Prevalence and risk factors of human papillomavirus infection in asymptomatic women in a Venezuelan urban area.

José Núñez-Troconis¹, Mariela Delgado², Julia González²_, Raimy Mindiola³, Jesvy Velásquez³, Betty Conde⁴, Denise Whitby⁴ and David J. Munroe⁵.

¹ Manuel Noriega Trigo Hospital, Department of Obstetrics and Gynecology, Faculty of Medicine, University of Zulia, Maracaibo, Venezuela

² Maracaibo Policlinic, Maracaibo, Venezuela

³ Regional Laboratory of Virological Reference, Faculty of Medicine, University of Zulia, Maracaibo, Venezuela,

⁴ Viral Oncology Section (VOS) Core Laboratory,

⁵ Laboratory of Molecular Technology, SAIC-Frederick, Inc., National Cancer Institute at Frederick, Frederick, MD, USA.

Abstract. The purpose of this study was to investigate the prevalence and risk factors of genital human papillomavirus (HPV) infection in asymptomatic women, using the HPV DNA Hybrid Capture 2 (HC2) test. Three hundred and two women who attended the Out-Patient Gynecological Clinic of a tertiary level hospital, in a Venezuelan urban area, were selected for the study. A pap smear, a cervical swab for HC2 and gynecological exam were performed to each patient. The HC2 testing showed that 47 samples (15.6%) were positive to HPV. Forty patients (13.2%) were positive to high risk-HPV (HR-HPV) and 11 (3.6%) were positive to low-risk-HPV (LR-HPV). The prevalence of HPV infections was higher for women under 35 years (51.1%; p < 0.02), and decreased to 6.4% for women ³ 65 years old. Women who had not finished high school had a higher prevalence of HPV infection (p < 0.035). Twenty six (42.6%) of 61 pathological Pap smears were positives to HPV infection. A statistically significant difference was found when HPV infection was compared in normal and abnormal Pap smear (HSIL+LSIL; p<0.0001). Twenty four of 56 (43%) women with diagnosis of LSIL, and 2(40%) of 5 with diagnosis of HSIL were positive for HPV infection. A statistically significant difference was found when we compared HPV infection in negative Pap smears and those with LSIL (p<0.001). The present study found that the prevalence of HPV infection in asymptomatic Venezuelan women who attended a tertiary level hospital was 15.6%. HPV infection was more frequent in young adult, and in women with low educational level.

Key words:  Prevalence, Human Papillomavirus, hybrid capture 2, Venezuela.

Prevalencia y factores de riesgo en la infección del virus  del papiloma humano en mujeres asintomáticas en un área urbana venezolana.

Resumen. El propósito de este estudio fue investigar la prevalencia y factores de riesgo que influencia la presencia de la infección por el virus del papiloma humano (VPH) en pacientes asintomáticas que asistieron a un hospital nivel 3 en un área urbana venezolana. Se estudiaron las pacientes que acudieron a la Consulta de Patología del Cuello Uterino del Hospital Manuel Noriega Trigo. A cada paciente se le realizó una historia clínica, toma de citología cervico-vaginal y una muestra del cérvix para captura de híbridos 2(CH2). Se incluyeron 302 pacientes. La CH2 mostró 47 muestras (15,6%) positivas al VPH. Cuarenta mujeres (13,2%) fueron positivas a VPH de alto riesgo (VPH-AR) y 11 (3,6%) a VPH de bajo riesgo (VPH-BR). La prevalencia de la infección por VPH fue más alta en mujeres £ 35 años (51,1%; p < 0,02) y disminuyó a un 6,4% en mujeres ³ 65 años. Las pacientes que no habían terminado los estudios de bachillerato presentaron un prevalencia más elevada del VPH (p < 0,035). Veinte y seis (42,6%) de 61 CCV patológicas fueron positivas a la infección del VPH. Una diferencia estadísticamente significativa fue encontrada cuando se comparó la presencia del VPH en las CCV normales con las CCV anormales (Lesión Intraepitelial Escamosa de Alto y Bajo Grado-LIE-AG y LIE-BG; p < 0,0001). Veinte y cuatro de 56 (43%) mujeres con diagnostico de LIE-BG, y 2(40%) de 5 con diagnóstico de LIE-AG fueron positivos a la presencia del VPH. Se encontró una diferencia estadísticamente significativa cuando se comparó la presencia de infección por el VPH en CCV normales y CCV con LIE-BG (p < 0,001). El presente estudio encontró una prevalencia de la infección por el VPH en mujeres asintomáticas que asisten a un hospital nivel 3 de 15,6% en área urbana venezolana. Fue más frecuente en mujeres jóvenes y de bajo nivel educacional.

