Infection with human papillomavirus (HPV) is a necessary cause of cervical cancers worldwide, and is increasingly recognized as an important cause of a subset of other anogenital cancers, including some vaginal, vulvar, penile, and anal cancers.1 HPV is also associated with a subset of head and neck cancers occurring specifically in the oropharynx.1 HPV type 16 (HPV16), the most prevalent of the approximately 13 cancer-associated types in cervical cancer, is also the most common type present in HPV-positive oropharyngeal cancers (of which, approximately 90% are positive for HPV16).2,3
Recent case-control data suggest that detection of prevalent HPV16 infection in oral exfoliated cells increased the odds of oropharyngeal cancer more than 13 fold.3 There is limited information about the natural history of oral HPV infection, but since oral HPV16 infection is associated with this cancer, it is important to estimate the proportion of healthy individuals with oral HPV infection. The aim of this work was to review existent literature to estimate the prevalence of HPV DNA detected in oral specimens collected from cancer-free individuals.
MATERIALS AND METHODS
The National Institutes of Health “PubMed” search engine was employed to search for citations published from January 1997 to June 2009, using the keywords “Papillomavirus” and “Oral” and limiting to English publications in humans. Using these terms, we identified 729 manuscripts. After reviewing their titles and abstracts for relevance, we identified 47 studies that appeared to evaluate HPV DNA in oral specimens collected from healthy individuals. We excluded studies that focused on individuals with precancerous lesions (n = 12) and studies of immunosuppressed populations (renal transplant or HIV-positive) (n = 1); although if a study included HIV-negative and HIV-positive individuals and HPV data were provided stratified by group, we utilized available data. We also excluded case-control studies, commentaries and systematic reviews (n = 11), studies that focused exclusively on infants/children (n = 2), studies with HPV test results on less than 50 people (n = 4), and studies that did not use DNA-based testing methods on oral specimens (i.e., serologic assays and secretory immunoglobulin) (n = 1). Using these criteria, 16 manuscripts were identified; two additional manuscripts were identified from manuscript references for a total of 18 manuscripts included in the final data abstraction. Informed consent was assumed to have been obtained by the individual studies from their respective subjects.
Data were abstracted (by A.R.K. and R.K.B.) and confirmed (by P.G. and A.L.M.) on the following variables: first and last author, year of publication, country, population, year of data collection, sample size, age of population, gender distribution, oral specimen collection method, laboratory methods for DNA extraction and HPV detection, as well as HPV types detected, and are presented in Table 1.
The HPV prevalence of overall, carcinogenic, and HPV16 was abstracted from each study when available. If information was missing from the publication, the author was contacted to obtain additional details and data when available.
Overall HPV prevalence was defined as persons testing positive for any oral HPV type, divided by the total healthy population tested for HPV. Carcinogenic HPV, defined as HPV 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, and 66,1 and HPV16 alone were measured only among those tested for the specific HPV type in question, and therefore, the sample size and number of contributing studies varied between analyses. Composite estimates (percent prevalence) and exact binomial 95% confidence intervals (CI) were calculated.
To begin to look at oral HPV by world region, we stratified by economic status using the definition put forth by the United Nations Public Administration Network.22 The following countries or regions were considered developed: Europe (excluding the east European transition economies), Canada, the United States of America, Japan, Australia, and New Zealand.
Categorizations were also made based on the study population to determine if the ‘healthiness’ of the population influenced the findings. Specifically, healthier populations “low-risk,” such as those from dental clinics and the general population, were compared to higher-risk groups “high-risk,” such as female sex workers. An unknown risk category was used for studies with mixed populations or for those whose ‘riskiness' could not be determined.
Evaluating the age-specific oral HPV distribution and comparing oral HPV prevalence among men and women were aims of this work. While most of the studies provided data on any HPV prevalence stratified by gender, many did not provide data in a manner that could be used for the age-analysis. The raw data from individual studies is displayed in tabular format (Table 1) and a gender-stratified analysis of any HPV was conducted.
Although understanding the influences of methods of specimen collection and laboratory analysis (specifically DNA extraction and HPV testing methods) is important for comparing results between studies, such methods work is best conducted within individual studies using head-to-head comparisons, as have been previously published.10,23
The relationship between study sample size and oral HPV prevalence was evaluated. We tested for heterogeneity between studies using the I2 statistic, which represents the approximate proportion of total variability in point estimates that can be attributed to heterogeneity.24 Formal testing using both the Begg and Mazumdar25 and Egger et al26 tests indicated publication bias was present (P = 0.04 and P < 0.001, respectively).
