Introduction {#sec1-1}
============

*Chlamydia trachomatis (C. trachomatis)* is a ubiquitous, obligate, intracellular Gram-negative bacterial pathogen \[[@ref1]\].  Humans are the only natural host of *C. trachomatis* \[[@ref2]\].  This bacterium remains a significant public health burden worldwide \[[@ref3]\].  It is estimated that 100 million cases of *C. trachomatis* infection occur annually \[[@ref4]\]. * C. trachomatis* is the pathogen that is most often transmitted by sexual contact \[[@ref5]\].  The clinical spectrum of infection with *C. trachomatis* varies from asymptomatic to several acute or chronic, local, and systemic diseases such as trachoma, oculo-genital, and neonatal diseases \[[@ref6]\].  Infections with *C. trachomatis* of the lower female genital tract are frequently asymptomatic, undiagnosed, and untreated \[[@ref4]\].  Pelvic inflammatory disease attributed to ascending genital *C. trachomatis* infections can result in ectopic pregnancies and infertility in women \[[@ref4]\].  In addition, a recent meta-analysis demonstrated that individuals infected with *C. trachomatis* have a higher risk of cervical cancer \[[@ref8]\]. * C. trachomatis* causes inclusion conjunctivitis of the newborn, with the female birth canal being the reservoir \[[@ref9]\].  In addition, infection with *C. trachomatis* causes pneumonia and sepsis \[[@ref10]\].  In men, *C. trachomatis* causes urethritis and epididymitis \[[@ref11]\]. 

Little is known about the epidemiology of *C. trachomatis* infection in Mexico.  The prevalence of *C. trachomatis* in women in Mexico has been determined in several population groups; for instance, 4% prevalence was reported in women in Cuernavaca City \[[@ref12]\], 3.3% prevalence was found in women with leucorrhea in Mexico City \[[@ref13]\], and 7.3% prevalence was reported in women in rural and suburban Oaxaca State \[[@ref14]\].  However, female sex workers have shown higher prevalence of *C. trachomatis* infection.  Uribe-Salas et al.  \[[@ref15]\] reported 11.1% prevalence of *C. trachomatis* in female sex workers in Mexico City.  Whereas in 3 northern Mexican states, prevalence between 12.4% and 16.6% were found in female sex workers \[[@ref16]\].  In the present study, by using a different study design (case--control) and laboratory method (polymerase chain reaction \[PCR\]) compared to those used in previous studies in female sex workers in Mexico, we aimed to determine the following: (1) the association between *C. trachomatis* infection and female sex work and (2) the sociodemographic, obstetric, and behavioral characteristics of female sex workers associated with *C. trachomatis* infection. 

Materials and Methods {#sec1-2}
=====================

Study Design and Women Studied {#sec2-1}
------------------------------

Through a case--control study design, we studied female sex workers (cases) and women with occupations other than sex work (controls) in Durango City from November 2014 to May 2016.  Female sex workers were enrolled at the Sanitary Inspection Clinic of the Municipal Government in Durango City, Mexico.  The inclusion criteria for enrollment of female sex workers were female sex workers (1) registered in the Sanitary Inspection Clinic, (2) worked in the sex industry for at least one year, and (3) aged ≥18 years old.  Control subjects were women without sex work.  Controls were sexually active women and enrolled at the Clinic for Family Planning in the Institute for Scientific Research.  This public clinic is in Durango City, Mexico, and is part of Juárez University of Durango State. 

Sociodemographic, Obstetric, and Behavioral Characteristics of Cases {#sec2-2}
--------------------------------------------------------------------

We recorded the sociodemographic, obstetric, and behavioral data of cases with the aid of a questionnaire.  Data about age, residence, birthplace, education, socioeconomic status, history of pregnancies, cesarean sections, miscarriages, and deliveries from all sex workers were obtained.  Behavioral factors included duration (years) in the sex industry, area of work (urban, suburban, or rural), sex work in Mexican states other than Durango or abroad, contact with semen during vaginal intercourse, condom use, practice of oral or anal sex, injuries at sex work, and consumption of alcohol, tobacco or drugs. 

