MONDAY, Aug. 27 (HealthDay News) -- Human papillomavirus (HPV), which is believed to be responsible for most cervical cancers, may also be at the root of many cancers of the mouth and throat, new research suggests.

Although the rate of most head and neck cancers has been declining over the past 30 years because more people have stopped smoking, the rate of certain cancers in the throat and mouth hasn't dropped, according to research published in the Aug. 27 online issue of Cancer.

"Smoking prevalence has dropped dramatically, and, likewise, most head and neck cancers have declined in incidence. Cancers at the base of the tongue and tonsil are increasing or have remained stagnant. We're not seeing the reduction in incidence that we would have expected," said study author Dr. Erich Sturgis, an associate professor of head and neck surgery and epidemiology at the University of Texas M.D. Anderson Cancer Center, in Houston.

The study authors suspect the reason may be orally transmitted HPV infections.

"Just as cervical cancer is the outcome of a sexually transmitted disease, as are most anal and penile cancers, people need to be aware that they can get throat or tongue cancer as the consequence of a sexually transmitted disease," said Sturgis. "Oral sex can't be considered safe sex."

Head and neck cancers aren't common; they account for about 3 percent of all cancer cases in the United States, according to the study. Each year, there are about 45,000 new cases of head and neck cancers. Cigarette smoking and excessive alcohol consumption have long been considered the most significant risk factors in the development of these cancers, said Sturgis. Between 80 percent and 90 percent of head and neck cancers can be attributed to tobacco or alcohol use.

In 2005, about 21 percent of Americans were smokers, down from 42.5 percent in 1965. The decline in smoking prevalence began in the 1970s, and about 10 to 15 years later, the incidence rates of most head and neck cancers began to decline, according to the study.

But the rate of some of these cancers didn't go down as quickly as others, and, in the case of cancers of the tongue, the rate has gone up. In 1995, there were fewer than 6,000 cases of tongue cancer. By 2005, that number was more than 8,000, according to the study.

Previous research has found that up to 50 percent of nonsmokers with throat and mouth cancers were infected with HPV, according to the study.