Palabras clave: Prevalencia, virus del papiloma humano, captura de Híbridos 2, Venezuela

Recibido: 02-07-08. Aceptado: 09-10-2008.

INTRODUCTION 

Cervical cancer (CC) is the second most common cancer in women worldwide (1). Eighty percent of cases are diagnosed in developing countries (2). In most developing countries, CC is the leading female malignancy and the leading cause of death by cancer, especially among middle-aged women (3). 

A number of epidemiological risk factors have been identified as contributing to the development of cervical carcinoma (4). Infection with human papillomavirus (HPV) is one of them. HPV infection of the uterine cervix is the most common viral sexually transmitted disease (5), which is usually acquired around the time sexual activity begins. HPV infection is more common in young women and adolescents than older women (6-8) and it frequently resolves spontaneously (6). Several types of HPV are now recognized as one of the main causes of cervical cancer and its precursors (9). The prevalence of genital HPV has steadily increased over the past twenty years and varies among different countries and even between different populations within countries (3). Prevalence of HPV infection in African countries such as Zimbabwe and Mozambique is over 40% (3). Some European countries, Spain, Italy, United Kingdom, Netherlands, report prevalence below 10% (3, 10). Bosch et al. (3) reported 22.5% and 13.3% of HPV infection in USA and Canada, respectively. In Latin-America, Clifford et al. (10) reported that the prevalence of HPV was 14.3% in cytologically normal women. In Venezuela, there are few reports about the prevalence of HPV infection using DNA testing (11, 12). 

The recognition that HPV infection is necessary for the development of CC has been important (13). This knowledge has led to the development of new CC screening strategies which incorporate HPV de-oxyribonucleic acid (DNA) testing. One of these tests is Hybrid Capture^® 2 HPV DNA (HC2) testing. 

This study was carried out to determine the prevalence and risk factors of genital HPV infection in asymptomatic women who attended a tertiary level hospital in a Venezuelan urban area using HPV DNA HC2. 

MATERIALS AND METHODS 

Study population 

A total of 302 women who attended the Out-Patient Gynecologic Clinic for annual Pap smear check-up, were studied during the period of August 2 through August 19, 2005. Patients with previous hysterectomy and treatment of premalignant or malignant lesions of the cervix were excluded from the study. 

This study was performed at the Manuel Noriega-Trigo Hospital, which is a tertiary and urban referral hospital, serving middle and low socio-economic classes, in the southern part of the city of Maracaibo, Venezuela. 

The study was approved by the ethics committees of the Manuel Noriega Hospital and Faculty of Medicine, University of Zulia. All participants read and signed an informed consent agreement before enrolment in the study. Patients were also informed of the anonymity and confidentiality of the study. 

Each patient provided a medical history including obstetric and gynecological information, and then had a Pap smear, a cervical swab for Hybrid Capture 2 and gynecological examination. Pap smear was taken and processed by the conventional way. 

The DNA testing, HC2 ( Digene Co., Gaithersburg, MD, USA) was processed at Viral Oncology Section (VOS) Core Laboratory, National Cancer Institute, Frederick, MD, USA. Each cervical swab sample was studied for High Risk probe (HR-HPV types: 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 68) and Low Risk probe (LR-HPV types: 6, 11, 42, 43, 44). Both HC2 probes were performed in order to know the prevalence of High and Low Risk HPV types. 

