Asymptomatic anal HPV infection more common than thought in heterosexual men

Anal infection with human papilloma virus (HPV) is more prevalent in heterosexual men than previously thought, according to a US study published in the June 15^th edition of the Journal of Infectious Diseases.

As with genital HPV, previous estimates of prevalence of anal HPV have varied widely. Although some studies have reported between 1.2% and 8% prevalence in asymptomatic men who report no sex with other men, prevalence of anal HPV was found to be 46% in a 2003 cross-sectional study of 50 HIV-positive heterosexual male injection drug users with no reported history of anal intercourse.

US investigators studied a cohort of men aged 18-40, recruited from both the community and sexual health clinics, who reported having sex with a woman in the preceding year. Participants completed a questionnaire including potential risk factors, and researchers used rigorous sampling techniques to check genital and anal sites for HPV. The infection was detected by PCR to identify DNA and then genotyped for 37 types of which 13 are associated with cancer. Any HPV not fitting these types was labelled as unclassified.

Out of the study population of 463 men, 253 either admitted sex with other men or did not answer the question, and were excluded from this analysis. The remaining 222 men were divided into a case group of men with anal HPV and a comparison group of men with or without genital HPV only. Men with only unclassified HPV at anal sites were also excluded.

Anal HPV was found in 24.8% of men, and 33% of these men had infection with a strain of HPV that has been linked with an increased risk of anal cancer. Although nine men had visible warts or lesions at genital sites none of the participants had these signs at anal sites.

In a multivariate model, lifetime number of female sexual partners (odds ratio 3.66, 95% CI 1.06-12.62, for 11-20 compared with 1-5 partners) and frequency of sex in the preceding month (OR 3.20, 95% CI 1.03-14.63 for 2-4 times compared with 0-1 times) were independently and significantly associated with presence of anal HPV.

However, participants who reported a greater frequency of sex in the past month were found to be not at increased risk of anal HPV, which might suggest that they were in a monogamous relationship. The analysis also found that circumcision was marginally associated with a reduced risk of anal HPV.

These findings parallel research in women and in gay men and other men who have sex with men. However in gay men, anal HPV affects all age groups equally, whereas the current study found that the prevalence of anal HPV is lower in those aged 30-40 compared with younger men.

Although this is the largest study of its kind, the authors acknowledge that the study was not large enough for a clear understanding of the full complement of risk factors. They also acknowledge the possibility of false-positive results; that DNA might have been detected from the skin surface without true infection; and the possibility that some participants may not have reported sexual behaviours with other men.

They suggest that future studies should collect sexual and non-sexual exposure information that may clarify transmission routes – for example, behaviours that involve self- or partner-initiated anal-digital contact.

The authors conclude that the findings should not be used to change clinical practice currently, but further research is necessary, especially to determine factors linked with the persistence of cancer-causing anal HPV in younger and older men.