An international team of investigators have found a possible explanation for the higher rates of anal cancer observed in men who have sex with men (MSM) compared to men who have sex with women (MSW). In the December 1st edition of the Journal of Infectious Diseases, researchers from the HIM study report that anal human papilloma virus (HPV) infections were significantly more likely to persist in MSM than MSW.
“Compared with MSM, we consistently observed a lower prevalence, incidence, and persistence of anal HPV among MSW,” comment the investigators.
Cigarette smoking was independently associated with the persistence of the infection in MSM.
Anal cancer can be caused by persistent infection with certain strains of HPV. Overall rates of anal cancer in men are low (approximately 1 per 100,000, but are significantly higher in men who have sex with men (36 per 100,000 before the HIV epidemic).
Prevalence of anal HPV infection alone does not appear to be a sufficient explanation for these different rates of cancer. For instance, observational studies suggest that prevalence of the infection is only four times higher in MSM compared to MSW (47% vs. 12%).
Investigators from Brazil, Mexico and the US hypothesised that the explanation for the higher rates of anal cancer in MSM was the greater persistence of anal HPV infections in MSM compared to more transient infections in MSW.
They therefore designed a prospective observational study involving 156 MSM and 954 MSW. None were HIV-positive.
The patients were screened for HPV infection at baseline and again after six months. Individuals were also asked to complete a questionnaire enquiring about their demographics, sexual behaviour, smoking habits and substance use.
Baseline screening found a similar prevalence of anogenital warts in MSM and MSW (5% vs. 6%). However, MSM were significantly more likely to have anal infection with the cancer-associated HPV-16 strain (10% vs. 3%).
At the six-month follow-up visit, incidence of new HPV-16 infections was 6.5 times higher in MSM than MSW (5 per 1000 person months vs. 0.7 per 1000 person months).
Analysis individuals with type-specific anal HPV infection at baseline showed that these infections persisted in 32% of MSM but in only 4% of MSW.
Moreover, MSM were more likely to experience persistence of infection with multiple cancer-associated HPV strains than MSW (16% vs. 2%, p < 0.001).
A total of eleven MSM had anal HPV-16 infection at baseline and three (27%) had cleared the infection by the six-month visit. In contrast all 21 MSW with anal HPV-16 at the start of the study had cleared the infection at the time of follow-up.
“These findings support our original hypothesis that anal HPV would be transient among MSW and more persistent with MSM,” write the authors.
Cigarette smoking significantly increased the risk of anal HPV persistence for MSM (PR = 1.73; 95% CI, 1.19-2.50).
“The association between smoking and anal carcinogenesis is plausible, given the potential for immune dysregulation and increased DNA mutations in anogenital epithelium,” comment the researchers.
They conclude: “Data in the current study begin to illustrate starkly different natural histories of anal HPV among MSM and MSW that helps explain the disparate anal cancer incidence among these groups observed in Western countries.”
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