This study recruited most of the women aged 18–69 (and most of the incidental consultations for the 14–17 age group) who attended routine or scheduled pap screening over an approximately one year time period.
The prevalence of overall HPV infection was 25.2% which was higher than in most countries [24
] but similar to Denmark [28
] and the US [29
]. HPV prevalence rates in both the Aboriginal (31.6%) and non-Aboriginal women (18.9%) in this study were slightly higher than those in other studies in Aboriginal [2
] and non-Aboriginal [30
] populations in Canada. The proportion of HR types among the HPV-positive women was lower than in British Columbia and Ontario studies but higher than in a Quebec study [2
Similar to most European and Canadian studies the two most common HPV types detected in Northern Canada were HPV-16 and 31 [28
]. The HPV 16/18 prevalence in the study was relatively lower than all similar studies in Canada [2
], but comparable to studies in Europe and the US [25
]. In this study HPV 16/18 comprised about 31% of all HR-HPV infections. Several HR HPV types other than HPV 16/18, such as HPV 31, 39, 45, 51, 59 and 66 were relatively common in Northern Canada.
The prevalence analysis shows that HPV prevalence was highest among women aged less than 20 years, which is consistent with other studies [27
]. This finding confirms what is already known on the natural history of HPV infection, which is characterized by higher infection rates after sexual initiation. In agreement with published data [2
], the age adjusted HPV prevalence rate in the Aboriginal group was higher than that in the non-Aboriginal group. Inconsistent trends in HPV prevalence by age were noted in older women, with a decrease or plateau of HPV prevalence among the non-Aboriginal group, whereas the Aboriginal group showed an increase of HPV prevalence (U-shaped age-specific HPV prevalence distribution). This has been observed in the other studies in Canada [2
] and from Costa Rica and other Latin American countries [41
], with the lowest prevalence among the age group 40–49. However, from most of the European literature, with the exception of the former Soviet Union [33
], a decrease in HPV prevalence after age 20 and a levelling off after age 45, has been reported [25
]. Further study needs to appreciate what role that social, cultural and lifestyle factors, as well as environmental determinants such as access to care may play in explaining this difference in age distribution.
Noteworthy is that, despite the difference in prevalence of HPV types and cytological abnormalities between Aboriginal and Non-Aboriginal women, the proportion of HPV 16/18 in high grade lesion is about 50% in both groups. This is very similar to the average proportion of HPV 16/18 in HSIL in North America and worldwide [13
]. Since HSILs are the precursors of invasive cancer, these results suggest that the current HPV vaccines should be equally applicable to Aboriginal and Non-Aboriginal women.
Currently routine Pap testing is one of the pillars to prevent cervical cancer. Other studies have found similar Pap test abnormality rates [2
]. HPV prevalence was higher in those women aged < 30 than in older women across the spectrum of abnormal Pap results. This is consistent with reports showing that HPV infection is common in young women and most often transient [43
]. It is expected that women aged 30 years and older experience more persistent HR infections than younger women and that HPV-DNA testing would be able to triage this group for further follow-up.
Among participants with a normal pap test result, 26% of any type HPV infection among the Aboriginal group and 15% among the non-Aboriginal women (Table ) translated into a 28 times higher risk of cervical abnormality than those with HPV negative (Table ). By the same token, among 5.4% of the Aboriginal women and 3.4% of the non-Aboriginal women with both normal Pap test results and HPV 16/18 infection, they have a 50 times higher risk of cervical abnormality than those who are HPV negative. Interestingly, similar to our previous NWT report [47
], results seem to indicate an additive risk model for multiple HPV infections. This information may be useful in considering HPV testing in cervical cancer screening programs in Northern Canada.
There are potential limitations in the study. Firstly, no PCR protocol is ideal for all types of HPV detection. Any of the above comparisons in prevalence rates must be taken with caution as we have used the most recent IARC classification for HR HPV and a novel (Luminex) detection method as sensitive as Linear Array but able to detect more types than other previously reported technologies such as GP5+/6+, Elisa, or Hybrid Capture [17
]. Secondly, this study represents HPV infection at various time points in the different regions, and does not represent the proportion of women at risk for disease or cancer related to HPV infection. Of note, there was no full regional coverage of the targeted populations during the study period. Different regions had different recruitment methods and therefore rates. Due to an “opt-in” process, the coverage for the Yukon sites lower than the other regions, and one thus should be cautious in interpreting the prevalence rates of the study population for Yukon. In addition, the Aboriginal status was not available in Labrador. To estimate HPV prevalence by ethnic group in the entire North in Canada, we used residence for Aboriginal or non-Aboriginal area instead as the purpose of the study was to understand the severity of HPV prevalence in Northern Canada and each northern region. Although we provided both crude rates and age adjusted rates for HPV infection, it still needs to be cautious to make comparison by region due to substantially different data collection processes. Finally, it’s worth noting that routine screening is not recommended by standard guidelines for young women aged 14–18. In addition Pap screening is not mandatory for anyone; e.g., NWT has their own guidelines [48
] and Yukon follows a guideline from British Columbia. Therefore, there may be a selection bias toward the young age group in data analysis. Nevertheless, the overall analysis was conducted with and without the 14–18 years age group and the results remained similar. We suspect also that there may be a potential bias affecting participation by Aboriginal versus non-Aboriginal people.