Oxford American handbook of oncology. Second Edition. Oxford University Press (2015)
It is estimated that .6% of the worldwide incidence of cancer is due to infection. For developed countries the proportion is 9%, and for developing countries the proportion is >20%.
Most tumor viruses are ubiquitous; prevalence of infection is much higher than the incidence of the respective form of tumor. Development of associated tumors requires many years of infection.
Viral infection plays a significant role in the initial phases of carcinogenesis. However, other cofactors are necessary for development of virally linked tumors, including genetic, immunological, and environmental factors. Examples of viruses directly linked to human tumors include the following.
- Small, double-stranded DNA viruses (Papovaviridae family)
- They specifically infect squamous epithelial cells
- >100 different genotypes identified
- The strongest evidence for carcinogenicity is for HPV types 16 and 18 (cervical cancer).
- HPV infection accounts for >80% of cervical cancers worldwide
Hepatitis B and C virus
- 81% of cases of hepatocellular carcinoma (HCC) are attributable to chronic infection.
- 5% with primary hepatitis B virus (HBV) and upwards of 80% with acute hepatitis C virus (HCV) infection develop lifelong hepatic infection, hepatocellular injury, and chronic hepatitis.
- Chronic HBV infection is associated with a 100-fold risk of HCC.
- Prevalence of HBV carriers in Southeast Asia, China, and Sub-Saharan Africa may be >20%.
- An endemic herpes virus.
- Epstein-Barr virus (EBV) may be associated with up to 60% of Hodgkin’s disease in developed countries, and up to 80% in developing countries.
- EBV is thought to be causative in >90% of Burkitt’s lymphoma in equatorial Africa, where Burkitt’s is the most common childhood malignancy.
- EBV has a lesser role elsewhere, with <25% of cases outside Africa, the Middle East, and South America.
- Carcinogenic potential in EBV-endemic areas may be facilitated by coincidental infection with Plasmodium falciparum (malaria) depressing cytotoxic T-cell function. EBV infection escapes T-cell surveillance.
- There is greater uncertainty about its role in other types of non-Hodgkin’s lymphoma.
- There is a highly consistent association with nasopharyngeal carcinoma.
RNA retroviruses associated with human malignancies
- Human T-cell lymphocytotrophic virus (HT LV)-1: adult T-cell leukemia
- HTLV-2: hairy cell leukemia variant
- Human immunodeficiency virus (HIV): Kaposi’s sarcoma, lymphoma, and other cancers linked to the acquired immunodeficiency disorder (AIDS)
- May be associated with >60% of cases of gastric adenocarcinoma in developed countries
- Also likely to have a role in the development of gastric lymphoma
- Different strains of Helicobacter pylori may have different carcinogenic potential.
Schistosoma haematobium (Bilharzial bladder disease)
- Linked to hyperplasia, metaplasia, dysplasia, and invasive carcinoma of the bladder.
- 8% of cases of bladder cancer in the developing world, most commonly squamous cell carcinoma, may be attributable to infection.
- It is not associated with bladder cancer seen in the developed world.