Some wild populations on mainland Australia have a 100 per cent infection rate while experts predict about half of all koalas have the disease.
Scientists led by the University of Adelaide have discovered that, unlike every other large population in Australia, Kangaroo Island koalas are free from infection by Chlamydia pecorum.
The bacterial infection is the most significant disease-causing death in koalas, and a key factor in koalas being under threat in north-eastern Australia.
“The impact of chlamydia on populations of koalas in Queensland and New South Wales is devastating, with high levels of severe disease and death, and common infertility,” said researcher Jessica Fabijan, PhD candidate with the University of Adelaide’s School of Animal and Veterinary Sciences.
“This last large, isolated Chlamydia-free population holds significant importance as insurance for the future of the species. We may need our Kangaroo Island koalas to re-populate other declining populations.”
The strain of chlamydia that infects koalas is not the same as the one sexually transmitted by humans but it can cause blindness through conjunctivitis, pneumonia and infections leading to female infertility.
The Australian Koala Foundation lists chlamydia as among the major threats to koalas along with habitat loss, dogs, traffic and bushfires.
Kangaroo Island is Australia’s third biggest offshore island and has long been regarded as one of the world’s most pristine natural environments.
Koalas are not naturally occurring on Kangaroo Island but 18 were introduced in the 1920s amid fears they were facing extinction on the mainland from habitat destruction and hunting.
The absence of predators such as dingoes and foxes as well as a lack of human encroachment – koalas outnumber people by more than 10-to-1 on the island – have allowed them to thrive.
Their population on the island known for its pristine environment and abundant wildlife has boomed with a 2015 survey putting their numbers at about 50,000 – about half of which are in the wild and half living in blue gum plantations on the island.
The latest chlamydia study was undertaken by the University of Adelaide’s School of Animal and Veterinary Sciences in collaboration with the South Australian Department for Environment and Water (DEW), the University of the Sunshine Coast and koala care volunteers. The research has been published in the Nature journal Scientific Reports.
The researchers captured and released 75 wild koalas from the Mount Lofty Ranges, east of the South Australian capital Adelaide, and 170 koalas from Kangaroo Island.
Each koala was checked by a veterinarian and tested for Chlamydia pecorum and koala retrovirus.
The researchers found that 46.7 per cent of koalas from the Mount Lofty Ranges were positive for Chlamydia, although the vast majority did not show signs of disease.
All Kangaroo Island koalas tested negative for Chlamydia and no disease was observed. There were also no definitive cases of chlamydial disease in the 13,000 previous records of koala examinations on KI.
“This is a very important finding because chlamydial disease is so prevalent and efforts to fight it have so far been unsuccessful,” said project leader Dr Natasha Speight, also from the University of Adelaide’s School of Animal and Veterinary Sciences.
“Although, South Australian and Victorian koala populations are stable, this is believed to be at least partly due to current lower prevalence and severity of chlamydial disease.”
Research is underway to understand the difference in chlamydial disease severity between southern and northern koalas.
The furry marsupials have been listed as Vulnerable by the International Union for Conservation of Nature.
While koala estimates of Australia’s wild koala population range from less than 100,000 to 330,000, the boom in numbers on Kangaroo Island prompted calls for a cull in 2017, which was quickly ruled out by the South Australian Government.
A koala management project on the island has been running since 1997 and involves fertility control through sterilisation and the relocation of thousands of koalas to their historic range in the South East of mainland South Australia.
DEW koala spokesperson Brenton Grear said the new research meant the strong Kangaroo Island population was one of the few, and likely the largest, Chlamydia-free population of koalas in Australia.
“Future-proofing South Australia’s koala health is paramount to ensuring the survival of the species in Australia, given the marked decline in the eastern states,” he said.
Kangaroo Island is located 13 km off shore from the Fleurieu Peninsula, 100 km south of Adelaide. It is an internationally renowned eco-tourism destination with a total area of 438,000 ha (140 km long and 55 km wide). Almost half of the island’s native vegetation remains intact, primarily at the western end.
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