Tasmanian devils may survive cancer threat

There’s more possible good news for Australia’s troubled Tasmanian Devil (Sarcophilus harrisii) – and the implications may be even broader.

Last month Cosmos reported that researchers had learned more about the evolutionary dynamics of the facial tumour that disfigures then kills many of the already endangered marsupials.

Now, an international collaboration is suggesting the transmissible cancer is unlikely to spell doom for the species.

A team led by Washington State University (WSU), US, analysed small changes in the genetic code to reconstruct the epidemiology and spread of Tasmanian Devil Facial Disease (TDFD) since its emergence in the 1990s.

They discovered that it is becoming endemic and exhibiting a pattern of transmission rate decline. This suggests, they say, that if left to evolve naturally, TDFD may go extinct or even co-exist in devil populations. Either way, devil extinction is unlikely.

“Because of the COVID-19 pandemic, people are becoming familiar with the R number of a pathogen, the number of people that one infected person will pass the virus on to,” says Hamish McCallum from Australia’s Griffith University, co-author of a paper in Science.

“Our analysis shows that R has decreased for [TDFD] and the disease has now slowed. One infected devil initially passed the disease on to 2.5 other animals, but now each infection leads to only one or fewer additional infections.”

“It is cautiously optimistic good news,” adds lead author Andrew Storfer from WSU. “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 despite predictions a decade ago.”

The researchers screened more than 11,000 genes from TDFT samples to find 28 genes that changed in a “clock-like” manner, showing mutations that were accumulating rapidly over time.

“To appreciate the scale of this work, the 28 identified genes are 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 of RNA,” McCallum says.

The screening technique, known as phylodynamics, is typically used to track how viruses such as SARS-CoV-2 and influenza spread and evolve over time, based on detailed knowledge of changes in their genetic information.

The new study demonstrates that the technique need not be limited to viruses and can be applied to a wide range of other emergent pathogens across species, the researchers say. To date, its application to non-viral pathogens has been limited by the challenges associated with their larger genome sizes

The findings also suggest the active management practice of releasing captive-bred devils into the wild may not be necessary and could actually be harmful, Storfer says.

“This study suggests that Tasmanian devils have rapidly evolved in the wild and changed genetically to tolerate or resist the cancer. 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.”