The human papillomavirus is contributing to the growing number of head and neck cancers in the United States, according to a new study Monday in the Journal of Clinical Oncology.

The study found that the number of cases of oropharyngeal cancer - cancers of the tonsil, back of the mouth (throat) and base of the tongue - has been on the rise since the mid-1980s. The study suggests that one reason could an increase in the number of people having oral sex resulting in oral human papillomavirus exposure.

Researchers say these cancers fall into two categories–those caused by tobacco and alcohol and those caused by the sexually transmitted virus, HPV. They now believe approximately 70% of all oropharyngeal cancers are caused by HPV infection.

"We used to think of oropharyngeal cancer as one cancer," said senior author Dr. Maura Gillison, The Ohio State University Comprehensive Cancer Center in Columbus. "Now we know the disease is comprised of two biologically and epidemiologically distinct cancers. This new understanding will increasingly enable us to improve and better personalize care for patients with each form of the disease."

Researchers tested cancer tissue samples from almost 6,000 patients in Hawaii, Iowa and Los Angeles between 1984 and 2004. They found the HPV-positive cancers increased 225% while HPV-negative oropharynx cancers dropped 50%–most likely because of a reduction in smoking and tobacco use. Even so, patients with HPV-positive cancers live longer.

"Patients with HPV positive cancers have better survival rates," said principal investigator Dr. Anil Chaturvedi of the National Cancer Institute. "The precise reasons for the survival benefits are not clear, but tumors in HPV-positive patients tend to have less genetic damage. Because of that, they are more responsive to cancer therapies like radiation treatment."

"The HPV status of a patient's tumor is the single greatest determinant of whether a person lives or dies after a diagnosis of local-regionally advanced oropharynx cancer," Gillison said. "HPV-positive patients have an approximate 60% reduction in risk of death after their diagnosis when compared to HPV-negative patients."

Gillison says about 95% of the HPV-positive oropharynx cancers were caused by HPV16, a strain targeted by Gardasil and Cervarix, the two vaccines currently on the market to prevent cervical cancer. Gardasil is manufactured by Merck & Co. and Cervarix is made by GlaxoSmithKline.

Gardasil was approved in 2006 for use in females ages 9-26 to help prevent cervical cancer and genital warts. It was approved in 2009 for males9-26 to reduce the risk of genital warts. It has not been recommended for use in preventing HPV-associated cancers, according to the National Cancer Institute. Cervarix was approved in 2009 for use in females 10-25. It is not approved for males.

The findings are important because the increase in these cancers has been relatively rapid compared with other HPV associated cancers like cervical cancer, Chaturvedi says. With cervical cancer, incidence rates have declined over time because of screening, but currently there is no screening in place for oropharyngeal cancers.

"Should the observed declines in cervical cancer and the observed increases in HPV positive oropharyngeal cancers continue into the future," Chaturvedi said. "HPV positive oropharyngeal cancers will be the leading HPV associated cancers over the next decade–by around 2020."

There were almost 35,000 new cases of oral and oropharyngeal cancers this year, according to the American Cancer Society. While about 7,000 Americans will die this year, death rates associated with these cancers has actually been declining over the last 30 plus years.

"The burden of invasive HPV-caused cancers will shift from women to men in the US, largely due to the rise of HPV-positive oropharynx cancers among men," Gillison said. "HPV infection and its consequences have long been considered a women's problem, and women will continue to bear the brunt of the morbidity associated with the infection. However, gender equity is being achieved with regard to the burden of HPV-caused cancers. HPV infection is therefore a problem for both men and women."