A baby with a rare fatal disease that was dissolving her brain has been saved after doctors used a treatment that had been tested only on mice.

Baby Z was born with a progressive neurodegenerative disorder in which a build-up of toxic sulphite causes fits, brain damage and death in infancy.

Until now there has been no known cure for the illness, called molybdenum cofactor deficiency type A, which kills about 100 babies a year globally.

A team from the Monash Medical Centre in Victoria, Australia, scoured medical literature looking for a cure when she began suffering seizures 60 hours after she was born in May last year.

Her doctors discovered a medical paper written by a German plant biologist, Günter Schwarz, who had worked on the experimental drug cyclic pyranopterin monophosphate (cPMP) for 15 years but had tested it only on 20 laboratory mice. Dr Schwarz sent his entire stock of the compound — which he had found negated the levels of sulphite in the body — on dry ice from Cologne to Melbourne.

The unapproved experimental procedure then had to be cleared by a bio-ethics board and the Family Court in Melbourne before doctors were able to administer it to Baby Z, whose sulphite levels had risen so high that they were dissolving her brain and her condition was deteriorating by the hour.

Approval was given on June 6 last year and within 90 minutes doctors began injecting the baby with cPMP. Within three days Baby Z, who had been in a comatose state, woke up, according to her neonatal paediatrician, Alex Veldman. “It was really like an awakening. It was incredible to see,” he told The Times.

Baby Z’s mother described it as “the most difficult thing anyone could ever go through”. She added: “It was the most challenging and most traumatic time of our lives.”

Dr Veldman said that the procedure carried many risks. “There was no precedent. We had no way of knowing if there would be any side-effects, if it would even work or even how much of the cPMP to administer,” he said.

Baby Z will need to receive an injection of cPMP every day for the rest of her life, and she suffers from developmental delay. Her condition has improved remarkably, however, and she is now beginning to move and talk.

As a result of Baby Z’s successful treatment, a baby in Germany has also been saved. The boy, who was born four weeks ago, has received similar treatment and, according to Dr Veldman, is also recovering. “It’s like a carbon copy. It’s incredible,” he said.

Dr Veldman and Dr Schwarz will head to the US next week with a team of doctors for discussions with the Food and Drug Administration in the hope of developing the treatment for use worldwide, along with a global trial of the therapy. They will also seek approval from the European Medicines Agency in London.

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