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Of warts and cancer
August 04, 2010The human papilloma virus was little more than a footnote when I studied virology in college. They caused warts: common warts, plantar warts, flat warts, anal warts, and genital warts. The warts came and they went. Disgusting, but not interesting. And though genital warts were contacted through sexual contact, there were much more "sexy" STDs to talk about. Then, in 1983, the year I graduated and started medical school, two strains of HPV were implicated in cervical cancer.
Today we know that HPV is responsible for nearly all cervical cancers, 85% of anal cancers, 70% of vaginal cancers, 40% of vulvar and penile cancers. And in just the last couple of years we have discovered that HPV causes over 25% of head and neck cancers, nearly all of which aren't related to tobacco. Four years ago the FDA approved the first of two HPV vaccines designed to prevent cervical cancer. This article is a fact sheet about this now interesting virus.
Most HPV infections do nothing. Of over 120 identified HPV types, 11 cause warts and 12 cause cancer. The rest of them seem to do nothing at all. Infections are temporary and insignificant. Even most of the HPV infections that can cause warts or cancer come and go without notice or consequences. Only about 5% of infections with cancer causing HPV become persistent infections capable of promoting cancer.
Different types of warts are caused by different types of HPV. Only genital warts are a sexually transmitted disease and you cannot get genital warts from someone with plantar warts no matter what they do with their feet. Further, though they may live in the same neighborhood - moist areas like the groin and throat - HPV types that cause warts do not cause cancer.
HPV is the most common sexually transmitted infection in the United States. 80% of sexually active Americans will be infected with HPV at some point in their lifetime. In a 2004 study of women between ages 14 and 59, it was determined that at any one point in time, 27% of them were infected with at least one type of HPV and 15% were infected with one of the types that can cause cancer. Condoms don't seem to offer much protection against this STD.
HPV infections caused 11,000 cases of cervical cancer and 3900 deaths in the U.S. in 2008. Worldwide there are 490,000 cases and 270,000 deaths each year. The potential benefit of effective prevention is huge. Pap smears detect late HPV infections and early cervical cancers. A newer test, one that looks for HPV DNA in the specimen, is much more sensitive for early infections and much more expensive. It is replacing the Pap smear in some US practices, but until a cheaper version of the test is available it will not be a global solution. It has been proposed that we screen high risk groups with anal and oral Pap smears, but we don't have data to demonstrate benefit yet.
Two commercial vaccines, Gardasil and Cervarix, can prevent infection by HPV types 16 and 18, responsible for 75% of cervical cancers. The Gardisil vaccine also prevents infections with types 6 and 11, responsible for 90% of genital warts. They only work by preventing infection and have no benefit for women already infected with HPV. Because they will not prevent all cervical cancers, these vaccines are not a replacement for Pap smears.
In the United States either vaccine is recommended for all girls age 13-17. Nevada has made it mandatory and has required health insurance companies to pay for it. New Hampshire provides it free of charge to girls ages 11-18. Similar legislative efforts are active in 27 states, but not in Washington State. These efforts are complicated by the high cost of the vaccine and by the opposition of anti-vaccine groups and elements of the religious right who fear it will promote promiscuity. As of late 2007, about one quarter of US females age 13-17 had received at least one of the three injections required for full benefit.
Nearly the entire developed world has approved at least one of the vaccines for use. Much of the developing world, with the notable exception of the Muslim Middle East, is following suit, though it is often an empty gesture. In Kenya, where both vaccines have been approved, the cost of one course of vaccine is more than the average families annual income.
No longer a footnote, HPV is both an example of how far medicine has come in 30 years, and how far it has yet to go.