Thursday, July 23, 2020

DEET Works

If you're not from the United States, you might not know about DEET. It's like a weapons-grade insect repellant, better than citronella (sorry to the naturopathic chemophobes out there). And if you're not from the U.S., you might not care about DEET, because you don't get mosquitos. As one living in the Northeast specifically, DEET is a household name (and sure, citronella plants abound in carefully defended backyards).

Since its widespread use in the 1940-50's, nobody has had a good idea of how it casts that magic spell. Does it smell really bad to mosquitos and actually repel them? Does it deactivate their smell receptors? Does it lower the rate of evaporation of our smelly sweat vapor, giving us a kind of odor-invisibility?

This is hard work you know. Mosquitos are pretty small, so getting inside their little brains is a bit difficult.

But scientists, man, they're smart. Instead of picking apart tiny insect brains, they use genetic engineering to tell how the bugs are being affected. To clarify, they're not using the DEET to genetically alter the bugs. Instead, they're making a genetic change to the mosquitos which makes it really easy for them to record their odor experience.

In fact, you may have already heard of this one -- fluorescent neurons. That's right, they genetically alter the mosquitos so that when/if their smell receptors activate in the presence of a human-sweat-odor-molecule, they literally light up.

And it turns out that the odor-masking theory is looking pretty good. I'm not sure how they do this, but they measured the actual amount of human odor molecules reaching the insect's antennae, and found way less after having sprayed the person first with DEET.

So DEET works, and this is why. And yes, citronellol works too, although its effect is not as potent.

And you know what is the absolute best mosquito-repellant, 100% effective? A fan. That's right. Next time you're sitting in your backyard, and the bug spray is wearing off, grab an oscillating fan and an extension cord. Problem solved, and no lingering smells afterwards.

image source link

Oct 2019,

Post Script
Citronella oil has been a registered insect repellant in the United States since 1948.

Post Post Script
General Mosquito was a brutal and feared leader in the Sierra Leone Civil War at the turn of the 21st century. He has his own wikipedia page. General Mosquito Killer, however, does not have his own page. He was even scarier than General Mosquito, as you might guess.

Thursday, July 2, 2020

Healthy Brains

An interest in olfaction has me following the anosmic effects of this disease. A career as an industrial hygienist has me posting public health information on this olfactory awareness weblog:

Our sense of smell is a secret weapon in the fight against the 2019 novel coronavirus.

1. Loss of smell (anosmia) is a primary symptom of Covid-19.
2. Covid-19 may cause neurological damage.
3. This is not a coincidence.

Most people don't realize it but your sense of smell offers very sensitive diagnostics for complex brain activity, like how it can predict Alzheimer's decades in advance. This is because our sense of smell has special connections with the brain that other senses don't have. This also gives us an opportunity to monitor our own "brain health," and that's the main reason I'm sending this.

I'll let this recent BBC article on the topic fill-in the details; it's not long:
How Covid-19 can damage the brain - BBC Future


If you don't read that article, and if you're about to stop reading this, please:

  • Pay attention to changes in smell or taste, both for you and those around you.
  • Changes in smell and taste are a primary symptom for Covid-19.
  • Changes in smell and taste, in general and aside from a Covid infection, could be a sign of neurological issues and may warrant attention from a professional.
  • The sooner you recognize neurological issues the better.


Monell Anosmia Project - US Organization studying smell and taste

AbScent - UK Organization raising public awareness of smell loss

National Institute on Deafness and Other Communicable Disorders (NIDC) - Smell Disorders

ENT UK - Loss of Smell as Marker of Covid-19 Infection

[x] image source link

Post Script:
The ability to detect smells predicts recovery and long-term survival in patients who have suffered severe brain injury, a new study has found. A simple, inexpensive 'sniff test' could help doctors to accurately diagnose and determine treatment plans for patients with disorders of consciousness.

Published in the journal Nature, ...

Simple sniff test reliably predicts recovery of severely brain injured patients
May 2020,

Olfactory sniffing signals consciousness in unresponsive patients with brain injuries, Nature (2020). DOI: 10.1038/s41586-020-2245-5 ,

Eric Song et al. Neuroinvasion of SARS-CoV-2 in human and mouse brain, Journal of Experimental Medicine (2021). DOI: 10.1084/jem.20202135

David H. Brann et al. Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID-19-associated anosmia, Science Advances (2020). DOI: 10.1126/sciadv.abc5801

Trying to Make Sense of Long COVID Syndrome, Dr. Francis Collins. NIH Director's Blog, January 19th, 2021.

Harvard Medical School, coming with all the hot smell data these days, figures out what's going on with the covid smell loss problem.

I'm pretty sure you already know this, but just in case -- covid-19 can cause changes in smell or taste. It's a primary symptom, and the number one symptom for identifying infection (also the earliest; others may not show up until days later). The problem is that most people don't even know they have a sense of smell in the first place, so they don't notice that it's missing.

We thought it was the actual neurons themselves getting attacked and damaged by the virus. This is certainly possible, because your olfactory neurons, which are like little brain fingers with special neuron cells at the very tips, and are the only part of your brain that reach across the blood-brain barrier, outside the body envelope, and into the world. 

Imagine a piece of your brain reaching, creeping, crawling outside your skull to sniff the world firsthand. That's your olfactory neurons. That's where they stick the swab when taking your covid test sample. Sounds pretty vulnerable. I can see the line of reasoning here.

Well we were wrong (and when I say we, I mean them, because I'm not a scientist). It's not the neurons themselves being attacked, but their "supporting cells." These nonneuronal cells are described as sustentacular cells which wrap around the neurons for structural and metabolic support, and also basal cells which are regenerative stem cells responding to damage.

The thing is, I'm not sure of the implications here. Do we actually know what causes non-obstructive virus-induced anosmia prior to this? And what are the associations between people who have it and people who later develop neurodegenerative symptoms like Alzheimer's etc.? I don't think the new information from this study lets us put our guard down. Using the precautionary principle, I would say that your sense of smell is deeply connected to core areas of your brain, and any dysfunction there should prompt you to investigate potential neurological damage. With the brain, as plastic as it is, and important to quality of life, anything out of the ordinary should be taken seriously, and as quickly as possible. 

David H. Brann et al. Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID-19-associated anosmia, Science Advances (2020). DOI: 10.1126/sciadv.abc5801

This lab also just released one of the more groundbreaking reports on how olfaction works,  basically in the same week: Stan L. Pashkovski et al, Structure and flexibility in cortical representations of odor space, Nature (2020). DOI: 10.1038/s41586-020-2451-1