Type-specific prevalence of human papillomavirus in women screened for cervical cancer in Labrador, Canada.

BACKGROUND
A higher incidence of cervical cancer and human papillomavirus (HPV) infection has been reported in northern Canada and in First Nation, Métis and Inuit women, with some evidence to suggest that the HPV type distribution in these populations may be different from the rest of Canada.


OBJECTIVE
The objective of this study was to measure the HPV type prevalence in Labrador women to determine if significant differences in HPV types could reduce the effectiveness of HPV vaccination.


DESIGN
The prevalence of HPV types was determined in 1,370 women presenting for routine pap screening in Labrador between February and November 2010. Cervical cytology and HPV genotyping were performed on the same liquid-based cytology specimens.


RESULTS
The overall prevalence of HPV was 21.4%; cytological abnormalities were found in 8.8% of the participants. HPV 16 and 18 were the most common high-risk HPV types. These two types were found in 52.4% of high-grade lesions. The prevalence in HPV infections was comparable across the Labrador regions.


CONCLUSIONS
The present results support the potential effectiveness of the HPV immunization program in Labrador.

very year in Canada, approximately 1,300 women are diagnosed with cervical cancer and 450 die from it (1). Aboriginal women living in Canada (Inuit, Métis and First Nations) have been reported to suffer a higher burden of cervical cancer (2Á5). Not surprisingly, higher prevalence of HPV infection has also been reported in these women (6Á8), with one study suggesting that the distribution of HPV types in Inuit populations may differ compared to the rest of Canada (6).
Over 50 different human papillomavirus (HPV) types infect the anogenital tract, some of which are referred to as high-risk (HR) types for cancer (9). These HR-HPV types are a necessary cause for developing cervical cancer and are involved in a sizable proportion of other anogenital and oropharyngeal malignancies. In particular, HPV types 16 and 18 have been found in over 70% of cervical cancers worldwide (10,11), 90% of HPV-positive anal cancers and 10Á60% of oropharyngeal malignancies (12). These findings have been confirmed in many studies and summarized in recent meta-analyses (11,13).
Labrador is a vast region of 294,330 km 2 inhabited by approximately 26,000 people with a predominantly Aboriginal population. The Aboriginal population living in Labrador is composed of Métis, Inuit and First Nations. We were interested in determining the distribu- Cytology results were reported according to the Bethesda system (14) (normal (including benign), atypical squamous cells of undetermined significance (ASCUS), low-grade squamous intraepithelial lesion (LSIL), atypical squamous cells in which high-grade lesion could not be excluded (ASC-H), atypical glandular cells (AGC) and high-grade squamous intraepithelial lesion (HSIL)). The cytological outcomes ASC-H, AGC and HSIL were combined in the category ''high-grade lesions'' and the cytological outcomes ASCUS and LSIL were combined in the category ''low-grade lesions''. HPV types were grouped into species, according to the most recent classification (15).
HPV typing was performed at the NML using an ''in-house'' Luminex-based method that detects 45 HPV types (16 HPV genotyping was carried out as previously described (17,18).
Cytology and HPV typing results were linked to demography data using a study identification number. For women who had more than one Pap test over the study period, results from the first specimen were taken. Many of the 17 communities in this study have a small population and therefore, in order to build up sufficient numbers for statistical analyses and to avoid identification of specific communities, the participants were grouped in three geographical regions, namely the North coast (Nain, Hopedale, Makkovik, Postville, Rigolet and Natuashish), the South coast (Cartwright, Black Tickle, Charlottetown, St. Lewis, Port Hope Simpson, Mary's Harbour, L'Anse au Loup and Forteau) and the Central region (Happy Valley-Goose Bay, Sheshatshiu and North West River). According to the 2006 Canadian census, approximately 40% of the population in the Central region self-identified as Aboriginal, while at least 87% self-identified as Aboriginal in the North coast and South coast. Univariate comparisons were made using x 2 . Prevalence rates are presented in percentages.
Regions were compared using logistic regression and the p-values reported are the type 3 analysis of effect. Analyses were performed using SAS † .
Ethics approval was obtained from the Public Health Agency of Canada ethics boards. Labrador-Grenfell Health facilitated the project and community stakeholders reviewed the manuscript before being submitted for publication.

