A freakish infectious cancer has brought Tasmanian devils to the brink of extinction, but the pugnacious marsupial is evolving rapidly as it fights for survival
From New Scientist – 3-1-2018
TWO decades ago, at the north-eastern tip of Tasmania, a single cell near the whiskers of a Tasmanian devil mutated and turned cancerous. That animal was bitten in the face by another devil, which was then bitten by another, and the cancer has been spreading ever since. It has taken a once-common species to the brink of extinction.
To date, the contagious cancer known as devil facial tumour disease (DFTD) has annihilated 85 per cent of the population. There may be fewer than 20,000 Tasmanian devils left in the wild. Worse still, the species has been hit by another deadly disease. Yet these animals are living up to their feisty reputation. With a combination of remarkably rapid evolution and some groundbreaking work by conservationists, they seem determined to survive.
An adult devil is about the size of an obese domestic cat, but like other marsupials, they are tiny at birth – about the size of a grain of rice. As they grow, so do their appetites. A full-sized devil can consume 15 per cent of its body weight in a single day. Luckily, they are unfussy eaters. They scavenge and hunt, happily devouring any creature including maggots, tadpoles, frogs, wombats, wallabies and possums. Their unusually large heads provide extra space, not for bigger brains, but for the massive jaw muscles that enable them to crack and crush bones. When they feast on a carcass, nothing goes to waste.
Devils are a combative bunch, frequently biting each other’s faces and jaw-wrestling to secure a mate or defend a meal. It is during these fights that cancer cells get dislodged from one animal and infect another. Then, as the cancer proliferates, bulging black and red lumps begin to bloom on their cheeks and jaws, over their eyes and in their mouths. These growths can hinder feeding so severely that afflicted animals starve to death.
Transmissible cancers are exceedingly rare. Cells normally have proteins on their surfaces known as the major histocompatibility complex (MHC), which are like labels saying “I belong here”. These allow the immune system’s T-cells to identify and attack foreign cells, including cancerous ones. But DFTD is believed to have originated in the nervous system, where cells often hide their MHC labels to prevent immune attacks that might damage the brain. “Because the cancer started off in these cells, it already had those immune escape mechanisms in place,” says immunologist Gregory Woods at the University of Tasmania. “Then it just amplified those mechanisms by turning off MHC completely.”
“It is as though the devils have been struck by lightning twice”
The devils may be gaining resistance, but David Pemberton, who manages the government-run Save the Tasmanian Devil Program, isn’t prepared to simply hope for the best. As the number of devils drops, so does their genetic diversity, he says, meaning fewer opportunities for natural selection to find ways to resist the cancer. Low numbers also increase the risk from other threats that could lead to extinction, including inbreeding and road deaths. So Pemberton is on a mission to bring the devils’ numbers up, and fast. To that end, he and his colleagues have started dipping into an insurance population – about 700 disease-free devils held in captivity across Australia – and transferring some of them to Tasmania’s wilderness.
But the programme is controversial. McCallum worries that these captive-born devils could slow down or even reverse the evolution of resistance. “It’s a bit like if you had a herd of pedigree dairy cows,” he says. “You wouldn’t just throw in a random bull from somewhere.” At the very least, he thinks susceptible devils should be released into disease-prone regions only once an effective vaccine against DFTD has been developed.
Woods is working on this. When his team discovered that a handful of devils can survive DFTD, he became convinced that the others simply needed an immune boost. For the past few years, he has been testing immunotherapies by inoculating captive devils before their release. As of August 2017, three of the 85 vaccinated and released animals had developed tumours. It is a small percentage, but suggests that the formulation needs tweaking.
Woods was recently tinkering with the vaccine when he discovered something quite remarkable. He grew tumour cells in a liquid spiked with cytokines, which are molecules that turn on a cell’s MHC labels. Then he injected the cells into afflicted devils. Over time, their tumours began to shrink and, in some cases, disappeared completely. “It’s a bit odd, treating cancer with cancer,” says Woods. Still, he can’t argue with the striking results.
You might think that means things are looking up for the devils. However, a second contagious cancer has struck. This slowly spreading strain has caused curiosity and concern since its discovery in 2015. Given that only two other contagious cancers have ever been seen – one in dogs, the other in shellfish – it is as though the devils have been struck by lightning twice. The second strain may indicate that they are highly susceptible to these diseases. Then again, if they have a history of encountering and overcoming contagious cancer, that’s an encouraging sign.
How quickly this second cancer will burn through the population is still unknown. “We just don’t know what it is going to do,” says Woods. “We don’t understand much about it at all.” However, according to the latest estimates, the original strain is finally beginning to wane. What’s more, many of the devil populations predicted to be extinct by now haven’t succumbed.
McCallum expects that one or both cancer strains will persist at some level for the foreseeable future, suppressing devil numbers. That means Tasmania’s ecosystems will continue to suffer. Devils are the top predator and with so few of them around, feral cats have flourished, devouring the island’s small native animals such as bilbies and bandicoots. Still, all the signs suggest that Tasmanian devils will ultimately survive. In fact, McCallum believes they are already out of the woods: “It’s fairly clear now that what we thought was a real possibility of extinction is no longer likely to happen.”
This article appeared in print under the headline “Luck of the devil”