It's rare for men to contract an oral HPV infection, but single men and smokers face a relatively greater risk, a new study suggests.
The study, published online recently in The Lancet, followed more than 1 600 men to chart rates of oral infection with HPV, or human papillomavirus. HPV, which can cause genital and anal warts, is the most commonly transmitted sexual infection in the United States. Some strains of the virus can eventually lead to cancer.
But it has not been fully clear how often HPV infects the mouth and throat. The answer, at least in healthy men, is not very often, based on the new findings.
However, being single or being a smoker were risk factors for initial infection. Smokers had nearly three times the risk of a cancer-linked HPV infection, versus non-smokers. Singles were about three to four times more likely to contract a cancer-linked infection than men who were married or living with someone.
Overall, less than 2% of the study participants contracted an HPV strain linked to an increased cancer risk in one year. And for most men, the immune system cleared the virus within a year.
The findings are "reassuring", partly because it's persistent infections that present a cancer risk, said Dr Edgar Simard, a researcher with the American Cancer Society who was not involved in the work.
Cervical cancer is the best-known HPV-linked cancer. But HPV infections of the mouth and throat can promote oropharyngeal cancer which affects the back of the throat, base of the tongue and tonsils.
It's a rare cancer, but the number of cases tied to HPV is on the rise in the United States. No one knows why, Simard said.
HPV-linked throat cancer recently came to the public's attention when the British newspaper The Guardian reported that actor Michael Douglas' recent bout with the disease might have been caused by oral sex. Douglas is also a longtime smoker.
To figure out how to prevent HPV-linked oropharyngeal cancer, "we need to improve our understanding of the risks associated with oral HPV infection and persistence," said researcher Christine Pierce Campbell, a postdoctoral fellow at the Moffitt Cancer Center in Tampa, Fla.
The reasons aren't clear, according to Pierce Campbell. But she speculated that single men tend to have riskier sexual behaviours. As for smoking, it's possible that inflammation in the oral cavity, and a dampened immune system, make people more vulnerable to an HPV infection.
"That's a plausible explanation," Simard agreed. "It makes sense biologically." He added, though, that smokers may also just happen to have different sexual practices than non-smokers. "Is smoking a proxy for some risky sexual behaviour?" he said.
Regardless, smoking is a bad idea so the fact that it's linked to oral HPV infection is another strike against it, according to Pierce Campbell. "If you smoke, quit. If you don't smoke, don't start," she said.
But a big question this study doesn't answer, Simard said, is, what are the risk factors for a persistent oral HPV infection? "It's the persistent infections we're worried about," he said.
Since persistent oral HPV infections are thankfully rare, it will take a large, long-term study to figure out why some people continue to harbour the virus, according to Simard.
There are two vaccines against the most common cancer-linked HPV strains. Experts recommend that all children ages 11 and 12 be vaccinated, which involves a series of three shots. 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 vaccines Merck's Gardasil and GlaxoSmithKline's Cervarix are known to ward off genital and anal HPV infections. But studies have not yet shown whether they prevent oral infections.
But, Pierce Campbell said, "We have no reason to believe that these vaccines will not be effective against oral HPV infection."
Learn more about HPV and cancer from the U.S. National Cancer Institute.