DNA Extraction and Amplification {#sec2-3}
--------------------------------

Cervical swab samples were obtained from participants and stored in a home-made 2SP (sucrose--phosphate) medium at --70 °C until analysis.  DNA was extracted from cervical samples by proteinase K digestion and phenol--chloroform extraction. * C. trachomatis* DNA was detected by PCR using primers of the *omp* A gen of *C. trachomatis* \[[@ref18]\].  These primers generate the amplification of a 129-base pair fragment.  The amplification mixture contained 1.5 mM MgCl~2~, 0.3 µM primers, 1 unit of Taq polymerase (Promega, Madison WI, EUA), 0.16 mM dNTPs, 2 ng DNA, and buffer in a 50-µL reaction volume.  This amplification mixture was denatured at 95 °C for 10 min.  Then, samples were amplified in 34 cycles.  Each cycle consisted of the following: denaturation at 95 °C for 1 min, annealing at 61 °C for 2 min, and extension at 70 °C for 1 min.  Samples underwent a final extension at 70 °C for 10 min.  Amplification products were analyzed by electrophoresis in 2% agarose gel and stained with 0.1% ethidium bromide. 

Statistical Analysis {#sec2-4}
--------------------

Data were analyzed with the software SPSS version 15.0 (SPSS Inc. Chicago, Illinois).  We calculated the odds ratio (OR) with 95% confidence interval (CI) to assess the association between *C. trachomati*s infection and sex work occupation.  In addition, the association between *C. trachomatis* infection and the sociodemographic, obstetric, and behavioral characteristics of the female sex work were analyzed by the Pearson\'s chi-squared test and the Fisher exact test (when values were 5 or less).  Variables of sex workers with a *P* value ≤0.20 obtained by bivariate analysis were further analyzed by multivariate analysis.  We calculated the ORs and 95% CIs by regression analysis with the Enter method.  Statistical significance was set at *P* value \<0.05. 

Ethics {#sec2-5}
------

The Ethics Committee of the Institute for Scientific Research of the Juarez University of Durango State, Mexico, approved this project.  Participation in the study was voluntary.  Written informed consent was obtained from all cases and controls before enrollment. 

Results {#sec1-3}
=======

Four hundred and two women were enrolled in this study.  Half (*n* = 201) of them were cases (female sex workers), and 201 were controls (age-matched women).  Mean ages in cases and controls were 33.06 ± 9.76 (range: 18--67) years old and 33.08 ± 9.79 (range: 17--67) years old, respectively (*P* = 0.98). * C. trachomatis* DNA was detected in 32 (15.9%) of the 201 cases and in 6 (3.0%) of the 201 controls.  The difference in the prevalence of *C. trachomatis* infection found in cases and controls was statistically significant (OR = 6.15; 95% CI: 2.5--15.0; *P* \< 0.001).  Bivariate analysis of socioeconomic characteristics of female sex workers showed no association (*P* \> 0.05) between *C. trachomatis* infection and age, birthplace, residence, educational level, or socioeconomic status ([Table 1](#table001){ref-type="table"}).  The frequency of infection with *C. trachomatis* in female sex workers did not vary *(P* \> 0.05) regardless of the history of pregnancies, deliveries, cesarean sections, or miscarriages.  With respect to behavioral characteristics, the variables, namely, duration in the sex industry, practice of oral sex, and alcohol consumption, showed *P* values ≤0.20 by bivariate analysis ([Table 2](#table002){ref-type="table"}), whereas other behavioral characteristics including area of work, sex work in Mexican states other than Durango or abroad, contact with semen during vaginal intercourse, condom use, practice of anal sex, injuries at sex work, consumption of tobacco, or drugs showed *P* values \>0.20 by bivariate analysis.  Regression analysis of the 3 behavioral characteristics with *P* values ≤0.20 obtained by bivariate analysis showed that the infection with *C. trachomatis* was associated only with consumption of alcohol (OR = 2.39; 95% CI: 1.0--5.71; *P* = 0.04) ([Table 3](#table003){ref-type="table"}). 

