Can we catch cancer?

Many years ago, while in college, a classmate – an intelligent, perceptive young woman – complained about her mother spending too much time with a friend who was in the terminal stages of cancer. “And,” she added, “we don’t even know whether it could be contagious.” And I thought, contagious cancer? That is just inconceivable.

Is it really? True, we have never heard of health care professionals cautioning people to avoid contact with cancer patients. And, one glance at the American Cancer Society’s site should reassure us. “Cancer is NOT contagious” comes the message loud and clear. That is, except under some extraordinary circumstances.

Nevertheless, some of us may have heard about the Tasmanian devil being in danger of extinction because of a contagious facial tumor.

o Why should this happen to animals but not us?

o What are the unusual circumstances under which we may become infected with cancer? o Is infection with human papilloma virus included in these unusual cases?

I will explain why contagious cancer develops in animals under some conditions and why its occurrence in humans is extremely unlikely.

Cancer associated with infections is fundamentally different from contagious cancer

Let us first make a clear distinction between cancers that are frequently associated with infections and cancers that can be contagious in themselves.

In the case of human papilloma virus, which is linked to cervical and other cancers, it is the microorganism that is transmissible, not the cancer itself. The microorganism is transmitted from person to person, and then may initiate development of cancer after infection has taken hold.

Contagious CancerNews of contagious animal cancer, such as the Tasmanian Devil’s disease, sometimes reaches the general press. Yet, - luckily - we never heard of the occurrence of this terrifying disease in human populations. Are we protected from it?

Similarly, after first infecting the liver, the hepatitis C virus can transform normal liver cells into cancerous ones. In all cancers initiated by microorganisms, the cancerous cells, like normal cells, carry specific markers (in the form of small differences in various molecules such as proteins, DNA and others) characteristic for each individual.

In contrast, the cells of a contagious cancer arose in a single event in one individual, and therefore are clones of that particular individual’s cancer cells. These cells are directly transferred from one host to another through many generations. Because of this, the cancer cells display characteristics of the original (diseased) organism. Thus, cancer cells are similar in all those individuals that harbor the disease, and differ from the cells of the host in which they reside. The high similarity of cancer cells among different individuals is the characteristic whereby contagious cancer is identified.

Although infections with viruses or bacteria that are associated with increased risk of cancer are not the main issue of this article, they are of the greatest medical importance. The American Cancer Society’s site lists several examples of viruses contributing to the development of cancer such as the Epstein-Barr and hepatitis B viruses, in addition to the human papilloma virus and hepatitis C virus mentioned above. The best-known bacterial infection that leads to an elevated risk of cancer –stomach cancer – is from Helicobacter pylori.

Three examples of naturally occurring contagious cancer are known in animals

Returning to the issue of direct infection by cancerous cells: how often does it occur in animals? - Very rarely. We only know of four examples, of which one is somewhat questionable, in the whole animal kingdom.

Of the three well-studied examples, two occur in vertebrates: one is the Tasmanian devil mentioned above, and the other one is our favorite companion, the dog. Scientists have verified the clonal nature of the cancer cells in both cases by molecular methods. The reason why many of us may not

be not familiar with the dog’s cancer, called canine transmissible venereal tumor (CTVT), is that it is a much milder, self-limiting disease than the one afflicting and decimating the devils.

Recently, a third example of the disease emerged, which is a contagious leukemia of clams. (Yes, clams have blood! They have a three-chambered heart that pumps their equivalent of blood.) People have known about leukemia in mollusks like clams for a while, but scientists discovered only recently that, as in the case of the dog and the devil, the leukemia cells are similar to each other in all clams but different from the host. We can therefore conclude that the clam cancer is also contagious.

The high variability of our immune system shields against contagious cancer

Returning to the issue of transmissible cancer in humans: why does it not happen? And why does it happen in animals? It is our immune system that recognizes and destroys cells derived from another individual, as well as some of our own diseased cells. Thus, the answer to the above questions lies in the structure of the immune system - though some characteristics of the tumor are also important.

A powerful component of the immune reaction against foreign or sickly cells is the so-called Major Histocompatibility Complex (MHC). The molecules of the MHC sit on the outside of all cells and pick up snippets of cellular proteins for ‘presentation’ to the immune system’s cells. These immune cells continuously scrutinize cell surfaces for signs of abnormality. If any of the fragments indicate a diseased state or the presence of foreign material – such as abnormal cancer proteins, viruses that penetrated the cell, or a transplant – the immune cells destroy the cells that carry the unusual or ‘faulty’ snippets.

