We observed, in the largest study to date, that oral HPV infection is present in a subset of healthy men (~4%) and that oral HPV16 infection, a risk factor for oropharyngeal cancer, is rare in our cohort (<1%). However, oral HPV infection was present in 6% of healthy men over the age of 55 years. This finding is in contrast to what is typically observed at the cervix, where cervical HPV infection decreases with increasing age into the 4th
decades of life throughout most of the world (13
) but is similar to the epidemiology of HPV infection at the anus (14
) and the penis (16
), where HPV prevalence remains constant across the age span. Because these were prevalently detected infections, a function of both incidence and duration, it is not possible to determine whether new infections were either occurring more commonly or were more likely to persist in older age groups, thereby creating the observed age-effect. Immune waning over the aging process could explain both increased acquisition and persistence at older ages, but does not account for the differences between the trends observed for oral compared to cervical HPV infection. Instead, it may be that immune surveillance differs at different anatomic sites and perhaps other co-factors—such as oral hygiene and microbiome (19
)-- may play an important role in the immune response to HPV infection in the oral region. Oral HPV natural history is not well studied or understood. More research is needed that evaluates rates of HPV persistence across multiple anatomic sites within a cohort.
Current tobacco use was significantly associated with detection of oral HPV infection and was the factor most strongly associated with oral HPV infection; a finding previously shown in another study of oral HPV infection among healthy individuals (20
). Cigarette smoking alters a wide range of immunological functions in the oral cavity, including adaptive and innate immune responses (21
). It may be that current tobacco use inhibits immune function in a way that allows for increased HPV persistence in addition to the direct genetic damage to cells; persistence would in turn increase the opportunity for detection cross-sectionally. From the cervical literature, tobacco smoking consistently increases the duration of HPV infections and the risk of progression to cervical precancer and cancer, even after controlling for the effects of HPV infection (22
). This effect may be more profound in the oral cavity, where the mucosal epithelium is directly exposed to the carcinogens in tobacco compared to the indirect path to the cervical mucosa (although the 2-fold elevation in risk of infection is comparable to what is reported in the cervical cancer literature). Prospective studies of oral HPV infection that investigate acquisition and persistence can best address the association between tobacco use and the natural history of oral HPV infections.
Performing oral sex on a partner, and other sexual behaviors, did not appear to play a significant role in detection of prevalent oral HPV infection. When we stratified by sexual practices, men who had sex with men had no cases of oral HPV infection, whereas men who have sex with both men and women had non-significantly higher oral HPV infection than men who have sex only with women. Despite the hypothesis that HPV is transmitted to the oral region via sexual behaviors, there has been reporting by some (9
) but not others (6
) of an association between oral sex and oral HPV infection among HIV-negative individuals. In our study, it may be that individuals are mis-reporting their sexual behaviors, although previous work in this cohort looking at sexual behaviors and risk of penile and anal HPV infections show clear associations and therefore good internal validity of the survey instrument. In addition, the reliability of reporting sensitive sexual behaviors has been demonstrated for this questionnaire (25
). Behaviors not presently queried, such as kissing (20
), may be more relevant to the transmission of HPV infection in this setting.
To our knowledge, this is the first study to systematically collect and test oral specimens from healthy men from multiple countries. The epidemiology of oral HPV infection appeared quite similar across the three countries. Specifically, HPV16 and other carcinogenic HPV infections were similarly prevalent in each country, and the associations between age and tobacco held for each country. However, there was an unexpectedly high prevalence of HPV55 in men from Mexico (18 of the 19 HPV55 infections detected), which explains the significantly higher overall HPV prevalence observed for Mexico compared to the US and Brazil. HPV55 is in the α10 clade, which contains non-carcinogenic HPV types such as HPV6 and 11 (26
). Although contamination at either the clinic or laboratory cannot be definitively ruled out, recent data presented by Chaturvedi et al
showed a similarly high prevalence of this previously unreported HPV type in oral specimens collected from HIV infected men and women (27
). It may be that in the changing landscape of the HPV epidemic, types not previously reported as having infected the oral region will be detected there in the future.
Finally, co-infection with multiple HPV types among men with oral HPV infection was a rare phenomenon (5%) compared to that which was observed at male external genital (26%) (9
) and anal (39%) (14
) samples from the same cohort or cervical (32%) samples from the published literature (28
). Multiple HPV types may be less likely to infect the oral mucosa compared to genital epithelia as a function of the rarity of any type of HPV infection. Alternatively, immune surveillance in the oral region may contribute to an overall lower prevalence as well as fewer multiple infections.
One important limitation to note for this study is the generalizability of our findings to other populations. While men from Mexico and the US were recruited from either the general population or factories, specialized populations were also targeted, such as college students and military recruits. Further, men from Brazil were enrolled from a clinic specializing in the diagnosis and treatment of sexually transmitted diseases and HIV/AIDS; although enrolled men were presenting for non–sexually transmitted infection–related conditions, we cannot dismiss the fact that these individuals were at a different risk of oral HPV infection compared to the population at large. It was reassuring to note that despite the differences in the populations by country, the country-specific oral HPV prevalence as well as the associations with age and behavior variables were consistent across countries.
HPV16 is one of the most common sexually transmitted infections detected in the anogenital region (29
) yet is relatively rare in the oral region. It remains unknown why oral HPV infection is rare especially considering the similarity of the mucosal epithelium in the oral and anogenital regions—it may be partly explained by differences in specimen collection, in that oral specimens are often collected in a large volume (via oral rinse), thereby diluting the HPV DNA; this may be compounded by lower viral load in oral specimens despite the sensitive PCR-based assay used for oral HPV detection. Alternatively, it could be a function of exposure, although in this population, oral sexual contact was common. Finally, it may be that the oral region is resistant to this infection. Understanding the intervening steps in the natural history of oral HPV to cancer in the oropharynx is important; specifically, the rates of clearance and progression will provide insight into the carcinogenic process. This is especially timely in light of recent reports suggesting that the incidence of oropharynx cancer is increasing (30