Humans can be considered an “oncogenic species” — meaning our activity induces the development of cancerous tumors. According to a recent meta-analysis, our activity might increase the rate of cancer occurence in wild populations through numerous processes.
Cancer is the second leading cause of death both worldwide and in the United States after heart disease, and its incidence is currently increasing worldwide. The surge in cancer has been mainly attributed to our lifestyle choices, such as poor diet, smoking, alcohol consumption, but also exposure to pollutants. The association between environmental contaminants and tumor development is well established by numerous studies carried on humans and animal models.
But our species is not the only one exposed to the compounds we have introduced in natural environments.
Humans: a cancer-causing species
Based on this link, an international team of researchers decided to investigate whether environmental contaminants resulting from human activity are also pushing up the rate of cancer incidence in wild animals. Their analysis suggests that this is indeed the case.
The researchers conclude “that the impact of cancer in wild populations is currently underestimated and that cancer prevalence should be exacerbated by rapid environmental changes caused by human activities.” They also propose several mechanisms by which human activity is promoting cancers in wildlife populations.
Pollution is one of the prime drivers of neoplasia — the uncontrolled growth of cells that is not under physiologic control — in wildlife. A study on beluga whales (Delphinapterus leucas) from the Saint Lawrence Estuary, which is highly polluted by waste from aluminum smelting, found that a staggering 27% of the whales had cancer. Similarly, high cancer rates among adult individuals have been recorded in freshwater, marine, and estuarine fish that swim in highly polluted areas. One recent example involves California sea lions (Zalophus californianus), which are experiencing high cancer rates due to chlorine-based pesticide exposure.
Pollutants can cause cancer by inducing somatic mutations, DNA damage, immunotoxic effects, and through interference with the production, release, metabolism, and elimination of natural hormones.
Another type of contaminant identified by the authors of the new study is the accidental release of radiation, such as in the wake of the 1986 Chernobyl disaster or the 2011 Fukushima nuclear accident. One study found an association between Chernobyl background radiation and increased cancer rates in birds. Previous studies performed on humans found that rates of thyroid cancer drastically increased in Ukraine after the Chernobyl nuclear accident, suggesting that both humans and wildlife are equally affected.
The world’s oceans are becoming increasingly choked with microplastics — tiny pieces of plastic (5 mm or smaller). Plastic cluttered beaches, massive landfills, and sea animals with clogged stomachs are common sights around the world. The new study suggests that we can also add cancer to the many environmental problems associated with microplastics. Microplastics, which are ingested by a wide range of species, increase cancer incidence through their intrinsic toxicity because they contain various organic contaminants. One of these contaminants (bisphenol A) possesses endocrine disruption properties and may contribute to the development
of breast cancer and prostate carcinoma in adult humans, as well as hepatic tumors in rodents.
Animals living in habitats close to farmlands and other agricultural activities are exposed to pesticides and herbicides, which, like microplastics, disrupt the hormonal system.
Light pollution is another source of endocrine disruption. Previously, a study of female employees working a rotating night shift found that exposure to artificial light at night (ALAN) was associated with elevated breast cancer risk. It’s believed that such cancers arise due to the suppression of pineal melatonin production. Melatonin is a hormone with anti-tumor properties, which is present in all vertebrates and is rhythmically secreted by the pineal gland. It peaks at night, causing us to feel the urge to sleep. However, even a minimum light contamination can disrupt this normal circadian production of melatonin, making rats raised in captivity more prone to cancer.
Highly processed foods have also been associated with more cancer, and this food is often consumed by wild animals as well. Animals are exposed to processed food either by intentional feeding or unintentional provisioning, typically involving waste. One of the most common causes of wildlife feed contamination is fungal growth, which can produce toxic metabolic by-products known as mycotoxins, along with toxins from certain antibiotics that become carcinogenic due to sunlight exposure. For example, 92% of tested Eurasian griffon vultures (Gyps fulvus) showed variable concentrations of residues of the antibiotic fluoroquinolone,
which is considered to be a photochemical carcinogen. Sometimes, even unaltered food can cause cancer due to its nutrient deficiency that decreases immune health and potentially alters gut microbiota. A recent review on the nutritional effects of supplementary food on wildlife demonstrated that nearly half of the studies (42%) found negative effects of provisioning on protein or micronutrient deficiencies
Finally, anthropogenic perturbations can, directly and indirectly, affect the genetic diversity of populations and therefore influence the capacity of individuals to respond to pathogens. Studies suggest that reduced genetic diversity and inbreeding are linked to cancer — both in humans and animals. Loss of genetic diversity can lead to malignant transformations due to two main effects: the accumulation of oncogenic homozygous mutations (direct effect) and through the increased susceptibility to oncogenic pathogens (indirect effect). For example, reduced genetic diversity has been associated with increased susceptibility of endangered wildlife species to cancer-causing pathogens, such as papillomatosis and carcinomatosis syndrome in western barred bandicoots (Perameles bougainville), and viral papilloma and squamous cell carcinomas in snow leopards (Uncia uncia).
The study published in the journal Nature Ecology and Evolutionconcludes that cancer in wildlife caused by human activity is greatly underestimated and call for more research to better understand our impact.
“It is therefore urgent to develop and conduct more research in this direction because — more than ever — ecosystems are being altered by human activities, and this tendency is unlikely to decrease in the future.”