Syphilis

Background

Syphilis, a genital ulcerative disease caused by the bacterium Treponema pallidum, is associated with significant complications if left untreated and can facilitate the transmission and acquisition of HIV infection.1-3 Additionally, historical data demonstrate that untreated syphilis in pregnant women, if acquired during the four years before delivery, can lead to infection of the fetus in up to 80% of cases and may result in stillbirth or infant death in up to 40% of cases.4

In 2000 and 2001, the national rate of reported primary and secondary (P&S) syphilis cases was 2.1 cases per 100,000 population, the lowest rate since reporting began in 1941 (Figure 35, Table 1). However, the P&S syphilis rate has increased almost every year since 2001. This rise in the rate of reported P&S syphilis has been primarily attributable to increased cases among men and, specifically, among gay, bisexual, and other men who have sex with men (MSM). MSM account for the majority of P&S syphilis cases and estimated rates are substantially higher among MSM compared with women or men who have sex with women only (MSW).5 The number of cases among MSM has continued to increase, but within the last five years, cases among MSW and women have increased substantially as well. The increase in syphilis among women is of particular concern because it is associated with a striking and concurrent increase in congenital syphilis. These recent trends highlight the importance of national syphilis surveillance to understand the current epidemiology of syphilis in the United States and to focus prevention efforts.

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Interpreting Rates of Reported Cases of Syphilis

Left untreated, infection with syphilis can span decades, progressing through multiple stages of infection. Case counts of all stages of syphilis represent the total burden of disease in the United States. The primary and secondary syphilis stages are the earliest stages of infection, reflect symptomatic disease, and are indicators of incident infection.6 For these reasons, trend analyses of syphilis primarily focus on reported cases and rates of reported cases of P&S syphilis. When referring to “P&S syphilis,” case counts are the sum of both primary and secondary cases, and “rate of P&S syphilis” refers to this sum per unit population.

Changes in reporting and screening practices can complicate interpretation of trends over time. To minimize the effect of changes in reporting over time, trend data in this report are restricted to jurisdictions that consistently report data of interest (e.g., sex of sex partners) for each year of a given time period. Details of these restrictions are provided in the pertinent text and figures.

Additionally, the case definitions for syphilis stages have changed over time, including a revision that took effect in January 2018. See Appendix A1.9 for more information on syphilis morbidity reporting and Appendix C1.4 for current syphilis case definition.

Syphilis — All Stages (P&S, Early Non-Primary Non-Secondary, Unknown Duration or Late, and Congenital)

In 2018, the total case count of reported syphilis (all stages combined: P&S, early non-primary non-secondary, unknown duration or late, and congenital) was the highest recorded since 1991. The total number of reported cases of syphilis (all stages) increased 13.3% during 2017–2018 (from 101,584 cases to 115,045 cases) (Table 1). The number of reported cases of P&S syphilis increased 14.4% (from 30,644 to 35,063), the number of reported cases of early non-primary non-secondary syphilis increased 13.3% (from 34,013 cases to 38,539 cases), the number of reported cases of unknown duration or late syphilis increased 11.5% (from 35,992 cases to 40,137 cases), and the number of reported cases of congenital syphilis increased 39.7% (from 935 to 1,306) (Table 1).

P&S Syphilis — United States

In 2018, a total of 35,063 cases of P&S syphilis were reported in the United States, yielding a rate of 10.8 cases per 100,000 population (Figure 35, Table 1). This rate represents a 14.9% increase compared with 2017 (9.4 cases per 100,000 population), and a 71.4% increase compared with 2014 (6.3 cases per 100,000 population).

P&S Syphilis by Region

In 2018, the West had the highest rate of reported P&S syphilis cases (15.0 cases per 100,000 population), followed by the South (11.1 cases per 100,000 population), the Northeast (8.7 cases per 100,000 population), and the Midwest (7.1 cases per 100,000 population) (Table 27). During 2017–2018, the P&S syphilis rate increased 16.4% in the Midwest, 15.6% in the South, 15.4% in the West, and 10.1% in the Northeast (Figure 36, Table 27).

