US scientists have said there is strong evidence linking oral sex to cancer, and urged more study of how human papillomaviruses may be to blame for a rise in oral cancer among white men.

In the United States, oral cancer due to HPV infection is now more common than oral cancer from tobacco use, which remains the leading cause of such cancers in the rest of the world.

Researchers have found a 225-percent increase in oral cancer cases in the United States from 1974 to 2007, mainly among white men, said Maura Gillison of Ohio State University.

"When you compare people who have an oral infection or not… the single greatest factor is the number of partners on whom the person has performed oral sex," said Gillison, who has been researching HPV and cancer for 15 years.

"When the number of partners increases, the risk increases," she told reporters at the American Association for the Advancement of Science meeting in Washington.

Previous studies have suggested that people who have performed oral sex on six or more partners over a lifetime face an eight-fold higher risk of acquiring HPV-related head or neck cancer than those with fewer than six partners, she said.

But even though the link between HPV and cervical cancer has been well known for many years, and vaccines now exist to provide some protection, much study remains to be done to confirm observational links and establish causes, Gillison said.

"The cervical cancer field is 20 years ahead," she said.

"We can’t demonstrate definitively that certain behaviors are associated with risk of acquiring an infection," she said.

"The rise in oral cancer in the US is predominantly among young white males and we do not know the answer as to why."

Researcher Diane Harper of the University of Missouri said such studies will take time, but the oral cancer field may move more quickly by using technology already developed for detecting HPV in cervical cancer patients.

"One of the scientific technologies that have evolved over time is the way that we detect HPV," said Harper.

"I think that the head and neck cancer area will benefit from that because we have gone through all kinds of different laboratory techniques to make sure we are actually finding what we think is HPV and getting type-specific information to go with that."

There as many as 150 different types of human papillomaviruses, and about 40 of those can be sexually transmitted, according to the National Cancer Institute.

Some may cause genital warts, while other more high-risk varieties can cause oral, anal, vaginal and penile cancers.

Sexually transmitted HPV infections are common and often asymptomatic, and untreated cases in women are the main cause of cervical cancer.

Half of all sexually active Americans will get HPV at some point in their lives, the US Centers for Disease Control and Prevention has estimated.

Two vaccines, Gardasil and Cervarix, were approved by the US Food and Drug Administration in 2006 for HPV types that cause cervical cancer and genital warts.

However, only 40 percent of US girls have received one dose and just 17 percent have received all three doses in the regimen, said researchers.

A study published earlier this month in the New England Journal of Medicine found that the HPV vaccine could prevent 90 percent of genital warts in men, and the vaccine has also been approved against anal cancer in men and women.

Harper said she was not recommending the general population get the HPV vaccine because research has not yet established its effectiveness past five to eight years for cervical cancer.

"We know from all of the very good modeling studies that have been done throughout the world that if the vaccine does not last for a minimum of 15 years, cervical cancer will not be prevented, it will only be postponed," she said.

For now, Harper and fellow presenter Bonnie Halpern-Felsher of the University of California San Francisco recommended that patients discuss HPV with their doctors.

"If you talk to health care providers and certainly parents and other educators, they are not talking to teens about oral sex, period," said Halpern-Felsher, who has studied teenagers’ attitudes and sexual behaviors.

"Teens really have no idea that oral sex is related to any outcome like STIs (sexually transmitted infections), HPV, chlamydia, and so on."