The world's last male northern white rhino is recovering from an infected leg that raised fears over the past week he might have to be put down, a veterinarian at a conservancy in Kenya says.

Sudan lives with the last two females of the same species in Ol Pejeta Conservancy, about 250 kilometres north of Nairobi.

After all attempts at getting him to mate naturally failed, conservationists last year put Sudan on dating app Tinder, hoping to raise enough money to pay for a $11.5 million fertility treatment.

The 45-year-old had spent most of the past two weeks lying in his pen due to discomfort from a deep wound on his right hind leg.

His keepers had wondered whether it might be time to put him down.

But Stephen Ngulu, Ol Pejeta's veterinarian, said they had managed to bring the infection under control with painkillers and antibiotics and Sudan had regained his healthy appetite.

"He is an animal that is showing the will to live," Mr Ngulu told Reuters at the conservancy, as he struggled to walk in his pen while his companions Najin, 27, Fatu, 17, played in the mud a short distance away.

Fatu and Najin graze at the Ol Pejeta Conservancy in Laikipia national park, Kenya. ( Reuters: Baz Ratner )

While there are thousands of southern white rhinos still roaming the plains of sub-Saharan Africa, decades of rampant poaching have drastically cut numbers of northern whites.

Poachers can sell northern white rhino horns for $50,000 per kilo, making them more valuable than gold or cocaine.

Kenya, whose tourism sector is a huge source of foreign exchange, had 20,000 rhinos in the 1970s, falling to 400 in the 1990s.

It now has 650, almost all of which are black.

Scientists are now working to help Sudan reproduce via in-vitro fertilisation using eggs taken from Najin.

The embryo would be implanted in a surrogate southern white, Mr Ngulu said.

Reproductive experts from Kenya, Europe and South Africa hope to have designed a means of extracting the eggs from Najin by the end of this year, he said.

With the old male nearing the end of his life, Zachary Mutai, who has cared for him at Ol Pejeta for the past eight years, said the ravages of age were a source of sadness.

"Sudan is my great friend," he said.

"He's a very gentle rhino, people used to stroke him, people used to come very close to him, he's a very gentle one."

Reuters