Contrary to what’s traditionally been believed, anal infection with the virus that causes genital warts is common in heterosexual men, a new study confirms.

Until recent years, there had been little interest in studying the prevalence of anal infection with the human papillomavirus (HPV) in men who have sex exclusively, or primarily, with women - the belief being that they were unlikely to harbor an anal infection.

However, in the new study, researchers found that among 902 men from the U.S., Mexico and Brazil, 12 percent had an anal HPV infection. All had had sex exclusively with women in recent months, though some said they had had sex with a man in the past - ranging from 6.5 percent of the U.S. men to 17 percent of the Brazilian men.

The findings confirm those of an earlier, smaller study suggesting that anal HPV infection is common among men who have heterosexual sex, said lead researcher Dr. Alan G. Nyitray, of the Moffitt Cancer Center and Research Institute in Tampa, Florida.

The precise public-health implications are not yet clear, however, he told Reuters Health in an email.

There are more than 100 strains of HPV, some of which cause genital or anal warts. In most people, the immune system clears the infection fairly rapidly. However, persistent infection with certain HPV strains can eventually lead to cancer in some cases.

HPV is best known as the primary cause of cervical cancer, but it can also cause certain other tumors, including anal cancer. Anal cancer is relatively rare - being diagnosed in about 5,300 Americans per year - but its incidence has been on the rise in the U.S. for several decades.

The implications of the current findings are unknown, in part, because the men were tested at one time point. So it’s not clear how many may have had or may develop persistent HPV infection.

“Persistence is a critical step in the HPV (cancer-causing) process that may lead to anal cancer,” Nyitray said. He and his colleagues are now conducting a study that is following men over time to try to shed light on the persistence of anal HPV infection in men.

In the meantime, the current findings “should put to rest the myth that only men who have had anal sex can have anal HPV infection,” writes Dr. Stephen E. Goldstone, of Mount Sinai School of Medicine in New York, in an editorial published with the study.

The study included 902 men between the ages of 18 and 70 from Tampa; Sao Paulo, Brazil; and Cuernavaca, Mexico. Overall, 12 percent had a current anal HPV infection, with the prevalence being similar in each country; genetic testing showed that 7 percent of all men carried a cancer-related HPV strain.

Certain factors appeared to raise a man’s risk of infection. Men who had had 10 or more female sex partners in their lives had a three-fold greater risk than those who’d had one or two partners. And men who’d ever had sex with another man were twice as likely to have an anal HPV infection as those who reported only female partners.

However, both Nyitray and Goldstone say the findings highlight how very common HPV appears to be among sexually active people in general.

“We must stop viewing HPV infection as a disease that only promiscuous males and females acquire,” Goldstone writes.

“I think one message (of the findings) is that HPV is just very, very common - part of the human condition,” Nyitray said.

He added that the study also “points out that we have much to learn about HPV, and some of it may be very surprising.”

As for how men who have only heterosexual sex may acquire anal HPV, one possibility, according to Nyitray’s team, is that the virus is transmitted via their female partners’ hands. It’s also possible that the virus was originally present in the men’s genitals and was passed on to the anal canal from there, they say.

Two vaccines against HPV have been developed - Merck’s Gardasil and GlaxoSmithKline’s Ceravix. In the U.S., Gardasil was initially approved only for girls and young women between the ages of 9 and 26. Last year, the Food and Drug Administration approved it for boys and men in the same age group.

The current findings, according to Goldstone, support the notion that HPV is an easily transmitted, and probably “unavoidable,” infection in sexually active people.

“We must destigmatize this infection and understand how important prophylactic vaccination may be,” he writes.

Goldstone is the principal investigator for Merck’s Gardasil in Men clinical trial, and serves on the company’s advisory board for the product.

SOURCE: Journal of Infectious Diseases, May 15, 2010.