Sniffing for Disease

Anyone who has been to an American hospital has probably noticed its smell. It's hard to describe it exactly, because it's unique, but personally I'd use words such as "sterile," "plasticky," and "nauseating." Clearly I'm not a fan. When I or someone in my household needs to stay home from work because of sickness, I've noticed that there's a smell that hangs around for a couple of days. I chalked it up to simply not showering for a little while.

It turns out there's a lot more to the story than that.

In the days before blood tests and lab results (which is most of human history), doctors had only their senses and logic to treat patients. While most doctors today use sight, hearing, and/or touch to diagnose patients, most of the doctors who came before had to rely on smell and, in some cases, taste as well! In fact, the use of smell as a diagnostic tool goes back thousands of years, and can actually be quite effective.

The Sushruta Samhita is an ancient Sanskrit text that has one of the first mentions of using smell as a tool. In the document, it states: "[B]y the sense of smell we can recognize the peculiar perspiration of many diseases, which has an important bearing on their identification." The history of this document is a fascinating one, and probably deserves its own blog post, but what is clear is that the Sushruta Samhita was written sometime in the middle of the first millennium BCE (though age estimates range from 1000 BCE to 500 CE). This means that the technique of identifying diseases based on smell was probably discovered and passed down through many generations of doctors through oral teachings before it was written down by the original author of the Sushruta Samhita. Later on, more doctors added to its contents, and today it survives as one of only two foundational Hindu texts on the medical profession from ancient India.

From Penn and Potts, 1998. TREE vol. 13, no. 10, pg. 391–396.

When humans began to concentrate more in cities, public health became an important concern for city officials. If a sickness broke out, it could mean the deaths of thousands, if not millions, of people. Bubonic plague, typhoid fever, cholera, and smallpox were (and in some places, still are) serious diseases that kill quickly. If doctors could identify the illness, then they are better able to treat those afflicted and stop the spread of the disease, saving lives. The scent of a patient, especially a strong and unmistakable one, gave doctors the chance to identify the disease quickly. The figure shows the scents of some diseases, though there are quite a few that aren't on the list.

Skip ahead thousands of years to the modern day, and smell is still the defining characteristic of certain illnesses. One disease in particular, called maple syrup urine disease (MSUD), is identified by the unmistakable smell of maple syrup (or fenugreek, if you're not from an area where maple syrup is common). The pleasant smell belies the seriousness of the disease, however. The sweet smell comes from the body's inability to break down long branched chains of amino acids, which causes them to build up to toxic levels in the body. Infants who have this disease initially start out healthy, but then quickly deteriorate. If left untreated, MSUD will lead to permanent brain damage, and in extreme cases, death within a few months. Older adults can develop this disease, and without proper recognition and treatment, may eventually cause the patient's death. If you ever notice that your urine smells like maple syrup, definitely tell your doctor!

Many service animals are trained to alert their charges if they are about to experience an episode or have an attack. Experts think dogs are actually detecting a change in the scent of their charge, as dogs have a much stronger sense of smell than humans do. Some dogs are even trained to sniff out cancer (though their effectiveness at accurately diagnosing cancer has been criticized).

In some cases, smell was not enough, and the doctors actually had to taste the excretions of their patients. This was the most common way for diabetes mellitus (which means "passing through sweet") to be diagnosed--the doctors had to taste their patient's pee! This knowledge was actually fairly well-known even among people outside of the medical profession, as the disease was also called "pissing evil" for centuries.

Today we rely more on technology than scent to tell us about our health and diseases. To be fair, the medical knowledge of medieval Europe certainly got a lot of things wrong, so it's overall a very positive thing that medical practice has changed so much. What hasn't changed is how much humans rely on their senses to explain the world around them. Smell may not be the most glamorous sense, but it was certainly an effective one, and remains an important tool to this day.

Sources:
Brown, R. (1995) What is the role of the immune system in determining individually distinct body odours? Int. J. Immunopharmacol. 17, 655–661.
Liddell, K. (1976) Smell as a diagnostic marker, Postgrad. Med. J. 52, 136–138.
https://www.popsci.com/problem-with-cancer-sniffing-dogs#page-2