Sunday, February 14, 2016

"It's not because of a virus, it's those nasty chemicals"***

Another couple of ideas have been presented to try and explain the apparent rise in microcephaly in NE Brazil. I have seen people point to glyphosate, atrazine, and now pyriproxyfen. The biggest problem with blaming these chemicals is that they've been used worldwide and other areas are not seeing an increase. There is also a big issue that needs to be addressed before we discuss the weak evidence against pyriproxyfen. It looks like the number of cases of microcephaly have historically been underreported in NE Brazil. With the new numbers, there has just been a slight increase in microcephaly cases above what would be considered the baseline. This underreporting has had the benefit of helping to identify the possible role of Zika virus in birth defects as it has been closely examined. As a result, new light on the biology and potential for it to cause birth defects in utero has been uncovered.

Pyriproxyfen is an insect growth regulator which generally has low oral toxicity for mammals. For pyriproxyfen, the LD50 was found to be >5000 mg/kg in rats. Pyriproxyfen has also been used for almost 20 years worldwide. The biggest fear here is that villainizing pyriproxyfen will lead to other control measures being taken. DDT is out of the question due to bioaccumulation and biomagnification. Citronella is a repellent that lasts for less time than DEET and it doesn't kill mosquitoes. Pretty much we would be left with organophosphates, and those compounds can be highly toxic (there is a reason why insect growth regulators replaced them). 

We can use a logic tool here to look at this situation. Which is more likely: that the surge in microcephaly cases are due to underreporting combined with an emerging viral pathogen that could cause birth defects, or that the surge is due to an insecticide that has been widely used for many years suddenly causing an issue. Occam's Razor would dictate that the simplest answer is likely the correct one. Human error in underreporting in a small region is far more likely than hundreds of scientists working for many different government agencies worldwide, from the WHO to the EPA, missing a link between pyriproxyfen and microcephaly. 

***Title is satirical