Abstract

This report summarizes incidence rates of the 5 most common sexually transmitted infections (STIs) among active component service members of the U.S. Armed Forces during 2010–2018. Infections with chlamydia were the most common, followed in decreasing order of frequency by infections with genital human papillomavirus (HPV), gonorrhea, genital herpes simplex virus (HSV), and syphilis. Compared to men, women had higher rates of all STIs except for syphilis. In general, compared to their respective counterparts, younger service members, non-Hispanic blacks, soldiers, and enlisted members had higher incidence rates of STIs. During the latter half of the surveillance period, the incidence of chlamydia and gonorrhea increased among both male and female service members. Rates of syphilis increased for male service members but remained relatively stable among female service members. In contrast, the incidence of genital HPV and HSV decreased among both male and female service members. Similarities to and differences from the findings of the last MSMR update on STIs are discussed.

What Are the New Findings?

The incidence of chlamydia and gonorrhea increased among male and female service members in the latter half of the surveillance period, while the incidence of genital HPV and HSV decreased. Among male service members, the incidence of syphilis increased sharply between 2012 and 2018.

What Is the Impact on Readiness and Force Health Protection?

STIs can adversely impact service members' availability and ability to perform their duties and can result in serious medical sequelae if untreated. Establishing standards for screening, testing, treatment, and reporting would likely improve efforts to detect STI-related health threats. Continued behavioral risk-reduction interventions are needed to counter the increasing incidence of STIs among service members.

Background

Sexually transmitted infections (STIs) are relevant to the U.S. military because of their relatively high incidence, adverse impact on service members availability and ability to perform their duties, and potential for serious medical sequelae if untreated.¹ Two of the most common bacterial STIs are Chlamydia trachomatis (chlamydia) and Neisseria gonorrhoeae (gonorrhea). Rates of chlamydia and gonorrhea have been steadily increasing in the general U.S. population among both men and women since 2000.² A Sept. 2017 MSMR report documented almost 180,000 incident infections of chlamydia and more than 29,000 incident infections of gonorrhea among active component U.S. military members between 2007 and 2016, with no overall decrease in incidence rates during the 10-year period.³

Another important bacterial STI is syphilis, which is caused by the bacterium Treponema pallidum. Rates of primary and secondary syphilis in the U.S. increased 72.7%, from 5.5 cases per 100,000 persons in 2013 to 9.5 cases per 100,000 persons in 2017.² These trends are mirrored in the active component of the U.S. Armed Forces, in which the incidence of syphilis (of any type) doubled between 2007 and 2016, with most of the increase occurring among men.³ Although these 3 relatively common bacterial STIs are curable with antibiotics, there is continued concern regarding the threat of multidrug resistance.⁴

Common viral STIs in the U.S. include infections caused by human papillomavirus (HPV) and genital herpes simplex virus (HSV). HPVs are DNA viruses that infect basal epithelial (skin or mucosal) cells. HPV genotypes 6 and 11 are responsible for 90% of all genital wart infections.⁵ HSV can cause genital or oral herpes infections that are characterized by the appearance of 1 or more vesicles that can break and leave painful ulcers. Most genital herpes infections are caused by type 2 (HSV-2); however, type 1 (HSV-1), which is most often associated with oral herpes infection, is estimated to be responsible for 50% of new genital herpes infections.⁶ Neither HPV nor HSV viral infections are curable with antibiotics; however, suppression of recurrent herpes manifestations is attainable using antiviral medication, and there is a vaccine to prevent infection with 4 of the most common HPV serotypes. From 2007 through 2016, the overall incidence rates of genital HPV and HSV in the active component were 60.1 and 23.3 cases per 10,000 person-years (p-yrs), respectively.³

The current analysis updates the findings of previous MSMR articles on STIs among active component service members.^1,3 Specifically, the current report summarizes incident cases and incidence rates of 5 of the most common STIs among active component military members during 2010–2018 by demographic and military characteristics.

Methods

The surveillance period was 1 Jan. 2010 through 31 Dec. 2018. The surveillance population consisted of all active component service members of the U.S. Army, Navy, Air Force, or Marine Corps who served at any time during the period. Diagnoses of STIs were ascertained from medical administrative data and reports of notifiable medical events routinely provided to the Armed Forces Health Surveillance Branch and maintained in the Defense Medical Surveillance System for surveillance purposes. STI cases were also derived from positive laboratory records in the Health Level 7 (HL7) chemistry and microbiology databases maintained by the Navy and Marine Corps Public Health Center at the EpiData Center.

