Sexual intercourse is the main route of transmission of HPV infection, which is known as one of the most common infections around the world. According to this view, FSWs are amongst the most vulnerable group to acquire HPV infection and consequently, to develop precursors of cervical cancer. In part, this arises from the fact that they are constantly being exposed to a large number of risk factors facilitating the spread of sexually transmitted diseases . Previous studies among the general population have reported that the prevalence of HPV ranged from 9–13% in the world . As expected, our findings indicated that the number of HPV-positive cases are significantly higher among FSWs compared to the general population, and the prevalence varied from 13–82% across the world.
Concerning the overall prevalence and genotype distribution of cervical HPV infection among FSWs, to date only one meta-analysis has been published by Peng etal. in 2012, which was conducted on 4198 FSWs from nine Asian countries . Their study found a high HPV prevalence in different regions of Asia, so that the overall HPV prevalence in East, South-east, and South Asia were 49.6%, 42.9%, and 29.3%, respectively. Consistent with this, the results of our meta-analysis also indicated that FSWs in most Asian countries, like Bangladesh, China, Cambodia, India, Japan, Philippines, South Korea, and Vietnam had a prevalence of HPV infection greater than 40%.
The wide range variations between the different studies can be attributed to differences in the socio-demographic and behavioral characteristics of FSWs. As an example, we found that Mexican prostitutes exhibit low levels of HPV infection. It may be related to implementation of preventive programs such as primary cervical cancer screening, condom promotion, and HPV vaccination, which were effective to reduce the prevalence of HPV infection . Application of vaccines against HPV infection in 11-year-old girls is a part of the Mexico’s national immunization program .
There are two available HPV vaccines licensed by the U.S. Food and Drug Administration (FDA): quadrivalent HPV vaccine, including HPV types 6, 11, 16 and 18 (Gardasil®, produced by Merck); and bivalent HPV vaccine, including HPV types 16 and 18 (Cervarix™, produced by GlaxoSmithKline) . HPV types 6 and 11 are known to be responsible for 90% of genital warts, and types 16 and 18 together cause up to 70% of invasive cervical cancer worldwide. Along with cervical cancer, HPV types 16 and 18 are responsible for 40%-50% of invasive vulvar cancer and 70% of vaginal cancer . HPV-16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 66, 68, 73 and 82 are considered high risk genotypes, whereas low-risk genotypes include HPV-6, 11, 34, 40, 42, 43, 44, 54, 61, 70, 72, 81, and 89 . Our results showed that HPV-16 and HPV-52 are the most commonly identified genotypes in FSWs. HPV-16 is considered as the most prominent type involved in the development of cervical cancer and other HPV‐associated malignancies. HPV‐52 is also an oncogenic HPV type, which is closely related phylogenetically to HPV‐16. Previous studies have shown that HPV-52 is the sixth most frequently detected HPV high-risk type in cervical intraepithelial neoplasia grade 3 (CIN3) and invasive cervical cancer . According to our findings, FSWs are strongly susceptible to the development of cervical intraepithelial neoplasia and cervical cancer.
Similar to Asian countries, the prevalence of HPV positivity was significantly high among European FSWs. Despite high coverage HPV vaccination among females in Europe, our meta-analysis indicated that HPV infection is very common among FSWs in the Netherlands, Belgium, Bulgaria, Denmark, Hungary, and Spain, with a prevalence of HPV infection between 30% and 80%. Our explanation is that vaccination does not protect against HPV types other than 6, 11, 16, and 18. For instance, the majority of FSWs in the Netherlands was infected with types different than what covered by the current vaccines . To overcome this problem, we recommend the use of a nine-valent vaccine (9vHPV) instead of the bivalent and quadrivalent vaccines. The 9vHPV vaccine contains type 6, 11, 16, 18, 31, 33, 45, 52 and 58 which was approved by the FDA in December 2014, and by the European Medicines Agency (EMA) in June 2015 .
Based on the results of the study, we found that the variations in the prevalence of HPV infection in FSWs across the studies could not be explained by the difference in detection methods. This is due to that the detection rates were similar for HPV using PCR, hybridization, and PCR-hybridization, which were applied in 60 (93.7%) studies. However, sample type may be one of the factors leading to differences in prevalence rates. To confirm this finding, our meta-analysis demonstrated that the detection rate of HPV using vaginal samples was significantly higher than cervical, endocervical, and cervico-vaginal samples. We concluded that the vaginal sample is more sensitive for detecting HPV and has a higher level of HPV DNA than the other genital specimens in FSWs. Furthermore, vaginal sampling is less invasive method, and is easily available for all women at the time of a regular HPV test. Owing to the high prevalence of HPV in vaginal samples, vaginal douching with disinfectants after sex with clients seemed to be an effective practice in reduction of HPV transmission.
In some countries, such as Thailand, Singapore, and Iran, the HPV prevalence is unexpectedly low, and we believe that this is due to several reasons, like limited HPV screening practices, low socioeconomic status, illegality of sex work, severely limited support systems, unsafe workplaces, fear of stigmatization, and lack of education or skills. Thus, it is so likely that the obtained results in our meta-analysis may not be a precise estimate of the HPV prevalence in these regions.
The present study has some limitations that need to be considered during the interpretation of our results. First, a significant part of the studies investigating the HPV prevalence among FSWs did not perform analysis of HPV genotype distribution, and thus we could not include their results in our meta-analysis of genotype distribution of HPV. Second, despite the subgroup analyses, significant heterogeneity still existed, suggesting that it arises from other sources that we could not characterize. Finally, there were no published data on the prevalence of HPV infection among FSWs in so many countries such as the United States, Canada, Russia, France, Germany, Italy, the United Kingdom, Nigeria, South Africa, Cameroon, and Arabian Peninsula.