How Do Viruses Cause Cancer and What Cancers Do They Cause?

Types of Cancer Causing Viruses You Should Know

Are there viruses that cause cancer?  If so, what viruses, how do they cause cancer, and what cancers do they cause?  Is there any way to prevent this from happening?

Viruses Are a Common Cause of Cancer

You may think of most viruses as the nuisance that causes the common cold, but some of these microorganisms do much more.  In fact, it's thought that worldwide, roughly 20% of cancers are caused by viruses.

  In the United States that number is lower, but viruses are still thought to cause between 5 and 10% of cancers.

It's important to note that most viruses do not cause cancer.  In addition, even when viruses cause the genetic mutations needed for a cell to become cancerous, most of these damaged cells are removed by our immune systems.  When a viral infection leads to a cancer, which in turn is able to escape the immune system, there are often other factors at work, as seen below.

How Does a Virus Cause Cancer?

A virus is nothing more than DNA or RNA wrapped in a protein coat. What makes them unique is that they do not contain the necessary materials to function on their own. They are forced to invade a host cell (can be plant, animal, or bacterial) in order to thrive and reproduce. There are several ways that a virus can cause cancer.

  • Some viruses can cause chronic inflammation.  The resulting inflammation causes increased cell division in the process of making cells to replace damaged cells.  Whenever cells divide, there is a risk that a genetic mutation will take place.  Therefore, inflammation caused by some of these viruses leads to increased cell division, which leads to a greater chance that errors in genetic material will take place, eventually leading to cancer.
  • Some viruses may directly damage DNA in cells resulting in cancer.
  • Some viruses may alter the immune system, so that it is less able to fight off cancer cells.

Viruses Known to Cause Cancer

Cancer viruses can be either DNA or RNA viruses.  Viruses known to cause cancer are listed below, though it's likely that others will be found in the future.

  Note as well that there are also some bacteria and parasites that are linked with the development of cancer.

Human Papillomavirus (HPV) and Cancer

The human papillomavirus (HPV) is sexually transmitted virus affecting over 20 million Americans. It is the most common type of sexually transmitted infection. There are currently over one hundred known strains of HPV, but only around 30 of these are thought to cause cancer.

The strains of HPV most commonly associated with cancer include HPV 16 and HPV 18.

Vaccination for HPV - a shot that protects against HPV 16 and HPV 18 - is available for children between the ages of 11 and 12, and can be given starting at age 9 and as old as age 26.

Cancer currently associated with HPV infections include:

  • Cervical cancer - High risk strains of HPV are responsible for almost all cervical cancers
  • Vulvar cancer - 69% of vulvar cancers are caused by HPV
  • Anal cancer - Roughly 91% of anal cancers are caused by HPV
  • Vaginal cancer - 75% of vaginal cancers are due to HPV
  • Penile cancer - 63% of penile cancers are related to HPV.
  • Head and Neck cancer - It's thought that 72% of cancers near the back of the throat are related to HPV.

In some other cancers, the data is less certain.  For example, HPV is linked with lung cancer, but it's not know if HPV contributes to the development of lung cancer, if instead, having lung cancer increases the chance of contracting HPV, or if it is just a random occurrence and they are unrelated.

Thankfully, it appears that some of the cancers have a better prognosis when related to HPV infections.  For example, throat cancers that are thought to be caused by a combination of smoking and alcohol have a much poorer prognosis than those felt to be caused by HPV.

Hepatitis B and Cancer

Infection with Hepatitis B virus (HBV) increases the risk of developing liver cancer.

These viral infections are extremely contagious are are spread through the transmission of blood, semen, and other bodily fluids from one person to another. Common means of exposure include unprotected sex, mother to infant transmission during childbirth, and the sharing of intravenous needles (most often by dug use, but can also occur during tattooing).

Most people recover from an acute hepatitis B infection (roughly 70% have symptoms and the other 30% are asymptomatic), but some people go on to develop a chronic infection with hepatitis B, most commonly those who contract the disease in early childhood and those who do not have any symptoms.  Liver cancer occurs much more commonly among those with chronic hepatitis B (hepatitis B carriers.)

Most children born since the 1980's have been immunized against hepatitis B, and adults who have not been immunized should consider doing so.

Hepatitis C and Cancer

Hepatitis C infection also increases the risk of developing liver cancer.  Until the 1980's, hepatitis C infection (HCV) was known as non-A non-B hepatitis.  The initial infection may have symptoms, but a significant number of people do not have symptoms.  Unlike hepatitis B, in which the disease does not often become chronic, around 80% of people with hepatitis C develop a chronic infection.

