Toxic leaves may be reason Lumholtz’s tree kangaroos are losing their eyesight, researcher believes


Rare, rainforest-dwelling mammals have been turning up in hardware stores, service stations and school classrooms in the Atherton Tablelands.

The Lumholtz’s tree kangaroos appear to be lost, dazed and blind, but scientists are baffled as to why.

It seems more than one species of kangaroo has been afflicted by blindness, with evidence of eastern grey kangaroos as well as the Lumholtz’s tree kangaroos experiencing vision impairment.

Veterinary experts would like to see more funding provided to look into the reasons why.

Lumholtz’s tree kangaroos are one of just two tree kangaroo species in Australia, found only in Far North Queensland between Cardwell and the Daintree.

They have been observed to be suffering from blindness for 10 years.

Karen Coombes, who has a PhD in Lumholtz’s tree kangaroos and founded the Tree Roo Rescue and Conservation Centre in Malanda, west of Cairns, said 18 out of 20 tree kangaroos in her care had vision impairment.

“We don’t say they are blind because they can see a little bit, but it’s their focus and depth perception that is decreasing for some reason,” she said.

Dr Coombes said the tree kangaroos had been found in some “really crazy places” such as a hardware store, service stations and school classrooms.

“This is not normal behaviour for a tree kangaroo,” she said.

“They are turning up lost and miles away from the rainforest.”

Specialist veterinary ophthalmologist Tony Read, who treats eye disorders in animals, has been examining the tree kangaroos in Dr Coombes’s care for the past 10 years and has concluded the actual eyes appear fine — but the animals’ behaviour suggests otherwise.

“The eyes look normal on examination, meaning there are no structural abnormalities like cataracts or retinal changes, but we are concerned about central blindness,” he said.

“They seem to have light sensation which we call the dazzle effect, where the optic nerve still has a reaction to light but it doesn’t mean they have full vision.”

Dr Read said the tree kangaroos were also showing behavioural vision problems, behaving as though they were blind and acting confused and dazed when rescued.

Dr Coombes has been sending samples to Andrew Peters, a veterinary pathologist and associate professor in wildlife health and pathology at Charles Sturt University, in NSW, for several years and he has confirmed the animals are sight-impaired.

“The clinical assessment is that the animals had central blindness and we found pretty subtle, mild inflammation in the optic nerves,” he said.

“The conclusion was optic nerve and brain damage which unfortunately is irreversible.”

How common is blindness in kangaroos?

This is not the first time the issue of blindness in some kangaroo species has caused concern.

Dr Peters said a student at Charles Sturt University investigated blindness in eastern grey kangaroos in Wagga Wagga years ago.

He said her findings showed the blindness was caused by an introduced exotic grass.

“It was the first time toxic plants were linked with blindness in Australian wildlife,” he said.

“What was interesting is it was linked with the particular climate at the time, rainfall after a prolonged dry period, all leading to the invasive grass taking over pastures.”

Dr Coombes believes climate also may be behind increased toxicity in the leaves the Lumholtz’s tree kangaroos are eating.

“I believe the rainforest is facing stress from lack of rain and hotter temperatures and with less water in the leaves, the toxins are more concentrated,” she said.

“The roos rely on the leaves for water and when they eat them, the neurotoxins are making their immune system compromised.”

Dr Peters said it was difficult to conclusively say if toxic leaves or a virus were causing vision impairment, because the effects of toxic plants could also masquerade as other kinds of diseases such as viruses.

“Toxic plants and viruses can both be associated with habitat change, climatic change and invasive species that can lead to diseases appearing in wildlife that weren’t there before,” he said.

Dr Peters said there had been many cases where viruses caused blindness in kangaroos and all were mosquito-borne, but to test if a virus had come from mosquitoes, there needed to be large sample numbers to study.

Dr Coombes said whatever was causing the vision impairment did not appear to be genetic.

“The babies born to sight-impaired adults can see and don’t show any signs of blindness,” she said.

Joeys rescued from a dead mother’s pouch stay at the Tree Roo Rescue and Conservation Centre until they are at least two and a half years old, as this is how long they would stay with their mothers.

They are then released back into the rainforest to live healthy lives.

Tree kangaroos that are vision-impaired and cannot be released are cared for by Dr Coombes and her volunteers, until they have been cleared to go to a zoo as part of a captive management and breeding program.

“These animals get to live out the rest of their lives in massive enclosures,” she said.

“In turn they also educate people about the issues affecting tree kangaroos and become part of the breeding program.

“You can’t save a species unless people know about them and a lot of Australians don’t even know that tree kangaroos exist.”

ABC News, 23 April 2023