Double-blind study suggests humans have olfactory defense against contagious disease

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(Medical Xpress)—A European team of researchers working at Sweden's Karolinska Institutet has found evidence that suggests that humans have an olfactory defense against contagious diseases. In their paper published in Proceedings of the National Academy of Sciences, the group describes experiments they conducted with volunteers undergoing fMRI scans while viewing photos and sniffing body odor samples from people experiencing induced immune response.

Humans have developed a wide assortment of survival skills over a long evolutionary period, which by most accounts has been quite successful for species survival. One of those survival skills may have gone unnoticed until now, however—the ability to smell sickness in other people so as to avoid them and thus prevent infection. In this new effort, the researchers tested this theory by enlisting the assistance of volunteers to serve in one of two main roles—a person made to appear sick, or as someone attempting to judge the health of another person by either looking at them or by sniffing a sample of their body odor.

The experiments consisted of asking a group of 22 volunteers to allow the researchers to inject them with a type of harmless bacteria that would activate their immune systems. The researchers then obtained body odor samples from each, and photographed their faces Another group was injected with a placebo. A second group of volunteers was then asked to undergo fMRI analysis as they looked at the photographs and sniffed the body odor samples.

In the experiment, the brains of the volunteers activated when sniffing body odors from subjects with activated immune systems, in what the researchers described as a multisensory reaction. Furthermore, they also found that the volunteers rated the people in the photographs with activated immune systems as less attractive. They did so even when sniffing body odor samples of "sick" people, but were shown photographs of "healthy" people. The therefore suggest that humans have a disease avoidance model that includes olfactory mechanisms.


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More information: Christina Regenbogen et al. Behavioral and neural correlates to multisensory detection of sick humans, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1617357114

Abstract
Throughout human evolution, infectious diseases have been a primary cause of death. Detection of subtle cues indicating sickness and avoidance of sick conspecifics would therefore be an adaptive way of coping with an environment fraught with pathogens. This study determines how humans perceive and integrate early cues of sickness in conspecifics sampled just hours after the induction of immune system activation, and the underlying neural mechanisms for this detection. In a double-blind placebo-controlled crossover design, the immune system in 22 sample donors was transiently activated with an endotoxin injection [lipopolysaccharide (LPS)]. Facial photographs and body odor samples were taken from the same donors when "sick" (LPS-injected) and when "healthy" (saline-injected) and subsequently were presented to a separate group of participants (n = 30) who rated their liking of the presented person during fMRI scanning. Faces were less socially desirable when sick, and sick body odors tended to lower liking of the faces. Sickness status presented by odor and facial photograph resulted in increased neural activation of odor- and face-perception networks, respectively. A superadditive effect of olfactory–visual integration of sickness cues was found in the intraparietal sulcus, which was functionally connected to core areas of multisensory integration in the superior temporal sulcus and orbitofrontal cortex. Taken together, the results outline a disease-avoidance model in which neural mechanisms involved in the detection of disease cues and multisensory integration are vital parts.

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May 24, 2017
I wonder if the sniffers disliked the infected people's body odors because they smelled infection products (or their metabolites) directly, or because they smelled the infected people's immune systems in unusual activity.

Previous studies have shown, for example, childbearing age women said, when sniffing bagged T-shirts that exercising men had worn a few minutes earlier, that they felt sexiest when sniffing the shirts of those men whose immune system genes were most different from the women's own genes. So it's established that (at least female) humans are deeply attuned by smell to basic immune system status of other (at least male) humans.

So I wonder if this study is the same sense at work, detecting more transient immune system status, not directly the infection itself. I'd like to see a study where people given antigens but not actual pathogens were sniffed and compared to this study's results.

May 24, 2017
In the case of the dog sensing cancer it would seem to eliminate immune system evaluation, - and I would vote for metabolites resulting from disease.

Experimentation as suggested would be interesting...

May 24, 2017
My wife of 30 years smelt something wrong with me long before the diagnosis, and she liked me.

May 24, 2017
gzurbay:
In the case


Well, it could be both smelling pathogens (or their direct products) and smelling the immune response (independent of the pathogens). But even "smelling cancer" could be smelling the immune response to cancer, not the cancer itself (or its direct products).

I do expect to see research distinguishing the pathogen smell from the immune system smell. Making therapies from it depends on the distinction, at least for the better therapies.

May 25, 2017
During the depression, my father, then a child, had appendicitis. The family doctor was called, came into the house, and without even going upstairs to see my father, called the hospital to set up surgery. This was Thanksgiving weekend, and finding a surgeon was hard, but they found a retired surgeon who could make it in quickly. He got to the hospital a few minutes before my father, and was cutting within a few minutes of my father's arrival. The appendix had not ruptured, and was drained and removed through a small incision. For decades doctors seeing my father's scar knew who had done the operation, how, and why. All this happened with no examination by either doctor. They could smell, the problem, and knew it was urgent.

Years ago, we had an amazing diagnostician with a practice near our house. Doctors from all over Philadelphia would send their puzzling cases to him, and at least 99% of the time, Dr. Eisenberg's diagnosis before even talking to the patient was correct.

May 25, 2017
"Humans have developed a wide assortment of survival skills over a long evolutionary period"

-Oh I'm sure this skill developed a lot farther down the evolutionary chain.

May 25, 2017
eachus:
During the depression


I knew someone at work a few years ago who we called "the fetus sniffer". He knew women were pregnant often before they'd even taken the test themself, and (we believed) it wasn't because he was the father :).

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