More than 20 years after the Ottawa Treaty banning landmines came into effect, 29 previously mined countries have been declared mine-free, and yet millions of the lethal devices remain in more than 50 nations.

Many are made almost entirely of plastic to escape detection by machines that search for metals. But talented rodents are increasingly finding the mines anyway.

Because they sniff explosives, rather than detecting metal, African pouched rats aren't distracted by coins, nails, and other debris. They also don't need to go slow for fear of setting off undetected mines.

Rats work fast, following a rope grid, and are cheap. They're not only good at sniffing out mines, they're also light, unlike humans, and don't set mines off.

"Essentially we work about 40 times faster than a human with a metal detector," says Charlie Richter, who oversees their work.

Picture a former battlefield, he said, where there would be lots of scrap metal in the ground. Detectors are constantly giving off false alarms, Richter said.

"Every time there's an alarm they have to stop everything, dig carefully around that area, and if they don't find a mine they continue," he said. "With a rat, they ignore all the scrap metal, so there are far fewer occasions when they have to stop and do that."

A student's idea

As a young Belgian graduate student, Bart Weetjens remembers watching a documentary about landmines at the time the Ottawa Treaty was being negotiated, and thinking of his own pet rats.

He had already trained them to find hidden objects in exchange for treats, and it occurred to him that they might be able to do something similar with landmines. With a group of friends he approached the Belgian government for a grant to develop his idea. That was 20 years ago this month.

Although human handlers keep a safe distance from the rats, they are generally too light to set off an anti-personnel mine. (APOPO)

The organization that Weetjens formed to promote his idea, APOPO (the Dutch acronym for Anti-Personnel Landmines Detection Product Development), has demined in seven countries, including Mozambique, which was declared cleared in 2015 after the removal of about 170,000 mines.

Without the rats, Mozambique would still be working towards that goal, says Richter.

From nuts to high explosives

Mines not only kill and maim, but also deny rural people the ability to use arable land, making it difficult for them to produce food.

"Having one landmine in a soccer field-sized area scares the local community and keeps that field as unproductive as having 50 landmines," said Richter, who works with APOPO.

That's the kind of situation where the rats excel. In densely packed minefields like the ones in the Korean demilitarized zone, their comparative advantage is weaker, said Richter.

Like any business, we need to specialize in what we're good at. And for us, it's the rats. - Charlie Richter

"Most minefields aren't like that, most are maybe one or two mines in an area the size of a soccer field," he said.

African giant pouched rats, also known as Gambian pouched rats, use their keen sense of smell to rediscover caches of nuts and other foods they bury for a rainy day. It takes about nine months to train them to sniff out TNT instead.

Once trained, their relatively long lifespan means they generally have careers of about five years.

Not everyone in the demining business initially warmed to the rats, which can grow up to 90 centimetres long, says Richter.

"There was general skepticism for a long time. This business is kind of conservative, because if you make a mistake people get blown up. But if you're too conservative, the mines stay in the ground for longer, and that also means more people die," he said.

Now that big demining NGOs have seen the rats' work in Mozambique, they are coming around to rodent solutions, said Richter.

Rats mostly follow a rope grid laid out over a minefield. The pole technique pictured here is also used. (APOPO)

APOPO would like to stop being a full-service deminer, he said, and focus on its niche of training rodents in order to spread their use through the world's biggest demining organizations.

"Like any business, we need to specialize in what we're good at. And for us, it's the rats."

Sniffing out pathogens

Richter said they've also revealed a talent for detecting tuberculosis, a disease that lingers in many of the same poor and underdeveloped corners of the world that are still blighted by landmines.

The rat's keen sense of smell can detect the disease on a human patient, or in a mucus sample in a petri dish.

Where a lab worker might need 20 minutes to analyze a sample under a microscope, a rat needs only about 12 seconds. (Any positives detected by the rat are confirmed by a human tester).

APOPO rats have screened nearly half a million sputum samples and over 90,000 human patients.

Because they are less likely to miss a case than human testers, APOPO estimates the rats have caught 12,000 diagnoses that might otherwise have been missed.

New rodent frontiers

Even as they gnaw their way through their existing tasks, the rat handlers are looking for new challenges, and they believe they've found them.

A rat is presented with human sputum samples to test for tuberculosis. It's faster and often more accurate than a human lab technician. (APOPO)

The rats are now going to work for the U.S. Fish and Wildlife Service, using their noses to detect smugglers trying to move endangered animals and hardwoods around the world. The focus is on finding one of the most trafficked of all animals — the pangolin, an endangered mammal poached for their scales and meat.

The next challenge for the rats will be to learn how to find people in collapsed buildings.

Equipped with GPS and a camera, the small rodents can penetrate far deeper into rubble than a rescue dog. Ritcher is hoping to train them to pinpoint survivors.

"We need to find out if the rats are reliable enough to go down, find someone and come back. There's been very little work done with off-leash detection. I mean you can imagine if there's a kitchen in the rubble and a whole bunch of cheese … you can see how that might be a distraction."