The world's last male northern white rhino is on death watch

(CNN) With three northern white rhinos left in the world, the only male is gravely ill, raising fears the subspecies is getting closer to extinction.

At 45, considered elderly in rhino years, the animal lives at the Ol Pejeta Conservancy in Kenya with two female northern white rhinos -- Fatu and Najin. They are the last three northern white rhinos.

But Sudan's ill health is a cause for concern as experts scramble to ensure the subspecies does not go extinct.

"We don't think he will last for much longer," said Elodie A. Sampere, a spokeswoman at Ol Pejeta.

Sudan developed an age-related infection on his back right leg last year, Ol Pejeta said. It was assessed and treated, and he resumed normal movement in January.

A secondary infection developed beneath the initial one, and it's not responding to treatment.

"Euthanasia will be explored if we feel he is suffering too much and won't recover," Sampere said. "We do not want him to suffer unnecessarily. Right now he is still feeding and walking around ... albeit very little."

Even before his illness, Sudan was a crucial part of ensuring the beloved animal does not go extinct. So much so, he was protected from poachers by 24-hour armed guards.

Photos: The great white rhino hope In December 2013, the northern white rhino Nabire walks around in her enclosure at a zoo in Dvur Kralove, Czech Republic. Nabire died of a ruptured cyst in July 2015, leaving only a few northern white rhinos left in the world. Hide Caption 1 of 7 Photos: The great white rhino hope In Kenya, where three of the remaining northern white rhinos live, armed guards prevent poaching. Hide Caption 2 of 7 Photos: The great white rhino hope Sudan is one of three northern white rhinos left worldwide. Hide Caption 3 of 7 Photos: The great white rhino hope As the only male, the fate of the subspecies depends on Sudan. Though he is too old to mate, scientists are hoping to impregnate other females using his sperm. Hide Caption 4 of 7 Photos: The great white rhino hope In addition to round-the-clock security, the conservancy has put radio transmitters on the animals and dispatches incognito rangers into neighboring communities to gather intelligence on poaching. Hide Caption 5 of 7 Photos: The great white rhino hope Rhinos are targeted by poachers, fueled by the belief in Asia that their horns cure various ailments. Hide Caption 6 of 7 Photos: The great white rhino hope Many extreme measures have been adopted to save the rhino. The Rhinos Without Borders moved 100 black rhinos from South Africa -- which holds 80% of Africa's rhino population -- to Botswana, which has the lowest poaching rate in the continent. Hide Caption 7 of 7

Rhinos are targeted by poachers, fueled by the belief in Asia that their horns cure various ailments. Experts say the rhino horn is becoming more lucrative than drugs.

With only three left, there's a race against time to try to sustain the northern white rhino. A committee at the conservancy is looking at various alternative reproduction techniques, including in vitro fertilization.

The western black rhino was declared extinct seven years ago as a result of poaching. All five remaining rhino species worldwide are considered threatened, according to the conservation group Save the Rhino

Experts say if poaching continues, rhino deaths could surpass births.