Clam cancer may be contagious, but human cancer is not

An interesting research article was published last month on cancer in clams. It turns out that there has been an outbreak of leukemia-like cancers that are devastating the clam population along the East Coast. Analyses of a variety of clams have shown that the responsible cancer cells are clonal i.e. they likely are derived from a single clam and did not arise from the host animal. Somehow these rogue cancer cells have been able to spread in the water to new clams, causing cancer in their new hosts as they continue to divide. This is one of the first examples of widespread horizontal transfer (i.e. contagious cancer cell transmission) of cancer in any animal organism.

This is a remarkable scientific discovery. From the article I learned a couple of interesting facts: 1) Yes clams can get cancer, and 2) Clam cancer can be spread through the water. However, clams are not humans, and so the results are not directly relevant to humans. There are two important points to make about human cancer:

1. Humans cannot get cancer from clams.
2. Human cancer is not "contagious" and cannot be transmitted through water.

Human cancer starts from abnormal cells created by your own body. Any cancerous clam cells from eating a clam would be immediately digested in your stomach and intestines. If clam cancer cells by some miracle got into your blood stream, they would be quickly attacked and destroyed by your immune system. The susceptibility of clams to this mode of cancer transmission is due to the more exposed nature of the organism, and the fact that their immune system is not well-developed enough to distinguish self from non-self to ward off foreign invader leukemia cells.

Some cancers can be caused by a pathogenic agent like human papilloma virus (HPV) in which infection leads to a higher risk of cervical cancer. HPV is sexually transmitted, and not transmitted through the water. In addition, hepatitis B is a virus whose chronic infection of the liver can lead to liver cancer. It is transmitted through contact of the blood or fluid of an infected individual.

In summary, clams cannot give you clam cancer, but they are a tasty treat if you happen to attend a clam bake (Figure 1).

Figure 1. Humans are not like clams and cannot get cancer from the water.