Eighteen studies including 4581 healthy individuals were identified and are presented in the Table 1. 1.3% (95% CI: 1.0%–1.7%) of 3977 healthy subjects had oral HPV16, 3.5% (95% CI: 3.0–4.1) of 4441 subjects had carcinogenic HPV, and 4.5% (95% CI: 3.9–5.1) of 4070 subjects were positive for any HPV (Table 2). HPV16 accounted for 28.0% (51/182) of all HPV infections detected in the oral region amongst individuals (and 32.9% [51/155] of carcinogenic HPV infections). Other carcinogenic types detected included: HPV 18, 31, 35, 39, 52, 56, 58, and 66.
Studies that occurred in developing nations (n = 4) had a greater prevalence of HPV16 compared to those from developed nations (n = 12) (4.3% vs. 0.7%, respectively); similar patterns were observed for carcinogenic and any HPV detected (Table 2).
By risk strata, studies (n = 6) that recruited lower-risk populations, such as people from the general population or college students, had a lower HPV16 prevalence (0.4%) compared to the unknown (n = 5) and higher (n = 2) risk categories (4.1% vs. 2.1%, respectively); a similar trend was observed for any HPV. An anomaly was noted for carcinogenic HPV, where low and high risk populations had similar prevalence of oral HPV (2.7% vs. 2.6% respectively). For all categorizations of HPV, studies of individuals of unknown risk had the highest oral HPV prevalence (Table 2).
Thirteen studies provided oral HPV data stratified by gender. Men (47 of 1017) and women (117 of 3690) had similar prevalence of any oral HPV detected (4.6% vs. 4.4%, respectively).
The relationship between any HPV prevalence and study size was assessed (Fig. 1). The majority of studies had around 100 to 200 individuals and showed broad variation in detection of any HPV (ranging from 2.6%–20.7%). The long right tail showed that six studies included over 200 people; the largest study had a sample size of 662 people. These larger studies tended to show overall HPV prevalence lower than the average (Fig. 1). By economic status, the few studies that occurred in developing countries tended to be smaller (all but one had less than 200 individuals).
Significant heterogeneity was present between studies: I2 for HPV16 of 89.3%, for carcinogenic HPV of 92.1%, and for overall HPV of 88.7% (all P < 0.001).
HPV16 infection has been established as a cause for a subset of oropharyngeal cancers1,3; therefore, quantitating the prevalence of oral HPV16 among healthy individuals is important. The present review included 18 studies that investigated HPV DNA, as measured by PCR-based assays, in over 4500 oral specimens collected from healthy individuals: more than one percent had HPV16 detected in their oral region. HPV16 accounted for 28% of all HPV infections detected in the oral region amongst individuals. Correspondingly, HPV16 accounts for approximately 25% of cervical infections detected among cytologically normal woman.27 Paradoxically, the proportion of HPV16 to all HPV types detected appears similar in the oral and cervical regions yet HPV16 causes a greater proportion of HPV-associated oropharyngeal cancers (approximately 90%)2,3 compared to cervical cancers (approximately 50%).28 While detection of oral HPV DNA does not translate to productive HPV infection, these data argue for large, well-designed studies that aim to understand the natural history and epidemiology of oral HPV infection. Given that HPV persistence is necessary for cervical cancer development,29 it is likely important for the oral region as well. Further, as differences were observed in the prevalence between nations with differing economic statuses, inclusion and comparison of multiple countries within one study will help determine if these differences are real or due to differences in study methodology.
To obtain an estimate of the burden of oral HPV infection in the general population, we focused on studies of healthy adults and chose to exclude certain populations. For instance, healthy controls from case-control studies were excluded because more than half of the control populations from these studies were recruited from either a hospital or clinic based setting and were typically matched to the case-distribution by at least age and gender, thereby potentially skewing results towards older ages and male gender, given the distribution of head and neck cancers. Additionally, studies conducted exclusively among children were excluded as the route of transmission is in most instances nonsexual, and the prevalence may accordingly differ as well. While our work is not necessarily generalizable to the population at large, especially considering the inclusion of higher-risk populations such as female sex workers and STD clinic attendees, we wanted to avoid further skewing the work in known ways. Lastly, because we restricted to healthy adults, we excluded populations at increased risk of infection, including HIV-positive, renal transplant patients, and cancer patients. Each of these special populations is of interest and warrants further investigation.
