Since then, several other human cancer viruses have been discovered; together, they account for an estimated 12% of malignancies worldwide.
How are Viruses Causes of Cancer?
There are two ways in which infection with a virus can lead to cancer, termed “direct oncogenesis” and “indirect oncogenesis.”
Direct oncogenesis occurs when the virus either inserts genes that cause cancer into the DNA of an infected cell, or stimulates the cell’s own genes in a way that causes that cell to become malignant. In both cases, the infected cell begins to grow uncontrollably, leading to a tumour.
In contrast to direct oncogenesis, which can be rapid, indirect oncogenesis can take years or even decades. In this latter process, the viral infection causes a chronic inflammation that, over a long period of time, predisposes to tumour growth.
Not everyone infected with an oncovirus will develop cancer. In fact, in most people an oncoviral infection causes no symptoms at all, a mild illness only, or a non-cancerous condition (e.g hepatitis in the case of the hepatitis viruses).
HPV and Other Types of Cancer Virus
Seven viruses are currently known to cause cancer in humans.
Human papillomavirus (HPV) is the cause of almost all cases of cervical cancer. There are over 100 different types of HPV, but HPV-16 and HPV-18 are those most often found in cervical tumours. Various HPVs have been associated with other malignancies of the anogenital region, including cancers of the vulva, anus and penis. The virus is transmitted by sexual contact.
Both hepatitis B virus (HBV) and hepatitis C virus (HCV) can cause cancer of the liver, the former by direct oncogenesis and the latter by indirect oncogenesis. With HCV infection, cancer is the final stage of a long process of liver damage that begins with the inflammation of hepatitis and progresses over years or decades to cirrhosis. Both types of virus-caused liver cancer are more common in Asia and Africa than in Western countries. Together, HBV and HCV account for about 5% of all cancers worldwide.
EBV has been identified as a cause of lymphoma, Hodgkin’s disease and nasopharyngeal cancer. As a member of the human herpesvirus family, EBV exhibits latency; that is, it can lie dormant in the body for many years after first infecting its host. The initial EBV infection usually occurs in childhood and causes merely a mild illness; EBV can trigger cancer only if it is reactivated later in life.
Human T cell lymphotropic virus type 1 (HTLV-1) is the only virus known to cause leukaemia, though only 1−3% of infected people develop the disease. The incubation period lasts several decades.
Human herpesvirus 8 (HHV-8), like EBV, is a cancer-causing human herpesvirus. HHV-8 is responsible for Kaposi’s sarcoma (multiple tumours of the skin, mouth and gastrointestinal and respiratory tracts most commonly seen in people with AIDS) and certain rare types of lymphoma.
Merkel cell polyomavirus (MCV) is thought to cause an aggressive form of skin cancer called Merkel cell carcinoma.
The recent discovery of MCV, suggests that other, currently unknown oncoviruses are likely to be identified in future studies.
Fighting Viral Causes of Cancer – HBV and HPV Vaccine
The number of cases of cancer each year, particularly in developing countries, would be greatly reduced if these viral causes could be eliminated. Two vaccines against oncoviruses are currently available – HBV vaccine (given routinely to infants in many countries) and HPV vaccine (recommended for girls or young women, though also available for males). Creating vaccines against the other oncoviruses is the subject of major research worldwide.