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Blinded ... a koala being treated for chlamydia at the Wildlife Health and Conservation Centre. One strain of the disease affects eyesight, the other fertility. Photo: Edwina Pickles

ONE of the few koala populations unaffected by debilitating chlamydia is at imminent risk of infection, researchers warn.

The colony at Campbelltown contains up to 500 marsupials that carry unique genetic diversity in their immune systems that potentially makes them important in combating future diseases.

While the numbers in this colony had increased steadily over the past two or three decades, they are now being threatened by two types of chlamydia carried by koalas in the nearby southern highlands.

''Koalas have had some hard times in the past,'' the director of the University of Sydney's Wildlife Health and Conservation Centre, David Phalen, said.

''Historically, because of fires, because of development, because of the fur trade … the koala populations were severely impacted.

''They're getting to the point where they're really starting to recover now.''

While both the Blue Mountains and southern highlands populations were riddled with chlamydia - a highly infectious bacterial disease that causes painful blindness or infertility, depending on the strain - Campbelltown's group had until now been free of infection.

But the centre was now treating a female koala, found in a field about 20 kilometres west of Mittagong, for chlamydia that had made her blind.

''She had probably been moving around the forest for some time blind,'' Dr Phalen said. ''She ended up out in the middle of a field and might have been chased out by a dog or got disoriented.''

With treatment, her poor eyesight had improved somewhat.

But if chlamydia took hold among the koalas in Sydney's south - many of which lived inside the Holsworthy military reserve - their important genetic advantage, which could be used to develop a cure for diseases such as chlamydia, might be lost.

''Whenever we're working with animal populations, we want to maximise genetic diversity,'' Dr Phalen said.

Chlamydia is widespread among koala groups in NSW, Victoria and Queensland.