Viruses

A virus is an infectious agent that is so small it cannot be seen with an ordinary microscope. They are made up of a small group of genes composed of DNA or RNA surrounded by a protein coat.

Viruses cannot reproduce by themselves, they need help. They enter a living cell and use the cell's machinery to make copies of themselves. Different cancer causing or oncogenic viruses have different mechanisms for both copying their DNA or RNA genomes and directing the production of viral proteins needed to make complete copies of themselves.

As the role of viruses as a cause of cancer is better understood it may lead to vaccines that prevent or treat certain human cancers in the future.

The following viruses are either now known or suspected of being linked to cancer in humans.

Epstein-Barr virus (EBV)

EBV is a type of herpes virus with a worldwide occurrence best known for causing infectious mononucleosis, or mono . Most people become infected with EBV sometime early in childhood and gain adaptive immunity, preventing repeated sickness from re-infection of EBV.

It is believed that impaired immunity provides an opening for development of the EBV to take hold with certain types of fast-growing lymphomas such as Burkitt lymphoma found predominately in the malaria-belt of Africa in children. Also, in parts of Southeast Asia, infection with EBV increases a person's risk of getting nasopharyngeal cancer (cancer of the area in the back of the nose), as well as Hodgkin disease and some types of stomach cancers. . However, very few people who have been infected with EBV will ever go on to develop these cancers.

Human papilloma viruses (HPVs)

HPVs are a group of over 100 related viruses that can cause warts on the skin, mouth, genital organs, larynx, and some types can cause cancer. Both harmless and cancerlinked high-risk HPV types pass by skin-to- skin contact.

		By the age of 50 more than 80% of American women will have contracted at least one strain of genital HPV. Certain types of HPV are the main cause of cervical cancer, which is the second most common cancer among women worldwide.
Image courtesy of The National Cancer Institute

Most genital HPV infections are cleared over time with the help of the body's immune system and do not cause problems. However when infections are not naturally cleared by the body the only effective treatments for HPV are removing or destroying infected cells. Over 99 percent of cervical cancer cases are linked to long-term infections with high-risk HPV. (NCI)

HPVs are also linked to cancers of the penis, anus, vagina, vulva, mouth, throat, head, and neck. Most people infected with HPV never go on to develop cancer. Recent studies strongly suggest that smoking and drinking are linked with these cancers and may work in synergy with HPV to increase cancer risk.

Cervical cancer is much rarer in the United States because of the routine use of the Pap test. The Pap test is designed to detect pre-cancerous changes in cells in the cervix that might be due to HPV infection. If these cells are found, they can be treated early and this treatment can often prevent the development of cancer.

Most women infected with HPV will not go on to develop cervical cancer, however nearly all women who develop cervical cancer are have an HPV infection. Women with a history of HPV infection may be checked for abnormal cells more often than those who don't have it.

The American Cancer Society estimated in 2008 that about 11,070 women in the United States will be diagnosed with invasive cervical cancer, and about 3,870 US women will die from this disease.

Image courtesy of The National Cancer Institute		An example, a vaginal sore or sex can abrade the cervical lining and may provide a point of entry for the papillomavirus. Once inside the cervical lining, the virus attaches to epithelial cells. As these cells take in nutrients and other molecules that are normally present in their environment, they also take in the virus. This virus must penetrate deeply into the lining of the cervix to establish a chronic infection.

"Recently Dr. Hildesheim and his colleagues from the NCI reported on the link between cervical cancer and single nucleotide polymorphisms (SNPs)-one-letter changes in the human genetic code-in genes involved in the immune response to infections and in the repair of DNA damage.

They noted that some genetic polymorphisms are associated with increased risk of persistent HPV infection, a prerequisite for an HPV-induced cancer, while others are associated with the tendency for HPV-infected cells to progress to precancer or cervical cancer. Parallel studies have also confirmed findings by other researchers that certain variations in immune regulation genes known as HLA genes appear to affect cervical cancer risk." Quote from the NCI Newsletter, Charting the Path from Infection to Cancer: http://www.cancer.gov/ncicancerbulletin/092209/page5

A Vaccine For HPV

An HPV vaccine named Gardasil® has been developed for use against the most prevalent high risk types of HPV linked to cervical cancer. The vaccine protects against four HPV types, which together cause 70% of cervical cancers. It is now approved for use in females aged 9 to 26.

		If an exposure occurs, the vaccinated person's antibodies coat the virus and prevent it from releasing its genetic material. Because Gardasil is still fairly new, it is not yet known how well it will protect against cervical cancer. Further studies of this vaccine and others like it are still under way as doctors track it's record.
Image courtesy of The National Cancer Institute

Hepatitis B virus (HBV) and hepatitis C virus (HCV)

HBV and HCV cause viral hepatitis, a type of liver infection that was originally known as "serum hepatitis". Following the discovery of HBV and the subsequent HBV diagnostic test cases of non-A non-B hepatitis were still observed. Through additional research hepatitis C virus was discovered.

Both HBV and HCV cause liver inflammation, vomiting, jaundice and rarely even death during the acute phase. If the infection is not cleared by the immune system chronic infection occurs and the person becomes a life-long carrier. This may lead over time to liver cirrhosis and liver cancer or hepatocellular carcinoma (HCC). HCC has a very poor response to current chemotherapy and death occurs rapidly.

About 30% of liver cancers have been found to be related to either HBV or HCV infection in the United States. This number is much higher in other countries, where both the hepatitis and liver cancer are much more common. In developing countries intake of aflatoxin on contaminated grain and co-infection with HBV can accelerate the development of cirrhosis and HCC.

		HBV and HCV are spread among people in much the same way as HIV by sharing needles, unprotected sex, and blood transfusions. In the United States and most developed countries blood products are screened for HBV and HCV. The infection is preventable by vaccination. In the United States, a national strategy was implemented in 1991 to eliminate HBV infection through vaccination of infants and children As a result, the rate of new HBV infections has declined by approximately 82 percent, with the greatest decline among children born since 1991.
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