Study Participants

The study was conducted at the University Health Services of the University of Hawaii at Manoa from February 2005 through November 2006. Promotional efforts included flyers and email invitations, which were part of larger ongoing studies of HPV. The study was approved by the Committee on Human Studies of the University of Hawaii. All study participants provided written informed consent. Eligible participants were at least 18 years of age, English-speaking, not currently pregnant, and in a monogamous relationship with the index partner.

Specimen Collection

Couples attended concurrent study visits at 2-month intervals. Trained clinicians collected exfoliated cell samples for HPV DNA detection. For men, separate genital specimens from the penis glans/corona, penis shaft, scrotum, and inner foreskin (uncircumcised men) were collected by using textured paper and a saline-moistened swab (8,9). Anal canal specimens were collected by using a saline-moistened swab. A cytobrush was used to collect oral specimens (buccal cavity, tongue). Specimens from the dominant hand (palm, fingertips, under the fingernails) were collected by using a saline-moistened swab. Participants self-collected first-catch urine samples (30 mL) at the clinic. Using latex gloves, men collected semen specimens at home during masturbation within 24 hours of each visit.

For women, a cervical cytology (Papanicolaou [Pap]) smear was collected, and a swab and cytobrush were used to consecutively sample the ectocervix and endocervix, including the transformation zone. The same methods used for collection of anal, oral, hand, and urine specimens from men were used for collection of specimens from women.

HPV DNA Testing and Genotyping

DNA was extracted from specimens by using commercial reagents (QIAGEN, Valencia, CA, USA). The PCR used PGMY09/PGMY11 primers to amplify a 450-bp region of the L1 HPV genome (10). Amplification of the human β-globin gene was included as an internal control for sample sufficiency. HPV-positive specimens were subsequently genotyped by using commercial reagents (Roche Molecular Systems Inc., Branchburg, NJ, USA) originating from a prototype line blot assay (11). The assay detects 37 different HPV types, including oncogenic/probable oncogenic types (HPV 16, 18, 26, 31, 33, 35, 39, 45, 51, 52, 53, 56, 58, 59, 66, 68, 73, 82, and IS39, a subtype of HPV 82), nononcogenic types (HPV 6, 11, 40, 42, 54, 61, 70, 72, 81, and CP6108, also known as candidate HPV 89), and types with undetermined risk status (HPV 55, 62, 64, 67, 69, 71, 83, and 84) (12,13). This PCR-based assay has demonstrated a high degree of sensitivity and reproducibility (14–16). HPV testing and genotyping procedures have been given in detail previously (9).

Statistical Analysis

The main objective of the statistical analysis was to evaluate HPV transmission between partners. Each type-specific HPV infection was assigned a status of transmitted or not transmitted by time period and anatomic site. For some statistics, the penis anatomic site was further divided into foreskin, glans, shaft, and urine, which is a proxy for urethral infection. HPV in female urine, which has demonstrated type-specific concordance with cervical measurements (17), was considered to be a proxy for cervical and other lower genital tract infections. Therefore, cervix and urine were combined as 1 anatomic site. When there were >1 possible source sites, sites were grouped and evaluated as 1 transmission event. When there were >2 destination sites, each was counted as a separate transmission event.

Partner transmission was defined as the presence of a specific HPV genotype at an anatomic site in 1 partner and its absence in all sites of the other partner at a given visit, along with the presence of this HPV type in the unaffected partner at the subsequent visit. Auto-inoculation was defined as the presence of a particular HPV type at an anatomic site in 1 partner and its absence in all sites of the other partner at a given visit, and the presence of this HPV type in a different anatomic site in the affected partner at the subsequent visit. An event was defined as self-inoculation only after possible transmission from the partner was ruled out, i.e., when the partner was negative for the HPV genotype at prior and concurrent visits.

The rate of HPV transmission was calculated as the number of HPV type-specific transmission events divided by the number of person-months of exposure × 100 and expressed as the rate per 100 person-months. Exact confidence intervals (CIs) for transmission rates were calculated by assuming a Poisson distribution for the number of events (18). Person-months of exposure for each HPV infection by anatomic site were computed based on the period of time between successive visits. When HPV was detected at a given visit and HPV type was absent at the successive visit, the exposure period was estimated at half of the visit interval. Comparisons between couples by transmission status were made by using the t test, χ² statistic, and linear rank statistic.

Study Participants

Thirty-eight couples (38 men, 38 women) were enrolled. Six couples left the study, including 2 whose relationships ended. The present analysis focuses on 25 couples with at least 2 visits. Couples were followed up at ≈2-month intervals over an average of 7.5 months.

The mean age was 28 years (range 18–59 years) for men and 26 years (range 18–57 years) for women. Participants comprised Caucasians (52%), Asians (8%), Pacific Islanders (6%), and persons of other single- or mixed-race backgrounds (34%). Couples were single/never married (48%), living together as married (32%), separated/divorced/widowed (12%), and married (8%). All participants indicated that the index partner was their only current sexual partner, and all reported vaginal intercourse with penile penetration during the follow-up period.

Five men were uncircumcised. At study entry, 1 woman had cervical low-grade squamous intraepithelial lesions, and 2 men had genital warts. Cervical atypia was diagnosed in 2 women during follow-up.

HPV Transmission

Overall, 957 (94%) of 1,015 specimens were adequate based on β-globin detection and were included in the present analyses. At study entry, 7 couples were HPV negative at all anatomic sites, 4 couples had 1 HPV-positive partner (3 were anal infections in the man), and 14 couples had both partners HPV positive. Of the 14 couples positive for HPV, 3 couples were positive for different HPV types, and 11 were positive for the same HPV type(s) at 1 or more anatomic sites (Table 1). All 11 HPV type-specific concordant infections involved the penis in the male partner and, with 2 exceptions, the cervix and/or urine in the female partner.