The methods employed in these studies were heterogeneous. Sources of variability between studies that we were unable to address with the current review included method of specimen collection and processing methods. In fact, inadequate sample purification due to PCR inhibition has been shown to importantly underestimate the prevalence of oral HPV infection.23 In addition, studies of oral HPV among cancer-free people have been, with rare exception, small (less than 200 cases); the HPV prevalence seemed to be inversely proportional to the study sample size. Hence, this work appeared to be susceptible to publication bias, in which certain studies may have been published because of higher oral HPV prevalence.
We determined from existing studies that a small but significant proportion of healthy individuals have HPV types detected in their oral region that are known to cause cancer in the oral region. While the fraction of these infections in healthy people that will lead to cancer will be small, HPV16 appears to cause approximately 40% to 60% of oropharynx cancers.3,30 There theoretically exists the possibility that the prophylactic HPV16 vaccine could be protective in this context. While a direct evaluation of vaccine efficacy against oral HPV16 infection remains necessary because there are no published data on this topic, if the HPV vaccine protects against oral HPV infections akin to the near complete efficacy observed for cervical infections among HPV naïve women,31,32 a percentage of oropharynx cancers could be prevented. Little is known about transmission and natural history of oral HPV infections, but prophylactic vaccines directed against oral HPV16 would need to be administered before exposure and would need to provide long-term protection. However, before any programmatic vaccine implementation, cost-effectiveness analyses are necessary and should aid in the decision of whether to promote vaccination to protect against some oropharyngeal cancers.
Prevention of even a subset of head and neck cancers is important, and is highlighted by recent data suggesting that the incidence of oropharynx cancer is increasing and may be due to changes in HPV endemicity.33 Prospective, epidemiologic studies that focus on the age-specific and type-specific oral HPV distribution, oral HPV prevalence by gender, and evaluation of rates of oral HPV incidence and persistence will be an important next step in understanding the natural history of oral HPV and in the evaluation of population benefit if the vaccine is effective.
1. Cogliano V, Baan R, Straif K, et al. Carcinogenicity of human papillomaviruses. Lancet Oncol 2005; 6:204.
2. Kreimer AR, Clifford GM, Boyle P, et al. Human papillomavirus types in head and neck squamous cell carcinomas worldwide: A systematic review. Cancer Epidemiol Biomarkers Prev 2005; 14:467–475.
3. D'Souza G, Kreimer AR, Viscidi R, et al. Case-control study of human papillomavirus and oropharyngeal cancer. N Engl J Med 2007; 356:1944–1956.
4. Canadas MP, Bosch FX, Junquera ML, et al. Concordance of prevalence of human papillomavirus DNA in anogenital and oral infections in a high-risk population. J Clin Microbiol 2004; 42:1330–1332.
5. Coutlee F, Trottier AM, Ghattas G, et al. Risk factors for oral human papillomavirus in adults infected and not infected with human immunodeficiency virus. Sex Transm Dis 1997; 24:23–31.
6. D'Souza G, Agrawal Y, Halpern J, et al. Oral sexual behaviors associated with prevalent oral human papillomavirus infection. J Infect Dis 2009; 199:1263–1269.
7. do Sacramento PR, Babeto E, Colombo J, et al. The prevalence of human papillomavirus in the oropharynx in healthy individuals in a Brazilian population. J Med Virol 2006; 78:614–618.
8. Fakhry C, D'Souza G, Sugar E, et al. Relationship between prevalent oral and cervical human papillomavirus infections in human immunodeficiency virus-positive and -negative women. J Clin Microbiol 2006; 44:4479–4485.
9. Giraldo P, Goncalves AK, Pereira SA, et al. Human papillomavirus in the oral mucosa of women with genital human papillomavirus lesions. Eur J Obstet Gynecol Reprod Biol 2006; 126:104–106.
10. Kreimer AR, Alberg AJ, Daniel R, et al. Oral human papillomavirus infection in adults is associated with sexual behavior and HIV serostatus. J Infect Dis 2004; 189:686–698.
11. Kujan O, Desai M, Sargent A, et al. Potential applications of oral brush cytology with liquid-based technology: Results from a cohort of normal oral mucosa. Oral Oncol 2006; 42:810–818.
12. Kurose K, Terai M, Soedarsono N, et al. Low prevalence of HPV infection and its natural history in normal oral mucosa among volunteers on Miyako Island, Japan. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2004; 98:91–96.
