As far back as 1989, a pair of
dermatologists reported in the "Lancet" about a women whose dog
wouldn't stop sniffing at a mole on her leg, and once even tried
to bite it off. She went on to get a medical checkup, which
showed the mole was a malignant melanoma, already a couple of mm
thick. It was removed and she suffered no consequences.
Another similar case was reported in the "Lancet"
in 2001 by two British doctors. A 66-year old man had his
Labrador strangely attracted to a rough patch of skin on his
leg, diagnosed as eczema. The dog kept pushing its nose into it,
until the man went back to his doctor. The
"eczema" turned out to be a basal cell carcinoma, which was quickly
What does cancer smell like? We'll never know,
because the concentrations of abnormal gaseous residues of the
cancerous cell metabolism are far below our threshold of
detection. It wasn't known what was it, specifically, that
dogs could smell, but it should belong generally to a group of
chemicals called "volatile organic compounds".
We do know that, unlike healthy cell, whose
unbelievably complex metabolism takes place in the oxygen-rich
environment, cancer cell lives on a comparatively very primitive
process of burning glucose under oxygen-deprived conditions. As a
result, the cancer cell is acidic, and
its chemical waste is very different than that
of a healthy cell.
While it is hard to compare sense of smell
between different animals, since they tend to vary in strength with specific
smells from one animal to another, it is generally accepted that
the number of smell receptors' genes, or
OR genes, indicates how strong is the
sense. Turns out, rat has 50% more of OR genes than a dog.
Knowing that the sense of smell ability increases exponentially
with the number of genes - dogs, on the average, have only twice
as many as humans - implicates that rats have significantly
stronger sense of smell.
Being in addition smaller, easier to handle and cheaper, all
make them better a candidate for use as a smell sensor for medical
We know that rats are already being used to sniff
out tuberculosis, as well as land mines, in Africa (so they
already are saving human lives, although not from cancer). In
either case, what they can smell is known: with TB, it is the
tar-smelling chemical waste of the TB-causing
bacteria, and with
land mines it is the smell of TNT. These rats are simply
trained to react to the smell by being rewarded for it with their
favored food (they love bananas - and they don't cheat!).
What is now being worked at in Russia, however,
is much more sophisticated. They use a microchip as a detector to
determine whether bioelectrical impulses generated in rat's
brain following its olfactory system (sense of smell) response
indicate the presence of "target compounds", or not.
As Fedor Arsenyev, leading researcher of the "Advanced
Research Foundation", which
developed this so called
biohybrid cancer screening
technology, says: "Animal
has special pre-implanted microelectrodes, which allow to record
biorhythms resulting from exposure of its olfactory receptors to
cancer markers present in the exhaled air".
In effect, the rat is a sensor used by the chip.
In the first test at a two hospitals in
at the end of November last year, 1073 volunteers went through a
very simple procedure of exhaling into a pipe-like device
connected to a box with the rat. Test was designed for lung and
stomach cancer, and it took only 2-3 minutes per person.
Main intended benefit of the test was to detect
cancer in its very early stage, making the chances for recovery
much better. Out of 1073 symptom-free participants, 85 (or 8%)
were positive, i.e. at a risk of developing cancer. Out of them,
follow-up examination using
computed tomography of the lungs and fibrogastroduodenoscopy, in
order to double check the rat-test result.
It was confirmed in 40% of them
for lung cancer, and 20% for stomach (which does not necessarily
mean that all others were cancer-free).
Compared to the standard diagnostic tests
performance for the
region, with a 0.8 and 0.4 per 1000
detection rate for the lung and stomach cancer, respectively,
the new test is roughly
3-5 times more sensitive.
Apparently, it still has too high
false-positive rate, but it is not unreasonable to expect that
with further refinements it could be made better in both,
sensitivity and accuracy.
The plan is to complete the investigative phase
this year, with another experiment with twice as many participants.
Potential buyers, if all goes as expected, are the Russian
Ministry of Health and
the Federal Medical and
Extreme expansion of the olfactory receptor gene
repertoire in African elephants and evolutionary dynamics of
orthologous gene groups in 13 placental mammals, Niimura et al.