Conjunctivitis

Friends of the Koala photo

Chlamydia can cause conjunctivitis (eye inflammation). Chester was successfully treated for chlamydia at a koala hospital in Lismore, Australia, with antibiotics and eye drops. Using antibiotics is risky, though, because they can severely disrupt a koala’s gut flora and cause kidney failure.

Chlamydia, a bacterial infection best known as a sexually transmitted disease in people, has increased the risk of koalas going extinct. Now, the approval of the first-ever vaccine to protect the iconic Australian bear-like marsupial from chlamydia promises to arrest a rapid decline in their numbers.

The vaccine also is regarded by scientists as a potential blueprint for treating chlamydia in other animals, such as sheep, cattle and humans.

The milestone was announced last week by the University of the Sunshine Coast in Australia's Queensland state, which said the vaccine it has spent more than 10 years developing had been approved by the country's veterinary medicines regulator.

Koalas are found in the wild only in Australia's east and southeast. Exactly how many remain is unclear. Koalas are notoriously hard to count because they are active at night, reside high in trees during the day and exist in low densities, with a few individuals spread across large areas. The federal government lists koalas as endangered in the states of New South Wales and Queensland and in the Australian Capital Territory but not in the states of Victoria and South Australia.

Estimates of the koala population vary widely. The National Koala Monitoring Program, a federal government initiative established in late 2022, estimates there were 224,000 to 524,000 koalas in the country as of March 31, 2024. The Australian Koala Foundation, a nongovernmental organization, estimates the current population at 49,033 to 77,248.

The foundation, which wants koalas listed as endangered in all states, maintains that its count is based on a more rigorous methodology than the government's. Research published in 2016 estimated that Australia's koala population had fallen 24% over the previous 20 years, a drop worsened by devastating wildfires in the summer of 2019-20. Research published in December concluded that declining genetic diversity among koalas could lead to their extinction.

Koalas face threats other than chlamydia and wildfires, such as habitat destruction due to development, road accidents and encounters with wild and domestic dogs. But veterinarians are hopeful that tackling chlamydia infections could at least stop the decline in koala numbers, and might even boost them.

"Any tool that's available to try and help these animals is something that's really positive," said Dr. Julien Grosmaire, a veterinarian who helped trial the vaccine. "Getting regulatory approval means [the vaccine] can actually go outside the university's research trials and reach other koala populations. It can be tested more wildly and more widely, which can also help us try and improve the vaccine."

The vaccine antigen, or substance that stimulates the animal's immune response, is referred to as major outer membrane protein, which is found in the shell of the bacterial cells.

The koala vaccine has six components: proteins from three chlamydia strains plus three adjuvants, ingredients that enhance the immune response.

"There's about 20 variants of chlamydia out there, and we collected a lot of data to indicate the three strains we've chosen will offer pretty broad coverage across all strains," said Peter Timms, a professor of microbiology at the University of Sunshine Coast and leader of the vaccine's development team.

The adjuvants have been formulated to confer the need for only a single dose of the vaccine. "I'm pretty sure if you were able to provide a booster shot, it may well work better," Timms said. "But the point is, a single-dose vaccine is more practical, because it's hard to recapture koalas in the wild." (During one recent trial, the researchers used heat-seeking drones at night and detection dogs during the day to find koalas and their dung, respectively).

Data from five individual vaccine trials involving a combined 165 koalas in southeastern Queensland found the vaccine reduced chlamydial mortality by 64%, according to a research paper published last year.

The studies zeroed in on a specific community of koalas so they could be more easily monitored. Koalas can live for 10 to 15 years, but chlamydia cuts their life expectancy to seven years or fewer. Crucially, in the community monitored by the researchers, the vaccine was found to substantially reduce chlamydia risk during breeding age. Modeling suggests that just 20% of koalas in that or a similar community need to be vaccinated to stabilize a declining population.

A 'miserable' fate

Emaciated

Endeavour Veterinary Ecology photo

Advanced chlamydia infections can cause severe weight loss.

A telltale sign of chlamydia in koalas is conjunctivitis (eye inflammation), which, in its most severe form, keratoconjunctivitis, can cause blindness. Infected animals also commonly suffer from urinary tract infections.

"They can get massive bladder-wall thickening and can't retain urine, so you get urine scalding of their rump," Grosmaire said. "And then the other sign we really see is reproductive disease in both males and females that causes infertility. So you don't get new joeys in the population."

While a chlamydia infection might not kill a koala outright, it can make them more vulnerable to other causes of death. Vision impairment from chlamydia, for instance, makes them easier prey for natural predators, like carpet pythons and dingos, or less able to avoid domestic dogs or cars.

