Dr Taggart normally deals with gall bladder problems in humans from his East Melbourne practice and at the Royal Melbourne Hospital,  but got the job of finding  the best treatment for the young male monkey, Ovo,  who weighs about 900 grams.

But devising the best treatment for the seven-year-old monkey was not that different to caring for his human patients.

"You follow the same principles but the main issue here was that the bile duct was so small for this monkey," Dr Taggart said. "And the gallstones are incredibly tiny."

Dr Taggart said a golden lion tamarin monkey was too small for an endoscopy and shattering the gallstones with ultrasound wouldn't have worked because they were already in an obstructive location. In addition, if the stones were shattered, it was likely they would re-form.

He said surgery on the bile duct was also considered risky because the duct could further narrow as a result of post-surgical scarring.

"It would have been risky and very difficult," Dr Taggart said.

In the end, in collaboration with Melbourne Zoo head vet Michael Lynch and Annie Rose from Melbourne University's vet school, it was decided that young Ovo would be given a drug used in humans and dogs called ursodeoxycholic acid. Made from a synthetic bile acid, it dissolves the stones to a point where they can be passed.

Dr Lynch said Ovo was being monitored and while the gallstones are still there, they appear smaller.

"He's a good patient because he takes his medication willingly," Dr Lynch said.

The fact that it doesn't taste too bad and the liquid medication is injected into a tasty cricket also helps.

Dr Lynch said Ovo would continue taking medication twice a day for  six months.

"We're taking it quite seriously and we have to resolve it," he said. "If we get to the end of the year and we still haven't resolved it medically, we will have to consider surgery."

Ovo is also being put on a diet. Dr Lynch thinks he could lose 100 grams which would reduce the risks of surgery.

Gallstones are common in humans but not so much in other species.

Listed as endangered, golden lion tamarins can live for up to 25 years. They live in small family groups in the wild, consisting of a breeding pair and offspring.

Native to Brazil's Atlantic coastal forest, there were less than 200 of these small monkeys in the early 1970s. Captive breeding programs and conservation work has boosted the wild population to about 1600.

It's not unusual for Melbourne Zoo vets to call on human medical experts, particularly when a specialist technique such as dental or optical work is involved.

Dr Taggart said treating Ovo had added "another string to his bow". And another animal to the intriguing list of species he has advised on, including pythons, gorillas, seals and a South American marsupial called a coati.

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