The story of Mwajuma Abdalla Ngema is that of thousands of people. He went to a clinic in Dar es Sallam (Tanzania) with a persistent cough and the first thing they did was to tuberculosis test which tested negative. After being discharged and a few days had passed, he received a call: the test was positive for tuberculosis, and the result did not come from a laboratory machine, but from the sense of smell of a giant African rat.
The method. This scenario, which seems straight out of a science fiction movie, is the core of an innovative program led by the non-profit organization APOPO. In this case, using giant spider rats (Cricetomys ansorgei) have managed to create a tuberculosis detection system that is not only faster and cheaper, but in many cases is proving to be more effective than conventional methods.
Tuberculosis. It remains one of the deadliest infectious diseases in the world, causing 1.25 million deaths in 2023. One of the biggest challenges is detection, especially in those countries that have very limited resources to purchase reagents or appropriate machinery. And even if these possibilities are available, sputum analysis has limited sensitivity and some cases with a low bacterial load may occur.
This is where the rats come in. APOPO, which initially began training them to detect landmines, discovered that their extremely acute sense of smell could be redirected to identify the specific volatile organic compounds (VOCs) that emits the bacteria Mycobacterium tuberculosis in sputum samples. And the results speak for themselves.
Scientific support. A published study in BMC Infectious Diseases reveals the incredible effectiveness of this method. During 2022, the program analyzed 35,766 samples in patients in Tanzania. Of these, local clinics gave a negative result to 33,866 of these samples through classical microscopy or Xpert tests. And this is where the rats came in to re-evaluate the results, offering a shocking fact: the rodents identified 2,029 additional cases of tuberculosis that would otherwise have been missed.
This means that rats contributed to 52% of the total tuberculosis cases identified in the program, saving thousands of people from going undiagnosed and untreated. Speed is also a key advantage: a rat can analyze 100 samples in less than 20 minutes, a task that would take a lab technician days.
More effective. The true superpower of these “HeroRats,” as APOPO calls them, lies in their ability to detect the undetectable. The study showed that rats are six times more likely to detect tuberculosis in patients with a low bacterial load (“poor” or “1+” categories) compared to standard microscopy in clinics.
This sensitivity is especially crucial for children, whose diagnosis of tuberculosis is notoriously difficult due to the low concentration of bacteria and the difficulty in obtaining quality sputum samples. But this is not a problem for rats, which are twice as likely to identify a case of TB in a child than in an adult.
The training. Behind each correct diagnosis is a rigorous training process that lasts between nine months and a year at the APOPO center in Morogoro. Trainers socialize the pups from four weeks old to create a trusting rat-researcher bond.
Although coexistence is not easy, according to the APOPO coordinator himself, he states that “at first there are trust problems (…) The rat has to trust that I am not a threat, and I have to be sure that it will not bite me.”
Once the bond has been created, training is based on positive reinforcement. The rats are presented with several samples and are rewarded with food when they correctly identify a sample that is positive. And logically, before becoming a ‘diagnostic system’ they must have a score of 10/10 by correctly identifying positive samples.
Economy. In addition to being effective, it is also a very economical solution. The cost of analyzing a sample with a rat is about 2,600 Tanzanian shillings (about 0.90 euros), while a smear scan costs between 4,700 and 7,000 shillings. And if we talk about a molecular test like PCR, we are going up to 42,000 shillings. This means that after a useful life of seven years, the rats “retire” having saved a lot of money, saving lives and ending his days in the center of Morogoro.
Hundreds of thousands of lives. Since its inception, APOPO has analyzed more than 900,000 different samples and detected more than 30,000 cases of tuberculosis that health systems had missed. This is something that has prevented approximately 300,000 new contagion infections, because an untreated person can infect between 10 and 15 people a year.
The success in Tanzania and Ethiopia has prompted APOPO to plan to open more laboratories in northern Tanzania and even to transfer the idea to neighboring countries that also have a very high prevalence of this disease.
Images | National Institute of Allergy
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