Training rats to detect tuberculosis

In today’s technology driven medical world, disease diagnosis is relying more and more on sophisticated machinery and complex tests. Having received my medical training in this era when the physician’s diagnostic armamentarium is expanding with imaging modalities, biomarkers, and other technologies, I was astonished to learn of a new approach that utilizes the principles of behavior and concepts of olfaction to train rats to diagnose tuberculosis.  I set out to learn more about this novel method and understand how rodents can be used in the diagnosis of this disease. 

To start lets review tuberculosis.  This disease is caused by a bacterium, Mycobacterium tuberculosis to be precise, and transmitted in the air through respiratory droplets.  While tuberculosis most commonly attacks the lungs, it can cause disease in other organs as well.  Tuberculosis is a significant contributor to morbidity and mortality, especially in the developing world. According to the WHO,  in 2009 alone there were 9.4 million new cases of the disease and 1.3 million deaths as a result of it.  Tuberculosis poses an important public health problem because it can easily be transmitted.  The disease is more common in people with HIV and therefore, more prevalent in Sub-Saharan Africa, where rates of HIV are greater. 

The most common method for diagnosing tuberculosis in the developing world is smear microscopy, in which slide smears of sputum samples are stained with a special dye and visualized under the microscope. This approach to diagnosis is relatively inexpensive but requires trained technicians who can visualize the samples and look for mycobacteria.  Not all regions of the developing world have the facilities or the staff to perform microscopy for tuberculosis detection.  In addition, the capacity of this method to detect positive cases varies greatly.  Some studies have reported that only 20% of positive cases were detected using microscopy.  As a result, alternative methods of tuberculosis detection are needed, especially in resource-limited settings. 

The idea of training rats to detect tuberculosis first came to Bart Weetjens, a Belgian product engineer, who kept these animals as pets during his childhood.  When Bart learned that these creatures had experimentally been used in the laboratory to detect explosive agents, he set out to employ these animals for humanitarian purposes, namely the detection of tuberculosis and landmines.  He established APOPO, a Belgian non-governmental organization that currently operates in Tanzania, Mozambique, and Thailand.   For the last several years, APOPO has been training giant pouched rats from the species Cricetomys gambianus, and studying the feasibility of tuberculosis detection using these animals.  There are several advantages to using this species of rats for tuberculosis detection.  First, there is evidence that rats can more accurately detect positive sputum samples than microscopy.  Second, rats are very fast at detecting tuberculosis and can go through many samples in a short period of time.  Third, using giant pouched rats is not as costly especially because these animals are indigenous to much of Sub-Saharan Africa. 

You may ask how a rat can detect tuberculosis?  It turns out that mycobacteria emit volatile molecules that the rat’s strong olfactory system can detect in sputum samples.  However, training rats to signal when they have detected tuberculosis is no small feat.  To achieve this, APOPO has been collaborating with a team of behavior analysts at Western Michigan University, led by Dr. Alan Poling.  Drawing on principles initially introduced by the famous psychologist, B.F. Skinner, the APOPO team works with Amanda Mahoney, a graduate student from Dr. Poling’s lab, to train rats to detect positive samples and indicate these samples with a defined behavior.  To learn more about how this training takes place, I spoke with Mahoney, who is currently in Tanzania working with APOPO on this project. 

I learned that the process is referred to as ‘discrimination training’.  It takes rats a fraction of a second to detect the smell of Mycobacteria. The rats are taught to indicate a tuberculosis positive sample by pausing for five seconds after they have smelled the sample; they indicate negative samples by moving past it after they have smelled it.  There are several steps to discrimination training. The rats are initially trained in a small cage with one hole where they are rewarded with food for placing their nose in the hole and smelling.  Once this behavior is reinforced, tuberculosis positive sputum samples are placed in the hole and rats are rewarded for smelling the positive samples and pausing for five seconds. In the third step, rats are placed in a cage with three holes, containing both positive and negative samples.  When the rats smell the positive samples and pause, they receive the food reward, but when they smell the negative samples they are not rewarded.  Over time, the rats learn to smell the holes and pause over those with a positive sample.  After the rats have learned to detect at least 80 percent of positive cases correctly, and their false positive detection rates are no higher than 5 percent, they are moved to a larger cage with 10 holes where they are trained to achieve similar detection rates with larger samples.

Currently, research is ongoing by APOPO and Dr. Poling’s team to determine the effectiveness of this method of tuberculosis detection.  One study found that rats are able to detect greater than 70 percent of known positive cases.  Preliminary results of new research has confirmed these findings and also demonstrated greater than 90 percent accuracy in detection of negative samples.  Nonetheless, experts like Dr. Poling warn that the data must be interpreted with caution and more studies ought to be done to validate current findings.  If future studies with a greater number of rats and more sputum samples confirm the above results, APOPO plans to implement rat-based tuberculosis detection programs in their laboratories as an adjunct or substitute to microscopy to increase tuberculosis detection, provide treatments for individuals, and reduce transmission rates. 

APOPO is already working on applying the lessons learned from tuberculosis and landmine detection to other areas.  New projects are aimed at training rats to screen cargo ships for illegally transported tobacco and to discriminate salmonella, a serious bacterial infection of humans and animals, in equine fecal samples.  Remarkably, this creative approach does not require sophisticated man-made technology.  Yet as Mahoney points out, it does employ the highly intricate olfactory and nervous system of the giant pouched rat.  And it goes to show how multidisciplinary approaches may bring together seemingly unrelated topics such as behavior analysis, engineering, public health and microbiology to offer novel approaches to challenging global problems. 

References

Mahoney AM, et al. Using giant African pouched rats to detect tuberculosis in human sputum samples: 2010 findings.  Pan African Medical Journal. 2011;9:28, Epub 2011 Jul 18.

Weetjens, BJ.  African pouched rats for the detection of pulmonary tuberculosis in sputum samples.  International Journal of Tuberculosis and Lung Disease. 13(6):737–743