For each service member, the number of days in active military service was ascertained and then aggregated into a total for all service members during each calendar year. The resultant annual totals were expressed as p-yrs of service and used as the denominators for the calculation of annual incidence rates. Person-time that was not considered to be time at risk for each STI was excluded (i.e., the 30 days following each incident chlamydia or gonorrhea infection and all person-time following the first diagnosis, medical event report, or positive laboratory test of HSV, HPV, or syphilis).

An incident case of chlamydia was defined by having any of the following: 1) a case-defining diagnosis (Table 1) in the first or second diagnostic position of a record of an outpatient or in-theater medical encounter, 2) a confirmed notifiable disease report for chlamydia, or 3) a positive laboratory test for chlamydia (any specimen source or test type). An incident case of gonorrhea was similarly defined by having 1) a case-defining diagnosis in the first or second diagnostic position of a record of an inpatient or outpatient or in-theater encounter, 2) a confirmed notifiable disease report for gonorrhea, or 3) a positive laboratory test for gonorrhea (any specimen source or test type). For both chlamydia and gonorrhea, an individual could be counted as having a subsequent case only if there were more than 30 days between the dates on which the case-defining diagnoses were recorded.

Incident cases of HSV were identified by 1) the presence of the requisite ICD-9 or ICD-10 codes in either the first or second diagnostic positions of a record of an outpatient or in-theater encounter or 2) a positive laboratory test from a genital specimen source. Antibody tests were excluded because they do not allow for distinction between genital and oral infections. Incident cases of HPV were similarly identified by 1) the presence of the requisite ICD-9 or ICD-10 codes in either the first or second diagnostic positions of a record of an outpatient or in-theater encounter or 2) a positive laboratory test from any specimen source or test type. Outpatient encounters for HPV with evidence of an immunization for HPV within 7 days before or after the encounter date were excluded as were outpatient encounters with a procedural or Current Procedural Terminology (CPT) code indicating HPV vaccination, as such encounters were potentially related to the vaccination administration. An individual could be counted as an incident case of HSV or HPV only once during the surveillance period. Individuals who had diagnoses of HSV or HPV infection prior to the surveillance period were excluded from the analysis.

An incident case of syphilis was defined by having 1) a qualifying ICD-9 or ICD-10 code in the first, second, or third diagnostic position of a hospitalization, 2) at least 2 outpatient or in-theater encounters within 30 days of each other, with a qualifying ICD-9 or ICD-10 code in the first or second position, 3) a confirmed notifiable disease report for any type of syphilis, or 4) a record of a positive polymerase chain reaction or treponemal laboratory test. Stages of syphilis (primary, secondary, late, latent) could not be distinguished because the HL7 laboratory data do not allow for differentiation of stages and because there is a high degree of misclassification associated with the use of ICD diagnosis codes for stage determination.^7,8 An individual could be considered an incident case of syphilis only once during the surveillance period; those with evidence of prior syphilis infection were excluded from the analysis.

Results

Between 2010 and 2018, the number of incident chlamydia infections among active component service members was greater than that of any other single STI and approximately 3 times the total number of genital HPV infections—the next most frequently identified STI during this period (Table 2). With the exception of syphilis, the overall incidence rates of all STIs were markedly higher among women than men. For chlamydia, gonorrhea, and syphilis, overall incidence rates were highest among those aged 24 years or younger and decreased with advancing age. However, overall rates of genital HSV and HPV were highest among those aged 20–24 years and those aged 25–29 years. Rates of all STIs were highest among non-Hispanic black service members compared to other race/ethnicity groups. For chlamydia, gonorrhea, and genital HSV, overall rates were highest among members of the Army. The overall incidence rate of syphilis was highest among Navy members, and the overall rate of genital HPV was highest among Air Force members. Compared to their respective counterparts, enlisted service members and those with lower levels of educational achievement tended to have higher overall rates for all STIs. Married service members had the lowest incidence rates of all 5 STIs compared to service members who were single and never married or of other/unknown marital status. Overall rates of chlamydia, gonorrhea, and syphilis were highest among those working in motor transport. In contrast, genital HPV rates were highest among those in health care occupations, and the highest rates of genital HSV were among those working in communications/intelligence, health care, or motor transport (Table 2). Patterns of incidence rates over time for each specific STI are described in the subsections below.

Chlamydia

During the surveillance period, annual incidence rates of chlamydia among service women generally ranged from 3 to 5 times the rates among men. Annual rates among men and women combined increased 56.6% between 2013 and 2018, with rates among both sexes peaking in 2018 (men: 175.1 per 10,000 p-yrs; women: 513.1 per 10,000 p-yrs) (Figure 1). In both sexes, the increase was primarily attributed to service members in the youngest age groups (less than 25 years among women; less than 30 years among men) (data not shown).

