Human papillomavirus (HPV) infection is the single most important risk factor which is found associated with almost 85 to 95% of the cases of anal cancer. Although the incidence of HPV infection is very common, only a few cases progress to pre-cancerous anal lesions or invasive anal cancer. HPV is a group of about 150 DNA viruses, with high-risk subtypes including HPV-16 and HPV-18, reported in about 75% and 10% of all the cases of anal cancer, respectively. HPV consists of two proteins known as E6 and E7 which inactivate some tumor suppressor genes and lead to anal cancer.
Following is a list of anal cancer risk factors:
- A history of Anal Warts: Although anal warts are caused by different subtypes of HPV, individuals with a history of anal warts remain at high risk of developing the infection with high-risk HPV subtype, and thus, anal cancer.
- HIV Infection: People infected with HIV are at 15 to 35 times higher risk of developing anal cancer compared to the normal population.
- Weak immune system: Individuals with a weak immune system that may be due to any cause, for example, infection with human immunodeficiency virus (HIV), use of medicines that suppress the immune system after an organ transplantation, an autoimmune disorder, chronic steroid therapy, etc, are more prone to HPV infection and thus development of anal cancer.
- A history of receptive anal sex: Men or women who have receptive anal intercourse remain at high risk of developing anal cancer due to a high frequency of HPV infection reported in these people.
- Multiple sexual partners: Individuals who have multiple sexual partners or have partners with multiple partners are considered to be at higher risk of developing anal cancer due to higher chances of sexually transmitted HPV infection.
- A history of cervical, vaginal, or vulvar cancer: Women with a history of genital cancer that are mostly caused by HPV infection are also considered to be at increased risk of developing anal cancer.
- Tobacco/Cigarette Smoking: Chronic tobacco chewing or cigarette smoking exposes the body to various carcinogens that suppress the immune system to fight against HPV infection and increase the risk of anal cancer.
Consumption of diet low in fruits and vegetables, in utero exposure to diethylstilbestrol (DES), history of a sexually transmitted disease, and chlamydial infection are other reported risk factors for anal cancer.
Apart from the above-listed risk factors, certain factors which can reduce the risk of anal cancer have also been reported. Such protective factors mainly include the use of intrauterine devices for birth control and HPV vaccination.