Wild Muse just perused the latest issue of Conservation Biology and went foraging for substantive research to post about… Hold on tight because we are going to Tasmania. To the Forestier peninsula in southeastern Tasmania, to be exact – where Tasmanian devils are pinned down by a catastrophic disease.
Unfortunately, it will not be all fun and games on our trip Down Under (well, under Down Under, really). As everyone is likely well aware by now, the Tasmanian devil (Sarcophilus harrisii) faces a perilous future due to a nasty infectious disease plaguing its geographically-restricted population. Infectious facial tumors are killing off devils at an alarming rate. The devils most-often contract the infectious cancer cells when they nip and bite each other during fights. Males will often fight vigorously for breeding rights with females. All it takes is a direct transfer of living infectious nerve cells from one to the other and bingo!, transmission is complete.
While some research has focused on the disease origin, mechanism and transmission, other researchers have turned their attention to ways to manage the devil population and reduce Devil Facial Tumor Disease (DFTD) in the wild. A group of researchers led by Shelly Lachish and working out of the School of Integrative Biology, Univ. of Queensland, Australia set out to test whether or not selectively removing infected individuals from an experimental colony of devils would alter the rates of infection within that colony as compared to a control colony. The experimental colony was located on the Forestier peninsula, and the control was located on the Freycinet peninsula on Tasmania’s east coast. Both locales have similar climates and habitat (open, dry forests and low-lying heath).
Some scientists have posited that if the disease continues at this rate, then the devil could be extinct in the wild in 10 to 20 years.* Such predictions call for drastic actions and so the researchers noted that where vaccines or treatment were non-existent options, culling animals is often a first-choice method of managers to reduce the pathogen reproductive rate to below one. Now, I am typically not in favor of animal culling, especially when the culls are random to simply thin out dense populations of so-called nuisance animals (like coyotes or starlings or geese.) But, they argued four persuasive reasons why culling was a good choice:
First, DFTD is a single-host disease, so management of reservoir species is not necessary (Hawkins et al. 2006). Second, because the cancer is a transmissible cell line (Pearse & Swift 2006) and transmission requires the direct transfer of live cells from an infected to a healthy devil, individuals without visible tumors are not likely to be highly infectious (Jones et al. 2007). Third, tumors are distinctive and readily identified in the field, so infected individuals can be diagnosed and removed (Hawkins et al. 2006). Fourth, DFTD causes death within 3–6 months with no recovery, any immunity, or resistance to DFTD observed (Lachish et al. 2007).
The team trapped devils systematically between 2004 and 2008, and euthanized infected devils caught in the Forestier site, while merely tagging those caught at the Freycinet site. They used the number of tumors present in infected individuals as a proxy for a scale of infectiousness, based on the logic that direct transfer of tumor cells in needed for infections (think: total surface area scales to degree of infectiousness).
All in all, they trapped 448 devils at Forestier and euthanized 145 that were found to have DFTD. At Freycinet, they caught 633 and 115 were found to have facial tumors but were released. At the end of their study, they found that total tumor volume per individual stayed the same at the control site, while it decreased slightly at the experimental site. The average age of individuals at both sites also decreased over time, as older individuals died from the disease. At Forestier, this decline occurred more rapidly than at Freycinet, which would be expected given the interference of the culling and older animals being pulled from the colony. One to two year olds had lower frequencies of the disease at each site, and three year olds and older had higher frequencies. Again, this makes sense given that devils mature sexually in their second year and there is a lag time between infection and the physical emergence of facial tumors.
Unfortunately, culling did not halt the disease from stalking the managed population. Not great news for the eastern half of Tasmania, which is so far most affected by the 14-plus year old devil plague. The researchers concluded:
There was no evidence that selective culling of infected individuals either slowed the rate of disease progression or reduced the population-level impacts of this debilitating disease at Forestier in the period of this study. Many of the demographic changes and epidemiological patterns indicative of disease impact (change in population age structure, decline in adult survival rate, increase in infection rates, decline in population size) occurred more rapidly at the managed site than at the unmanaged site. Because selective culling immediately removed those individuals that would die anyway, culling mortality simply substituted for disease-induced mortality.
Still, negative data has its value. In the trial and error process of trying to save imperiled wildlife, finding out which efforts have no results is almost as valuable as figuring out what does.
Now, if only DFTD would jump to mosquitoes and quit bedeviling Tasmanian devils…
Lachish S, McCallum H, Mann D, Pukk CE, & Jones ME (2010). Evaluation of selective culling of infected individuals to control tasmanian devil facial tumor disease. Conservation biology : the journal of the Society for Conservation Biology, 24 (3), 841-51 PMID: 20088958
*Rev-1: This should have given a range to 35 years. I’ve seen different estimates, but truthfully I’m not sure how robust they are.