13. Lambropoulos AF, Dimitrakopoulos J, Frangoulides E, et al. Incidence of human papillomavirus 6, 11, 16, 18 and 33 in normal oral mucosa of a Greek population. Eur J Oral Sci 1997; 105:294–297.
14. Marais DJ, Sampson C, Jeftha A, et al. More men than women make mucosal IgA antibodies to human papillomavirus type 16 (HPV-16) and HPV-18: A study of oral HPV and oral HPV antibodies in a normal healthy population. BMC Infect Dis 2006; 6:95.
15. Montaldo C, Mastinu A, Quartuccio M, et al. Detection and genotyping of human papillomavirus DNA in samples from healthy Sardinian patients: A preliminary study. J Oral Pathol Med 2007; 36:482–487.
16. Ragin CC, Wheeler VW, Wilson JB, et al. Distinct distribution of HPV types among cancer-free Afro-Caribbean women from Tobago. Biomarkers 2007; 12:510–522.
17. Rintala M, Grenman S, Puranen M, et al. Natural history of oral papillomavirus infections in spouses: A prospective Finnish HPV Family Study. J Clin Virol 2006; 35:89–94.
18. Smith EM, Ritchie JM, Yankowitz J, et al. HPV prevalence and concordance in the cervix and oral cavity of pregnant women. Infect Dis Obstet Gynecol 2004; 12:45–56.
19. Smith EM, Swarnavel S, Ritchie JM, et al. Prevalence of human papillomavirus in the oral cavity/oropharynx in a large population of children and adolescents. Pediatr Infect Dis J 2007; 26:836–840.
20. Summersgill KF, Smith EM, Levy BT, et al. Human papillomavirus in the oral cavities of children and adolescents. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2001; 91:62–69.
21. Winer RL, Lee SK, Hughes JP, et al. Genital human papillomavirus infection: Incidence and risk factors in a cohort of female university students. Am J Epidemiol 2003; 157:218–226.
22. United Nations Public Administration Network. List of country groupings and sub-groupings for the analytical studies of the United Nations World Economic Survey and other UN reports. Available at: http://unpan1.un.org/intradoc/groups/public/documents/un/unpan008092.pdf
. Accessed 07 July 2009.
23. D'Souza G, Sugar E, Ruby W, et al. Analysis of the effect of DNA purification on detection of human papillomavirus in oral rinse samples by PCR. J Clin Microbiol 2005; 43:5526–5535.
24. Higgins JP. Commentary: Heterogeneity in meta-analysis should be expected and appropriately quantified. Int J Epidemiol 2008; 37:1158–1160.
25. Begg CB, Mazumdar M. Operating characteristics of a rank correlation test for publication bias. Biometrics 1994; 50:1088–1101.
26. Egger M, Smith GD, Schneider M, et al. Bias inmeta-analysis detected by a simple, graphical test. Br Med J 1997; 315:629–634.
27. de Sanjose S, Diaz M, Castellsague X, et al. Worldwide prevalence and genotype distribution of cervical human papillomavirus DNA in women with normal cytology: A meta-analysis. Lancet Infect Dis 2007; 7:453–459.
28. Munoz N, Bosch FX, de Sanjose S, et al. Epidemiologic classification of human papillomavirus types associated with cervical cancer. N Engl J Med 2003; 348:518–527.
29. Koshiol J, Lindsay L, Pimenta JM, et al. Persistent human papillomavirus infection and cervical neoplasia: A systematic review and meta-analysis. Am J Epidemiol 2008; 168:123–137.
30. Zumbach K, Hoffmann M, Kahn T, et al. Antibodies against oncoproteins E6 and E7 of human papillomavirus types 16 and 18 in patients with head-and-neck squamous-cell carcinoma. Int J Cancer 2000; 85:815–818.
31. Mao C, Koutsky LA, Ault KA, et al. Efficacy of human papillomavirus-16 vaccine to prevent cervical intraepithelial neoplasia: A randomized controlled trial. Obstet Gynecol 2006; 107:18–27.
32. Harper DM, Franco EL, Wheeler CM, et al. Sustained efficacy up to 4.5 years of a bivalent L1 virus-like particle vaccine against human papillomavirus types 16 and 18: Follow-up from a randomised control trial. Lancet 2006; 367:1247–1255.
33. Sturgis EM, Cinciripini PM. Trends in head and neck cancer incidence in relation to smoking prevalence: An emerging epidemic of human papillomavirus-associated cancers? Cancer 2007; 110:1429–1435.