Among service women in each race/ethnicity group, annual rates of chlamydia increased among those under 25 years of age during 2013–2018 but remained relatively stable among those aged 25–34 years and among those aged 35 years or older (Figure 2). Among service men, annual rates of chlamydia increased between 2013 and 2018 in all age and race/ethnicity groups less than 35 years old but remained relatively stable among those in older age groups (Figure 3).

Genital HPV

The annual incidence rates of diagnoses of genital HPV decreased 51.9% among all active component service members from the beginning to the end of the surveillance period, with the most dramatic decrease occurring among women (Figure 4). There was a dip in the overall incidence of genital HPV in 2013 at 55.6 cases per 10,000 p-yrs, but the lowest point was reached in 2018 at 41.9 cases per 10,000 p-yrs. Incidence rates among female service members declined by almost 50% during the surveillance period, from a high of 372.1 cases per 10,000 p-yrs in 2010 to a low of 191.3 cases per 10,000 p-yrs in 2018 (Figure 4). Rates among men also decreased, from 45.5 per 10,000 p-yrs in 2010 to 16.0 per 10,000 p-yrs in 2018. The decrease in the incidence among both men and women was attributable to a decrease in the rates in the youngest age groups (less than 30 years) (Figures 5, 6). Age-specific time trends were similar when stratified by race/ethnicity, in that the incidence of genital HPV decreased in the youngest age groups among service members in all race/ethnicity groups (data not shown).

Gonorrhea

Between 2012 and 2018, annual incidence rates of gonorrhea increased by 55.3% and 33.6% among male and female service members, respectively (Figure 7). The increase in gonorrhea incidence between 2012 and 2018 was primarily driven by increases among women less than 25 years of age and among men less than 30 years of age (Figures 8, 9). The ratio of the annual incidence rate for women compared to men was 2.1 in 2010 but dropped to 1.4 in 2018. The incidence of gonorrhea increased during the surveillance period among all race/ethnicities, with the sharpest increase occurring among non-Hispanic Black service members between 2015 and 2018 (data not shown). The incidence increased during the surveillance period among the youngest age groups for service members in all race/ethnicity groups (data not shown).

Genital HSV

Incidence rates of genital herpes infections decreased slightly from 25.3 to 20.4 per 10,000 p-yrs during the surveillance period. Rates among female service members ranged from a high of 74.8 per 10,000 p-yrs in 2010 to 64.0 per 10,000 p-yrs in 2018. Men’s rates also peaked in 2010 (17.2 per 10,000 p-yrs) and reached their lowest point in 2018 (12.1 per 10,000 p-yrs) (Figure 10). Among women, the highest rates were observed among those less than 25 years of age, while the highest rates among men were among those aged 25–29 or 20–24 years (data not shown). The incidence of genital HSV decreased among all age groups during the surveillance period, although the sharpest decrease occurred among service members aged 30 years and older (data not shown). In addition, the incidence decreased among all race/ethnicities during the surveillance period except for Asian/Pacific Islanders. The decrease was most notable among non-Hispanic Black service members, who saw a decline from a high of 49.0 per 10,000 p-yrs in 2011 to a low of 37.4 per 10,000 p-yrs in 2018 (data not shown).

Syphilis

The incidence rate for syphilis in the last year of the surveillance period was 2.7 times that observed in 2010, with the increase primarily driven by cases identified in male service members (Figure 11). Rates of syphilis steadily increased among men during the surveillance period, with the sharpest increase occurring after 2012. Among women, rates increased from 2010 to 2014 but leveled off during the remainder of the surveillance period. The incidence of syphilis increased with advancing age among both men and women (data not shown). Among men, the pattern of increasing incidence by age was consistent among all race/ethnicity groups; there were not enough cases to evaluate associations with age and race/ethnicity among women (data not shown).

Editorial Comment

During the last few years of the surveillance period, the annual incidence rates of chlamydia, gonorrhea, and syphilis increased among male service members, and the annual incidence of chlamydia and gonorrhea increased among female service members. Rates of syphilis remained relatively stable among female service members during the latter half of the surveillance period. In contrast, the incidence of genital HPV and HSV decreased among both male and female service members. Overall rates of STIs were higher among women when compared to men for HPV, HSV, gonorrhea, and chlamydia. Syphilis was the only STI in this analysis for which the incidence was, on average, higher among male compared to female service members.