As the immune system continues to attack the virus over time, fibrosis develops, eventually leading to cirrhosis.  This chronic inflammation can also lead to liver cancer.

The virus is spread through infected blood, such as with transfusions and IV drug abuse, but many people do not have obvious risk factors for the disease.  It is now recommended that adults born between 1945 and 1965 be tested for the disease, as well as others who may be at risk.

Epstein Barr Virus (EBV) and Cancer

The Epstein Barr virus is most commonly recognized for causing mononucleosis, but is also linked with the development of several different types of lymphoma.  These include

  • Posttransplant lymphoma - Between 1 and 20% of people develop a lymphoma after an organ transplant, and almost all of these are related to Epstein Barr virus infections.
  • HIV associated lymphoma.
  • Burkitt's lymphoma - In Africa, Burkitt's lymphoma is responsible for half of childhood cancers, and 98% of these are linked with the Epstein Barr virus.  The link between Epstein Barr and Burkitt's lymphoma is not as strong in children with this disease in the United States, and it's thought that malaria may weaken the immune system in African children allowing the virus to transform cells into cancer.
  • Hodgkin's lymphoma - It's thought that the Epstein Barr virus plays in a role in 40 to 50% of cases of Hodgkin disease in the U.S.

Epstein Barr virus is also known to cause nasopharyngeal carcinoma and gastric carcinoma.

Human Immunodefiency Virus (HIV) and Cancer

HIV and cancer are linked in a few ways.  Just as we've known for years that immunosuppressive drugs can weaken the immune system resulting in cancers, the immunosuppression caused by the HIV virus can predispose people with the disease to cancer.  Non-Hodgkin lymphoma, Hodgkin lymphoma, primary cns lymphoma, leukemia, and myeloma are all linked with the infection.  As noted above, it appears that HIV weakens the immune system (as does malaria) allowing the Epstein Barr virus to cause the transformation needed for lympohocytes to become a lymphoma.

In addition to lymphomas, HIV increases the risk of Kaposi's sarcoma, cervical cancer, lung cancer, anal cancer, and liver cancer.

T-Lymphotrophic Virus (HTLV-1) and Cancer

HTLV-1 is a retrovirus (similar to HIV) that causes adult human T-cell leukemia/lymphoma.

Human Herpes Virus 8 (HHV-8) and Cancer

HHV-8 can cause Kaposi's sarcoma, and is also known as KSHV - Kaposi sarcoma herpes virus.

Merkel Cell Polyomavirus

Merkel cell polyomavirus - known as McPyV - can cause a form of skin cancer known as merkel cell carcinoma.  Yet even though the virus is very common in the population as a whole, the cancer caused by it is uncommon.

Prevention of Cancer Caused by Viruses

An ounce of prevention is worth a pound of cure, and it's notable that many of these viruses that can lead to cancer are passed from person.  Practicing safe sex and not sharing needles are one way to lower risk.  The importance of being healthy in general - eating right and exercising - is reinforced in seeing how suppressed immune function can raise the risk of some of the viral induced cancers.

Prevention of cancer caused by viruses is an exciting area of research - especially the idea of being able to prevent some of these cancers through vaccines to prevent the virus from invading the body in the first place.

On a final note, scientists are working on a different combination of viruses and cancer and using some viruses to fight cancer instead of cause it.

Sources:

Centers for Disease Control and Prevention. The Link Between HPV and Cancer. Updated 09/30/15. http://www.cdc.gov/hpv/parents/cancer.html

Centers for Disease Control and Prevention. Testing Recommendations for Hepatitis C Infection. Updated 10/15/15. http://www.cdc.gov/hepatitis/hcv/guidelinesc.htm

Geng, L., and X. Wang. Epstein-Barr Virus-associated lymphoproliferative disorders: experimental and clinical developments. International Journal of Clinical and Experimental Medicine. 2015. 8(9):14656-71.

Grundhoff, A., and N. Fischer. Merkel cell polyomavirus, a highly prevalent virus with tumorigenic potential. Current Opinion in Virology. 2015. 14:129-37.

National Cancer Institute. Infectious Agents. Updated 04/29/15. http://www.cancer.gov/about-cancer/causes-prevention/risk/infectious-agents

Vedham, V., Verma, M., and S. Mahabir. Early-life exposures to infectious agents and later cancer development. Cancer Medicine. 2015. 4912):1908-22.

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