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Context Disease is increasingly becoming a driver of wildlife population declines and an extinction risk. Vaccines are one of the most successful health interventions in human history, but few have been tested for mitigating wildlife disease. The transmissible cancer, devil facial tumour disease (DFTD), triggered the Tasmanian devil’s (Sarcophilus harrisii) inclusion on the international endangered species list. In 2016, 33 devils from a DFTD-free insurance population were given an experimental DFTD vaccination before their wild release on the Tasmanian northern coast. Aim To determine the efficacy of the vaccination protocol and the longevity of the induced responses. Method Six trapping trips took place over the 2.5 years following release, and both vaccinated and incumbent devils had blood samples and tumour biopsies collected. Key results In all, 8 of the 33 vaccinated devils were re-trapped, and six of those developed DFTD within the monitoring period. Despite the lack of protection provided by the vaccine, we observed signs of immune activation not usually found in unvaccinated devils. First, sera collected from the eight devils showed that anti-DFTD antibodies persisted for up to 2 years post-vaccination. Second, tumour-infiltrating lymphocytes were found in three of four biopsies collected from vaccinated devils, which contrasts with the ‘immune deserts’ typical of DFTs; only 1 of the 20 incumbent devils with DFTD had a tumour biopsy exhibiting immune-cell infiltrate. Third, immunohistochemical analysis of the vaccinated devils’ tumour biopsies identified the functional immune molecules associated with antigen-presenting cells (MHC-II) and T-cells (CD3), and the immune checkpoint molecule PD-1, all being associated with anti-tumour immunity in other species. Conclusions These results correlate with our previous study on captive devils in which a prophylactic vaccine primed the devil immune system and, following DFTD challenge and tumour growth, immunotherapy induced complete tumour regressions. The field trial results presented here provide further evidence that the devil immune system can be primed to recognise DFTD cells, but additional immune manipulation could be needed for complete protection or induction of tumour regressions. Implications A protective DFTD vaccine would provide a valuable management approach for conservation of the Tasmanian devil.

ContextDiseases are increasingly contributing to wildlife population declines. Tasmanian devil (Sarcophilus harrisii) populations have locally declined by 82% largely due to the morbidity and mortality associated with two independent transmissible devil facial tumours (DFT1 and DFT2). Toxic baits are often used as a management tool for controlling vertebrate pest populations in Australia, but in other areas of the world oral baits are also used to deliver vaccines or pharmaceuticals to wildlife. Oral vaccine bait products have been distributed for more than 50 years at a landscape scale to protect wildlife from rabies virus and contributed to the elimination of fox rabies from more than ten European nations. An oral bait vaccine to protect against devil facial tumours has been proposed as a management tool to improve the population health, resiliency and fitness of wild Tasmanian devils.AimOur goal was to evaluate the potential use of edible baits as vehicles for vaccine delivery to Tasmanian devils.MethodWe tested placebo versions of baits that are already used in Australia.Key resultsCaptive devils consumed all types of placebo baits but exhibited a preference for ruminant- and fish-based baits. Captive devils also consumed inert capsules inserted into placebo baits. Bait fate trials in the field revealed that baits were generally removed within 24 hours. Tasmanian pademelons (Thylogale billardierii), brushtail possums (Trichosurus vulpecula), and Eastern quolls (Dasyurus viverrinus) were the most common nontarget bait competitors at six private properties in southern Tasmania; wild devils removed approximately 5% of ground baits at these sites. We also evaluated an automated bait dispenser, which reduced nontarget uptake and resulted in over 50% of the baits being removed by devils.ConclusionsThis study demonstrates that captive and wild devils will accept and consume placebo versions of commercial baits. Bait dispensers or modified baits or baiting strategies are needed to increase bait uptake by devils.ImplicationsBait dispensers can be used at a regional scale to deliver baits to Tasmanian devils. These could act as vehicles for preventive or therapeutic vaccines to mitigate the impacts of disease on devil populations.Short summaryThis study aimed to test oral baits as potential vaccine delivery vehicles for Tasmanian devils. Captive and wild devils consumed placebo versions of commercial baits used on mainland Australia. Abundant non-target species, such as brushtail possums, Tasmanian pademelons, and eastern quolls consumed most baits in the wild. Implementation of automated bait dispensers increased bait uptake by devils to over 50% at the same regional field sites.