i dream of seeing a saola. even if i got a glimpse, a two-second glimpse, it would make all of this, all of the years of hard work, it would make it worthwhile. reporter: andrew tilker knows the saola only from photos. here, 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

searching high and low for the saola, an extremely rare bovid in vietnam. and representing the millennium - our teenager this time around comes from the caribbean.