Super-smelling rodents

Scientists are battling the world’s deadliest disease with a little help from an unlikely ally… rats.

Among the animals that humans count as friends, we rarely think of rats. We usually consider them dirty, unwelcome creatures, hiding from view and living among rubbish. Yet, in Africa, rats are helping to save thousands of human lives – in more ways than one.

Since 2000, scientists from Tanzania, eastern Africa, have used rats to sniff out landmines. Years of war have left these deadly explosives buried just under the ground in sites across Africa and Southeast Asia. Thousands of people who accidentally tread on them are killed or injured every year. Humans with metal detectors can track down landmines, but it’s slow work – and very dangerous.  Rats, however, can work in safety, because their bodies aren’t heavy enough to set off the mines. What’s more, they work quickly, clearing an area of mines in 20 minutes that would take humans four days to complete.

These aren’t ordinary rats. They’re giant pouched rats that can grow to up to a metre long from head to tail – about the size of small dogs. They are nocturnal, which means they are active only at night. In the darkness, sight isn’t much use to them. In fact, they can’t see very well at all. But they have developed an extremely powerful sense of smell, which helps them to forage for food. It’s their super-sensitive noses that make them so good at detecting TNT, the explosive chemical found in landmines.

Starting from shortly after they are born, it takes months to train rats to find landmines. Their human trainers gradually teach them to respond to the clicking sound that a mine makes when it is stepped upon. Over time, the rats come to realise that this sound means they get a reward of food. It’s similar to the training methods we use to encourage dogs to behave and perform tricks. Rewarding certain behaviours can encourage animals to perform them.

Now, scientists are finding another way to train rats to use their amazing powers of smell: detecting tuberculosis. This is one of the world’s worst infectious diseases, killing more than a million people every year. Though it can be treated, it is hard to detect. Last year, around 4 million cases of the disease went undiscovered, according to World Health Organization estimates. The standard method of identifying tuberculosis – using powerful microscopes – can miss up to 80% of cases.

Rats can help to improve the success rate of detecting the disease. This is because, like all living matter, diseases have a smell. The bacteria that cause tuberculosis produce 13 chemicals that rats can distinguish from other substances. These chemicals are found in patients’ sputum – the mixture of saliva and mucus produced when we cough. Rats can analyse 100 sputum samples in 20 minutes – much quicker than other techniques – and they are particularly effective at finding evidence of tuberculosis in young children, who often produce too little sputum to be tested using a microscope. A rat signals that it has sniffed out the telltale chemicals by pausing at a sample and scratching the bottom of its cage.

Although the World Health Organization is still investigating the effectiveness of this technique, the scientists working with the rats report that it results in the detection of around 40% more cases of tuberculosis than other methods – potentially saving many lives.

For their life-saving work, the giant pouched rodents have been given the nickname HeroRats. The rats don't understand that they are heroes, of course. They have no idea what tuberculosis or landmines are. All they're aware of is that if they respond to the correct smells, they get tasty treats. But the work they do is incredibly valuable to humans. Without it, many landmines and tuberculosis cases would go undiscovered, causing suffering and death to thousands. In the animal kingdom, it turns out we have friends in unlikely places.

Watch Terrible TB: Part 1 to learn about another way that the changing seasons affect animals!