April 2 (Bloomberg) -- The number of people diagnosed with tonsil cancer has tripled in Sweden since the 1970s due to the rise of a virus that can be spread via oral sex, a study shows.

The cancer, which develops in the part of the throat just behind the mouth, has long been associated with smoking and drinking. Researchers at Sweden’s Karolinska Institute analyzed cells taken from 98 patients with tonsil cancer between 2003 and 2007 and found 83 of them were positive for a strain of the human papillomavirus. In the 1970s, just 23 percent of tonsil cancer cases were HPV-positive, they said.

New studies will determine if GlaxoSmithKline Plc’s Cervarix and Merck & Co.’s Gardasil, vaccines against HPV infections, protect against tonsil cancer and other malignancies in the mouth, researchers said. The shots will be given to girls in Sweden between the ages of 10 and 12 starting next year. Patients with HPV-positive tonsil tumors are generally diagnosed at a younger age and more likely to survive, studies show.

“What we’re seeing today is the result of infections that occurred roughly 20 years ago,” said researcher Tina Dalianis, a professor of tumor virology at the Karolinska Institute, in a statement. “The prognosis is obviously better for the HPV- positive patients, but the treatment is still arduous.”

Traditionally, patients get chemotherapy, radiation, then surgery for the cancer, a triple combination treatment that can carry heavy side effects. People with HPV-positive tumors may not need such aggressive care, Dalianis said in a telephone interview today.

The study, which focused on patients in Stockholm, appears in the International Journal of Cancer. The Swedish Cancer Society funded the work. A report in the May 2007 New England Journal of Medicine found people who had oral sex with at least six partners were three times more likely to get throat cancer as those who were less sexually active.

Tonsil cancer develops in a part of the throat known as the oropharynx, including the back portion of the tongue, the roof of the mouth and the tissue in front and behind the tonsils. Symptoms include difficulty swallowing, sore throat, blood in the saliva and weight loss.

To contact the reporter on this story: Michelle Fay Cortez in London at mcortez@bloomberg.net

Last Updated: April 2, 2009 12:18 EDT