Two pathologists (MD, JG) studied the Pap smears. The 2001 Bethesda System (TBS) for reporting cervical cytological diagnosis was used in the cytological analysis.

Statistical analysis 

Mean values and Standard Deviation (SD) of all continuous variables were calculated. Categorical variables were expressed as percentages of each group, and the simple frequencies were used for the categorical variables. To determine statistical relevance of the various parameters of the study Chi Square test, Fisher Exact test and logistic regression were performed. A p-value of less than 0.05 was considered statistically significant. 

RESULTS 

The mean age was 39.3±11.2 years old (mean ± SD) (range: 17-72). One hundred twenty seven women (42.1%) were married. One hundred thirty one (43.4%) were housewives. Two hundred seventy eight (92.1%) had pregnancies with 90.4% (n=273) reporting deliveries. Sexual and reproductive data are shown in Table I.

The HC2 testing showed that 47 samples (15.6%) were positive for HPV. Forty patients (13.2%) were positive for High Risk-HPV (HR-HPV) and 11 (3.6%) were positive for Low-Risk-HPV (LR-HPV). Four cases (1.3%) were positive for both. Table II shows that the prevalence of HPV infections was higher for women under 35 years (51.1%; p< 0.027; Odds Ratio, OR: 0.497; 95% Confidence Interval, CI: 0.265-0.930), and decreased to 6.4% for women ³ 65 years old. HR-HPV and LR-HPV prevalence was 47.5% (p=NS) and 72.8% (p<0.018; OR: 4.453; 95% CI: 1.157-17.141) for women under 35 years, respectively. HR/LR-HPV infection decreased as women got older.

Women who were £ 30 years old (p < 0.02) and that had not finished high school had a higher prevalence of HPV infection (p < 0.035). Other risk factors for having HPV infection are summarized in Table III. Table IV shows that higher number of sexual partners was associated with a higher HPV infection in different age groups. However, there was no statistically significant difference in the number of sexual partners when the different groups of age were compared. The average number of sexual partners was 1.57 for women younger than 30 years of age, 2.01 for women 30 to 39 years, 1.4 for women older than 40 years (p=NS).

Pap smear findings showed that 241 (79.8%) patients were negative for intraepithelial lesion or malignancy, 56 (18.5%) patients with low-grade squamous intraepithelial lesion (LSIL), and 5 (1.7%) with high-grade squamous intraepithelial lesion (HSIL). The median age of patients with cytological diagnosis of LSIL was 38 years (37.16 ± 9.65) (mean ± SD). The prevalence of this diagnosis was higher in women between 35 and 44 (33.9%) years old. The median age of patients with cytological diagnosis of HSIL was 32 years old (34.4 ± 10.6). Three of 5 (60%) women with a diagnosis of HSIL were between 25 and 38 years. 

Twenty six (42.6%) of 61 pathological Pap smears were positive for HPV infection detected by HC2. Twenty-one patients (8.7%) of 241 with Pap smear negative for intraepithelial neoplasia or malignancy were positive for HPV infection. A statistically significant difference was found when HPV infection was compared between normal and abnormal Pap smear (HSIL + LSIL; p < 0.0001; OR: 7.782; 95%CI: 3.956- 15.310). Twenty four of 56 (43%) women with diagnosis of LSIL, and 2(40%) of 5 with a diagnosis of HSIL diagnosis were positive for HPV infection, respectively, as shown in Table V. A statistically significant difference was found when we compared the presence of HPV infection in negative Pap smear and LSIL (p<0.001; OR: 7.857; 95% CI: 3.938- 15.715) and in normal Pap smear and HSIL was not statistically significant. (p < 0.07; OR: 0.1; 95% CI: 0.015-0.642).

Women with cytological diagnosis of LSIL, LR-HPV infection were statistically significant different (p < 0.037; OR: 0.260; 95% CI: 0.077-0.886).

HR-HPV infection was statistically significant in patients with Pap smear diagnosis of LSIL (p<0.0001; OR: 0.095; 95% CI: 0.045-0.201) and with diagnosis of HSIL (p<0.04; OR: 0.100; 95% CI: 0.15-0.642).

