Two interesting stories have reached my nose and I had to bring them to you because it smells like there’s something juicy here to discuss. Let’s get started.
It’s probably too soon to speak about the smell of spring, specially given that here in Munich we are still forcing our way out of winter BUT there is a sort of smell that, according to new research is surely around these days: the smell of disease.
Historically changes in body odour have been used to identify certain maladies like tifoid fever which smells of baked bread or the meaty odour of yellow fever. But also in rats it’s been shown that under an infection, be it intestinal worms or a virus, there’s a change in body odour that produces rejection to their mates. However, we humans don’t have the best of senses of smell, so until now there were doubts as to whether we were able to discern an early odour change associated to infectious processes that would serve as a cue to avoid contagion. A subtle way of saying: run away from the devil. Well, it seems there is.
What researchers at the Karolinska Institute in collaboration with some other american groups did was the following: they activated the immune system of a group of subjects by injecting them with LPS (bacterial lipopolysaccharide), a bacterial cell wall component known to activate the innate immune response and to produce an inflammatory response and then they collected odour samples to see if this procedure had affected its smell. How did they test this? Apart from this samples, they also gathered some control samples from the same individuals only after injecting them with saline instead of LPS and they presented them to a group of people to differenciate. They also took blood samples and checked for the presence of certain inflammatory factors like TNF-alpha and interleukins IL6 and IL8, which have also been involved in the expression of unpleasant smell in animals after infection.
The analysis of the data showed that after immune system activation, even shortly after (4 hours), smell already changes significantly: it becomes more intense and more unpleasant than that of subjects treated with placebo. Because even a pleasant smell can turn out otherwise if too intense -just picture those women surrounded by a cloud of pungent perfume- they also tested for this possibility but they found that actually the concentration of volatile substances in the placebo samples was higher than in the treated so it seems reasonable to suspect there’s a compound in the infected samples that alters the perception of their smell. Was they couldn’t find though was an association between the perception of this change in the smell and the sensation of disease, that is this bad smell only served as an indirect signal of something wrong calling for avoidance, even if the cause remains unknown.
By means of some statistical analysis not quite clear to me they discard a relationship between IL8 and these changes in body odous but IL6 and TNF-alpha do seem to have some role in the matter. Here I should mention that I miss some experimental data validating these points as well as some others to try and identify the compound that would putatively serve as a key signal for the avoidance reaction.
To sum up I find this an interesting -and easy- way of getting a proof of principle. however, now remains what’s -to me- the most enticing part: identifying the compound, describing the pathways involved and to know how general the process is: is the same compound for every kind of infection? I’d guess not given what we know about yellow or tifoid fever but I WANT TO SEE MORE, also at which time in the infection does it start being produced? etc etc
The Scent of Disease: Human Body Odor Contains an Early Chemosensory Cue of Sickness Psychological Science