A new study showing an estimated 7% of American teens and adults carry the human papillomavirus in their mouths may help health experts finally understand why rates of mouth and throat cancer have been climbing for nearly 25 years. The evidence makes it clear that oral sex practices play a key role in transmission.
The new data, published online Thursday by the Journal of the American Medical Assn., are the first to assess the prevalence of oral HPV infection in the U.S. population. The findings 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, which many Americans mistakenly view as a safe practice.
“There is a strong association for sexual behavior, and that has important implications for public health officials who teach sexual education,” said Dr. Maura L. Gillison of the Ohio State University Comprehensive Cancer Center, who led the study and presented the findings Thursday at a meeting of head and neck cancer researchers and doctors in Phoenix.
Though herpes, HIV and other diseases can be transmitted via oral sex, the practice is often considered a safer alternative to sexual intercourse. A survey released last year by theU.S. Centers for Disease Control and Prevention found that about 90% of adults have had oral sex, along with 27% of 15-year-old boys and 23% of 15-year-old girls.
“I don’t think people think of oral sex in the same way they do with traditional intercourse,” said Fred Wyand, director of the HPV Resource Center at the American Social Health Assn. in Research Triangle Park, N.C. “Sometimes younger people engage in oral sex so they don’t have to worry about pregnancy. They may not even make the link between oral sex and STDs.”
Suspicion among researchers that the behavior could cause oral cancers by transmitting HPV to the mouth has been mounting over the last decade. Initial studies found that patients with oral cancer were far more likely than healthy controls to have engaged in oral sex. And a 2007 study in the New England Journal of Medicine found that the more oral sex partners a person has had, the greater their risk of developing throat cancer.
Most oral HPV infections are harmless, and oral cancers are still relatively uncommon. But given the new information, doctors should encourage their patients to use protection during oral sex, Dr. Hans Schlecht, assistant professor of medicine at Drexel University College of Medicine in Philadelphia, wrote in an editorial accompanying the study.
“It’s something people are not comfortable talking about, but it is protective,” he said in an interview. “If you are going to be intimate with someone, there are some adult conversations you need to have.”
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% 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% 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.
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 survey includes a detailed questionnaire and a physical examination, including the first large-scale use of a 30-second oral rinse from which researchers were able to extract cells to test for HPV infection. The test, which can detect the virus in the mouth as accurately as in the cervix, was 10 years in the making.
Gillison’s team found that the overall prevalence of oral HPV was 6.9% — far less than the rate of genital HPV infection in reproductive-age women, which can be as high as 42% among women in their 20s.
The infection rate varied substantially among different groups. For instance, 10.1% of men in the study had oral HPV, compared with 3.6% of women. The reason for the difference is unknown but it could have to do with oral sex practices, Gillison said.
Among people who had more than 20 sexual partners, the prevalence of oral HPV was 20%. But the researchers found it in fewer than 1% of people who said they were virgins and in fewer than 4% of people who said they had never performed oral sex.
Researchers also noted age differences: Those in their early 60s had the highest prevalence at 11.4%. 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 grew up during an era of sexual permissiveness that preceded public health messages about safe sex.
“People who came of age during the sexual revolution may have had more sexual partners than other age groups, such as groups that came of age during the HIV epidemic,” 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% 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% 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, Schlecht said.
It’s unclear whether the HPV vaccine will protect against oral cancers. That question that could take years to answer, experts said.
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.”