Successful embryo transfer could save rhinos from extinction

BioRescue, an international consortium of scientists and conservationists, has achieved what is understood to be the world’s first pregnancy of a rhinoceros after an embryo transfer.

The successful embryo transfer and pregnancy of a southern white rhino are a proof of concept and allow for the team to safely move to the transfer of northern white rhino embryos — a cornerstone in their mission to save the subspecies from extinction.

Currently, there are only two northern white rhinos left in the world: the female Najin and her daughter Fatu, who are cared for day and night at their home in the Ol Pejeta Conservancy, Kenya. Additionally, the BioRescue conservation science program has produced and cryopreserved 30 northern white rhino embryos that are currently stored in liquid nitrogen at -196°C in Berlin, Germany, and Cremona, Italy, awaiting embryo transfer into southern white rhino surrogate mothers. The successful transfer of a southern white rhino embryo will allow researchers to take this crucial step — an embryo transfer with a northern white rhino embryo — for the first time.

On 24 September 2023, the BioRescue scientists and veterinarians, led by the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW), transferred two southern white rhino embryos into surrogate mother Curra, a southern white rhinoceros living at the Ol Pejeta Conservancy. The oocytes used in producing the embryos were retrieved from Elenore, a southern white rhinoceros living in the Pairi Daiza Zoo in Belgium. The sperm used for fertilisation originated from the male Athos from the Zoo Salzburg in Hellbrunn, Austria. The oocytes from Elenore were fertilised in vitro by intracytoplasmic sperm injection (ICSI) and developed into blastoscysts at Avantea’s laboratories in Cremona. For the embryo transfer in Kenya, the BioRescue scientists transferred two embryos to increase the chance of a successful outcome.

A vasectomised, sterile teaser bull named Ouwan mated with Curra on 17 and 18 September, signalling the ideal timing for the embryo transfer, which took place on 24 September. After the procedure, Curra was monitored on a daily basis in the enclosure at the Ol Pejeta Conservancy. During this period, Ouwan showed no further interest in Curra, a first sign of a successful embryo transfer resulting in pregnancy. The BioRescue team was scheduled to perform a pregnancy check on 28 November, but Ouwan was found dead on 22 November and Curra followed on 25 November. Apparently, extremely heavy rains had led to a flooding of the surrogate enclosure and set free dormant clostridian bacteria spores.

Dissection of the animals revealed a severe systemic infection by a clostridian bacterial strain and resultant intoxication by the bacterial toxin. It also revealed that Curra was pregnant with a 70-day-old male foetus that was 6.4 cm long. Tissue samples of the foetus were collected and transported to the Max Delbrück Center for Molecular Medicine and Leibniz-IZW in Berlin — and in January 2024, it was confirmed through the analysis of the foetus DNA that the pregnancy resulted from the embryo transfer.

Having swiftly implemented measures to protect all current semi-captive rhinos at the Ol Pejeta Conservancy — including a vaccination program, quarantine of affected areas and fencing of new emergency enclosures — the next steps for the BioRescue team include the selection and preparation of a new teaser bull that will allow the scientists to know when a possible surrogate female is ready to receive an embryo implantation. The team also has to select the next surrogate mothers. After these steps, which will take several months, an embryo transfer with a northern white rhino embryo will be attempted.

“The embryo transfer technique is well established for humans and for domesticated animals such as horses or cows — but for rhinos, it has been completely uncharted territory and anything from the approach over procedure protocols to required equipment had to be invented, developed, tried and tested to be safe for use,” said Thomas Hildebrandt, BioRescue Project Head, Leibniz-IZW.

“Together with the team and many professional partners, I developed the devices that can actually find and access the required location where to insert the tiny embryo into a 2-ton animal. It took many years to get it right and we are overwhelmed that we now have proof that this technique works perfectly.

“It is bitter that this milestone is confirmed under such tragic circumstances with the death of the surrogate Curra and her unborn calf, but I am certain that this proof of concept is a turn of the tide for the survival of the northern white rhino and the health of Central African ecosystems. It comes just in time to achieve a pregnancy for northern white rhinos: we want the offspring to live together with Najin and Fatu for years to learn the social behaviour of its kind.

“Although embryos can be stored in liquid nitrogen for a very long time, we are in a rush to bring a northern white rhino baby to the ground. With this proof of concept, it can become a reality in two to three years.”

Image caption: Najin and her daughter Fatu live in the Ol Pejeta Conservancy in Kenya. Image credit: Jan Stejskal, Safari Park Dvůr Králové.