Isolated population of chlamydia-free koalas even more vulnerable to the disease

New research from the University of Sydney highlights the unique existential dilemma faced by Sydney’s koala population.  

These populations in south-western Sydney are among the very few in New South Wales (NSW) still free of chlamydia, a highly contagious disease causing infertility that has severely diminished populations elsewhere in the continent’s eastern states.  

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However, analysis of these koalas led by Dr Elspeth McLennan and Professor Carolyn Hogg at the University’s School of Environmental and Life Sciences shows how vulnerable they are to environmental threats and outbreaks of disease. Highly inbred and with low genetic diversity, they are less likely to adapt to the disease should it arrive on their doorstep.  

The findings have been published in Conservation Genetics. 

Low genetic diversity 

Tissue samples from 111 koalas were collected by NSW Government staff from seven sites in the south-western Sydney suburbs of Liverpool, Campbelltown, Heathcote and Wollondilly, and from Wingecarribee in the Southern Highlands.  

Genetic analysis showed a high level of interrelatedness, inbreeding and worryingly low genetic diversity across Sydney koalas.   

Low genetic diversity means populations cannot always adapt to change, making them highly susceptible to environmental threats and disease outbreaks.  

“On average, koalas in the Sydney populations have cousin or half-sibling relationships,” said Dr McLennan.  

Classic Catch-22

Living in highly urbanised areas limits opportunities for Sydney’s koalas to move around and breed with populations further afield and increase the diversity needed to build resilience. However, there is a chance that koalas from the neighbouring Wollondilly Shire, where chlamydia is present, may find their way to the Sydney populations. Gene flow analysis showed koalas are moving between Wollondilly and Campbelltown, the southern-most chlamydia-free site. 

“It’s a classic Catch-22 situation,” said Dr McLennan. “If the Wollondilly koalas breed with those elsewhere in Sydney they could increase genetic diversity. But they may bring chlamydia with them. If the latter happens, individual koalas are unlikely to have enough genetic variation to adapt to the threat. 

“Instead of some individuals being able to naturally clear chlamydia without it progressing to blindness and infertility, it is possible all individuals will contract the infection whereby it progresses to the later stages of the disease.” 

 No clear path

Dr McLennan says there is no ready solution to addressing the threats, including anthropogenic threats from climate change and ongoing urbanisation, to koalas in south-west Sydney. Simply improving habitat connectivity to increase genetic diversity may promote chlamydia spread, she said.  

“Beyond south-west Sydney, the results show the importance of managing koala populations and their surrounding landscapes. We need to ensure ongoing connectivity between all koala populations to maintain their health and resilience to threats.” 

Koalas in Queensland, New South Wales and the Australian Capital Territory were listed as endangered in 2022. Their populations have decreased 24 per cent in the last 20 years.  

The research highlights an issue faced by conservationists worldwide too.  

“Without diversity, endangered species risk succumbing to disease outbreaks and environmental threats,” the authors said.