A koala made famous by a photograph of her drinking from a firefighter's water bottle as wildfires ravaged Australia this year is about to undergo a risky surgery, animal shelter officials said Wednesday.

"Sam" the koala suffered second- and third-degree burns to her paws in the February fires and is recovering at the Southern Ash Wildlife Shelter in Victoria state.

But shelter spokeswoman Peita Elkhorne said Wednesday that Sam had developed abdominal cysts due to a disease called urogenital chlamydiosis, which affects up to 50 percent of Australia's koala population. The disease can cause infertility, urinary tract infections and blindness and can be life-threatening.

Sam will undergo surgery on Thursday and the prognosis for surviving the operation is not good, Elkhorne said.

"We're telling the public to brace for the worst-case scenario," she said.

Elkhorne said Sam was "comfortable" but that a decision was made to try to remove the cysts because the disease can kill koalas.

She said Sam's prospects would be known by Friday.

In February, Sam was gingerly making her way on scorched paws past a fire patrol north of Melbourne when one of the firefighters spotted her. The firefighter was photographed holding a bottle of water to her lips.

Record temperatures, high winds and forests dried by years of drought set off infernos that swept a vast area of Victoria state, killing more than 170 people and destroying thousands of homes.