DISCUSSION 

Human papillomavirus is a necessary cause for the development of carcinoma of the cervix and its precursors (13-15). The prevalence of genital HPV infection has steadily increased over the past 20 years, which varies among different countries and even between different populations within countries (16, 17). We investigated the prevalence of HPV infection in a population of 302 urban, asymptomatic, sexually active women and its relationship to a variety of biologic, demographic, and behavioral risk factors. 

Among asymptomatic women in the general population, the prevalence of HPV infection ranges from 2 to 44% (5). The worldwide prevalence of HPV infection is estimated to be 10.41% (95% CI: 10.2-10.7) (5). The HPV prevalences estimated by continents are: Africa: 21.12% (95% CI: 20.87-23.43), America: 12.95% (95% CI: 12.41-13.51), Europe: 8.08% (95 CI: 7.77-8.41) and Asia: 7.95% (95% CI: 7.53-8.4) (3, 5, 10). Burchell et al. (5) mentioned that the number of women who harbor HPV-DNA worldwide is estimated in 291 million, and around 105 million will have a HPV 16 or 18 infections (16). Clifford et al. (10) found a worldwide HPV prevalence of 9.2%. Trottier et al. (18) reported that there are 6.2 million new cases of HPV infection cases each year in USA, and approximately 20 million Americans are infected with HPV at any one time. 

This study found that the overall prevalence of HPV infection in asymptomatic women was 15.6%. This prevalence was similar to the prevalence reported in several Latin-America countries for different authors (3, 19). Herrero et al. (19) reported a prevalence of HPV infection of 16% in Costa Rica. Bosch and de Sanjosé (3) reported that HPV infection prevalence in the general population in Canada, Mexico and USA was 13.3%, 14.5% and 22.5%, respectively. In South America countries, such as Argentina and Colombia, the prevalence was 16.6% and 14.9%, respectively (3). In 2000, Mendoza et al. (11) found a prevalence of HPV infection of 9.9% in 101 patients using HC in another Venezuelan city. Scucces et al. (12) found a prevalence of 51% of HPV infection using PCR in Venezuela in 2001. Theses authors studied 39 patients. 

The present study found that the HR-HPV infection (13.2%) in asymptomatic women was more common than the infection produced by LR-HPV (3.6%). Mendoza et al. (11) reported 7 cases of HR-HPV infection, 2 cases of LR-HPV infection and 2 cases positive for both types of infection. Scucces et al. (13) found 12.8% of prevalence of HR-HPV infection. Nyári et al. (16) reported 16% of HR-HPV type infection. Herrero et al. (19) found a prevalence of HR-HPV infection of 7.6% in more than 8,000 women. Clifford et al. (10) reported HR-HPV and LR-HPV prevalences of 6.1% and 2.5% in 15,613 sexually active women, respectively. Other studies (20, 21) have also shown HR-HPV prevalences ranging from 5 to 15%. 

The present study found that women younger than 35 years of age (p < 0.02) had a higher prevalence of HPV infection. Other authors (16, 19, 20, 22, 23) have reported similar results. Different authors (19, 23, 24) have found an age-related decline in HPV prevalence. The decreasing prevalence of HPV infection with increasing age, even after adjustment for sexual behavior, could happen because the genital HPV infection is cleared by local cell mediated immunity and protection against a further infection by the same genotype of HPV (22, 25). Herrero et al. (19) found an increase of HPV infection to almost 20% for women 65 years old or older. Burchell et al. (5) reported that the prevalence of HPV infection is highest for young women, decreases in the middle age groups and increases at age 65. A two peak pattern is observed in many studies all over the world, with the exception of Asia, where the age-specific curves decrease smoothly with increasing age. The reasons for the second peak and its geographic variation are unclear, but may be influenced by one or more non-mutually exclusive mechanisms, for example, reactivation of previously undetectable infections acquired earlier in life could occur due to a gradual loss of type-specific immunity or to a sudden loss due to hormonal influences during the post-menopausal years(5, 18, 26). The second peak could also originate from acquisition of new infections due to sexual contacts with new partners later in life (5, 18, 26). Because the changes in sexual morals or patterns over the last several decades have affected some cultural groups more than others, this explanation cannot be ruled out. Further, other cofactors that may affect HPV progression or clearance (e.g., smoking, parity, oral contraceptives) and competing risks (e.g., mortality due to other causes) could also be involved (5). Trottier et al. reported (18) that the decrease in risk of HPV infection with increasing age seems to be independent of changes in sexual behavior, suggesting a role for immune response. No second peak was observed in this investigation and found that the number of partners declined with increasing age. 

