A first-of-its kind study released Thursday estimates that about one in 14 Americans carries in their mouths and throats a sexually transmitted virus that can cause a virulent form of cancer.

The study, published in the Journal of the American Medical Association, has broad implications for head and neck cancers, which already strike 50,000 people a year and are increasingly being caused by human papillomavirus (HPV).

The new study finds that the “most prevalent” HPV strain found in people’s mouths is HPV-16, a type that is particularly likely to cause cancers, Dr. Maura L. Gillison and her colleagues said.

An estimated 2.13 million Americans have an oral HPV-16 infection, the authors said in their study, published in the Jan. 26 issue of JAMA’s Online First edition. Men are almost three times as likely as women to have oral HPV, with 10.1 percent of men and 3.6 percent of women testing positive for it.

While most HPV infections do not cause cancers, whether of the head/neck or the cervix, oral cancers are particularly dangerous once they develop, and doctors until now had not looked at HPV as a cause and ascribed most such cancers to tobacco use.

Oral HPV is almost always transmitted through sexual activity, added Dr. Gillison, who is associated with Ohio State University Comprehensive Cancer Center. Transmission “by casual, nonsexual contact is likely to be unusual,” she and her colleagues wrote.

Dr. Paul Harari, department chairman of human oncology at the University of Wisconsin School of Medicine and Public Health, said the new study is “very fascinating new work.”

“I’ve been involved in the care of head/neck cancer patients for 25 years, and it is only in the last five to seven years that we are realizing that HPV is associated with a substantial proportion” of these cancers, Dr. Harari said.

Head and neck cancers, which involve the mouth, lips, tongue, throat and voice box, have about a 50 percent mortality rate, Dr. Harari said. Oropharynx cancer, which pertains to cancers in the back part of the mouth, the tonsils and skin down to the middle part of the throat, is fatal for about 8,000 people a year.

In contrast, cervical cancer, virtually all of which is caused by HPV, kills about 4,300 women a year.

In their new study, Dr. Gillison and her colleagues note that the number of cases of HPV-caused oropharyngeal cancers have been steadily rising for several years, but “little is known about the epidemiology of oral HPV infection.”

For decades, oropharyngeal cancers were linked to chronic alcohol use, smoking and use of chewing tobacco.

But these kinds of cancer cases have been dropping, from 2.0 cases per 100,000 population in 1988 to 1.0 cases per 100,000 in 2004.

At the same time, HPV-related oropharyngeal cancers have jumped 225 percent, from 0.8 cases per 100,000 to 2.6 cases per 100,000, the new study found.

The data comes from oral-fluid samples collected from some 5,000 people ages 14 to 69 as part of the National Health and Nutrition Examination Survey.

They found that infections were associated with being a smoker, a heavy drinker, and current or past marijuana use. Also, because oral HPV transmission is spread “predominantly” through sexual activity, prevalence rose with the number of lifetime or recent sex partners.

An ongoing study should help clarify how oral HPV is transmitted. There are questions, for instance, about whether “deep kissing” can spread the virus, Dr. Gillison said. While HPV-16 is not necessarily easier to acquire than other strains, it is more likely to develop into cancer.

There are HPV vaccines on the market, but Dr. Gillison and her colleagues said that the impact of the shots on oral HPV infections are unknown, “and therefore vaccination cannot currently be recommended for the primary prevention of oropharyngeal cancer.”

However, because cases of HPV-related oropharyngeal cancers are expected to surpass HPV-caused cases of cervical cancer by 2020, “perhaps such vaccine trials are warranted,” they added.

In a related editorial in JAMA, Dr. Hans P. Schlecht noted that the 6.9 percent prevalence of oral HPV infections is still “much lower” than HPV infections at other bodily sites - cervicovaginal HPV in women is 42 percent; penile/scrotal HPV in men ranges from 14 percent to 51 percent; and anal HPV ranges from 42 percent to 57 percent in homosexual men, 25 percent in heterosexual men and 27 percent in women.

In light of the new study, health-care professionals “should encourage their patients who engage in oral sex to use barrier protection,” Dr. Schlecht advised.

The study, released at Thursday’s Multidisciplinary Head and Neck Cancer Symposium in Phoenix, is supported by the Ohio State University cancer center, HPV-vaccine-maker Merck & Co., John and Nina Cassils, and Intramural Research Program of the National Cancer Institute.

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