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"This doesn't mean it is present in all (esophageal cancers), but it may be a factor in a certain proportion of cases," said Dr. Surabhi Liyanage, the study's lead author.

HPV is a very common sexually transmitted virus that is known to cause cervical cancer, anal cancer and some cancers of the reproductive organs and the upper throat.

Liyanage, a graduate student at the University of New South Wales in Sydney, Australia, said there's been a lot of debate among researchers about the role of HPV in cancer of the esophagus because most of the studies to date have been small and used disparate methods that make them hard to compare.

According to the National Cancer Institute, nearly 18,000 people - roughly four out of every 100,000 - are diagnosed with esophageal cancer each year in the U.S. and 15,000 Americans die from it annually.

Worldwide, esophageal cancer is the eighth most common cancer and responsible for some 400,000 deaths a year, according to World Health Organization data.

To get a better handle on the relationship between HPV and esophageal cancer, Liyanage and her colleagues gathered results from all of the studies that have compared patients with the cancer to people without it.

The studies focused on esophageal squamous cell carcinoma, one type of esophageal cancer that affects the lining of the esophagus.

In each study, esophageal tissue samples from cancer patients and from patients without the cancer were examined to see if HPV was present.

The 21 studies Liyanage's group analyzed included 1,223 people with esophageal cancer and 1,415 people without cancer.

HPV was found in the esophageal tissue of 35 percent of the cancer patients, compared to 27 percent of the people without esophageal cancer.

Taken together, the studies link HPV infection of esophageal tissue to a three-fold greater risk of esophageal cancer, the researchers report in the online journal PLOS ONE.

If the general population's rate of esophageal cancer is 4.4 out of every 100,000 people, a three-fold risk increase would raise those chances to 13.2 out of every 100,000.

Two vaccines, Cervarix and Gardasil, have been developed to prevent infection with HPV.

Some of Liyanage's co-authors serve on advisory boards for the companies that make these vaccines and have received research funding from them.

If HPV indeed causes esophageal cancer - and that remains to be determined - it's possible that the vaccines could help prevent the cancer, Liyanage said.

"However, this needs to be studied further. The benefits of cancer-preventing vaccines are not seen immediately, but after many years following vaccination," Liyanage wrote in an email to Reuters Health.

U.S. health officials would like for 80 percent of teenage girls to receive the HPV vaccine, but last week they reported that rates of vaccination appear to be lingering closer to 53 percent (see Reuters story of July 25, 2013 here:).

Liyanage said there are also other ways to reduce the risk of developing esophageal cancer, including avoiding smoking and excessive drinking.

SOURCE: bit.ly/15s7YV8 PLOS ONE, online July 24, 2013.