Anal Cancer Causes, Risk Factors, and Prevention

The cause of a disease plays a direct role in the development of the disease. A risk factor makes that disease more likely, but it isn’t the only reason it develops.  Researchers have found some risk factors that increase a person’s risk of anal cancer, but the exact cause of anal cancer is not known. It’s also important to remember that some people with anal cancers do not have any known risk factors.

Causes of anal cancer

• HPV infection: Human papillomavirus (HPV) infection seems to be important in the development of anal cancer, but the majority of people with HPV infections do not get anal cancer. There is evidence that HPV causes many anal squamous cell carcinomas, but its role in causing anal adenocarcinomas is less certain.

• Lowered immunity: When the body is less able to fight off infections, viruses like HPV can become more active, which may allow anal cancer to develop.

• Smoking: Most people know that smoking is the main cause of lung cancer. The cancer-causing chemicals in tobacco smoke can also travel from the lungs to the rest of the body, causing other types of cancer as well.

Risk factors for anal cancer

A risk factor increases your chance of getting a disease, such as cancer. Different cancers have different risk factors. Some risk factors, like smoking or diet, can be changed. Others, like a person’s age or family history, cannot.

Several factors can affect your risk of developing anal cancer. But having a risk factor, or even several risk factors, does not mean that you will get cancer. Many people with risk factors will never develop anal cancer, while others with anal cancer may have few or no known risk factors.

HPV (human papillomavirus) infection

Infection by the human papillomavirus (HPV) is the most important risk factor for anal cancer. Most squamous cell anal cancers are linked to infection with HPV. HPV is a group of more than 150 related viruses, the same group of viruses that causes cervical cancer, as well as other kinds of cancer. In fact, women with a history of cervical cancer (or cervical pre-cancer) have an increased risk of anal cancer.

• HPV can infect cells on the surface of the skin, and those lining the genitals, anus, mouth and throat, but not the blood or internal organs such as the heart or lungs.
• HPV can spread from one person to another during skin-to-skin contact. One way HPV spreads is through sexual activity, including vaginal, anal, and even oral sex.
• Different types of HPV cause warts on different parts of the body. Some cause common warts on the hands and feet; others tend to cause warts on the lips or tongue.

Infection with HPV is common, and in most cases, the body can clear the infection by itself. Sometimes, however, the infection does not go away and becomes chronic. Chronic infection, especially when it is caused by certain high-risk HPV types, can eventually cause certain cancers, such as anal cancer.

Certain types of HPV are high-risk because they are strongly linked to cancers including anal cancer, as well as cancer of the cervix, vulva, and vagina in women, penile cancer in men, and cancers of the anus, mouth, and throat in both men and women. The high-risk subtype most likely to cause anal cancer is HPV-16. Another high-risk type is HPV-18, but this is seen less often with anal cancer.

Other types of HPV may cause warts in or around the anal area and on the female and male genital organs. These are called low-risk HPV because they are seldom linked to cancer.

Although there is currently no cure for HPV infection, there are ways to treat the warts and abnormal cell growth that HPV causes. Also, HPV vaccines are available to help prevent infection by certain types of HPV and some of the cancers linked to those types.

Anal warts

People who have had anal warts are more likely to get anal cancer. This is because people who are infected with HPV subtypes that cause anal and genital warts are also more likely to be infected with HPV subtypes that cause anal cancer.

Having certain other cancers

Women who have had cancer of the cervix, vagina, or vulva are at increased risk of anal cancer. This is probably because these cancers are also caused by infection with HPV.

HIV infection

People infected with HIV (the human immunodeficiency virus), the virus that causes AIDS, are much more likely to get anal cancer than those not infected with this virus.

Learn more: HIV Infection, AIDS, and Cancer.

Sexual activity

Having multiple sex partners increases the risk of infection with HIV and HPV. It also increases the risk of anal cancer. Receptive anal sex also increases the risk of anal cancer in both men and women. Because of this, men who have sex with men have a high risk of this cancer.

Smoking

Smoking increases the risk of anal cancer. The higher a person's pack-year history of smoking, the higher their risk of developing anal cancer.  People who currently smoke are more likely to have cancer of the anus compared with people who do not smoke or have quit smoking. Quitting smoking seems to reduce the risk.

Lowered immunity

Higher rates of anal cancer occur among people with reduced immunity, such as people with HIV/AIDS or people who have had an organ transplant and must take medicines that suppress their immune system.

Sex and race/ethnicity

Anal cancer is more common in White women and Black men. 

Can anal cancer be prevented?

Since the cause of many cases of anal cancer is unknown, it’s not possible to prevent this disease. However, there are things you can do that might lower your risk of anal cancer.

HPV vaccines

Infection with HPV increases the risk of anal cancer. HPV infection can be present for years without causing any symptoms. Even when someone does not have warts (or any other symptom), they can still be infected with HPV and pass it on to somebody else.

Vaccines are available that prevent certain HPV infections, including HPV-16 and HPV-18. Some can also protect against infections with other HPV subtypes, including some types that cause anal and genital warts.

These vaccines can only be used to help prevent HPV infection – they do not  treat an existing infection. To work best, the vaccine should be given to young children before they become sexually active.

Learn more: HPV Vaccines.

Not smoking

Smoking is a known risk factor for anal cancer. Stopping smoking greatly reduces the risk of developing anal cancer and many other cancers.

Treating HIV

For people with HIV, it’s very important to take medicines (known as highly active antiretroviral therapy, or HAART) to help keep the HIV infection under control and prevent it from progressing to AIDS. This can also lower the risk of long-term HPV infection and anal intraepithelial neoplasia (a kind of anal precancer), which might help lower the risk of anal cancer. Compared to other HIV-related cancers, the number of people with HIV who have developed anal cancer while being treated with HAART has increased over the years. The reasons for this are unknown, but may be because people with HIV treated with HAART are living longer.

Condom use

Condoms may provide some protection against infection with HPV and HIV, but they do not prevent it completely. Given this, it is unclear if condom use can reduce the risk of anal cancer.  

One study found that when condoms are used correctly, they can lower the genital HPV infection rate in women – but they must be used every time sex occurs. This study did not look at the effect of condom use on anal HPV infection.

Condoms cannot provide complete protection because they do not cover every possible HPV-infected area of the body, such as skin of the genital or anal area. HPV can still be passed from one person to another by skin-to-skin contact with an HPV-infected area of the body that is not covered by a condom. Still, condoms may provide some protection against HPV. Male condom use also seems to help genital HPV infections clear (go away) faster.

Condom use is also important because it can help protect against AIDS and other sexually transmitted diseases that can be passed on through some body fluids.