LOS ANGELES — An estimated 7 percent of American teens and adults carry the human papilloma virus in their mouths, an infection that puts them at heightened risk of developing cancer of the mouth and throat, researchers said Thursday.
Their study, the first to assess the prevalence of oral HPV infection in the U.S. population, may help health experts understand why rates of oropharyngeal cancer — a type of head and neck cancer — have skyrocketed in recent years, increasing 225 percent between 1988 and 2004.
The findings also indicate that the virus is not likely to spread through kissing or casual contact and that most cases of oral HPV can be traced to oral sex.
“There is a strong association for sexual behavior, and that has important implications for public health officials who teach sexual education,” said Dr. Maura Gillison of the Ohio State University Comprehensive Cancer Center, who led the study.
Gillison presented the data Thursday at a meeting of head and neck cancer researchers and doctors in Phoenix. The results were also published online by the Journal of the American Medical Association.
HPV is best known as the cause of cervical cancer, which kills 4,220 women in the U.S. each year, according to the National Cancer Institute. The virus can also cause vulvar, anal, penile and various head and neck cancers. A study published in October in the Journal of Clinical Oncology traced more than 70 percent of new cases of oral cancers to HPV infection, putting it ahead of tobacco use as the leading cause of such cancers.
If present trends continue, HPV will cause more cases of oral cancers than cervical cancer by 2020, according to the October study.
HPV infection is common — an estimated 80 percent of Americans have contracted the virus, Gillison said. It usually produces no symptoms and is typically cleared from the body through natural processes.
But persistent infections can cause cancer. Vaccines are now available for children and young adults to prevent cervical and anal cancers caused by the most troublesome HPV strains.
Among people who had more than 20 sexual partners, the prevalence of oral HPV was 20 percent. But the researchers found it in less than 1 percent of people who said they were virgins and in less than 4 percent of people who said they had never performed oral sex.
To get a handle on HPV’s role in oral cancers, Gillison and her colleagues analyzed data from 5,579 people ages 14 to 69 who participated in the National Health and Nutrition Examination Survey in 2009 and 2010. The NHANES survey includes a detailed questionnaire and a physical examination; volunteers provided a 30-second oral rinse from which researchers extracted cells to test for HPV infection.
They found that the overall prevalence of oral HPV was 6.9 percent — far less than the rate of genital HPV infection in reproductive-age women, which can be as high as 42 percent among women in their 20s, Gillison said.
The infection rate varied substantially among different groups. For instance, 10.1 percent of men in the study had oral HPV, compared with 3.6 percent of women. The reason for the difference is unknown but it could have to do with oral sex practices, Gillison said.
Very little is known about how the virus is transmitted to the oral cavity and triggers the growth of cancer cells. But the findings strongly suggest that oral sex is to blame more than kissing, experts said.
“There is no evidence to suggest casual transmission of oral HPV infection,” said Gypsyamber D’Souza, a cancer prevention expert at Johns Hopkins Bloomberg School of Public Health in Baltimore who was not involved in the new study. But anyone who is sexually active appears to be at risk, she said: “It only takes one partner who has had another (infected) partner.”
Researchers also noted age differences; those ages 60 to 64 had the highest prevalence at 11.4 percent. That’s in marked contrast to cervical HPV infection, which is most common among women in their early 20s.
It’s unclear why the prevalence of oral HPV peaks much later in life, Gillison said. One possibility is that the immune system weakens with age, making people more vulnerable to latent infections. Another theory is that study participants in their 60s came of age during an era of sexual permissiveness that preceded public-health messages about safe sex, Gillison said.
The study also linked heavy smoking to oral infection. It’s possible that smoking weakens the body’s immune response, making it easier for an infection to persist.
The most common high-risk HPV strain, HPV-16, infected 1 percent of the participants. That strain raises the risk of oral cancer fiftyfold and accounts for most cases of squamous cell cancers of the mouth and pharynx. Squamous cell cancers, which arise in the mucous membranes that line the mouth and throat, are diagnosed in 2.6 per 100,000 people and are the most common type of oropharyngeal cancer.
Even with only 1 percent of people infected by HPV-16, that still translates to “hundreds of thousands of people” who will contract the virus and be unable to clear it, said Dr. Hans Schlecht, assistant professor of medicine at Drexel University College of Medicine in Philadelphia.
The data raise a number of important issues, experts said. For example, while Pap tests can detect HPV infection and precancerous cells in the cervix, there is no such screening test for precancerous cells in the mouth. Researchers also wonder whether the HPV vaccine will protect against oral cancers, a question that could take years to answer.
In the meantime, the new data should give parents more to think about as they consider whether to vaccinate their children — especially their sons, Gillison said. HPV vaccination is recommended for females ages 9 to 26 and males ages 9 to 21.
“Some parents may have felt that the risk of HPV infection wasn’t relevant to them,” she said. “But this study shows 1 in 10 boys has an infection that can lead to a cancer. And HPV-16, which causes most of the cases of cancer, was almost six times more common in the men than in women.”
The research also raises the delicate issue of educating people on safe-sex practices, Schlecht said. Based on the findings, doctors should encourage their patients who engage in oral sex to use barrier protection, he wrote in an editorial accompanying the study.
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