A virus behind brain tumors in racoons

Racoons rarely get cancer. Can you imagine the surprise of scientists when, during March 2010-May 2012, 10 racoons in California and Oregon suddenly developed brain tumors? Everything suggested that the unexpected “epidemics” had an infectious origin. In fact, racoon polyomavirus was found in the tumor tissue of all affected animals, but not in 20 unaffected animals. This is not only about racoons: viruses can be the etiologic agents of many human cancers as well. The case of racoons has probably something to teach us.

There are many causes for cancer, ranging from genetics to environmental factors. Sometimes, the responsible are infectious agents, like viruses. Oncogenic viruses induce the transformation of infected cells into malignant cancer cells. They do not simply press a magic switch, but rather they trigger a multitude of molecular mechanisms, like interference with DNA repair system, modification of gene expression, induction of chronic inflammation.  Viruses account for 20% of human cancers and most importantly, they are contagious. Everyone has probably heard about the human papilloma virus or hepatitis B virus, against which we fortunately realized two anti-cancer vaccines.

In the meantime, in racoons…

Viruses do not mess up with humans only. Polyomavirus are DNA viruses icosahedral in shape. They can cause a rare form of skin cancer in humans and tumors in various animals, including mice and birds. Even if infection rarely causes cancer under natural condition, its oncogenic potential has been confirmed in laboratory. In 2012, racoons were added to the list of its hosts. The association between a previously unknown form of the virus and the sudden outbreak of brain tumors in these animals is strong. As far as we know, racoon may not even be the unique host of this particular form; but, because it was discovered under this unfortunate circumstance, the new virus was called racoon polyomarivus.

Humans’ handiwork? Maybe.

Is it really all virus’ fault? Racoons are at the interface between human and wildlife environment and therefore are frequently exposed to our waste, garbage, sewage. Maybe these environmental stresses weaken the racoons’ immune system, which becomes more vulnerable to the infection. There is also another hypothesis: where do racoon polyomavirus come from? Someone has noticed that it is very similar to some human polyomavirues. Occasionally, viruses can jump across species: it cannot be excluded that the virus responsible for brain tumors in racoons has evolved from a human strain.

One Health Approach

If it had really jumped from men (or some other species) to racoons, could it also make the leap back in the opposite direction? Quite unlikely, according to experts. Racoon polyomavirus does not represent a threat for human health, at the moment; and thank goodness, since we already need to take care of our own viruses. At the same time, the animal world can be of great help in the study of oncogenic viruses in general and polyomavirus in particular. In fact, in humans the infection may take even decades to cause cancer, while in animals with shorter lifespan studies on oncogenic viruses may proceed faster and clarify important details about their mechanism of action. In addition, wild animals, like racoons, are often exposed to the same mix of environmental toxins and pathogens as humans, with the difference that they develop any symptoms in a shorter time: monitoring the status of wildlife populations allows to identify in advance any potential threat for human health as well.  

The discovery of racoon polyomavirus offers new insights into the biology of all polyomaviruses, including those infecting humans, and of oncogenic viruses in general. This underlines the importance of One Health approach in science: animals and humans are deeply interrelated and any scientific discovery in one species can directly or indirectly benefit the others.

Erika Salvatori

References:

Florante N. Dela Cruz, Jr., et al. Novel Polyomavirus associated with Brain Tumors in Free-Ranging Raccoons, Western United States. Emerg Infect Dis. 2013 Jan; 19(1): 77–84.

Patricia A. Pesavento et al. Polyomavirus and Naturally Occuring Neuroglial Tumors in Raccoons (Procyon Lotor). LAR Journal, 56(3), 24 February 2016, Pages 297–305