At the turn of the 20th century, the concept of viruses being cancer-causing agents emerged.

 

 

 

As early as the 1950’s, a link between the virus and cancer has been established. In 1981, only two viruses were thought to cause human cancers. Now, the oncogenic roles of seven to eight viruses are well-established. According to a survey, it was estimated that nearly 20 percent of the cancer cases worldwide are due to viral infections.

Here is a review of the seven, known human tumour viruses that have been implicated in the etiology of human cancers and contribute to nearly 10–15% of the cancers worldwide.

 

Types of human cancer viruses

 

Oncogenic viruses can be either DNA or RNA viruses. To date, the DNA viruses which have been implicated in the etiology of human cancers include human papilloma viruses (HPVs), Epstein-Barr virus (EBV), Kaposi’s sarcoma-associated herpesvirus (KSHV), hepatitis B virus (HBV) and the Merkel cell polyomavirus (MCV). Among the RNA viruses, hepatitis C virus (HCV) and the human T-cell leukemia virus type 1 (HTLV- 1)- retrovirus are associated with human malignancies.

 

1. Epstein-Barr virus (EBV)

Associated cancer- Nasopharyngeal carcinoma, lymphomas such as Hodgkin’s and non-Hodgkin’s lymphoma, and gastric carcinomas.

Epstein-Barr virus was the first human viral carcinogen discovered. It is a DNA virus which causes infectious mononucleosis. Several research studies have revealed the association between Epstein-Barr virus and cancer.

 

2. Human papilloma virus (HPV)

Associated cancer – Papilloma, warts, cervical cancer, penile cancer, head and neck cancer, anogenital cancers

Human papilloma viruses are a group of more than 150 related viruses and cause sexually transmitted diseases. Although HPV is linked with the development of cancer, it is not very common as that of HPV infection. HPV, known to cause cervical cancer, is the second most common cancer in women worldwide. Most of the women diagnosed with cervical cancer show signs of HPV infection.

 

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

Associated cancer – Heaptocellulr carcinoma

It is estimated that 0.47 % of the patients infected with hepatitis B virus may develop liver tumor. But, the exact mechanism of oncogenesis by the HBV and HCV are still unclear. Hepatitis virus can be transmitted through sexual contact, blood transfusions, sharing needles of the infected person, and transmission from mother to infant during childbirth.

With the advent of a vaccine against hepatitis B virus, the incidence of cancer with HBV infection has greatly reduced. In contrast to this, the development of a vaccine against HCV infection was unsuccessful due to the high mutation rate of hepatitis C virus.

 

4. Human T lymphotropic virus type I (HTLV-1)

Associated cancer – Adult T-cell leukemia/lymphoma.

Once the person is infected with HTLV-1, the risk of developing cancer is about 5 percent. Although the infection has long latency period, there is a rapid progression of the tumor once it is formed.

 

5. Human immunodeficiency virus -1 (HIV)

Associated cancer – Immunosuppression mediated enhanced risk of other cancers like Hodgkin and non-Hodgkin lymphomas, Kaposi sarcoma, liver cancer, anogenital cancers, non-melanoma skin cancer, eye cancer, oral cancer and lung cancer.

Although the science involved in viral oncogenesis is not yet established, knowing the association between virus and cancer will help to prevent these types of cancers.

 

6. Merkel cell polyomavirus (MCV)

Associated cancer – Merkel cell carcinoma

Merkel cell virus is a newly discovered virus that is commonly found on the skin and may cause a type of skin cancer called Merkel cell carcinoma. The cause of MCV infection remains unclear but it has been found in various places of the body including skin and saliva. Experts believe that the virus affects the DNA of the cell and causes Merkel cell carcinoma.

 

7. Human herpes virus-8 (HHV-8)

Associated cancer – Kaposi’s sarcoma (a type of skin cancer), primary effusion lymphomas and multicentric Castleman’s disease.

The oncogenesis of herpes virus 8 is well established and targets the individuals with compromised immune function.

 

 

The viruses reviewed here have been best established as human tumour viruses and these seven viruses including Epstein-Barr virus (EBV), hepatitis B virus (HBV), hepatitis C virus (HCV), Kaposi's sarcoma herpes virus (KSHV), human immunodeficiency virus, type-1 (HIV-1), human T cell lymphotrophic virus, type-1 (HTLV-1), and human papillomavirus (HPV) have been classified as Group 1 human carcinogens by the International Agency for Research in Cancer (IARC).

It is likely that many other viral infections that may be oncogenic in humans remain to be unveiled. Because oncogenic viral infection can be prevented with vaccines (safe and effective HBV and HPV vaccines have been developed), it is imperative that we redouble our efforts to identify and characterize viral etiologies of human neoplasias.