In a statement released on February 25, Kathryn England, chief operating officer at London Zoo, said the cub—Loki—had suffered several unexplained seizures, “each worse than the last.”

The cub was facing “an increasingly limited quality of life” leading vets to take the “heart-breaking” decision to euthanize him, the statement said.

The rare Sumatran tiger cub was born in December to 10-year-old Gaysha. Loki was born nearly a year to the day Gaysha arrived at the zoo from Denmark. Sumatran tigers are a critically endangered species. They are the rarest subspecies of tiger in the world and there may be as little as 400 remaining in the wild.

Loki had shown no prior signs of illness before suffering his first seizure on February 18. The zoo said he had been playing just hours before. The cub was rushed to a veterinary hospital and appeared better after receiving antibiotics. Veterinarians ran tests to determine the cause but the results were inconclusive.

While under 24/7 monitoring, zookeepers grew concerned when Loki appeared lethargic, with little appetite.

He was once again rushed to the hospital for more tests. Late on February 20, he suffered two more seizures.

Loki went for an emergency MRI on February 22, where once again, there was no “obvious cause” for what had triggered the seizures.

When back in his enclosure, the cub enjoyed a day with his mother where there was no sign of any further seizures.

But “devastatingly,” the zoo said, he then suffered a far more serious seizure confirming “the vets’ worst fears.”

“Loki’s fits were not going to stop and his health would only decline,” the zoo said. “We left no stone unturned. Through our tests we ruled out so many things; a brain tumor, infections, parasites – frustratingly we still had no answers. We were considering a presumptive diagnosis of epilepsy – something that can only be confirmed through a process of eliminating all other possibilities.”

The cub was euthanized on February 24.

Tigers have been known to suffer seizures before, but it is rare at a young age. It can sometimes be down to tapeworm infections, neurological conditions, or epilepsy.

Vets will continue efforts to understand Loki’s condition and carry out a post-mortem evaluation, England said. The results will be shared with other zoos, in hopes it will give them a better understanding of the critically endangered species.

“We had all fallen in love with little Loki and know that many of you had too. We wish we had different news for you but are so comforted by the outpouring of kindness and compassion you’ve shown us already this week,” England said.