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 2011–2019. 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 black service members, soldiers, and enlisted members had higher incidence rates of STIs. The incidence of chlamydia and gonorrhea generally increased among both male and female service members in the latter half of the surveillance period but may have begun to level off or decrease in 2019. Rates of syphilis increased for male service members through 2018 but decreased slightly in 2019; the rate among female service members increased from 2011 to 2014 but leveled off through 2018 before increasing in 2019. The incidence of genital HPV generally decreased among both male and female service member but rose slightly among women in 2019; HSV incidence 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 generally increased among male and female service members in the latter half of the surveillance period; however, the rates may have begun to decrease in 2019. The incidence of genital HPV and HSV continued to decrease. The incidence of syphilis increased among female service members in 2019.

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 some STIs and maintain any decreases.

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 caused by 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 March 2019 MSMR eport documented more than 212,000 incident infections of chlamydia and more than 32,000 incident infections of gonorrhea among active component U.S. military members between 2010 and 2018, with increasing incidence rates among both males and females in the latter half of the surveillance period, mirroring trends in the general U.S. population.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. have risen steadily from a historic low in 2001 and increased 71.4%, from 6.3 cases per 100,000 persons in 2014 to 10.8 cases per 100,000 persons in 2018.² This upward trend is mirrored in the active component of the U.S. Armed Forces, in which the incidence of syphilis (of any type) increased steadily between 2010 and 2018, 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.^4–6

Common viral STIs in the U.S. include infections caused byhuman 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,⁷ while genotypes 16 and 18 cause most HPV-related cancers.⁸ 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.⁹ 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 as well as 5 additional cancer-causing types.⁷ From 2010 through 2018, the overall incidence rates of genital HPV and HSV in the active component were 61.1 and 23.7 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, this report summarizes incident cases and incidence rates of 5 of the most common STIs during 2011–2019 and their distribution by demographic and military characteristics.

Methods

The surveillance period was 1 Jan. 2011 through 31 Dec. 2019. 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 (DMSS) for surveillance purposes. STI cases were also derived from positive laboratory test results recorded 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 any of the following: 1) a case-defining diagnosis (Table 1) n 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 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 International Classification of Diseases, 9th or 10th Revision (ICD-9 or ICD-10, respectively) 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 before the surveillance period were excluded from the analysis.

An incident case of syphilis was defined by 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.^10,11 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 2011 and 2019, the number of incident chlamydia infections among active component service members was greater than the sum of the other 4 STIs combined and 3.4 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 infections 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 infections, 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 infections 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 infection rates were highest among those in health care occupations, and the highest rates of genital HSV infections were among those working in communications/intelligence, health care, or motor transport (Table 2). Patterns of incidence for each specific STI are described in the subsections below.

Chlamydia

During the surveillance period, annual incidence rates of chlamydia among service women were generally 3 to 4 times the rates among men. Annual rates among men and women combined increased 58.6% between 2013 and 2019, with rates among women peaking in 2018 (518.2 per 10,000 p-yrs) and rates for men peaking in 2019 (176.5 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 generally increased among those under 25 years of age during 2013–2018 but leveled off or decreased slightly in 2019. Rates 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). However, in 2019, rates stabilized for many age and race/ethnicity groups less than 35 years old, particularly among non-Hispanic black service members less than 25 years old (Figure 3). Rates decreased for non-Hispanic white service members less than 25 years old but continued to rise for non-Hispanic black service members 25–34 years old, Hispanic service members less than 35 years old, and those in the other race/ethnicity group less than 25 years old (Figure 3).

Genital HPV

The crude annual incidence rates of genital HPV infections decreased 44.5% 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 incidence of genital HPV infections among all active component service members in 2013 at 55.6 cases per 10,000 p-yrs, but the lowest points were reached in 2018 and 2019 at 43.5 and 43.8 cases per 10,000 p-yrs, respectively. Incidence rates among female service members declined by a little more than 35% during the surveillance period, from a high of 313.2 cases per 10,000 p-yrs in 2011 to a low of 198.1 cases per 10,000 p-yrs in 2018 (Figure 4); rates increased slightly to 199.6 per 10,000 p-yrs in 2019. Rates among men decreased from 44.6 per 10,000 p-yrs in 2011 to 16.0 per 10,000 p-yrs in 2019. The slight increase in rates of genital HPV infections among women overall was attributable to small increases in the rates among women in each of the age groups younger than 35 years old (Figure 5). The decrease in the genital HPV infection rates among men overall was driven by decreases in the rates in the youngest age groups (less than 30 years) (Figure 6).

Gonorrhea

Between 2011 and 2019, the crude annual incidence rate of gonorrhea increased by 49.2%; however, after increasing steadily from 2011 through 2018, the rate decreased in 2019 (by 4.6%). The annual rates among women declined between 2011 and 2015 then increased through 2018 before decreasing in 2019 (by 3.1%). After increasing steadily between 2011 and 2018, the rate among men also decreased in 2019 (by 5.3%) (Figure 7). These trends in gonorrhea incidence were primarily driven by similar trends among women less than 25 years of age and among men less than 30 years of age (Figures 8, 9). The ratio of the incidence rate for women compared to men was 2.1 in 2011 but dropped to 1.4 in 2019. The annual rates of gonorrhea increased during the surveillance period among all race/ethnicity groups through 2018, but the rates fell in 2019 for all groups except non-Hispanic black service members, whose rate continued to increase (data not shown).