Risk determinants for HPV infection that have been identified in various cross-sectional and prospective cohort studies include number of sexual partners (lifetime and recent), age at first intercourse, smoking, long-term use of oral contraceptive, other STDs (e.g. Chlamydia and herpes simplex virus), multiparity, vaginal infection, malnutrition, chronic inflammation, immunosuppressive conditions including HIV infection, and parity(5, 10, 18, 26-28). Results have been inconsistent partly owing to the fact that different populations have been studied. Furthermore, risk factor profiles have been found to differ depending on whether HR or LR-HPV types were considered (29). The present study did not find any correlation between civil status, number of partners, age of first intercourse, number of pregnancies and deliveries, and HPV infection. Other authors have found the civil status (unmarried) and nulliparous woman or few pregnancies as statistically significant predictor factors of the presence of HPV infection (5, 10). This investigation found a significant difference according to the educational level of women (p > 0.035, Table III). Most of the women studied were from low and middle low socioeconomic level. Low socioeconomic level is usually associated to low educational level and promiscuous sexual behavior (30). 

The median age of patients with LSIL and HSIL were 38 and 32 years old in this study, respectively. Herrero et al. (19) reported a prevalence of LSIL highest among the youngest patients, with a median age of 29 years old. The same authors (19) found the median age of 34 years in patients with HSIL. 

The present study found that the presence of HPV infection in women with cytological findings of SIL was statistically significant when compared with patients with normal Pap smears. Different authors (16, 18, 22) reported similar results. HPV infection was statistically significant when LSIL was compared with normal Pap smear but not HSIL and normal Pap smear. The low number of patients with HSIL could explain this result, and this is a limitation of this investigation. Forty three percent of women with LSIL Pap smear had HPV infection as observed in Table V. Twenty three of 24 (95.8%) patients with LSIL on Pap smear were positive for HR-HPV types. Herrero et al. (19) found cancer associated HPV types present in more than a 50% of LSILs. They (19) reported that LSILs having cancer associated HPV types are probably the most likely to persist and progress. Clifford et al. (31) reported that overall HPV positivity in LSIL varied from 29 to 100%. 

Our results indicated that the prevalence of HPV infection in asymptomatic Venezuelan women was 15.6%, similar to other Latin-American countries. This study confirmed that HPV infection was more frequent in young adults, and was found more commonly in women with a low educational level. Also, HPV infection was statistically significantly associated with SIL, and more than 40% of the LSIL were positive to HR-HPV. 

ACKNOWLEDGMENTS 

This project has been funded in whole or in part with Federal Funds from the National Cancer Institute, National Institutes of Health, under Contract N01-C0-12400. The content of this publication does not necessarily reflect the view or policies of the Department of Health and Human Services, nor does the mention of trade names, commercial products, or organizations, imply endorsement by the U.S. Government. Dr. Núñez-Troconis is supported by a scholarship from Fogarty Foundation/NCI/ PAHO. 

We thank all the member of Social Service and nurses who work at Gynecological Out-Patient Clinic of Manuel Noriega Trigo Hospital for their assistant and help. Also, we thank Thomas Parks, Michael Walters and Wendell Miley for technical assistance with HC2 testing. 