"When they feel sick, they'll sometimes come down the tree and sit at the bottom — and that's when a dog could grab them," Grosmaire said. "Some of them will start losing weight just from the infection and become so emaciated, they can't cope anymore."

Dr. Kate Thomas sees up to 350 koalas per year as superintendent veterinarian at the Northern Rivers Koala Hospital in the northern New South Wales town of Lismore — including one to seven per day during the September-to-December breeding season.

"It's our busiest time of year, as males travel in search of mates and young joeys begin dispersing from their mums, leading to more movement across roads and backyards," Thomas said. "Sadly, this results in a spike in car strikes and dog attacks."

Still, the prevalence of chlamydia is very high in the Northern Rivers region. Thomas estimates around 80% of her patients are infected, making it by far their biggest problem.

"Koalas with chlamydia can look pretty miserable," she said, alluding to their eyes, swollen and weepy from conjunctivitis, and rumps stained and ulcerated from incontinence. Females also develop ovarian cysts that can cause infertility. By the time many cases reach the hospital, the disease is too advanced for treatment. "Unfortunately, a large number of them require euthanasia," Thomas said.

Chlamydia in other animals typically is treated with antibiotics. But for koalas, there's a catch. Antibiotics can cause them severe dysbiosis, a disruption of the microorganisms in their gut, hampering digestion and immune system function. One potential outcome of this disruption is a condition called oxalate nephrosis. The eucalyptus leaves that koalas chew for moisture and nutrients contain a lot of oxalates, which are usually processed by bacteria in the animal's gut. "We theorize that the antibiotics we use seem to kill those bacteria off, which leads to an accumulation of oxalates in the kidneys and then, renal failure," Grossmaire said.

Even so, doctors can be left with little choice but to use antibiotics. The Lismore hospital's general treatment regime involves a 28-day course of injectable antibiotics, along with topical eye medications two to three times a day. "It's quite an intensive regime," Thomas said.

Stain

Endeavour Veterinary Ecology photo

Chlamydia often causes urinary tract infections, leading to incontinence and brown staining around the rump.

Her hospital assisted in the vaccine's development by providing cases and data to the university. "When I heard the vaccine was finally approved, I was relieved and excited," Thomas said. "It feels like a real turning point. If it does what we hope, it could stop many koalas from getting sick in the first place — which would be massive, both for the species' survival and for those of us treating them every day. It definitely gives me hope."

Another vaccine under development

Hospitals that treat koalas are already asking for large consignments of the new vaccine. They'll have to be patient.

The University of the Sunshine Coast's research so far has been supported by federal and state government funding, and that's about to run out.

"At the moment, government have been strong supporters," Timms said. Provided more funding materializes, including potentially from philanthropic donations, he hopes the vaccine will start being delivered to hospitals in early 2026, free of charge.

"We stopped for 10 seconds to celebrate getting approval, and we've just moved on," Timms said. "It's not a cheap vaccine, and that's why we're already working on the next version."

Theirs is not the only effort for a koala chlamydia vaccine. Another is underway at the Queensland University of Technology (QUT). It is a two-dose vaccine — but that doesn't mean koalas will need to be handled twice. Its creators have invented a device that will release the booster shot inside the animal about four to five weeks after being implanted under its skin. Slightly larger than a pet microchip, the "booster vaccine" resembles drug-delivering implants used in humans.

"With all the vaccines we have developed, we find immunity lasts a lot longer following a booster immunization," said Kenneth Beagley, the QUT vaccine developer and an immunology professor.

The device is still at the experimental stage. Study results so far have been positive: The immune response in koalas given the implant has been identical to those given two separate injections, Beagley said. As to the vaccine's efficacy, two shots given 30 days apart to koalas in a high-chlamydia-prevalence area of Queensland's Gold Coast produced encouraging outcomes. The proportion of koalas admitted to a wildlife hospital there with chlamydia fell to 28% last year, down from 69% in 2020.

There is also evidence that immune responsiveness stimulated by the vaccine is passed on through the generations. "We've had more than 50 young koalas born to vaccinated females, and we're now seeing animals in the second and third generation, and they've all been chlamydia-free two years after we followed them," Beagley said. "There is transfer from mother to joey."

The investment case for both vaccines goes beyond koalas. Chlamydia infects many animals, such as pigs, sheep, cattle, cats, dogs and birds. In adult humans, there were an estimated 128.5 million new chlamydia infections in 2020, according to the World Health Organization.

The koala research is respected in global health circles because trials have been conducted in the wild, not just in a laboratory, Timms said. "The human vaccine world is pretty interested in the koala model because it's a real-world model, and you can learn things that you cannot learn from mice. So we're pretty hopeful that the lessons of what we're doing are useful in human vaccinology."