Higher rates of most STIs among women can likely be attributed to implementation of the services’ screening programs for STIs among female service members as they enter active service and during the subsequent annual screenings for women under age 26. Because asymptomatic infection with chlamydia, gonorrhea, or HPV is common among sexually active women, widespread screening may result in sustained high numbers of infections diagnosed among young women. Rates of chlamydia and gonorrhea increased among both male and female service members during the latter half of the surveillance period. This trend is similar to the increasing rates in the civilian population. In the U.S., rates of chlamydia have been increasing among both men and women since 2000, and rates of gonorrhea have been increasing among both sexes since 2013.² These increases in both the civilian and military populations could reflect true increases in the incidence of infections as well as improved screening coverage in men, particularly extragental screening in men who have sex with men.⁹ 

No data on sexual risk behaviors were available in this study, but prior surveys of military personnel have indicated high levels of risk behaviors. The 2015 Department of Defense Health Related Behaviors Survey (HRBS) documented that 19.4% of respondents reported having more than 1 sex partner in the past year and that 36.7% reported sex with a new partner in the past year without using a condom; these percentages were almost double those reported from the previous survey in 2011.¹⁰ A pattern of continued increases in such reported risk behaviors would further suggest a true increase in the incidence of STIs like chlamydia and gonorrhea; however, data from the 2018 HRBS were not available at the time of this report.

The downward trend in genital HPV incidence rates observed during the surveillance period may be related to the introduction of the HPV vaccine for women and girls in 2006 and for men in 2010. Among civilian women aged 14–24 years, cervical/vaginal prevalence of HPV types 6, 11, 16, and 18 decreased by approximately 6% from the period 2003–2006 to 2009–2012.⁸ The HPV vaccine is currently not a mandatory vaccine for military service, but it is encouraged and offered to service members. Because the HPV vaccine (Gardasil®) is approved for use among males and females beginning at age 11 years, it is possible that an increasing number of members who entered military service during the surveillance period may have been vaccinated for HPV prior to entering service. This prior vaccination may account for the decrease in genital HPV incidence during the surveillance period even as the number of service women initiating HPV vaccine is decreasing.¹¹However, the reason for the increased incidence of genital HPV after 2013 among women aged 30 years and older is unknown.

The trends in the incidence of HSV and syphilis in the U.S. military are also similar to what is observed in the civilian population. Data from the Centers for Disease Control and Prevention's (CDC's) National Health and Nutrition Examination Survey indicate that the seroprevalence of both HSV-1 and HSV-2 has decreased in the U.S. population since 1999.2 In contrast, the incidence of primary and secondary syphilis reported to the CDC has increased markedly since 2001, with men accounting for the majority of cases.^2,12

This report has several limitations that should be considered when interpreting the results. First, the results presented here are not comparable with the prior MSMR update on STIs because the case definition employed in the 2017 analysis did not include the results of laboratory tests for any of the STIs. In addition, the case definition for syphilis was revised for the current analysis to limit misclassification of diagnoses recorded during outpatient encounters and of diagnoses by syphilis stage.^7,8 However, diagnoses of STIs may still be incorrectly coded. For example, STI-specific "rule out" diagnoses or vaccinations (e.g., HPV vaccination) may be reported with STI-specific diagnostic codes, which would result in an overestimate of STI incidence. Cases of syphilis, genital HSV, and genital HPV based solely on laboratory test results are considered "suspect" because the lab test results cannot distinguish between active and chronic infections. However, because incident cases of these STIs were identified based on the first qualifying encounter or laboratory result, the likelihood is high that most such cases are acute and not chronic.

STI cases may not be captured if coded in the medical record using symptom codes (e.g., urethritis) rather than STI-specific codes. In addition, the counts of STI diagnoses reported here may underestimate the actual numbers of diagnoses because some affected service members may be diagnosed and treated through non-reimbursed, non-military care providers (e.g., county health departments or family planning centers) or in deployed settings (e.g., overseas training exercises, combat operations, or aboard ships). Laboratory tests that are performed in a purchased care setting, a shipboard facility, a battalion aid station, or an in-theater facility were not captured in the current analysis.

For some STIs, the detection of prevalent infections may occur long after the initial infections. As a result, changes in incidence rates reflect, at least in part, temporal changes in case ascertainment, such as a shift to more aggressive screening. The lack of standard practices across the services and their installations regarding screening, testing, treatment, and reporting complicate interpretations of differences between services, military and demographic subgroups, and locations. Establishing screening, testing, treatment, and reporting standards across the services and ensuring adherence to such standards would likely improve efforts to detect and characterize STI-related health threats. In addition, continued behavioral risk-reduction interventions are needed to counter the increasing incidence of STIs among military service members.

Author affiliations: Armed Forces Health Surveillance Branch, Silver Spring, MD (Dr. Stahlman and Ms. Oetting)

Acknowledgment: The authors thank Mr. Nicolas Seliga, MPH, with the Navy and Marine Corps Public Health Center, Norfolk, VA, for providing the laboratory data.

References

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