Each MHC protein can bind a range of different peptide snippets. MHC proteins show a high degree of variability among human individuals. This means that individuals of a population will show a varied immune response towards a pathogen, and some, at least, will survive an epidemic. Thus, high MHC diversity among individuals is advantageous for the survival of the species. The variability of the MHC system is also responsible for the rejection of foreign tissue, such as transplants or infectious cancer cells. In fact, people have suggested that one reason for the marked MHC diversity in vertebrates may be to avoid infection by cancer cells from individuals of the same species. Whereas this feature of our physiology may save us from infectious cancer, it becomes a major problem when we need a transplant.

The low variability of the immune system is a major reason why cancer became transmissible in some animals

In contrast to humans, the dog and the devil have a relatively low diversity in their MHC molecules. This may be because of recent reductions in population sizes. In addition, there are likely to be other effects: the cancer cells can hide efficiently from the immune system in the devil, and, at the start of the disease, in the dog. Later in the course of the disease, however, the dog is able to launch an immune attack on the cancer cells. The immune system attacks the cancer cells and saves the dog.

Clams – and mollusks in general – have relatively primitive immune systems, and therefore may be more susceptible to transmissible cancer.

Humans may become susceptible to the transmission of cancer cells under exceptional conditions

Based on the discussions above, it is probably not too difficult to guess what the unusual circumstances are under which transfer of tumor cells in humans can occur. If we need to have a transplant, we need to have treatments that suppress our immune system. These treatments may not only make the engraftment of a transplant possible, but also let some tumor cells survive. Luckily, the number of such cases is very small – only about four people among ten thousand who receive transplants will develop a malignancy.

Transmission of cancer cells to the fetus during pregnancy can occur, albeit at vanishingly low frequencies. There are about 3500 pregnancies per year involving women with malignancies in the United States, yet there are only 17 documented cases of maternal to fetal transmission.

In summary, we do not have to worry about a cancer itself being communicable. The extremely low tumor transmission rates during transplantation will probably be further lowered in the future: transplant donors will be checked more efficiently as scientists develop new ways to identify tumor cells.

Could an aggressive cancer line appear that is able to evade surveillance by the immune system and be transmitted among people? This, although possible, seems an immeasurably unlikely event. Also, with today’s technologies, such an occurrence would be promptly detected, allowing for speedy response from the healthcare system.

ResourcesIs Cancer Contagious? Cancer is NOT contagious. American Cancer Society. http://www.cancer.org/cancer/cancerbasics/is-cancer-contagious. Revised July 17, 2015. Accessed March 15, 2016

Belov K. Contagious cancer: lessons from the devil and the dog. Bioessays 2012; 34: 285-292.

Siddle HV, Kaufman J. Immunology of naturally transmissible tumours. Immunology 2015; 144: 11-20

Weiss RA, Fassati A. The clammy grip of parasitic tumors. Cell 2015;161: 191-192

Focosi D, De Donno M, Barbuti S, Davini S, Fornaciari S, Curcio M, Mariotti ML, Scatena F. Cancer transmissibility across HLA barriers between immunocompetent individuals: rare but not impossible. Hum Immunol. 2011;72: 1-4

ImagesAmerican Cancer Society site: Learn About Cancer » Cancer Basics » Is Cancer Contagious?http://www.cancer.org/cancer/cancerbasics/is-cancer-contagious

Tasmanian Devil in defensive stance, at Tasmanian Devil Conservation Park, Tasman Peninsula.https://commons.wikimedia.org/wiki/Sarcophilus_harrisii#/media/File:Tasdevil_large.jpgBy Wayne McLean ( jgritz) Taken with Nikon D100. - Own work, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=496088

A mixed-breed terrier. Mixed-breed dogs have been found to run faster and live longer than their pure-bred parentshttps://en.wikipedia.org/wiki/Dog#/media/File:Terrier_mixed-breed_dog.jpgBy Chris Barber - Flickr, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=2253341

Mya arenaria, the soft-shell clam, a bivalve from the family Myidaehttps://commons.wikimedia.org/wiki/File:Mya_arenaria_001.jpg#/media/File:Mya_arenaria_001.jpgBy Georges Jansoone (JoJan) - Own work - photo made at an sea aquarium, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=10599742

Schematic representation of MHC class I molecule, consisting of three -domains and one 2-α βmicroglobulin molecule. The peptide-binding groove is situated between domains 1 and 2.α αhttps://commons.wikimedia.org/wiki/File:MHC_Class_1.svg#/media/File:MHC_Class_1.svgBy User atropos235 on en.wikipedia - Own work, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=1805424