Cheap, smart and efficient: how giant rats are transforming the fight against tuberculosis
Mwajuma Abdalla Ngema went to the clinic in Dar es Salaam with a persistent cough and severe pain around her ribs. The mother of two children was tested for tuberculosis (TB), but the result was negative. “I was given medicine for the cough, but the pain was unbearable,” said Ngema, who went home frustrated by the lack of clarity about her health.
A few days later, Ngema was recalled to the Mbagala Kizuiani clinic – she subsequently tested positive. She was relieved when she was finally diagnosed: “I am currently on medication and on the road to recovery.”
Robert Ndoto, 55, tells a similar story. Because he has HIV, he is particularly susceptible to tuberculosis, but his tests were also negative. A few days later he was told that he indeed had tuberculosis. The diagnosis allowed him to start medications for the bacterial infection, which he was able to access through Mkutaa local health organization that focuses on tuberculosis and HIV.
“I was in bad shape when I met Salma Matamika (a volunteer at Mkuta). I had heard her give a lecture on tuberculosis. She became the link between me and TB treatment,” he says. “If you have HIV and tuberculosis, you can easily die.”
Unusually, Ngema and Ndoto’s revised TB diagnoses came after specially trained giant rats detected the disease in samples of the patients’ sputum, or mucus from the lungs and airways.
The screening programme, run by a charity called Apopo, is only used in Tanzania and Ethiopia. They are among the thirty countries listed by the World Health Organization countries with a high TB burden.
Samples judged negative using swab tests or molecular tests at clinics are sent to Apopo’s laboratory for secondary screening by the rats. Research this year shows that 52% of the initially negative tests are again assessed as positive by the rats. Samples are always clinically tested to confirm the rats’ judgment – and research shows that the rats are right at least 82% of the time compared to standard clinical testing.
Apopo started in 1995 as a Belgian project to use the acute sense of smell of rats to detect mines. In 2007, Tanzania became the first country to use rats for tuberculosis screening; Apopo now works with 81 clinics in the African state.
Next year the organization will open a second testing laboratory in Moshi, in the northern region of Kilimanjaro. It is also in discussions with health officials in Sierra Leone and Angola about introducing the program there.
At the Tanzania Veterinary Laboratory Agency in Dar es Salaam, a dozen rats screen up to 400 sputum samples per week. Since it started, Apopo has analyzed more than 900,000 samples and has detected more than 30,000 cases of tuberculosis that may have been missed, preventing at least 300,000 potential infections.
“One untreated person can infect 10 to 15 people, multiply that by 24,000 properly treated people, who have been missed through regular testing. These weren’t just samples, these were lives saved,” said Dr. Joseph Soka, manager of Apopo’s laboratory.
The rats are trained to detect the presence of Mycobacterium tuberculosis (MTB), the pathogen that causes the disease. “The highly developed olfactory receptor cells in these rats can detect the volatile organic compounds responsible for MTB,” says Soka. “The rats can isolate the specific strain even at low levels of these compounds.”
In the laboratory, a rat is placed in a glass enclosure with ten holes, each containing a sample. The rats are trained to associate positive samples with food, hovering over the positive samples and moving directly from the negative samples. It typically takes about three seconds to detect a positive sample and less than two seconds for a negative sample. The process must be repeated by at least five rats before technicians can determine a positive result.
Using rats saves time, as they can screen dozens of samples in minutes, and is much cheaper than a clinical test. A rat costs 2,600 shillings (90 cents) for one sample, compared to Sh4,700-7,000 for smear microscopy and Sh42,000 for molecular testing.
Apopo uses African giant possums – one of the largest rodent species in the world, which can grow up to 90cm (including the tail) – and breeds them at its base in Morogoro, 200km west of Dar es Salaam.
“We train them from the age of four weeks,” says Fidelis John, coordinator at Apopo. It takes nine months to a year to train a rat to detect MTB. But before it can become part of the team, he says, “it must ‘take’ the (TB detection) exam and score 100%.”
Before training begins, the rats are socialized to create a bond between them and their human handlers. “Initially there are trust issues,” says John, holding Louis, a two-year intern.
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“This rat has to trust that I’m not a threat, while I have to trust that the rat won’t bite me.” In captivity, most rats will live eight years, some longer. When they are about seven years old, rats retire to the center in Morogoro, where “he will just eat, sleep and wait to die,” says John.
A recently published article shows that rats are particularly sensitive to low levels of MTB; in children, for example, who are more difficult to diagnose. It found that “children, especially those with lower levels of bacteria, were more likely to be diagnosed with tuberculosis by rats” and suggested that “rat-based technology could be used to improve the detection of tuberculosis cases in children in countries with a high disease burden, such as Tanzania. .”
There have been cases in which rats have been detected with tuberculosis, but follow-up clinical tests have been negative; in some of these cases, patients later developed tuberculosis, suggesting that rats detect the disease before a clinical test can. Apopo is monitoring these patients to better understand rats’ susceptibility to low levels of bacteria.
The 40% reduction in tuberculosis incidence in Tanzania since 2015 can be partly attributed to public clinic screenings (a program in October found 49 cases). Those who test negative for tuberculosis during screenings are told they may be contacted again after further sample analysis.
Dr. Sultan Lusambi, tuberculosis and leprosy coordinator in the district, said: “The screened people are not surprised that their samples turned positive after further analysis as we had prepared them for such a possibility.”
These outreach programs rely largely on the efforts of volunteers trained to warn communities about the dangers of living with untreated tuberculosis.
Albertina Kisonjo, a volunteer at Mkuta who has had tuberculosis herself, describes how she helped one family: “One of my neighbors had the symptoms. I took her to a clinic, where it was confirmed that she was positive for tuberculosis. Follow-up visits found four other cases of tuberculosis among her relatives.
“If we had lost this family, the community would have said it was witchcraft. We have also prevented infections in the community by getting the family on treatment.”
The decline in tuberculosis is also due to Apopo’s work. Last year, his laboratory in Dar es Salaam identified more than 2,000 cases that would otherwise have gone unnoticed and helped prevent at least 24,000 new infections.
“I’m happy when I hear the sound of a motorcycle bringing in samples,” Soka says. “I see lives that could be saved – lives that could be lost if conventional testing methods are inconclusive.”