Discussion {#sec1-4}
==========

The epidemiology of *C. trachomatis* infection in female sex workers in Mexico has been scantly studied.  We therefore aimed to determine the association between *C. trachomatis* infection and the occupation of female sex worker in Durango City, Mexico.  We found a significantly higher frequency of *C. trachomatis* DNA in cervical samples from female sex workers than in those from control women without sex work occupation.  This finding indicates that *C. trachomatis* infection is associated with the occupation of female sex worker.  This age- and gender-matched case--control study thus demonstrates that female sex workers in Durango City have an increased risk of *C. trachomatis* infection.  Comparison of our results with those reported in studies of female sex workers in Mexico is limited and should be interpreted with care because differences in the study design and in laboratory methods among the studies exist.  There are few reports on the prevalence of *C. trachomatis* in female sex workers in Mexico.  In a study performed in 3 cities (Durango, Zacatecas, and Torreón) in the north of Mexico, prevalence values between 5.2% and 16.2% of *C. trachomatis* infection were found \[[@ref17]\].  However, in such study, a cross-sectional design and an enzyme immunoassay which detects *C. trachomatis* antigen in endocervix were used \[[@ref17]\].  In the present study, we used a case--control study design, and diagnoses of *C. trachomatis* infection were based on *C. trachomatis* DNA detection using PCR.  In a study of female commercial sex workers in Mexico City, researchers found a prevalence of 11.1% of *C. trachomatis* infection obtained by cultures of the bacterium in McCoy cells and identification with fluoresceinated monoclonal antibodies \[[@ref15]\].  In an additional study, the same research group found 14.4% prevalence of *C. trachomatis* infection in female sex workers at the Mexico--Guatemalan border by analyzing cervical swab samples using a nonradioactive nucleic acid hybridization assay \[[@ref19]\].  In a further study, 25.0% of female commercial sex workers in Mexico City had anti*-C. trachomatis* IgG antibodies \[[@ref20]\].  However, our results obtained by detection of *C. trachomatis* DNA cannot be compared with those of seroprevalence of *C. trachomatis* infection.  We compared our results with those obtained in studies of female sex workers in other countries using PCR methodology.  The prevalence (15.9%) of *C. trachomatis* infection found in our study is higher than the 6.6% prevalence of *C. trachomatis* endocervical infection reported in female sex workers in Hungary using plasmid PCR \[[@ref21]\].  In contrast, the prevalence found in our study is lower than a 28.5% prevalence of *C. trachomatis* cervical infection found in female sex workers in Dakar, Senegal, using PCR \[[@ref22]\].  The association between *C. trachomatis* infection and female sex workers found in our study agrees with the same association found in a study in Bangladesh, where researchers found a significantly higher prevalence value of *C. trachomatis* infection in 40 female sex workers (58%) than in 110 sexually active women (27%) using immunochromatography test and/or plasmid PCR \[[@ref23]\]. 

We found an association between *C. trachomatis* infection and alcohol consumption in female sex workers.  In a search of this association in the biomedical literature, we did not find any report.  It is possible that alcohol consumption in female sex workers might be linked to risky behavioral factors for *C. trachomatis* infection.  Further research about this association should be conducted. 

Conclusions {#sec1-5}
===========

We conclude that *C. trachomatis* infection is associated with female sex work in Durango City, Mexico.  This is the first age- and gender-matched case--control study on the prevalence of *C. trachomatis* infection in female sex workers in Mexico using detection of *C. trachomatis* DNA in cervical samples.  The association between *C. trachomatis* and consumption of alcohol deserves further investigation. 

**Funding Source**

This study was financially supported by Juarez University of Durango State, Mexico. 