REFERENCES 

1. Ferlay J, Bray F, Pisani P, Parkin DM. GLOBOCAN 2002 cancer incidence. Mortality and prevalence worldwide. IARC CancerBase Nº 5 version 2.0Lyon: IARC Press; 2004.          [ Links ]

2. Waggoner SE. Cervical cancer. Lancet 2003; 361:2217-2225.          [ Links ]

3. Bosch FX, de Sanjosé S. Human papilloma- virus and cervical cancer: Burden and assessment of casualty. J Natl Cancer Inst Monogr 2001; 31:3-13.          [ Links ]

4. Núñez-Troconis J. Cervical intraepithelial neoplasia: Chlamydia trachomatis and other co-factors. Invest Clin 1995; 36:101-116.          [ Links ]

5. Burchell AN, Winer RL, de Sanjosé S, Franco EL. Epidemiology and transmission dynamics of genital HPV infection. Vaccine. 2006; 24(Suppl 3): S3/52- 61.          [ Links ]

6. Snijders PJF, Steenbergen RDM, Heideman DAM, Meijer CJLM. HPV-mediated cervical carcinogenesis: concepts and clinical implications. J Pathol 2006; 208: 152-164.          [ Links ]

7. Zur Hausen H. Papillomaviruses and cancer: from basic studies to clinical application. Nature Reviews/Cancer. 2002; 2:342- 350.          [ Links ]

8. Cox JT. The development of cervical cancer and its precursors: what is the role of human papillomavirus infection? Curr Opin Obstet Gynecol 2006; 18(suppl 1): S5-S13.          [ Links ]

9. Wright TC, Bosch FZ, Franco EL, Cuzick J, Schiller JT, Garnett GP, Meheus A. HPV vaccines and screening in the prevention of cervical cancer: conclusions from a 2006 workshop of international experts. Vaccine 2006; 24(Suppl 3):S3:251-261.          [ Links ]

10. Clifford GM, Gallus S, Herrero R, Muñoz N, Snijders PJF, Vaccarella S, Anh PTH, Ferreccio C, Hieu NT, Matos E, Molano M, Rajkumar R, Ronco G, de Sanjosé S, Shin HR, Sukvirach S, Thomas JO, Tunsakul S, Franceschi S, and the IARC HPV Prevalence Surveys Study Group. Worldwide distribution of human papillomavirus types in cytologically normal women in the International Agency for Research on Cancer HPV prevalence surveys: a pooled analysis. Lancet 2005; 366:991-998.          [ Links ]

11. Mendoza JA, Muñoz M, Vielma S, Noguera ME, López M, Toro M. Infección cervical por el virus del papiloma humano: diagnóstico por citología y por captura de híbridos del ADN viral. Rev Obstet Ginecol Ven 2000; 60(2):103-106.          [ Links ]

12. Scuceces M, Paneccasio A. Lesión intraepitelial cervical asociada a virus de papiloma humano. Rev Obstet Ginecol Ven 2001; 61(2):101-107.          [ Links ]

13. Bosch FX, Lorincz A, Muñoz N, Meijer CJ, Shah KV. The causal relation between human papillomavirus and cervical cancer. J Clin Pathol 2002; 55:244-265.          [ Links ]

14. Walboomers JMM, Jacobs MV, Manos MM, Bosch FX, Kupek E, Shah KV, Snijders PJ, Peto J, Meijer CJ, Muñoz N. Human papillomavirus is a necessary cause of invasive cervical cancer worldwide. J Pathol 1999; 189:12-19.          [ Links ]

15. Muñoz N, Bosch FX, de SanJosé S, Herrero R, Castellsague X, Shah KV, Snijerds PJF, Meijer CJLM, for the International Agency for Research on Cancer Multicenter Cervical Cancer Study Group. Epidemiologic classification of human papillomavirus types associated with cervical cancer. N Engl J Med 2003; 348:518-527.          [ Links ]

16. Nyári TA, Kalmár L, Deák J, Szõllõsi J, Farkas I, Kovács L. Prevalence and risk factors of human papillomavirus infection in asymptomatic women in southeastern Hungary. Eur J Obstet Gynecol 2004; 115: 99-100.          [ Links ]

