Rats are fast, cheap TB detectors
August 2, 2010 1 Comment
An unpredictable killer continues to kill
Can you name the disease that killed Chopin, Keats, Descartes, Kafka, Florence Nightingale, Eleanor Roosevelt and 200 million more people in the last 100 years? It’s tuberculosis, formerly known as “consumption,” and now known as TB. The tuberculosis bacterium, Mycobacterium tuberculosis, is an airborne pathogen that can be passed on from people with active cases of TB and usually settles in the lungs, where it can flourish and cause infection. It is possible to have what is known as “latent TB,” a situation in which you harbor the bacteria, but do not manifest the disease and are not contagious. Worldwide, World Health Organization predicts that the numbers of people who die from TB will climb to 8 million by 2015.
Experts agree that generally, when TB is caught early and treated, it is curable. But with millions of people suffering from it—including people with suppressed immune systems, such as those with HIV infections—detecting every case of TB is a tough job. Current methods require three saliva samples taken over a two-day period to be prepped on a slide, stained, and examined by a trained technician for the presence of TB bacteria. A good technician can analyze about 20 samples per day; for the 8 million people who may have TB in 2015, it would take 1200 technicians working 365 days to identify them all. One thing desperately needed in nations without the funding for technicians is a fast, accurate, low-tech way to analyze samples for the presence of TB.
Enter the rat. Bart J.C. Weetjens, who is not a rat, but a scientist working for Apopo, a Belgian company based in Tanzania, had one of those “chance favors the prepared mind” moments. He realized that the Dutch word for tuberculosis, tering, means something along the lines of “that’s starting to smell like tar.” Given that traditional Chinese medicine includes using smell to diagnose TB, Weetjens concluded that a trained animal might be able to detect TB, much in the way a bomb-sniffing dog detects explosives. It just so happened that Weetjens’ company had already trained a native African giant pouched rat, Cricetomys gambianus, to use its olfactory faculties to sniff out land mines. Weetjens decided simply to substitute TB bacteria for explosives.
The rats were uniquely qualified for the job. Unlike most nocturnal predators, these animals have very small eyes, indicating a strong reliance on olfactory and aural senses. During the day, they seem blind, sniffing the air rather than looking around. They can grow as large as a cat, are omnivorous, easily tamed and trained, and great breeders; a single female can produce 10 litters of up to four young each year. Weetjens took his idea to the World Bank, which agreed to fund a complete study of the rats’ ability to sniff out TB.
Weetjens already reported preliminary results that indicate the rats may be a viable way to ID TB. The rats identified 77 percent of infected saliva samples, and 92 percent of cultured bacteria samples, with a false-positive (indicating bacteria where there were none) rate of 2 percent. The current human-based process has an accuracy rate of about 95 percent, but Weetjens figures that with several rats analyzing samples, the rats’ accuracy will match the humans’. According to Weetjens, the rats can analyze 126 samples in 20 minutes, making them a very cheap, fast diagnostic test. The analysis of 8 million samples that would have taken 1200 humans 365 days would take only two rats working the same period of time. The latest data indicate similar success rates.