Results
Of the 1,370 women that were enrolled in the study, 293 (21.4%) were infected with at least one HPV type and 218 (15.9%) tested positive for at least one HR-HPV type. The infected women had a total of 398 HPV infections; 77 of HPV-positive women had multiple infections. The age of the participants ranged from 13 to 86 years (mean: 36.9; median: 35). Overall, 90.4% of participants had a normal cytology, 3.5% had an ASCUS, 3.7% a LSIL, 1.5% a high-grade lesion (AGC, ASC-H, HSIL) and 0.8% had an unsatisfactory Pap test.
At least one HR-HPV type in the IARC Group 1 (carcinogenic) was detected in 12.5% of participants; 4.6% were positive for at least one IARC Group 2 type (probably or possibly carcinogenic); and 5.6% tested positive for multiple HPV infections (Table I). Study participants from the three geographical regions had comparable risk of having a HPV infection with any HPV types or with a HR-HPV type (Table II). The probability of being diagnosed with a cervical abnormality was comparable in the three geographical areas.
Infections with species A9, which includes types closely related to HPV 16, were the most frequent (32.2% of infections), including HPV 16 (12.3%), HPV 33 (4.5%), HPV 58 (3.5%) and HPV 67 (3.8%) (Table III). Species A7, which includes types closely related to HPV 18, was the second most common species (18.8% of infections), of which HPV 18 (6.3%) was the most prevalent type. HPV types belonging to species A9 and A7 account for almost all cases of cervical cancer (15). HPV 16 and/or 18 were found in 2.7% of women with normal cytology, 27.3% of women with low-grade cervical lesions (ASCUS, LSIL) and in 52.4% of those with high-grade lesion (Table IV). HPV 16 and HPV 33 were the most frequent types detected in high-grade lesions.     (24). A study conducted in a population of Inuit women living in northern Quebec reported a prevalence of 29% (6) and our own study in the Northwest Territory reported a prevalence of 24.2% (25). The prevalence of any cytological abnormality (8.8%) is also in keeping with the reported rates of cytological abnormalities in Canada (7% in Nunavik, Manitoba and British Columbia) (6,21,24). HPV 16 was the most prevalent HPV type observed in the present study, which is similar to that previously reported worldwide (26); HPV 18 was the second most prevalent type. HPV31 has been found as the second most prevalent type in some areas of Canada (6), but not in Labrador. The prevalence of HPV 16 in high-grade cervical lesions (48%) was in the range of that reported worldwide (45%) (11), as well as in Canada (65%) (27). As expected, the prevalence of HPV infections was higher in younger women and decreased markedly with increasing age, which reflects the natural history of the infection (28).
The prevalence of multiple HPV infections in this study was 5.6%, or 26.2% of HPV-positive women.   This proportion of multiple infections is in the middle of the worldwide range of 11.5Á42.4% for women with a normal cervix (25). It is, however, lower than in other studies in Northern regions (see for example 40% relative prevalence in Nunavik, Quebec (6) or 45% in rural aboriginal women in the United States) (29). Detection of multiple infections may be influenced by the performance of the genotyping method. In comparison with the Roche LinearArray kit (a widely used HPV genotyping method), the Luminex method used in this study was shown to detect fewer HPV types in multiple infections with 3 or more types, but not fewer infected women (17). It is therefore possible that the percentage of multiple infections may be slightly, but not drastically, underestimated. A similar study by our group using the same Luminex method has detected 35.2% multiple infections in HPV-positive women (25). Labrador has a predominantly Aboriginal population (Métis, Inuit, and First Nations), especially outside Happy Valley-Goose Bay. In the 2006 Canadian Census, 38% of the residents of Happy Valley-Goose Bay (the 95% of population of the Central Region) self-identified as Aboriginal, while at least 87% of the people living outside Happy Valley-Goose Bay self-identified as Aboriginal. Higher prevalence of HPV in Aboriginal groups living in Canada has previously been reported (6Á8). No significant difference was found between the Labrador regions in terms of HPV infection rates or rates of cytological outcomes.
Our study shows that the distribution of HPV types in women undergoing routine cervical cancer screening in Labrador is in accordance with observations reported in Canada and worldwide. These results indicate that the current HPV vaccine against HPV 16 and 18 is likely to have the expected effectiveness in preventing cervical cancer in Labrador. A careful investigation of the underlying risk factors would help guide strategies for the prevention of cervical cancer in the population of Labrador.