Tuberculosis (TB) is contagious and airborne – it is the top infectious killer worldwide claiming 1.6 million lives yearly. The WHO estimates that globally 3.6 million out of 10 million TB patients are left undiagnosed or missed every year and do not get TB treatment. In many sub-Saharan African countries, only about half of TB patients are diagnosed and start treatment. Poor access to health care, slow conventional diagnostic methods, underfunding of clinics and lack of trained staff prevent or delay TB diagnosis among affected populations. In turn, this delays treatment and facilitates the disease progression and transmission to patients’ family and community, which causes suffering and economic hardship.
APOPO’s TB detection rats are a diagnostic tool under ongoing research and development. To date, the rats have a higher diagnostic sensitivity (probability of correctly identifying a person with TB) than conventional microscopy. However the diagnostic specificity (probability of correctly identifying a person without TB) of the rats remains to be improved. The rats are also relatively low cost and much faster than conventional diagnostic methods: One TB detection rat screens 100 samples in 20 minutes, this would take a lab technician using conventional microscopy up to four days. The statistics of the rats clearly highlight the difference they make. APOPO’s ongoing research which uses the rats as a detection tool followed by internationally endorsed confirmation tests (additional to the ongoing routine diagnostics) has helped to increase the TB case detection rate of collaborating TB clinics by 40%.
APOPO inactivates the incoming samples using heat treatment in an autoclave. This ensures that the sputum is not contagious for the trainers and rats alike.
Our research suggests that they detect a bouquet of odours, namely volatile organic compounds, which are specific for Mycobacterium tuberculosis.
The rats re-test samples, which have already undergone routine testing, mainly conventional sputum microscopy in clinics. If the rats suspect TB they hold their noses in place for 3 seconds. The samples that were tested negative in clinics but positive by rats are then rechecked using WHO endorsed confirmation tests. Confirmed TB-positive results are sent back to clinics who oversee patient treatment. TB treatment is provided for free by the government. For privacy reasons APOPO is never aware of patient names unless they actively volunteer to tell their story. If the patient has not gone back to the clinic for their result, APOPO is collaborating with local patient organisations who work with the clinics to track patients and link them to care.
APOPO’s TB detection rats are under ongoing research and development and have not been endorsed by the WHO. Their accuracy remains to be improved before the rats can qualify as a stand-alone diagnostic tool. Interestingly, however, the rats’ accuracy is independent of the patients’ HIV status, while other diagnostic tests are less accurate among people living with HIV. The detection rats stand out through their high speed and throughput and relatively low cost, key attributes for initial screening tests. Considering the current gap in current TB diagnostics, APOPO is well positioned to contribute to finding the ‘missed’ cases by its research in the countries in which it works. Beyond our study of the TB detection rats’ performance under field conditions, our research on TB-specific odours and on scent-detection may also inform the development or refinement of synthetic devices, such as e-noses.
In many developing countries, TB is still detected through sputum microscopy a method dating back over 130 years to Robert Koch’s time, which is simple and inexpensive but has known limitations. It works best in cases with a high bacterial load (many bacteria easily visible with smear text microscopy used in clinic), which is also a sign of advanced disease. However its sensitivity is poor among children and people living with HIV.