablet form, we haven’t seen outbreaks of microcephaly (or brain abnormalities) in our pets[2].

Pyriproxyfen is poorly absorbed by humans and rapidly broken down so even the minute amounts humans would be exposed to via water treatment would be reduced even further. As well, pyriproxyfen is relatively rapidly removed from the environment, so overall exposure will be low (certainly thousands of times lower than what our pets are exposed to with no sign of ill health or brain developmental abnormalities).

What is the evidence for Zika virus?

The “Physicians in the Crop-Sprayed Villages’” only evidence for the role of pyriproxyfen is that spraying began in 2014.

Yes, that’s it.

While the evidence that Zika virus is responsible for the rise in microcephaly in Brazil is not conclusive, it is substantial. As well as the coincidence of the outbreak and the rise of microcephaly, there are several reports of the Zika virus being found baby and foetal tissues. The latest report, where the complete genome of the Zika virus was found in a microcephalic foetus’s brain, is here.

In contrast, the role of pyriproxyfen, with at least a decade of safe use, is simply not plausible.

The importance of mosquito control

If there is one thing the “Physicians in the Crop-Sprayed Villages” gets right, is that for mosquito control, you need to involve the local communities.

Mosquito control is a significant public health issue. In the Americas, the mosquitoes carry not only Zika virus, but dengue and yellow fever and others.

I grew up in Brisbane in the late 50s and 60s, when the would come around and fog the local waterways behind our house, and I remember the rainbow slick in the local waterholes from kerosene put on the water to control mosquito larvae (my father-in-law learnt about dosing the hard way when he put too much kerosene in his water tank, the water tasted of kerosene for a long time after). And I remember the mosquito fish released into the creeks to help control mosquito larvae.

Public involvement to reduce standing water sources where disease-carrying mosquitoes breed, and to prevent breeding in drinking water supplies is critical. But as well as mosquito fish you need spraying programs to control mosquitoes where people can’t reach.


Pyriproxyfen is simply not plausible as the cause of microcephaly seen in Brazil. If the current panic over pyriproxyfen interrupts mosquito control programs, this will only help spread disease and decrease the health of the local communities.

[1] In some interviews I used 300 litres, based on heavier body weights, here I’m using the international reference body weight.

[2] Seriously, our pets are exposed to around 10 milligrams of pyriproxyfen per kilogram body weight, exposures from drinking water (or spraying or other forms of administration) is on the order of nanograms per kilogram body weight, around 10,000,000 times lower. Did the “Physicians in the Crop-Sprayed Villages” not do any dose checking?

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