An article in npj vaccine in August 2024 presents an evaluation of a 10-year assessment of a Major Outer Membrane Protein-based vaccine in wild koalas from Southeast Queensland. The koala was listed as endangered by the Australian Government in 2022, facing threats such as habitat destruction, dog attacks, and Chlamydia pecorum disease. This extended study offers a “thorough evaluation” of vaccine efficacy, revealing that vaccinated koalas had “significantly lower disease incidence”.
“This vaccine demonstrated positive impacts on both male and female koalas, highlighting its crucial role in conserving the Australian koala population and mitigating the threats they face.”
Koala health
The Australian Government listed the koala as an endangered species in three of the five states/territories in Australia where the species naturally occurs: Queensland, New South Wales, and the Australian Capital Territory. This was a response to population declines, attributed to several factors: climate change, habitat loss, fragmentation and degradation, traffic strikes, dog attacks, and disease. The top three reasons for a koala being admitted to a wildlife hospital are infectious disease, motor vehicle trauma, and wasting.
Among the “wide range of infectious pathogens” that affect koalas, including Bordetella, Koala retrovirus (KoRV), and Gamma herpes virus, infections that involve the intracellular bacterium Chlamydia pecorum are “by far of the most concern”. They are “highly prevalent” and cause “premature mortality” as well as various “chronic, painful conditions”. Chlamydia pecorum infections can result in disease of the conjunctivae, urinary tract, and reproductive tract. If these infections go untreated, they can cause “significant discomfort, and premature death”; koalas with severe, untreated Chlamydia infections can have a life expectancy reduction of several years.
Although chlamydiosis can be treated with antibiotics, this approach is associated with a risk of “potentially fatal gastro-intestinal dysbiosis” and does not prevent future infection. Thus, “considerable effort” has been directed towards the development of a koala Chlamydia vaccine, with 14 separate koala vaccine trials conducted in South East Queensland.
The project
Another project also took place among a specific population of wild koalas in South East Queensland’s Moreton Bay region. Over 10 years the project involved “intensive telemetric monitoring and veterinary management” of around 150 koalas at any given time. The programme featured five separate vaccine trials, using the C. pecorum Major Outer Membrane Protein (MOMP) as the antigenic target and varying adjuvants and dosage regimens.
“Each of these trials identified that, in wild koalas, a MOMP-based vaccine can elicit a strong anti-Chlamydia systemic and mucosal antibody response that persists for more than two years.”
The trials also found that, with “careful selection of the adjuvant”, protection can be achieved with a single dose of vaccine in both healthy and diseased animals, reducing disease incidence by 42% and infection incidence by 82%.
The paper presents an analysis that comprehensively evaluates C. pecorum MOMP-based vaccine effectiveness across a large population of wild koalas over a 10-year period. The authors state that a MOMP-based vaccine for koalas can protect individuals “both from developing chlamydial disease and, crucially, from dying due to chlamydial disease”. Notably, it emerges that vaccination “extended the age at which disease will affect 50% of the population” by three years, during koala breeding ages, from 5 years to 8 years.
The vaccine also decreased the likelihood of a koala dying from chlamydial disease by 64.7%, which correlates with an improvement in survival probability for death due to chlamydial disease. While the analysis highlights the benefits of the vaccine against chlamydial disease in vaccinated koalas, the “primary goal” of a koala chlamydial vaccine is to “aid in the recovery of a declining population”.
“A recurring theme is that any koala management plan aiming to have a positive impact on declining populations should involve multiple strategies.”
Thus, the study suggests that the vaccine, although presenting “lower efficacy than desired”, could reverse population declines if implemented in conjunction with other strategies. The authors state that the next steps should focus on incorporating vaccination into koala management plans.
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