After two weeks of nursing, the rats begin click training twice a day, in approximately 5 minute sessions. Here they learn to associate the “click” sound with a food reward. This takes place in a square glass-walled cage, with a small food hole in one wall. Immediately after the rat hears the “click”, the trainer gives them a mouthful of mushed banana and crushed pellets. Click training takes one-two weeks. Once the rats learn that “click” means food, trainers can begin to teach the rats that they must now find something in order for the “click” and subsequent food reward. They have to move to sniff a hole with the target scent beneath. When the rat goes to the hole, they hear a “click” and must learn to return to the food hole to receive their reward. They do this twice a day for one week, for 5-10 minute sessions.

Once the rat has learned the discrimination of TB positive sputum samples from negative sputum samples, it will move to a lrge cage with 10 sniffing holes in a line setup.  Sample bars with positive and negative samples are placed under the sniffing holes, but during the first week, tha rat will mainly get used to the new cage configuration.  It has to learn to start with sample number one and sniff all the holes, and get its reward on the other side after hearing a click sound.  So initially, APOPO uses a limited amount of samples untill the rat catches up on the exercise.  Over time, the sample set is expanded up to a hundred samples at a go or even more.  This is ultimately the big advanatge of using rats for TB screening:  their capacity to sniff out large sample numbers in a short time frame.  Once the rat is up to speed, evaluating these hundred samples will take it only fifteen to twenty minutes, which would take a skilled lab technician more then two full days.