In this retrospective cohort study conducted among active duty soldiers stationed at Fort Bragg, NC, the crude incidence rate of Ct infection of 21.7 per 1,000 person-was nearly twice the rate (11.4 per 1,000 person-years) previously reported for active duty military personnel by the Armed Forces Health Surveillance Center during 2008 to 2009 , which raises a suspicion about the influence of high STI rates in the southeastern United States. National notifiable disease data indicate that the crude incidence rate of Ct per 1,000 population ranged from 3.02 in the New England area to 5.10 per 1,000 in the East South Central region of the U.S., though it should be noted that figures are not age- or sex-adjusted for direct comparison to military populations . In their Ct prevalence study of female Army recruits, Gaydos and colleagues found an association with origin from any of 5 southern states, including East (Alabama, Mississippi) and West (Louisiana) South Central states. The implicated South Atlantic states (South Carolina, Georgia) did not include North Carolina . However, state-level data show that during 2005 to 2009, the average annual incidence in Cumberland County—which surrounds Fort Bragg—was 7.99, and this was twice the rate for North Carolina as a whole during the same period (3.91) .
About 1 in 10 Fort Bragg soldiers who were found to have an incident Ct infection in this study were later discovered to have Ct again, on average within a year of the initial infection. There were 223 new positive Ct diagnoses overall, translating to a crude recurrent infection rate of 110.7 per 1,000 person-years of follow-up. This rate is generally in line with other studies, particularly if accounting for differences in population selectivity [16, 17]. Since some proportion of presumed incident infections at Fort Bragg likely represents prevalent infections that had not been treated, but were detected on screening, the rate of recurrent infection may serve as a better indicator of locally endemic disease, or of the behavioral and other risk factors influencing transmission.
Our analysis supports previously published findings regarding the association of Ct infection with young age , [12, 14, 16, 17, 19–25] black race [14, 16, 18, 20–23, 26, 27], not having more than a high school education  and being single . Young age was not found to be associated with recurrent infection in men, and this supports the findings of Dunne and colleagues.  In that study, however, men with less than a high school education appeared to be at greater risk of recurrent infection, whereas at Fort Bragg the lower recurrent infection rate associated with a higher education level was only found to hold in women after adjustment for significant covariates.
Overall, women were more than twice as likely to experience recurrent infection as men. Since directly observed, single-dose azithromycin treatment is a reliable procedure at Fort Bragg—with only rare cases of macrolide sensitivity prompting a doxycycline regimen instead—few if any of the presumed reinfections were likely to represent non-compliance, treatment failure, relapse, or persistent infection. The likelihood of screening despite no genitourinary symptoms was probably greater among women; and this may also underlie the significant difference in the time elapsed between initial and subsequent infection, which was a median of 6.7 months for females, but only 3.9 months for males, in whom symptoms may have prompted follow-up screening more often than in women. Time away from Fort Bragg (deployment as well as other breaks in duty) was associated with a lower Ct infection rate in both men and women. Non-deployment breaks in duty were also analyzed with respect to recurrent Ct infection, and were associated with a lower recurrence rate among women (though the adjusted hazard ratio did not reach statistical significance). The mobility-related findings were adjusted for several factors including age, pay grade, marital status, race, and education. While opportunity bias may have affected the findings to a degree, person-time from absence periods was not part of incidence rate calculations, and there were actually more person years of observation at Fort Bragg among those who experienced deployment (60,374) than among those who did not (40,776). It also happens that the number of soldiers who deployed during the study period (33,630) was similar to the number who did not (33,795).
Soldiers’ history of prior deployments, a longer term transience indicator than deployment or other breaks in duty during the Fort Bragg assignment, was associated with lower incident and recurrent infection rates only in women. During the study period, medical readiness screening of women prior to overseas deployment included Chlamydia testing. Thus there was increased compliance with periodic screening—and treatment as indicated—in women who deployed. This likely resulted in a smaller proportion of women having chronic, asymptomatic infection if they previously deployed, compared to women who had no deployment history. Similarly, women who are screened for Chlamydia may be more likely to take precautions to prevent infection or reinfection; and the screening encounter alone may raise sufficient awareness even among uninfected patients. Further study would be needed to bear this out.
