I smell a rat...

January 31, 2011

Eva Gusnowski

… and it smells your lung infection.

Tuberculosis is a bastard. Approximately 1/3 of the human population is infected with the mycobacteria that causes tuberculosis, Mycobacterium tuberculosis, with new infections thought to occur once every second. However, many of the people that are infected do not show disease symptoms, resulting in a latent (asymptomatic) infection.

Eva Gusnowski, M. tuberculosis, Giant African Rat

Sadly, the latent infection is due to our own immune system: M. tuberculosis cells are surrounded by immune cells (making a ball of cells called a granuloma), which creates a small microenvironment that protects the pathogen and gives it a nice little home to divide in. If the immune system is somehow compromised, the pathogen is released from the granuloma and can result in disease activation. Because M. tuberculosis is transferred from saliva or mucus that has been coughed/sneezed into the air, its favorite place to live is the lungs. Tuberculosis (TB) is therefore most likely to result in a lung infection, although it can spread to other parts of the body if left untreated for extended periods of time. The best way for a doctor to detect the active infection is to analyze sputum for the presence of M. tuberculosis…and yes, this means looking at your spit to see if the mycobacterial cells are there or not.

What’s interesting about TB, aside from what I’ve already mentioned, is that a characteristic odor is produced when the disease has been activated. This has led to the training of certain animals to identify this specific odor as a means of early disease detection. And the best detector is not just any animal; it’s the Giant African Rat.

Eva Gusnowski, M. tuberculosis, Giant African Rat

Giant African Rats were trained to identify M. tuberculosis-containing sputum by positive reinforcement; they were given a mouthful of banana every time they paused at a TB-positive sputum sample. After training, the rats were used in a study performed in 2009 that analyzed the sputum of 10,523 patients for the presence of M. tuberculosis. The sputum went through a primary screen by microscopists, followed by a secondary screen performed by the rats. 

The microscopists identified 13.3% of the patients as TB-positive. And then the rats absolutely owned the microscopists. In addition to confirming the 13.3% of the positive cases found by the microscopists, the rats were able to identify an additional 44% of the patients as TB-positive. All of the rat-detected cases were reanalyzed by the microscopists and confirmed as TB-positive.

Eva Gusnowski, M. tuberculosis, Giant African Rat


There were cases where the rats detected TB, but the presence of M. tuberculosis was not confirmed after reanalysis by the microscopists. However, because the smear test used to identify M. tuberculosis cells is very inefficient at low mycobacterial concentrations, it is thought that the rats might be able to detect TB even when the level of mycobacteria is extremely low. Essentially, this means the Giant African Rat might be one of the best means of early TB detection that we have so far. Therefore, because the rats are able to quickly smell and identify TB-positive samples, they offer an efficient, inexpensive and quick means for early TB screening and detection.  (Watch the rats in action here:

You know, they actually look kind of cuddly. And since they’re the size of a raccoon, maybe I should get a Giant African Rat as a pet instead of a dog. It’ll keep the nosy neighbors away and it will be able to tell me if I ever get TB if I give it bananas. Win-win.

P.s. The Giant African Rat is also currently being trained to smell landmines. Move over naked mole rat. Best. Rodent. Ever.

Eva Gusnowski, M. tuberculosis, Giant African Rat


Poling A, Weetjens BJ, Cox C, Mgode G, Jubitana M, Kazwala R, Mfinanga GS, & Huis In 't Veld D (2010). Using giant African pouched rats to detect tuberculosis in human sputum samples: 2009 findings. The American journal of tropical medicine and hygiene, 83 (6), 1308-10 PMID: 21118940



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