New research from a group of Australian veterinary scientists has identified a bacterium as the cause of mystery lizard deaths on Christmas Island and lays the groundwork for a treatment.
Invasive species like yellow crazy ants and feral cats have turned Christmas Island into a hotbed for modern extinctions in Australia. The Christmas Island forest skink was earlier this month formally listed as the first Australian reptile to have gone extinct since colonisation.
Adding to their existing challenges, two more Christmas Island lizard species currently balancing on the brink of extinction have a new hidden threat.
The Lister’s gecko and blue-tailed skink both number around 1000 individuals each. Both species are extinct in the wild on Christmas Island.
While captive breeding programs protect the lizards from these predators, it still leaves what is left of the two species susceptible to disease outbreaks.
An Elusive Disease
Lizards began presenting with facial deformities and lethargy in 2014 despite quarantine measures, with some dying from their symptoms. Microscopy and genetic testing identified the bacterium Enterococcus lacertideformus as the cause.
The bacterium grows in the lizard’s head where it forms large colonies that replace bone and soft tissue before spreading to the internal organs, at which point infection can lead to death.
The disease is spread through direct contact with other infected reptiles as they feed or when the lizards bite each other on the face during the breading season. This presents a problem for captive breeding programs where large numbers of the lizards may be kept in close confines.
Co-lead researcher and PhD candidate in the Sydney School of Veterinary Science says ‘healthy captive animals need to be kept apart from infected ones and should also be kept away from areas where infected animals have been’.
USING GENETIC SEQUENCING TO UNMASK THE DISEASE
To date, efforts to grow the bacterium in a laboratory setting have been unsuccessful and its origins are unknown.
In the study released last week, veterinary scientists from the University of Sydney, the Australian Registry of Wildlife Health, and the Taronga Conservation Society Australia not only confirmed E. lacertideformus as the cause of the lizard deaths, but identified genetic features that could be used to treat the infection.
Samples were collected from Asian house geckos displaying the same symptoms in September 2017.
Ms Agius said they ‘found that the bacterium can surround itself with a biofilm – a community of bacteria that can help it survive’. The biofilm is thought to mask the bacterium from a lizard’s immune system.
Unlocking Treatment Pathways
The genetic analysis also found limited evidence of antibiotic resistance, especially to recently developed clinical antibiotics.
‘This suggests that infected animals might be successfully treated. That’s what we need to determine now,’ said Professor David Phalen, co-lead author and Ms Aguis’ PhD supervisor.
However, the researchers note previous studies have shown bacteria with biofilms require up to 1000-times higher concentrations of antibiotics, making them difficult to eradicate. The authors recommend conducting experiments into common broad-spectrum antibiotics on infected reptiles.
They hope this work can guide infection control protocols and help conserve susceptible species including the Lister’s gecko and blue-tailed skink.
‘It’s critical we act now to ensure these native reptiles survive,’ Ms Agius said.
In 2019, 300 blue-tailed skinks bred in captivity over a decade were released onto the tiny island Pulu Blan, part of the nearby Cocos Islands archipelago. All individuals were tested to ensure they were free of E. lacertideformus before arrival.