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 papillomavirus (HPV) infections were significantly more likely to persist in MSM than
“Compared with MSM, we consistently observed a lower
prevalence, incidence, and persistence of anal HPV among MSW,” comment the
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
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
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 of individuals with type-specific anal HPV infection
at baseline showed that these infections persisted in 32% of MSM but in only 4%
Moreover, MSM were more likely to experience persistence of
infection with multiple cancer-associated HPV strains than MSW (16 vs 2%, p
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
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.”
However, as only a small number of MSM were enrolled in the
study they call for further research to validate their findings.