While the Covid-19 crisis is still bothering us, there is at least good news in the event of an animal pandemic: The infectious cancer that attacks the Tasmanian devils will probably not be able to completely eradicate the famous marsupials. This is the result of a study that, through genetic studies, uncovered the evolution of the spread of the disease. Accordingly, it has now slowed down so much that each infected animal only infects one more. This now provides important information for measures to recover the Teufel stocks, say the researchers.
The name has given him bizarre features: The Tasmanian Devil (Sarcophilus harrisii) has pitch black fur, a huge mouth and screams and smells like hell. This charismatic bully is the largest pouch predator still in existence today – but for how long? An infectious cancer has been decimating its populations since the 1990s. DFTD (Devils Facial Tumor Disease) is caused by tumor cells that are passed on through bites. They cause ulcers on the animals’ faces, metastasize throughout the body and ultimately lead to death. As a result, the populations have already collapsed by 80 percent and so there were fears that the Tasmanian devil might soon become extinct.
A cancer – as bizarre as the victim
Various studies have already been devoted to the unusual disease. What makes this form of cancer transferable is therefore a bizarre property: the tumor tissue does not have the genetic makeup of the infected animal, as is normally the case with degenerate cells. Instead, they are genetically still cells from the devil, in which the cancer first developed through mutations decades ago. This tissue retains its ability to divide even after transmission and, despite its strangeness, can escape the immune system of the infected animals and be passed on.
The genome of the contagious cancer has now been re-examined by Andrew Storfer from Washington State University in Pullman. In order to literally get on the track of cancer, they used the technique of so-called phylodynamic analysis. So far, this method has only been used for viruses or bacteria in order to draw conclusions about the dynamics of their spread. The genetic mutations that typically occur in the pathogen’s genome in the course of its spread can provide information on how a disease has spread in a population. In the case of infectious cancer, however, the researchers were breaking new ground, because its genome differs significantly from that of viruses or bacteria: In contrast to these, tumor cells have very large and complex genetics – because their DNA corresponds to that of the devilish pouch.
As part of the study, the researchers therefore initially looked for certain genes in the tumor genetic makeup that mutate particularly intensely and are therefore suitable as “documents” of the history of spread. To do this, they analyzed 11,000 genes from the genomes of DFTD tumor lines from various marsupial devil populations on the island of Tasmania. In the end, they identified 28 genes that display a strong, clock-like signal of the mutation. They then used them to carry out the phylodynamic analysis.
Spread has slowed significantly
As they report, the history of the spread of these genes is actually clearly reflected. Accordingly, the rate at which the disease spread declined rapidly after the initial epidemic phase. Specifically, it reached its peak in 1996 with an infection rate of around 3.5 and has now fallen to around 1. This means: an infected animal only infects one more on average. As epidemiologists put it, the disease has now become endemic – it persists, but can no longer eradicate it.
While the disease is still largely fatal to the Tasmanian Devils, its spread appears to have reached equilibrium levels. The scientists say that the animals’ adaptations may play a role. “It is cautiously optimistic news,” says Storfer. “I believe that the devils will continue to survive in fewer numbers and fewer densities than the original population size, but the feared extinction no longer seems likely”.
According to the researchers, this information could now be important for the release of captive-bred marsupial devils into the wild. “Active management may not be necessary and could even be harmful,” says Storfer. “It looks like the devil populations evolve naturally by creating resistance to cancer. If, however, you introduce many genetically unaffected individuals again, they could make the wild populations more susceptible again, ”says the scientist.
In addition to the concrete importance for the preservation of the Tasmanian devil, the scientists also see a fundamental success in their study: The phylodynamic analysis can be used more widely: “An important aspect of this study is the clarification of the possibility of applying this method to practically every pathogen “Says Patton in conclusion.
Source: Washington State University, Technical article: Science, doi: 10.1126 / science.abb9772