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How do Viruses Cause Cancer ?
- Chronic (long-term) inflammation caused by the infection is a key factor in the development of cancer.
- Some cancer causing viruses introduce their genetic material directly into the host DNA when they infect human cells, resulting in malignant change.
- RNA tumor viruses (e.g. HTLV-1) carry or alter key growth-regulatory genes (oncogenes).
- DNA tumor viruses (e.g HPV) cause cell transformation by encoding proteins of exclusively viral origin which are essential for the virus to replicate.
Understanding how cancers are caused by viruses has been one of the most important developments in cancer research in the last century, and has obvious implications for prevention, detection and treatment.
Cancer causing viruses are not the only infectious agents associated with malignancy. Over 70 percent of all stomach cancers worldwide are attributed to the bacterium H. pylori, and bladder cancer has been associated with the worm schistosoma haematobium.
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Human Papilloma Virus (HPV)
Persistent infection with certain types of HPV is involved in cervical cancer development. Of the 40 types of HPV found to infect the anogenital tract, HPV 16 and HPV 18 have the highest prevalence among cervical cancer patients. HPV transmission is mainly through sexual activity, so condom usage can give some degree of protection. However the most effective way of preventing cervical cancer is though detection of precancerous disease by Pap testing. More than half of the world's cervical cancer deaths occur in Asia, partly due to a lack of adequate cervical screening programmes. HPV infection has also been associated with cancer of the oral cavity, vulva, vagina, penis and anus.
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Hepatitis B and C (HBV and HCV)
Liver cancer, or hepatocellular carcinoma (HCC), is the third highest cause of cancer deaths worldwide. Roughly 80 percent of hepatocellular carcinoma is due to chronic infection with HBV or HCV. Transmission of these viruses from person to person is by blood or other body fluids. The most common methods of spread include injection, sexual activity and transmission from mother to baby. A vaccine for HCV is not yet available, but a safe, effective vaccine against HBV has been developed. It is estimated that effective vaccination programs could prevent nearly 300,000 deaths annually from liver cancer worldwide.
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Human Herpes virus 8 (HHV-8)
Kaposi's sarcoma, a connective tissue tumor, was common in central Africa, the Middle East and the Mediterranean. It was rarely fatal, occurring in older males. In the 1980s, a new aggressive form of KS was reported in the United States amongst AIDS patients. However, only one of the four subtypes of KS is associated with AIDS, and KS is now thought to be primarily caused by infection with HHV-8 (KS-associated herpes virus), rather than HIV. Co-infection with HIV makes a patient more at risk of KS due to their immunocompromised state. The main modes of spread are sexual transmission and exchange of blood.
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Epstein Barr Virus (EBV)
Hodgkin's lymphoma and some forms of non-Hodgkin's lymphoma (NHL) e.g. Burkitt's lymphoma are associated with chronic EBV infection. EBV is primarily transmitted in the saliva of healthy individuals. The majority of the population are infected at some point in their lives, EBV being the cause of infectious mononucleosis ('glandular fever') in young adults. In those with a healthy immune system infection is controlled and generally has no long-term effects. AIDS patients who are immunocompromised, have a signifiicantly higher risk of developing NHL.
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Human T Cell Lymphotropic Virus Type 1 / Human T Cell Leukemia Virus type 1 (HTLV-1)
Adult T cell Leukemia (ATL) is an aggressive malignancy associated with HTLV-1 infection. ATL has the highest incidence in Japan, central Africa, the Caribbean, Taiwan and Papua New Guinea, where HTLV-1 is most prevalent. In vitro experiments have shown that HTLV-1 infection of normal human T cells induces malignant change.
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The global burden of cancer: priorities for prevention by M.Thun, J.Delancy, M.Center, A.Jemal, E.Ward.. Carcinogenesis, 2009, Vol 31, P100-110.
Incidence and mortality rates of selected infection-related cancers in Puerto Rico and the USA by A.Ortiz, M.S-Salgado, W.Calo, G.T-Luna, C.Perez, C.Romero, J.Perez, N.F-Valles, E.Suarez. Infectious Agents and Cancer, 2010, 5:10 P1-10.
View and review on viral oncology research by V.Bergonzini, C.Salata, A.Calistri, C.Parolin & G.Palù. Infectious Agents and Cancer 2010, Vol 5:11