This study demonstrates that HPV is efficiently transmitted between sexual partners and that multiple transmission events can occur within a couple. The rates of genital transmission from women to men were substantially higher than from men to women. Greater rates of female-to-male transmission should imply higher HPV prevalence in men. Studies in men to date, including our own cohort, have reported male genital HPV prevalences at least as high as in women, with most reporting prevalences of at least 20% and up to 73% (9
). The penis shaft was the primary source of transmission to the cervix; the cervix and urine were the primary sources of infection to male genitals.
Sexual transmission also involved the scrotum, the anus of women, and the hands of both sexes. The oral cavity and semen were not involved in transmission.
The anus of women was both a major source and target of heterosexual transmission. We observed consistency between penis-to-female anus transmission and reported anal intercourse during the corresponding period. We previously demonstrated high genotypic concordance between concurrent cervical-anal infections in women, which indicates possible common sources of infection (20
Transmission through nonpenetrative sexual contact was demonstrated between the female anus and the scrotum, as well as the female hand and male genitals. The male anus was not a major source or a target of HPV transmission. However, 3 of the 4 couples with baseline infection in only 1 partner involved anal infection in men.
Male self-transmission frequently involved the scrotum, likely facilitated by passive contact between proximate genital sites. The scrotum may be an important reservoir of infection for penile infections that can subsequently be transmitted to partners. Hands may also serve as reservoirs of infection in both men and women. Auto-inoculation involving the hands may result from casual contact or masturbation.
To some extent, our study results suggest that HPV is relatively indiscriminate in its patterns of transmission. We observed the transmission of a given viral genotype to multiple anatomic sites in a partner and concurrent transmission of multiple genotypes to the same site.
Other observations suggest that HPV transmission is not entirely arbitrary and may reflect tissue or genotype differences or both. Rates of transmission of oncogenic types to the male genitals from the urine were higher than from the cervix. This may reflect differences in genotypes found in the vagina and vulva compared with those found in the cervix. All transmission events requiring extended periods of exposure involved the female anus target site.
A total of 15 genotypes were transmitted, including 13 which were transmitted through heterosexual means. HPV 16 and 18 and nononcogenic HPV 6 and 11, the 4 types included in the current quadrivalent vaccine, comprised <10% of transmitted types. Notably, we observed greater transmission of HPV 16 to the cervix than such transmission by other types, which underscores the possibility of selective transmission of some HPV types.
Compared with couples not experiencing HPV transmission, transmitting couples were more sexually active and were more likely to use certain nonbarrier forms of contraception. Few HPV-transmitting couples reported always using condoms during recent sexual activity, compared with over half of nontransmitting couples.
A major limitation of our study was the potential for misclassification of HPV transmission events. Variable detection of HPV could be due to natural fluctuation in viral levels or variable sampling of sites could confound the observation of viral transmission. For example, instances of apparent reinfection of sites may alternatively represent possible reactivation of latent infections.
Another potential source of misclassification was the lack of a priori knowledge of the time required for HPV to be acquired from an infected partner. Viral transmission could have occurred more frequently than the 2-month visit intervals used in the study, and transmission events could have been missed.
Another limitation of the study was the inclusion of couples who had already had sexual contact with one another; initial viral transmission was likely to have occurred before study entry. Indeed, nearly half of the couples had type-specific concordant infections at study entry, indicating previous transmission of HPV, which limited our ability to evaluate incident infections.
Because our study relied on self-reported sexual activity, it was subject to recall bias. Furthermore, although all persons reported monogamous relationships, some of the incident infections without a source, most of which involved the male genitals, could have been acquired through sexual activity with another partner. Despite these limitations, the present study included intensive follow-up of a well-characterized cohort, sampling of multiple genital and nongenital sites, and state-of-the-art HPV testing and genotyping methods.
The development of comprehensive HPV prevention and control strategies, which incorporate HPV vaccine usage and contraceptive practices, is impeded by lack of information on the risk and routes of sexual transmission between heterosexual partners and potential genotype-specific differences in transmission efficiency. The small size of the cohort and the diversity of genotypes precluded type-specific analysis of transmission.
This study contributes to a growing body of knowledge of HPV in men because we directly examined HPV transmission. However, study results are preliminary and need to be verified in larger cohorts. Future HPV transmission studies are critical to address major gaps in our knowledge of the natural history of this virus.