**Authors\' Contributions**

LFSA and NVH conceived the study protocol, performed the data analysis, and wrote the manuscript.  FMGI and JHT performed data analysis and wrote the manuscript.  JANF obtained the samples and clinical data.  MAD, ARPA, AASC, and EIAS performed laboratory tests and data analysis.  SEM performed statistical analysis.  CAE participated in the design of the protocol, performed data analysis and wrote the manuscript.  All authors read and approved the final version of the manuscript. 

**Conflict of Interest**

The authors declare that no conflict of interest exists. 

###### 

Sociodemographic characteristics of female sex workers and prevalence of *C. trachomatis* infection

  Characteristic          Women tested   Prevalence of *C. trachomatis* infection   *P* value   
  ----------------------- -------------- ------------------------------------------ ----------- ------
  Age groups (years)                                                                            
  ≤20                     10             0                                          0.0         
  21--30                  82             14                                         17.1        0.56
  31--40                  64             11                                         17.2        
  ≥41                     45             7                                          15.6        
  Birth place                                                                                   
  Durango State           163            24                                         14.7        
  Other Mexican state     38             8                                          21.1        0.33
  Residence place                                                                               
  Durango State           200            32                                         16.0        
  Other Mexican state     1              0                                          0.0         1.00
  Education (years)                                                                             
  No education            4              0                                          0.0         
  1 to 6                  58             11                                         19.0        0.38
  7--12                   129            21                                         16.3        
  \>12                    10             0                                          0.0         
  Socio-economic status                                                                         
  Low                     39             8                                          20.5        
  Medium                  160            24                                         15.0        0.4

###### 

Correlation between behavioral characteristics of female sex workers and *C. trachomatis* infection

  Characteristic                                      Women tested   Prevalene of *C. trachomatis* infection   *P* value   
  --------------------------------------------------- -------------- ----------------------------------------- ----------- ------
  Duration in the sex industry (years)                                                                                     
  1 to 5                                              20             1                                         5.0         0.14
  6 to 11                                             47             11                                        23.4        
  ≥11                                                 134            20                                        14.9        
  Area of work                                                                                                             
  Urban                                               196            32                                        16.3        0.74
  Suburban                                            2              0                                         0.0         
  Rural                                               1              0                                         0.0         
  Sex activity in Mexican states other than Durango                                                                        
  Yes                                                 58             9                                         15.5        1.0
  No                                                  142            23                                        16.2        
  Sex activity abroad                                                                                                      
  Yes                                                 3              1                                         33.3        0.4
  No                                                  198            31                                        15.7        
  Condom use                                                                                                               
  Yes                                                 115            19                                        16.5        0.77
  No                                                  80             12                                        15.0        
  Contact with semen during vaginal intercourse                                                                            
  Yes                                                 86             13                                        15.1        0.74
  No                                                  113            19                                        16.8        
  Practice of oral sex                                                                                                     
  Yes                                                 121            16                                        13.2        0.18
  No                                                  79             16                                        20.3        
  Practice of anal sex                                                                                                     
  Yes                                                 17             1                                         5.9         0.31
  No                                                  182            31                                        17.0        
  Injuries during sex activity                                                                                             
  Yes                                                 12             2                                         16.7        1.0
  No                                                  187            30                                        16.0        
  Alcohol consumption                                                                                                      
  Yes                                                 36             10                                        27.8        0.03
  No                                                  165            22                                        13.3        
  Tobacco consumption                                                                                                      
  Yes                                                 41             5                                         12.2        0.63
  No                                                  160            27                                        16.9        
  Drug use                                                                                                                 
  Yes                                                 13             3                                         23.1        0.43
  No                                                  188            29                                        15.4        

###### 

Results of the regression analysis of selected behavioral characteristics of female sex workers and infection with C. trachomatis

  Characteristic         Odds ratio   95% Confidence interval   *P* value
  ---------------------- ------------ ------------------------- -----------
  Duration in the sex                                           
  industry (≥5 years)    0.31         0.03-2.43                 0.26
  Practice of oral sex   0.59         0.27-1.28                 0.59
  Alcohol consumption    2.39         1.0-5.71                  0.04
