The findings, reported in the Journal of Infectious Diseases, confirm some well-established connections between the genital warts virus and certain cancers. And they hint that there could be additional risks.

Genital warts are caused by certain strains of the human papillomavirus (HPV). It's well known that some of those HPV strains -- some of which are the focus of vaccines -- can also promote tumors.

In most people, the immune system is able to clear HPV infection fairly quickly. But some people harbor persistent infections, and a chronic infection with a cancer-linked strain can eventually lead to cancer in some cases.

HPV is probably best known as a cause of cervical cancer -- a disease that experts say is nearly always caused by HPV. The virus is also blamed for most cases of anal cancer, and a large share of vaginal, vulvar and penile cancers.

Lesser known is the link between HPV and certain cancers of the mouth and throat. But a number of studies have found that a large portion of those cancers can be traced to HPV infection.

These latest findings, from a study of more than 49,000 Danish adults, confirm all those connections, said Dr. Susanne Kruger Kjaer of the Danish Cancer Society, who worked on the study.

They also hint that HPV might be involved in the risk of non-melanoma skin cancers -- the most common, and highly curable, types of skin cancer.

CANCER IN LESS THAN FIVE PERCENT

For the study, Kjaer and her colleagues looked at medical records for nearly 33,000 women and 16,000-plus men diagnosed with genital warts over 30 years.

As expected, they had higher-than-average rates of cervical, anal, penile and vaginal cancers, as well as certain mouth and throat cancers.

The vast majority remained cancer-free during the study period. In all, 2,363 people -- or less than 5 percent of the study group -- developed some form of cancer. And only 305 were diagnosed with any of the cancers known to be linked to HPV.

Still, some of those risks were higher than average.

Men with a history of genital warts had a 21 times higher-than-average risk of anal cancer, for instance. Among women, the risk was eight times higher than the norm.

The rates of cervical, penile, vaginal, and mouth and throat cancers were anywhere from 50 percent to eight-times higher, compared with the general population.

There was also some evidence tying genital warts to non-melanoma skin cancers, the researchers say.

Overall, 440 study participants, or less than 1 percent, developed non-melanoma skin cancer -- which is somewhat higher than expected, the researchers say.

It's possible there is some type of connection between HPV and non-melanoma skin cancer, Kjaer told Reuters Health in an email. Since the immune system fights both infections and cancer, it's possible that impaired immune function could underlie persistent HPV infection and skin cancer in some people.

But that's all "speculative" for now, Kjaer stressed.

Non-melanoma skin cancers are the most common type of cancer by far. In the U.S., about 3.5 million cases are diagnosed each year, according to the American Cancer Society.

Experts say the best way to cut your risk of skin cancer is to limit your sun exposure, especially if you have fair skin. No one knows whether preventing HPV infection has any effect.

COULD VACCINES PLAY A ROLE?

For now, there are no "concrete practical implications" from the current findings, Kjaer said.

But if further studies confirm that HPV is associated with a broader range of cancers, that could underscore the importance of preventing the infection, according to Kjaer.

There are two vaccines that can prevent infection with certain cancer-related strains of HPV: Merck's Gardasil and GlaxoSmithKline's Cervarix, both of which cost about $400 for three doses. The current study was funded by Sanofi Pasteur, which sells Gardasil in Europe.

Right now, public health authorities recommend that all girls and boys ages 11 and 12 receive the HPV vaccine. Older girls and young women up to age 26 are advised to get "catch-up" shots if they've never been vaccinated. The same advice goes for boys and men ages 13 to 21.

The HPV vaccine is generally considered safe. The most common side effects are pain at the injection site, fever, dizziness and nausea, according to the U.S. Centers for Disease Control and Prevention.

SOURCE: bit.ly/HmRmm2 Journal of Infectious Diseases, online March 15, 2012.

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