Genital HSV

Crude annual incidence rates of genital HSV infections decreased from 25.2 to 20.2 per 10,000 p-yrs over the course of the surveillance period. Annual rates declined steadily among both men and women from 2011 to 2019 (Figure 10); rates among female service members decreased from a high of 74.0 per 10,000 p-yrs in 2011 to a low of 60.8 per 10,000 p-yrs in 2019, and rates among men decreased from 17.2 per 10,000 p-yrs in 2011 to 12.2 per 10,000 p-yrs in 2019. The incidence rates of genital HSV infections decreased among all age groups over the course of the surveillance period (data not shown). The rates decreased between 2018 and 2019 among women in all age groups and decreased or remained relatively stable among men in all age groups during the same period except among those 25–29 years old (increase of 4.8%) (data not shown). In addition, the incidence rates decreased among all race/ethnicity groups during the surveillance period except for non-Hispanic black and Asian/Pacific Islander service members, whose rates increased slightly (data not shown).

Syphilis

The crude incidence rate of syphilis in the last year of the surveillance period (6.1 per 10,000 p-yrs) was 2.7 times that observed in 2011 (2.2 per 10,000 p-yrs). This increase in the rate was primarily driven by cases identified among male service members, though the rate among female service members increased sharply (43.1%) between 2018 and 2019 (Figure 11). Rates of syphilis steadily increased among men until 2018 and then decreased slightly in 2019. Among women, rates increased between 2011 and 2014, leveled off through 2018, and then increased in 2019. Among men, the pattern of decreasing overall incidence with increasing age was consistent among all race/ethnicity groups; there were not enough cases to evaluate associations between age and race/ethnicity group among women (data not shown).

Editorial Comment

As in previous reports, the crude annual incidence rates of chlamydia, gonorrhea, and syphilis generally increased during the surveillance period. However, from 2018 through 2019, the rate of chlamydia decreased slightly in women and the rates of gonorrhea decreased slightly in both men and women. From 2018 through 2019, the rate of syphilis remained relatively stable among male service members but increased in female service members. In contrast, the incidence rate of genital HSV continued to decrease through 2018 and 2019 among both male and female service members, and the rate of HPV continued to decrease in men and remained relatively stable, with a slight rise, for women. Overall incidence 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 incidence rates of most STIs among women compared to men 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. Although rates of chlamydia and gonorrhea increased among both male and female service members during the latter half of the surveillance period, mirroring the increasing rates in the civilian population,² the rate of chlamydia decreased slightly for women and the rates of gonorrhea decreased slightly for both men and women between 2018 and 2019. 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 2013²; however, no 2019 data on the rates of chlamydia or gonorrhea in the civilian population were available for comparison at the time of this report. The increases seen through 2018 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 extragenital screening in men who have sex with men.¹²

No data on sexual risk behaviors were available in this study, but prior surveys of military personnel have indicated high levels of such 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 2011.¹³ Data from the 2018 HRBS were not available at the time of this report, precluding any comparisons.

The general downward trend in incidence rates of genital HPV infections 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 2009–2012.¹⁴ 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 9 years, it is possible that an increasing number of members who entered military service during the surveillance period may have been vaccinated for HPV before entering service. This prior vaccination may account for the decrease in the annual rates of genital HPV infections during the surveillance period. However, a recent study has shown that the number of service women initiating HPV vaccine is decreasing.¹⁵ This decrease may partially explain the possible slight rate increase among women in 2019.

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 CDC has increased markedly since 2001, with men accounting for the majority of cases.^2,16
This report has several limitations that should be considered when interpreting the results. First, diagnoses of STIs may 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 infections 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 nonreimbursed, 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 Click to closePurchased CareThe TRICARE Health Program is often referred to as purchased care. It is the services we “purchase” through the managed care support contracts.purchased care setting, a shipboard facility, a battalion aid station, or an in-theater facility were not captured in the current analysis. Finally, medical data from sites that were using the new electronic health record for the Military Health System, MHS GENESIS, are not available in the DMSS. These sites include Naval Hospital Oak Harbor, Naval Hospital Bremerton, Air Force Medical Services Fairchild, and Madigan Army Medical Center. Therefore, medical encounter data for individuals seeking care at any of these facilities during 2017–2019 were not included 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.

References

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2. Centers for Disease Control and Prevention. Sexually transmitted disease surveillance 2018. https://www.cdc.gov/std/stats18/default.htm. Accessed 20 Feb. 2020.
3. Stahlman S, Seliga N, Oetting AA. Sexually transmitted infections, active component, U.S. Armed Forces, 2010–2018. MSMR. 2019;26(3):1–10.
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11. Armed Forces Health Surveillance Branch. Brief report: Use of ICD-10 code A51.31 (condylomalatum) for identifying cases of secondary syphilis. MSMR. 2017;24(9):23.
12. Centers for Disease Control and Prevention. 2018 Sexually Transmitted Disease Surveillance. National Overview of STDs, 2018: Chlamydia. https://www.cdc.gov/std/stats18/chlamydia.htm. Accessed 21 Feb. 2020.
13. Meadows SO, Engel CO, Collins RL, et al. 2015 Health Related Behaviors Survey. Sexual Behavior and Health among Active-Duty Service Members. RAND Corporation Research Brief. https://www.rand.org/pubs/research_briefs/RB9955z5.html. Accessed 16 Feb. 2019.
14. HPV Infections Targeted by Vaccine Decrease in U.S. [news release]. Bethesda, MD; National Cancer Institute; 9 March 2016. https://www.cancer.gov/news-events/cancer-currents-blog/2016/hpv-infections-decreased. Accessed 24 Feb. 2020.
15. Clark LL, Stahlman S, and Taubman SB. Human papillomavirus vaccine initiation, coverage, and completion rates among U.S. active component service members, 2007–2017. MSMR. 2018;25(9):9–14.
16. Centers for Disease Control and Prevention. 2018 Sexually Transmitted Disease Surveillance. National Profile Overview: Syphilis. https://www.cdc.gov/std/stats18/Syphilis.htm. Accessed 16 Feb. 2019.