Gay teenagers who have had at least four sexual partners are at increased risk of contracting human papillomavirus (HPV), a new study suggests.
At least half of sexually active people get HPV at some point in their lives, according to the Centers for Disease Control and Prevention (CDC).
Previous research has suggested most adult gay men have the sexually transmitted infection. HPV is usually cleared by the immune system but can cause genital warts and anal cancer, as well as cervical cancer among women.
"In this study we found rates of anal infection increased rapidly with increasing numbers of partners with whom they have received anal sex," senior author Marcus Y. Chen said. "The virus is presumably being transmitted from penis to anus."
Chen is an associate professor in the School of Population and Global Health at the University of Melbourne in Australia.
The CDC recommends boys and girls get vaccinated against HPV at age 11 or 12, before becoming sexually active. There are two versions of the HPV vaccine, one of which is available for boys.
The vaccine is very effective if given before a person is exposed to HPV but provides "diminishing protection" after that, Dr. Ross D. Cranston told Reuters Health.
"Thus if there is a high rate of HPV acquisition, as we also see in girls, there is a lost opportunity to provide protection if the HPV vaccine is not given early," he said.
Cranston, who was not involved in the new study, directs the Anal Dysplasia Clinic and Research Program at the University of Pittsburgh Medical Center in Pennsylvania.
Chen and his team tested 200 young gay men age 16 to 20 for HPV and genital warts and gave them a sexual history questionnaire.
One-third of the men tested positive for high-risk forms of the virus, and 11 percent tested positive for two or more forms.
Men who'd ever had vaginal sex or anal sex were more likely to test positive for penile HPV, according to results published in the Journal of Infectious Diseases.
Among men who had never received anal sex, 10 percent tested positive for anal HPV. That compared to nearly half of those who said they'd had at least four anal sex partners.
The finding that some young men who reported never receiving anal sex tested positive for anal HPV suggests the virus can be transmitted in other ways, the authors write.
About 7,000 people in the U.S. will be diagnosed with anal cancer in 2013, according to the American Cancer Society. Rates are higher among gay men than heterosexual people, Chen noted.
Of the many types of the HPV virus, HPV 16 is most commonly associated with anal cancers.
"Our study found that gay male teens acquire the HPV virus including HPV 16 very soon after they first become sexually active," Chen told Reuters Health.
"This means that the HPV vaccine, which has been shown to be effective in preventing HPV infection in males, including anal infection in gay men, needs to be given very early on, preferably before gay teens start to have sex."
Many countries routinely vaccinate all girls against HPV. But as of 2013, Australia is the only one to implement universal and free vaccination of boys at school, Chen said.
"This is great news for boys in Australia including those that are gay but in other countries the absence of such a program means gay males will miss out on anal cancer prevention," he said.
Some gay teens might be reluctant to admit their sexuality and ask for the vaccine, he said.
Gay men are no more susceptible to HPV than heterosexual men, but more often have anal infections, Cranston said.
He said doctors can increase awareness and the likelihood that boys will be vaccinated against HPV through conversations with their parents.