Scientifically Speaking | Sniffing out cancer with insect detectives
Scientists are now turning to the highly refined odour-detecting abilities of two insects – ants and locusts – to detect cancer
Ants are social insects that have been long studied for their complex communities. These creatures have thin sensory appendages crowning their heads which act as chemical sensors. Ants use smell to protect their young, locate food, and communicate with colony mates.
Publishing in the scientific journal Proceedings of the Biological Society B earlier this year, a team led by Baptiste Piqueret put the smelling ability of ants to a unique test. Building on their earlier work last year showing that ants could be trained to whiff out different volatile organic compounds (VOCs) given off by cancer cells, they tested if the same striking results could be obtained from diseased animals.
Since cancer is a complex disease, there isn’t a single smell to identify all types of cancer. Each cancer type has a unique pattern of VOCs. What’s important to note, however, is that different cancers have different odour fingerprints. In earlier studies, different researchers identified volatile organic compounds such as styrene, benzaldehyde, and dodecane associated with different cancer cell lines.
In this study, the team trained 35 ants to associate urine from mice bearing human breast cancer tumours with sugary food. Ants like sugar and they rush towards it. Over multiple training sessions, ants became quicker at finding the sugar reward associated with cancerous samples, which indicated that they were learning to smell the VOCs linked to cancer.
The ants were then tested in an environment containing two different smells: Urine from mice with cancer and urine from healthy mice. The ants showed a clear preference for the scent of cancerous urine by spending more time near tubes which contained it, compared to those containing urine from healthy mice. Effectively, this study turned the ants into mobile biological cancer detectors.
Parallel to these endeavours with ants, scientists also exploited the advanced sense of smell of locusts. Locusts use smell to transform from solitary grasshoppers into migratory swarms. Debajit Saha and his team at Michigan State University utilised this ability in locusts to sniff out the chemical signatures of various oral cancers. Their work appeared in the scientific journal Biosensors and Bioelectronics earlier this year.
Saha and his team implanted electrodes into the brains of locusts to capture neural responses when exposed to VOCs emitted by cancer cells. These locust cyborgs helped the team to create "scent fingerprints" associated with different kinds of oral cancer cells, which were distinct from those emitted by non-cancerous cells.
The team notes that the locust cyborgs can help to make distinctions between cancerous and non-cancerous cells within just 250 milliseconds with success rates between 76% and 100% for different oral cancer cell types. The next step will be to use these locust cyborgs to test breath samples from actual human cancer patients.
Both the ant and locust studies utilise the same underlying principle of harnessing the olfactory abilities of insects to detect volatile organic compounds emitted by cancer cells. There are differences, of course.
While the ant study relies on animal behaviour without making any modifications to their brains, the locust study takes an engineering approach to hijack a biological pathway in the insect’s brain.
Ants and locusts are not the only creatures that have an acute sense of smell. Mice can be trained to discriminate between healthy counterparts and those bearing tumours. Dogs are also being used to sniff out diseases. However, Piqueret believes that ants have an edge since they can be trained more quickly and easily for such tasks. Saha and Piqueret both note that dogs are expensive to train compared to insects such as ants and locusts. Trials can also be run on many insects at the same time, improving the robustness of the results.
Cancer detection methods currently in use such as biopsies, colonoscopies, and blood tests are often invasive and costly. They are also not always readily available in low-resource settings. Applying insect detectives to the task offers the prospect of a less intrusive, cost-effective solution. But it is still early days. Both teams are quick to point out that we are still far off from having practical cancer screening using ants and locusts.
For any screening test to be successful, there must only be a small number of false positives (those who test positive but don’t have the condition) and false negatives (those who are mistakenly tested as not having the condition when they, in fact, do).
Future testing will determine whether ant and locust-derived cancer tests have the accuracy that they need to be deployed in human populations. Sceptics may point out that there are understandably high regulatory barriers for medical tests and devices, but even if ultimately, these tests are found unsuitable, they’re a reminder that inspiration in medicine and engineering can come from the tiny creatures with which we share our planet.
Anirban Mahapatra is a scientist by training and the author of a popular science book on COVID-19. The views expressed are personal