Models predict that culling is not a feasible strategy to prevent extinction of Tasmanian devils from facial tumour disease

Abstract

Summary 1.Culling, either of all animals or infected animals only, is often suggested as a way of managing infectious diseases in wildlife populations. However, replicated experiments to investigate culling strategies are often impractical because of costs and ethical issues. Modelling therefore has an important role. Here, we describe a suite of models to investigate the culling of infected animals to control an infectious cancer in the Tasmanian devil Sarcophilus harrisii. 2. The Tasmanian devil is threatened by an infectious cancer, Tasmanian devil facial tumour disease. We developed deterministic susceptible, exposed and infectious (SEI) models with differing ways of incorporating the time delays inherent in the system. We used these to investigate the effectiveness for disease suppression of various strategies for the removal of infected animals. 3. The predictions of our models were consistent with empirical time series on host population dynamics and disease prevalence. This implies that they are capturing the essential dynamics of the system to a plausible extent. 4. A previous empirical study has shown that removals every 3 months did not appear to be sufficient to suppress disease in a semi-isolated infected population. Our models are in accordance with this observed result. The models further predict that while more frequent removals are more likely to be effective, the removal rate necessary to successfully eliminate disease may be too high to be achievable. 5. Synthesis and applications. Our results, in association with a previous experimental study, show that culling is unlikely to be a feasible strategy for managing Tasmanian devil facial tumour disease. Similar conclusions have been reached in studies of other wildlife diseases. We conclude that culling is rarely appropriate for controlling wildlife diseases and should only be attempted if models predict that it will be effective.

Age-Structured and Vaccination Models of Devil Facial Tumor Disease

“…The depletion of adult devils has decreased the competition for resources and hence enabled more juvenile devils to reach breeding size sooner [9]. Previous age-structured models did not incorporate the increased breeding by younger female devils in DFTD affected areas [10]. Our model accounts for this increased breeding of juvenile devils and supports Beeton's conclusion that this increase will not be sufficient to regain pre-DFTD population numbers [10].…”

“…For i = 1, 2, 3, devils enter the class S i from the class S i−1 by maturation and leave the S i class by natural death at a rate of d si , by frequency-dependent exposure to the disease with a transmission rate k, or, for i = 1, 2 only, by maturation to the next susceptible age class S i+1 . As juvenile devils have been observed to have fewer interactions than adults, they have a reduced transmission rate of bk where 0 < b < 1 for one-year-old devils [10]. Prior to the introduction of DFTD, the average lifespan of a devil was six years, but now, very few devils over three years old have been found where the disease has emerged [6,9].…”

“…Captive breeding is expanding in Australia and other countries, and captive-bred devils have been successfully released into the wild in populations in which DFTD is not present [11]. Selective culling of infected devils has been considered but previous models found this strategy would not be an effective means of disease control for DFTD [10,12]. Due to no documented vertical transmission of the disease, fertility control has not be considered.…”

A model for the dynamics of Ross River Virus in the Australian environment

“…We also chose not to include a latent stage (SEI models, e.g. Beeton & McCallum, 2011;Beeton & Forbes, 2012) as the latent period is relatively short for RRV and would not have qualitatively altered the results. Vertical transmission of the virus (between parent and offspring) is ignored here in both host and vector populations, and it is also assumed that infection occurs at a frequency-dependent rate β mk from mosquito to kangaroo and β km from kangaroo to mosquito.…”

Devil women

“…An effort to remove DFTD from a population by culling all individuals exhibiting signs of DFTD only served to remove those individuals who would die anyway, quicker (Lachish et al 2010), and changed the evolutionary trajectory of the disease (Ujvari et al 2014). Without a preclinical test to determine which animals were carrying the disease before they showed clinical signs, any hope of removing the disease from a population is currently futile (Lachish et al 2010;Beeton and McCallum 2011).…”