Do You Smell a Rat … Or Should the Rat Smell You?

African giant pouched rat

Some animals, such as this African giant pouched rat, are being used to sniff out disease in humans. (Photo credit: Penny Boyd/Alamy Stock Photo)

In 1989, a paper appearing in the British medical journal The Lancet made an astounding claim. Two dermatologists reported how a patient decided to come in for an exam because her dog kept sniffing a mole on her leg. The dog even tried to bite it off at one point. Tests proved it was a malignant melanoma nearly two millimeters thick. When removed, the woman survived, and the study would eventually become known as the ‘First Lancet Letter’ or the first time in a peer-reviewed medical journal that an animal’s senses had been linked to the detection of disease.

Certainly the idea was not entirely new. Ancient doctors would ask patients to supply samples of body fluids that they would then smell as part of their process for coming to a diagnosis. This, however, marked the first time modern medicine apparently had taken the idea seriously enough to publish. And while the letter didn’t result in a flood of similar anecdotes–the next such report appearing more than 10 years later (2001)–physicians have come to the realization that, although speculative, animal noses may provide a quick, inexpensive method for disease detection. More importantly, animals, especially dogs, are mobile enough that they could be deployed to locations to screen for cancer and other diseases that modern high-tech screening techniques could never reach.

A dog’s sense of smell is truly amazing. Tests show that dogs can detect the aromas of substances with concentrations of 0.001 parts per million. This is the equivalent of one teaspoon of sugar dissolved in two Olympic-sized swimming pools. Scientists believe dogs can smell cancer in particular, because, during their growth, cancerous cells cause peroxidation of cell membranes, resulting in the production of volatile organic compounds. These compounds are then excreted in urine or in the breath. In the first major clinical study (British Medical Journal, 2004), six trained dogs of various breeds smelled the urine of both healthy and known cancer sufferers to detect urine cancer (transitional cell carcinoma). The dogs’ diagnoses proved correct in 41% of the cases, as compared to a predicted 14% success rate if left to pure chance. A followup study in 2011 showed a much improved success rate if the samples came from younger patients (about 92%). Additional studies from Great Britain and Italy showed similar results. An outgrowth of these studies was the establishment of Medical Detection Dogs in Great Britain, a charity that trains dogs to screen for cancer as well as to be assistants to those with medical conditions such as diabetes.

Yet, while seemingly supported by the medical community at least in theory, adequate funding for cancer sniffing dogs hasn’t been forthcoming enough to make it go mainstream. However, another animal that seems to be meeting with greater mainstream medical success is the African giant pouched rat (Cricetomys gambianus), which has come into routine use in Tanzania for sniffing out tuberculosis. The idea to use rats came from Belgium product designer Bart Weetjens. Weetjens had rats as pets as a child and knew of their refined senses of smell. After seeing a documentary about the dangers of landmines in Africa, he wondered if he could use a rat’s keen sense of smell to sniff out small concentrations of TNT in the air, the “fingerprint” of a landmine. The African giant pouched rat was selected as the best test species because it was indigenous to the desired areas, was long lived, and could be raised easily. More importantly, weighing only about 1 kg, the rat wasn’t massive enough to set off a landmine. In 2003, his organization, APOPO, began operations in Mozambique, using rats to detect landmines. When Weetjens heard of the trials using dogs to detect cancer, he wondered if his trained rats could serve a similar purpose. He focused on TB, because the breath from an infected patient contains aromatic pathogens whose smell, if the disease is advanced enough, can even be detected by humans.

The trained African rats are invited to smell the saliva of both known and suspected TB patients. The “positives” they identify from the suspected cases then go to the lab for additional testing. What makes the TB studies with these rats different from the cancer studies with dogs is that researchers have a much better idea of what compounds the rats are sensing. This is especially significant in Tanzania where most TB sufferers also suffer from HIV. In short, researchers can be more certain the rats are responding to the specific markers for TB, not something associated with HIV. People with HIV also contract TB more easily because of their weakened immune systems. Thus, fewer of the TB bacteria in a cough-and-spit sample from an HIV infected person mean it’s less likely more traditional methods would detect the disease in its early stages. Since its start, the TB detection programs screened over 300,000 patients. It has identified over 9,000 cases of the disease in patients who have been told by other clinics that they did not have it. This represents an increase of 40% in the number of TB cases identified. Better early detection also means those sufferers are less likely to affect others.

More to Explore
Meet the dogs who can sniff out cancer better than some lab tests
Medical Detection Dogs
The rats who sniff out tuberculosis


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