Amid the global COVID-19 crisis, a study released this week has some good news about a wildlife pandemic, which may help scientists better understand how other emerging diseases evolve.
Griffith University researchers were part of an international collaboration that found the transmissible cancer decimating Tasmanian devil populations is unlikely to spell their doom.
The research team employed phylodynamics, which analyses small changes in the genetic code, to reconstruct the occurrence and spread of Tasmanian devil facial tumour disease since its emergence in the 1990s.
Phylodynamics is typically used to track how viruses such as influenza and SARS-CoV-2 spread and evolve over time, based on detailed knowledge of changes in their genetic information.
“This research has opened the door for applying this technique to genetically more complex pathogens, like the Tasmanian devil facial tumour disease for which the genome is far bigger and more complex than that of any virus,” said Professor Hamish McCallum from the Environmental Futures Research Institute.
The study published in Science indicated that the devils’ pandemic is shifting from an emerging disease to an endemic one.
“The COVID-19 pandemic has made people aware of the importance of the number of people that one infected person can pass a virus on to,” Professor McCallum said.
“One infected devil initially passed the facial tumour disease on to 2.5 other animals, but our analysis shows that now each infection leads to only one or fewer additional infections.”
“It is cautiously optimistic good news,” said Professor Storfer led researcher at Washington State University. “I think we’re going to see continued survival of devils at lower numbers and densities than original population sizes, but extinction seems really unlikely.”
This conclusion reinforces the findings of a Griffith-led mathematical model published in 2019 that used field recapture data rather than genetic information.
“When two very different approaches lead to the same result that devils are unlikely become extinct, we can have confidence in the conclusion” Professor McCallum said.
Since its identification in 1996, Tasmanian devil facial tumour disease has reduced populations of the iconic marsupial by more than 80%. The devils spread the infection when they fight and bite each other on the face.
“This study suggests that Tasmanian devils have rapidly evolved in the wild and changed genetically to tolerate or resist the cancer,” Professor McCallum said.
“If we were to release captive animals, bred from populations that haven’t been exposed to the disease, we run the risk of slowing down or even reversing these genetic changes. This is likely when captive-reared animals interbreed with those in the wild.”
This is the first time that phylodynamics, a tool that uses genetic sequencing to investigate evolutionary relationships among pathogen strains to understand and predict disease spreads across a population, has been used successfully to trace this transmissible cancer.
“What makes this disease much more difficult than a virus like COVID-19 to trace genetically, is that it is a type of cancer, which mutates the animals’ own cells,” Professor McCallum said.
“That means we have to trace the changes in the Tasmanian devils’ genes, of which there are thousands more than in a virus.
“To appreciate the scale of this work, we screened more than 11,000 genes from Tasmanian devil tumour samples to find 28 that changed in a “clock-like” manner, showing mutations that were accumulating over time.
“The 28 identified genes were made up of more than 430,000 base pairs, the fundamental units of DNA, compared to the entire genome of the SARS-CoV-2 virus that causes COVID-19, which has only 29,000 base pairs.”
Professor McCallum explained that this study opens the door to apply these types of approaches to the study of virtually any pathogen that impact humans as well as wildlife.