Cancer prevalence and etiology in wild and captive animals

Despite the evolution of numerous natural cancer suppressor mechanisms (DeGregori, 2011), neoplasia has been recorded in most metazoans (Leroi et al., 2003). Although, a few exceptional species, such as the naked mole-rat (Heterocephalus glaber) and sharks have been claimed to be resistant to cancer (Finkelstein, 2005; Tian et al., 2013). Recent studies have, however, shown that even these species may develop cancer (Delaney et al., 2016; Finkelsetein, 2005) strongly suggesting that the vast majority of multicellular organisms are indeed susceptible to cancer. The frequent occurrence of cancer in metazoans suggests that neoplasia, similar to pathogens/parasites, may have a significant negative impact on host fitness in the wild (Vittecoq et al., 2013). This is supported by a recent review of wildlife cancer by McAloose and Newton (2009) demonstrating that high prevalence of cancer in, for example, Tasmanian devils (Sarcophilus harrisii) and belugas (Delphinapterus leucas) resulted in concomitant significant increase in levels of mortality and reduction in fitness. Wildlife cancer statistics are, however, highly scattered in the scientific literature and hence challenging to access. Moreover, tumors in wildlife are most commonly detected during post-mortem examination and therefore hard to confirm without histopathological examinations. However, even such analyses can be inaccurate because of high levels of autolysis (organ disintegration) (McAloose and Newton, 2009). In addition, individuals harbouring tumors often display a decrease in body condition frequently resulting in higher levels of parasite/pathogen infections and concomitant increased levels in morbidity and mortality (Vittecoq et al., 2013) further impeding a correct analysis of the ultimate cause of death. The combination of the negative effects of cancer and/or pathogen/parasite infections has also been shown to result in increased levels of predation (Vittecoq et al., 2013). In our view, the combination of the problems involved in accurately recording wildlife cancer, the increased risk of succumbing to pathogens/parasites, and/or predation has often led to a somewhat erroneous assumption that although cancer is common in domestic animals, it remains rare in the wild. If, as we suggest, cancer may be a significant determinant of animal fitness it is therefore crucial to determine cancer prevalence in the wild. The etiology and prevalence of transmissible cancers are presented and discussed in Chapter 12; this chapter will therefore focus on the prevalence and etiology of nontransmissible cancers. Via thorough searches of the available literature we provide a comprehensive and an updated list of cancer prevalence in wild animals ranging from fish to whales. We also provide a list of cancer recorded in captive animals from French Zoological parks and compare our findings to that recorded at other zoological parks. Finally we provide an updated list of cancers recorded as single cases in the wild, as well as in captive animals demonstrating that cancer occurs in nearly every taxonomic order of the animal kingdom.