Putting zoo animal cancer into perspective

Abstract

As part of a comparative research agenda that promises insights that help extend the human lifespan and combat cancer, cancer prevalence in zoo animals has received recent attention. Here, we want to draw attention to a principle of cancer research that was introduced into the zoo world as early on as 1933, but that seems to have gone somewhat forgotten: Cancer is mainly a disease of old age, and therefore studies aiming at identifying taxa that are particularly susceptible or resistant to cancer must control for whether the respective zoo populations are ‘old.’ In a comparative context, ‘old age’ cannot be measured in absolute terms (e.g., years), but only in relation to a species' maximum lifespan: Species that achieve, across zoos, a higher mean lifespan as a percent of their maximum lifespan are ‘older.’ When applying this metric to former as well as more recently published data on cancer prevalence, it appears that those species that become relatively old in zoos—in particular, the carnivores—have a relatively high cancer prevalence. Any improvement in animal husbandry—which reduces premature deaths—should, by default, lead to more cancer. Cancer in zoo animals, like any other old‐age condition, might therefore be embraced as a proxy for good husbandry. Rather than following a sensationalist approach that dramatizes disease and death per se, zoos should be clear about what their husbandry goals are, what relative longevities they want to achieve for which species, and what old‐age diseases they should therefore expect: in the end, one has to die of something.

Gingival squamous cell carcinoma in 2 lions under managed care

“…This has contributed to an increase in the life expectancy of these animals, which in turn may in part have contributed to the higher prevalence of neoplasia observed over the past decades. 4 Neoplasia can represent up to 50% of diagnoses in nondomestic felids, and neoplasia of the digestive, reproductive, hemolymphatic, integumentary, and endocrine systems are among the most common reasons for euthanasia of Panthera spp. 6,12,14 In lions, lymphoma, mammary adenocarcinoma and carcinoma, reproductive leiomyoma, and mesothelioma are among the most frequently reported tumors.…”

“…1,21,27 Most animals live longer in captivity than they do in the wild, and hence have a wider window of opportunity to develop neoplasia than their wild free-ranging counterparts. 4 Dental disease has been proposed to be a predisposing condition for SCC based on the fact that the region immediately adjacent to the maxillary and mandibular incisor teeth is an important site for the development of both periodontal disease and oral neoplasms. 5,8,19,20 The formation of dental plaque, calculi, and periodontal disease appears to be more prevalent in captive tigers and lions fed a diet of raw beef, compared to free-living species.…”