January 10, 2011
The most commonly used TB detection method in developing countries is sputum smear microscopy. But while the 100-year-old technique can be employed in places with minimal facilities, it is not very sensitive: Without a high concentration of TB bacilli, some 60 percent to 80 percent of positive cases go undiagnosed. Preliminary new research suggests Gambian pouched rats can do better.
Dr. Alan Poling, a professor of psychology at Western Michigan University, and colleagues tested the rats' TB detection sensitivity using samples that were laboratory-confirmed as either positive or negative. The rats' sensitivity ranged as high as 86.6 percent and their specificity, or ability to detect the absence of TB, was more than 93 percent. When the team compared the rats' results to microscopy, the animals detected 44 percent more positive cases.
Cricetomys gambianus are omnivorous rodents that weigh 10-15 pounds and live in colonies of up to 20 all over sub-Saharan Africa. The same animals have been trained to sniff out landmines.
The rats are raised in captivity. When the pups are eight weeks old, trainers put TB-positive and -negative samples under "sniffing holes" in a specially designed cage. Using a reward technique, the rats learn to give a positive sample a longer sniff and to quickly skip over negative samples. By 26 weeks, the proficient ones have become experts.
The rats have been accepted as a reasonable tool in Tanzania, though "the medical community is still skeptical," said Poling. "We think that eventually there will be a place for them in first-line screening."
But Dr. Neil W. Schluger, a lung disease expert at Columbia University, has doubts. "They're a long way from demonstrating the robustness of their technique," he said. "These rats can do something amazing but even if you accept that it worked within their lab, are they still good at it a year later?"
The study, "Using Giant African Pouched Rats to Detect Tuberculosis in Human Sputum Samples: 2009 Findings," was published in the American Journal of Tropical Medicine and Hygiene (2010;83(6):1308-1310).