All transmission events occurred in couples with 1 or both partners positive for HPV at study entry. A total of 78 transmission events were observed in 16 couples, including 14 male-to-female, 39 female-to-male, 21 male auto-inoculation, and 4 female self-inoculation (Appendix Table). Overall, 15 different genotypes were transmitted, including oncogenic types and nononcogenic/undetermined risk types. Partner transmission involved 13 genotypes. Forty-one (53%) of the 78 transmission events involved multiple potential sources of infection.

Male-to-female transmission was observed in 7 couples. All infections transmitted from male to female partners originated in the penis with or without additional involvement of the scrotum. In particular, the penis shaft was a predominant source of infection either alone or with other genital sites. The cervix and anus were the most frequent targets of transmission from the men. Transmission of oncogenic HPV 16 comprised 1 of the 5 genital-to-cervix events. Male genital to female hand transmission was observed in 3 couples (D, E, and G).

Female-to-male transmission was observed in 12 couples. Transmission from the cervix and/or urine to the male genitals comprised most female-to-male events. Within penis subsites, the glans was targeted more frequently than the shaft. The anus was frequently an additional source site along with the cervix/urine. In addition, the anus was the sole source of transmission to the male genitals on 4 occasions, of which 3 targeted the partner’s scrotum (couples A, B, and J). The penis and scrotum, respectively, were the most frequent targets of infection from women. There were 4 instances of transmission from the woman’s hands to the man’s genitals, including 1 case in which it was the sole source (couple I). Oncogenic HPV types 16 and 18 were transmitted in only 2 of the 39 female-to-male events.

Male self-inoculation was observed in 11 men, including 3 for whom no heterosexual transmission was observed during the entire period of follow-up. Sixteen events involved transmission between different genital sites, 2 involved anal-to-genital transmission, and 3 involved genital-to-hand transmission. Most genital-to-genital events involved transmission between penis subsites. In 3 instances, male self-inoculation immediately preceded transmission of HPV to female partners (couples A and B).

Female self-inoculation was observed in 4 females. All involved urine as a source site and 3 of these targeted the hands.

Heterosexual transmission of the same HPV genotype to >1 anatomic site (excluding penis subsites) was observed on 13 occasions. Eight of these cases involved transmission from the cervix/urine to the penis and scrotum.

Heterosexual transmission of multiple genotypes to the same anatomic site during the same period was observed in 5 instances (couples A, B, D, F, and G); the scrotum was the target site on 3 of these occasions. In 1 case, different genotypes in the cervix and urine were each transmitted to both the penis glans and shaft (couple K).

Transmission of some HPV types in preference to others was also observed. In couple L, after cervical infection with HPV 16 and 31 was detected at study entry, only HPV 31 was transmitted to the penis (glans, shaft) and scrotum. Alternatively, the female anus may have been the source of infection because HPV 31, not HPV 16, was present in the anus at baseline. In couple D, HPV 16, 59, and 62 were present in the penis shaft at baseline. Subsequently, only HPV 16 was transmitted to the woman’s cervix/urine and anus by the second visit.

A number of instances indicated apparent reinfection after viral clearance. In 1 couple (A), the woman’s anus was positive for HPV 39 infection at baseline and at visit 2. By visit 3, HPV 39 had been transmitted to the man’s scrotum, and the woman was negative for HPV 39. HPV 39 had been transmitted (presumably by auto-inoculation) to the penis by visit 4, and the infection remained through visit 5. By visit 6, or 9 months after initial clearance in the woman, HPV 39 had been transmitted to the woman’s anus.

Four cases of transmission to the female anus required >4 months of exposure to an infected partner. In 27 incident infections, no source of infection could be ascertained; 17 of these infections were in the male genitals.

The rates of HPV transmission by source site are shown in Table 2. Overall, the rate of transmission from the penis to the cervix/urine was 4.9 per 100 person-months of exposure (95% CI 1.6–10.0). By contrast, the overall rate of transmission from the cervix/urine to the penis was 17.4 per 100 person-months of exposure (95% CI 10.6–25.8). Transmission of oncogenic types to the male genitals was greater from the female urine alone than from the cervix alone. Transmission of HPV from the penis to the female anus was higher than that to the cervix; this was particularly true for transmission of oncogenic types to the anus (12.2 per 100 person-months of exposure, 95% CI 3.9–24.9). The highest rates of transmission were observed from the female anus to the male genitals (47.1 per 100 person-months of exposure, 95% CI 30.2–67.7), followed by cervix to the male genitals (27.8 per 100 person-months of exposure, 95% CI 19.0–38.3). In men, the rate of transmission by auto-inoculation was comparable to that of transmission from women. For example, the rate of transmission from the scrotum alone to the penis (6.0 per 100 person-months of exposure, 95% CI 1.2–14.5) was comparable to that of cervix only to penis (5.0 per 100 person-months of exposure, 95% CI 1.0–11.9).

Behavioral Factors

All couples with genital-to-genital transmission reported vaginal intercourse during the period corresponding to the transmission event. Among the 5 couples with penis-to-anus transmission, 4 reported anal intercourse during the corresponding time period.

Table 3 compares baseline characteristics of couples with and without HPV transmission. Male partners in transmitting couples had had more sexual partners over their lifetime. Transmitting couples had more frequent sexual intercourse with one another, were more likely to have contact between the male’s mouth and the female’s anus, were more likely to use birth control injections and have withdrawal before ejaculation, and had fewer periods of abstinence. Over half of nontransmitting couples reported use of condoms 100% of the time during sexual intercourse within the previous 4 months, compared with only 3% of transmitting couples.