Natural history and epidemiology of HPV, anal warts, AIN, and anal cancer
Human papillomavirus (HPV)
There are over 200 different kinds of HPV but only about 20 or so commonly infect the genital region. HPV types 6 and 11 cause genital warts. Since warts do not usually progress to cancer, HPV 6 and 11 are considered to be "low-risk of cancer" HPV types. They are also called "non-oncogenic," meaning non-cancer-causing.
On the other hand, there are a variety of "high-risk," or oncogenic (cancer-causing) HPV types that also infect the anal canal. The most common of these is HPV type 16. Other common types include HPV18, HPV31, HPV33, HPV35, and several others. Many people have warts on their hands and feet, and these are usually caused by types 1, 2 or 4. These HPV types do not cause cancer and the good news is that they only rarely go the genital skin. Likewise, genital HPV types only rarely live in the skin of the hands and feet.
HPV lives only in skin cells (epithelium) and no other kinds of cells, so the only way to get it from another person is through skin-to-skin spread. That usually means contact between genital skin to genital skin. It is currently believed that HPV is rather easily acquired, and while wearing condoms may help some, it is by no means completely effective to prevent HPV spread. Many people get HPV within having their first few sexual partners. With a higher number of partners, the risk of acquiring new HPV types increases even more.
How common is anal HPV infection? Nearly all HIV-seropositive men with a history of receptive anal intercourse have anal HPV infection. About 60% of HIV-seronegative men with a history of receptive anal intercourse have anal HPV infection, and even 60% of HIV-seropositive men without a history of receptive anal intercourse have anal HPV infection. One recent study showed that the prevalence of anal HPV infection stays pretty consistent at that rate from ages 20 to 60. Very little is known about how common anal HPV infection is among HIV-seronegative men without a history of receptive anal intercourse. Many men and women also have multiple HPV types at one time. This tends to be more common among HIV-seropositive men and women, and especially among people with lower CD4+ T cell counts.
Women are at risk of anal HPV infection too. Most of the studies have been done on HIV-seropositive women and HIV-seronegative women at high risk of HIV infection either through heterosexual contact with a male with HIV infection or through injection drug use. These studies show that about 76% of HIV-seropositive women and 42% of HIV-seronegative women are at high risk of HPV infection. These figures were even more common than the rates of cervical HPV infection! Very little is known about other populations of women but one study of HIV-seropositive adolescent girls showed a 59% rate of infection. How do women get anal HPV infection? Some get it through anal intercourse but others may get it through spread from their vagina or vulva, or through anal insertion of fingers or toys during sex.
What is the natural history of anal HPV infection? We don't really know since long-term studies have never been done. What is clear though is that most people don't run into trouble from HPV infection. Some people develop AIN due to HPV infection, but most of the cases of AIN will not progress to cancer. In most people the AIN will go away by itself and they will not even have been aware of it. But for some people it can be a real problem.