17. Ponten J, Adami HO, Bergestrom R, Dillner J, Friberg LG, Gustafsson L, Miller AB, Parkin DM, Sparén P, Trichopoulos D. Strategies for global control of cervical cancer. Int J Cancer 1995; 60:1-26.        [ Links ]

18. Trottier H, Franco EL. The epidemiology of human papillomavirus infection. Vaccine. 2006; 24(Suppl 1): S1/4-15.        [ Links ]

19. Herrero R, Hildesheim A, Bratte C, Sherman ME, Hutchinson M, Morales J, Balmaceda I, Greenberg MD, Alfaro M, Burk RD, Wacholder S, Plummer M, Schiffman M. Population-based study of human papillomavirus infection and cervical neoplasia in rural Costa Rica. J Natl Cancer Inst 2000; 92(6):464-474.        [ Links ]

20. Arora R, Kumar A, Prusty BK, Kailash U, Batra S, Das BC. Prevalence of high-risk human papillomavirus(HR-HPV) types 16-18 in healthy women with cytologically negative Pap smear. Eur J Obstet Gynecol Reprod Biol 2004; 121:104-109.        [ Links ]

21. Gjooen K, Olsen AO, Magnus P, Grinde B, Sauer T, Orstavik I. Prevalence of human papillomavirus in cervical scrapes as analyzed by PCR, in a population based sample of women with and without cervical dysplasia. APMIS 1996; 104(1):68-74.        [ Links ]

22. Burk RD, Kelly P, Feldman J, Bromberg J, Vermund S, Dehovitz J, Landesman S. Declining prevalence of cervicovaginal human papillomavirus infection with age is independent of other risk factors. Sex Trans Dis 1996; 23(4):333-341.        [ Links ]

23. Ho GY, Bierman R, Beardslley L, Chang CJ, Burk RD. Natural history of cervicovaginal papillomavirus infection in young women. N Engl J Med 1998; 338(7):423-428.        [ Links ]

24. Baay MFD, Sunits E, Tjalma WAA, Lardon F, Weyler J, Van Royen P, Van Marck EAE, Vermorken JB. Can cervical cancer screening be stopped at 50? The prevalence of HPV in elderly women. Int J Cancer 2004; 108(2):258-261.        [ Links ]

25. Stanley M, Lowy D, Frazer I. Prophylactic HPV vaccines: Underlying mechanisms. Vaccine 2006; 24(Suppl 3):S3/106-113.        [ Links ]

26. Schiffman M, Castle PE, Jeronimo J, Rodriguez AC, Wacholder S. Human papillomavirus and cervical cancer. Lancet 2007; 370:890-907.        [ Links ]

27. Herrero R, Castle PE, Schiffman M, Bratti MC, Hildesheim A, Morales J, Alfaro M, Sherman ME, Wacholder S, Chen S, Rodriguez AC, Burk RD. Epidemiologic profile of type-specific human papillomavirus infection and cervical neoplasia in Guanacaste, Costa Rica. J Infect Dis 2005; 191:1796-1807.        [ Links ]

28. Winer RL, Lee SK, Hughes JP, Adam DE, Kiviat NB, Koutsky LA. Genital human papillomavirus infection: incidence and risk factors in a cohort of female university students. Am J Epidemiol 2003; 157(3): 218-226.        [ Links ]

29. Giuliano AR, Papenfuss M, Abrahamsen M, Inserra P. Differences in factors associated with oncogenic and nononcogenic human papillomavirus infection at the United States-Mexico border. Cancer Epidemiol Biomarkers Prev 2002; 11(9):930-934.        [ Links ]

30. Palacio-Mejias LS, Rangel-Gómez G, Hernández-Avila M, Lazcano-Ponce E. Cervical cancer, a disease of poverty: Mortality differences between urban and rural areas in México. Salud Publica Mex 2003; 45(supp 3):S3/15-25.        [ Links ]

31. Clifford G, Franceschi S, Díaz M, Múñoz N, Villa LL. HPV type-distribution in women with and without cervical neoplastic diseases. Vaccine 2006; 24(S3):S3/ 26-34.        [ Links ]