People who have had more than five oral-sex partners in their lifetime are 250% more likely to have throat cancer than those who do not have oral sex, a new study suggests.

The researchers believe this is because oral sex may transmit human papillomavirus (HPV), the virus implicated in the majority of cervical cancers.

The new findings should encourage people to consistently use condoms during oral sex as this could protect against HPV, the team says. Other experts say that the results provide more reason for men to receive the new HPV vaccine.

Maura Gillison at Johns Hopkins Blo****erg School of Public Health in Baltimore, Maryland, US, and colleagues collected *lo** and saliva samples from the throats of 100 patients diagnosed with cancers of the tonsils or back of the throat. The scientists also took samples from 200 healthy people for comparison.

By c****ining the *lo** and saliva samples with antibody molecules, Gillison's team could tell whether a person had ever had an HPV infection.
Cancer traps

All of the study participants provided information about their sexual history, including the number of people with whom they had engaged in oral sex.

After controlling for other risk factors for throat cancer, such as drinking and smoking, the analysis revealed that people who had prior infection with HPV were 32 times as likely to have this cancer as those with no evidence of ever having the virus. And those who tested positive for a particularly aggressive strain of the virus, called HPV-16, were 58 times more likely to have throat cancer.

By comparison, either smoking or drinking increases the risk of such cancer by about threefold.

The throat cancers analysed in the new study mostly started in the "crypts" of the throat the grooves at the base of the tonsils. This might be because the tonsil grooves trap infectious particles, suggests Mark Stoler of the University of Virginia in Charlottesville, US, who was not involved in the study.
High risk levels

The study also revealed a link between oral sex and throat cancer caused by HPV. People who had one to five oral-sex partners in their lifetime had approximately a doubled risk of throat cancer compared with those who never engaged in this activity - and those with more than five oral-sex partners had a 250% increased risk.

There was an even stronger link between oral sex and throat cancers clearly caused by HPV-16 (those tumours that tested positive for the strain). People with more than five oral sex partners had a 750% increased risk of these HPV-16-caused cancers.

"This study is important because it is putting all of the pieces together," says Gillison. "We need to add oral HPV infection to the list of risks for oral cancer," she adds.
Virus vaccine

A vaccine against several of the most aggressive strains of HPV linked to cervical cancer received approval from the US Food and Drug Administration in 2006. However the plan to vaccinate adolescent girls with this vaccine developed by Merck, called Gardasil, has received some criticism.

There have been no studies investigating whether the vaccine can also protect against throat cancer, but the new evidence linking HPV to throat cancer could lead to broader vaccination with Gardasil. "We will see a push for vaccination in men," says Stoler, who has been involved in the development of the vaccine.

Tonsil and throat cancers affect about two in every 100,000 adults in the US. The new results could promote the development of spit tests for HPV infection to help identify people at high risk for these cancers, researchers say.