A recently published scientific study examined cancer prevalence across multiple species of mammals in support of the hypothesis that elephants possess genetic mechanisms conferring resistance to the development of cancer.1 The study included cancer prevalence data abstracted from a summary of San Diego Zoo necropsy findings for several species housed in its facility from 1964 to 1978.2 The study used mortality data for elephants from an online database (Elephant Encyclopedia: www.elephant.se, accessed May 2017) compiled by self-report for a lay audience. The approach used for this study illustrates several potential pitfalls in the use of retrospective mortality data from zoo collections. To further examine findings, the team re-abstracted data from Griner and added new data from the San Diego Zoo’s database of mortality records with attention to important interpretation details that were not included in the original report.2,3 The study team calculated cancer prevalence for several species, and compared new estimates to those from the original report. Findings from the limited sample size show that elephants necropsied at the San Diego Zoo from 1987–2015 did acquire cancer in proportions similar to humans: 8/12 were diagnosed with neoplasia, one-third (4/12) had malignant neoplasia, and two geriatric elephants died of neoplasia. These refined cancer prevalence estimates across all of the evaluated species illustrate the biases that can be introduced in mortality reviews based on insufficient case definitions, inadequate identification of the at-risk population, temporal confounding, unreliable sources of data, and/or interpretative details important for drawing conclusions.
The authors thank Jere Stern for contributions to the original letter and San Diego Zoo pathologists and scientists for thoughtful comments on poster content.
1. Abegglen LM, Caulin AF, Chan A, Lee K, Robinson R, Campbell MS, Kiso WK, Schmitt DL, Waddell, PJ, Bhaskara S, Jensen ST, Maley CC, Schiffman JD. Potential mechanisms for cancer resistance in elephants and comparative cellular response to DNA damage in humans. J Am Med Assoc. 2015;314(17):1850–60.
2. Griner LA. Pathology of zoo animals. San Diego, CA: Zoological Society of San Diego; 1983.
3. Pessier A, Stern JK, Witte C. Letter to the editor. J Am Med Assoc. 2016;315(16):1789.