reporter: andrew tilker knows the saola only from photos.e, at the far right in the picture, the last image from a camera trap. the american biologist has been studying large mammals in vietnam for three years. scientists first discovered the saola in 1992. since then, there have been five photos. with his team from the world wildlife fund, tilker works in the forest for weeks. they're worried that the world's last saolas could die in traps like these at any time. andrew: nobody knows how many saola are left in quang nam or vietnam. it could be up to 100. it could be that we can count the number of saola on one hand. so it's really searching for a needle in a haystack. reporter: these leeches might help him. they're a bit like thousands of tiny camera traps. andrew: the leeches are feeding on these animals in the jungle. the leeches are easy to find. usually they come to us, because they want to feed on us. so we can collect the leeches and analyse the dna in the leech to see what they've fed upon. reporter: from the forest to the laboratory