The ‘healthy worker effect’ may have a role in the lower infection rates associated with deployment. Factors that medically or administratively preclude soldiers from deploying to a combat theater may also be associated with a higher risk of Ct infection. On the other hand, after deployed soldiers—particularly males--return to their home stations following a combat mission, their risk-taking behavior that was recently projected on the battlefield may be diverted toward their social interactions, including those with local civilian populations. This is supported by the preliminary findings of a behavioral survey conducted at Fort Bragg [personal communication, Womack Army Medical Center, Fort Bragg]. Behavioral factors may explain the diagnosis of approximately half of incident and recurrent infections within six months of return from a deployment.
The findings of this investigation should be interpreted with caution due to a number of factors. Firstly, the incidence of Ct may have been underestimated using RMEs alone. In a comparison of Ct laboratory results from 2010 to 2012 to RMEs, 36.1% (females) and 38.7% (males) who had a positive test result did not have a RME for Ct within 30 days of the result. However, the proportion of service members with RMEs and a positive test was >82% . Secondly, Ct infection may occur or become manifest while soldiers are deployed, or during rest and relaxation periods away from the combat zone. Such cases would be treated before soldiers’ return to their home garrison setting, and would thus not be captured as disease reports that would permit a direct comparison to garrison rates.
Thirdly, compared to soldiers who attend the Fort Bragg STI clinic, those who avoid military facilities for Chlamydia treatment and self-pay or use alternative insurance to keep their infections from appearing in transferred health records may be very different with respect to one or more of the covariates examined in this study. Rates may thus be underestimated, and the effect of risk factors either diluted or exaggerated. On the other hand, the high Chlamydia rate at Fort Bragg compared to other military installations may be attributable not only to endemic disease in the local community, but also enhanced case finding and reporting at Fort Bragg compared to other military locations. With respect to possible underreporting across the military, the degree to which patients seek care outside of the military treatment facility is not known. Still, administrative and patient care factors probably do not account for most of the variance in Ct rates within the Army, as higher rates are noted at locations where civilian care is readily available.
Finally, valid comparisons between women and men are limited by differences in regular screening practices. By U.S. military policy women under 30 years of age are regularly screened for Ct during annual well woman examinations and military women younger than 26 years of age are routinely screened for Ct infection during their initial entry training. Also, distinguishing reinfection from persistent infection is difficult; and two additional, related factors are the high proportion of Ct-infected persons without symptoms, and variability in clearance of the infection. Lechner et al. determined prevalence rates of 15% and 11% for females and males, respectively, when they screened sexually active young adults, as well as adolescents who were children of military personnel. These were asymptomatic patients attending military-run clinics in San Antonio . The relatively small difference in rates between asymptomatic females and males suggest that when male rates are calculated primarily from those seeking care for urethritis they may underestimate the incidence or prevalence of actual infections. Information regarding the reason for diagnostic testing, compliance with antimicrobial treatment, treatment of partners, and tests of cure could not be determined from the available data in the present study.
Despite its limitations, this descriptive study provides evidence of sufficient statistical power to influence ongoing surveillance and prevention efforts. The population observed for this study is substantially larger than sample populations in most other published reports about Ct risk factors and recurrent infection that use data from a single geographic location—with the notable exception of descriptive analyses based on national or multi-regional surveillance data [17, 23]. Moreover, the cohort represents a broad cross-section of the U.S. population as every region contributes to the military population [31, 32] and therefore may better permit generalizing results for comparison to national notifiable disease data, perhaps to a greater degree than other studies of predominantly young populations based on a specific state, city, college, or school district.