The age-specific pattern of HR-HPV prevalence can differ somewhat by geographic location. In most populations, incidence and prevalence of HR-HPV infection peak at young ages soon after sexual debut, followed by a decline as nearly all infections clear and new sexual encounters decrease [7,8]. This pattern motivates the use of HR-HPV screen-and-treat programs, targeted to women past the peak age of HR-HPV prevalence.

However, high HR-HPV prevalence at all ages has been reported in some, but not all, population-based studies conducted in sub-Saharan Africa, including areas of West Africa [9-11]. Recently, we have shown a similar pattern from our population-based study in Irun, a rural village in southwestern state of Ondo, Nigeria, where the prevalence of HR-HPV even among cervical cytologically normal women appears to be consistently elevated across all ages with a slight peak in older women [12]. The reasons for the high HR-HPV prevalence at older ages, which may negatively affect the performance of HR-HPV screen-and-treat programs, are unknown. Data on HR-HPV risk factors in among older women, particularly in West Africa, are very limited [13-15].

At the clinic visit, women completed a second informed consent and were given more detailed information of clinic procedures. Following consent, 1,420 women were administered a questionnaire that addressed potential risk factors associated with HR-HPV and cervical pre-cancer such as tar and tobacco exposure, number of malaria diagnoses within the past two years, menstrual history, sexual behavior, marital status, and birth control use. Nurses conducted a cervical exam involving the collection of cervical cells using a broom device and endocervical brush, and placed into PreservCyt buffer. On the day of collection, 1 ml was removed, frozen, and subsequently used for the HR-HPV DNA testing discussed here.

The presence of HR-HPV DNA was determined from the 1 ml of residual cytology specimens using MY09-MY11 PCR-based method as described [16]. We considered 13 HPV types to be carcinogenic types: 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, and 68. Of 1,282 non-virgins for whom HPV DNA test results were available, 14.7% were infected with one or more carcinogenic HR-HPV genotypes [12].

Table ​Table11 summarizes the clinical questionnaire data and shows the relationship between these risk factors and HR-HPV positivity. In terms of menstrual history, most women (70%) reported age of menarche to be between 15-19 years (average age 16.1 ± 2.5 years). The majority of women reported an average age at sexual debut of 20 years. Most women reported only one sex partner in the last two years (~80%). The average age of first pregnancy was 23.1 years and about 50% of women reported having between 5-9 lifetime pregnancies (average number 6.5 ± 3.2 pregnancies). Only a small percentage of women reported using birth control (14%) and of those women very few responded as to what type, although hormonal contraception appeared to be the predominant method in comparison to condom use (data not shown).

In terms of marital status and related variables, the majority of women were in union (either married or living with a man) and about 60% of these women reported that they resided in a household with co-wives, reflecting the over-sampling in our field effort. A relatively equal percentage reported having 1, 2, or 3+ co-wives and 45% stated that they were ranked first among other wives. This reflects that our recruitment efforts were particularly successful among older women. The average age of the youngest wife was 39.4 ± 10.7 years.

Most women reported that neither they nor their husbands used tobacco, thus the overall exposure to tobacco was relatively low (15.2%). A majority of women reported having at least one case of malaria in the past two years (average number of diagnoses 2.2 ± 1.5).

Overall, age was observed to be a risk factor, although a non-linear one, for presence of HR-HPV, as we observed an increased prevalence in women 15-29 (18.9%) and 56+ years (15.1%). An age of 10-14 years at sexual debut was associated with a nearly two and a half-fold risk for HR-HPV infection (OR 2.4; 95% CI 1.1-5.3) compared to late age. Early rather than late age of first pregnancy was also associated with increased HR-HPV prevalence with a 40% increased risk for ages 12-19 (OR 1.4; 95% CI 0.9-2.4) and a 50% increased risk for ages 20-24 years (OR 1.5; 95% CI 1.1-2.2) compared to late age. Having more than two sexual partners and use of birth control were also positively related to HR-HPV risk (OR 1.4; p-value = 0.07 and OR 1.6; p-value = 0.03).

With regard to our more novel hypothesized risk factors, we did not find associations of interest. Living in a household with co-wives was associated with a slightly decreased risk of HR-HPV positivity (OR 0.8; p-value = 0.24) although this relationship was not significant. The number of self-reported malaria diagnoses in the past two years was not associated with increased risk of HR-HPV prevalence (p-value = 0.85).

Table ​Table22 shows the distribution of key questionnaire variables determined by the analyses above, including those that we felt to be important risk factors to explore, stratified by age. Women 15-29 years reported younger ages at sexual debut and first pregnancy while women 50+ years recalled the highest ages for these measures. In addition, younger women were more likely to have greater than two sex partners in comparison to women aged 45-55 and 56+ years. The number of women using birth control was highest among the 46-55 year olds and lowest in women aged 56+ years. Women 30 years or older were the most likely to be in a household with co-wives, while most women aged 15-29 years did not live in a household with co-wives. Interestingly, self-reported malaria was most prevalent in women aged 56+ years, who were more likely to report three or more cases in the past 2 years and lowest among women 15-29 years of age, who frequently reported one or fewer cases.

Table ​Table33 shows the risk of HR-HPV infection for table ​table22 variables within age strata. We had hypothesized that the risk factors might help explain the high HR-HPV prevalence at older ages. However, age stratification did not reveal strong and consistent explanatory differences. The slight differences are as follows: An age of 10-14 years at sexual debut was associated with a five-fold risk for HR-HPV (10-14 years: OR 5.0; 15-24 years: OR 1.2; p-trend = 0.02) in women aged 30-45 years. Early age at first pregnancy was associated with a greater than two-fold risk for HR-HPV in women aged 56+ years (12-19 years: OR 2.4; 20-24 years: OR 2.2; p-trend = 0.04). Early age at sexual debut was also a risk factor among women in this age group, although the association was not quite as strong (15-24 years: OR 1.6; p-value = 0.15). Multiple sex partners and birth control use were risk factors for women aged 30-45 (OR 1.7; p-value = 0.08 and OR 1.8; p-value = 0.8, respectively). Living in a household with co-wives was not associated with HR-HPV in this age stratified analysis. We did observe somewhat of a dose response relationship with the number of recent self-reported malaria diagnoses and HR-HPV prevalence in women aged 15-29 years, although this relationship was not significant (p-trend = 0.31).

We hypothesized that being a co-wife might correlate with HR-HPV appearance at older ages since a woman in a polygamous marriage is theoretically engaged in sexual relations with more than one partner; however we did not observe this finding. We also hypothesized that women with HR-HPV at older ages might be at increased risk due to partial immunosuppression from to chronics parasitosis, however there was no association with recent self-reported malaria. Although self-reported malaria exposure was not associated with risk for HR-HPV infection, we did find an increase in recent reported malaria cases among women aged 56+ years compared to women in younger age groups. Even though it was not related to HR-HPV, this is an interesting finding given that prevailing research in endemic areas indicates the highest malaria burden to be among younger age groups [18,19]. It is possible that self-reported malaria exposure is inaccurate, attenuating our results. A recent study has shown that malaria self-report data is highly sensitive and specific when prevalence is measured over a 6-month time frame, but that specificity dramatically decreased when recalling after 18 and 30 months [20].