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P&S Syphilis by State

In 2018, rates of reported P&S syphilis cases per 100,000 population ranged by state from 1.8 in Vermont to 22.7 in Nevada (Figure 37, Table 26). The rate of reported P&S syphilis cases in the District of Columbia was 40.2 cases per 100,000 population. During 2017–2018, P&S syphilis rates increased in 74% (37/50) of states and the District of Columbia, and remained stable or decreased in 26% (13/50) of states (Table 27).

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P&S Syphilis by Metropolitan Statistical Area

The overall rate of reported P&S syphilis cases in the 50 most populous metropolitan statistical areas (MSAs) was 13.5 cases per 100,000 population in 2018, which represents a 12.5% increase since 2017 (12.0 cases per 100,000 population) (Table 30). Overall, in 2018, 69.2% of reported P&S syphilis cases (71.9% of male cases and 53.0% of female cases) were reported by these 50 MSAs. The rate among women in these MSAs was 2.9 cases per 100,000 females, while the rate among men was 24.5 cases per 100,000 males (Tables 31 and 32).

P&S Syphilis by County

In 2018, 61.5% of reported P&S syphilis cases occurred in 70 counties or independent cities (Table 33). Of 3,142 counties in the United States, 556 (17.7%) had a P&S syphilis rate of 8.8 cases per 100,000 population or greater,  557 (17.7%) reported a rate from 4.2 to 8.7 cases per 100,000 population, 531 (17.0%) reported a rate from >0.0 to 4.1 cases per 100,000 population, and 1,498 (47.7%) counties reported no cases of P&S syphilis in 2018 (Figure 38).

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P&S Syphilis by Sex and Sex of Sex Partners

As has been observed in previous years, in 2018, the rate of reported P&S syphilis cases among men (18.7 cases per 100,000 males) was much higher than the rate among women (3.0 cases per 100,000 females), and men accounted for a large majority (85.7%) of P&S syphilis cases (Figure 39, Tables 28 and 29). Among men, the rate of P&S syphilis has increased every year since 2000, and during 2017–2018, the rate among men increased 11.3% (Figure 40, Table 29). In contrast, the P&S syphilis rate among women fluctuated between 0.8 and 1.7 cases per 100,000 females during 2000–2013, but has increased substantially since 2013 (Figure 40, Table 28). During 2014–2018, the P&S syphilis rate among women more than doubled (172.7% increase). During 2017–2018, the P&S syphilis rate among women increased 30.4%.

These increases in male and female P&S syphilis rates were observed in every region of the country during 2017–2018. Among men, the rate increased 13.5% in the Midwest, 12.4% in the South, 11.5% in the West, and 7.2% in the Northeast (Table 29). Among women, the largest increases were observed in the West (41.2%), followed by the Northeast (40.0%), the South (30.8%) and the Midwest (30.8%) (Table 28).

MSM continued to account for the majority of P&S syphilis cases in 2018 (Figures 39 and 41). Of 35,063 reported P&S syphilis cases in 2018, 18,760 (53.5%) were among MSM, including 16,905 (48.2%) cases among men who had sex with men only and 1,855 (5.3%) cases among men who had sex with both men and women (Figure 39). Overall, 5,416 (15.4%) cases were among MSW, 4,995 (14.2%) were among women, 5,858 (16.7%) were among men without information about sex of sex partners, and 34 (0.1%) were cases reported with unknown sex. Among the 24,176 male cases with information on sex of sex partners, 77.6% occurred among MSM.

A total of 36 states were able to classify at least 70.0% of reported P&S syphilis cases as MSM, MSW, or women each year during 2014–2018 (Figure 41). In these states, during 2017–2018, the number of cases increased 5.3% among MSM, 16.3% among MSW, and 32.9% among women.

Figure 39 - Pie graph showing the distribution of primary and secondary syphilis cases by sex and sex of sex partners in the United States in 2018. Men accounted for 85.7% of reported primary and secondary syphilis cases. Of 35,063 reported primary and secondary syphilis cases in 2018, 18,760 (53.5%) were among men who have sex with men, including 16,905 (48.2%) cases among men who had sex with men only and 1,855 (5.3%) cases among men who have sex with both men and women. Overall, 5,416 (15.4%) cases were among men who have sex with women only, 4,995 (14.2%) were among women, 5,858 (17%) were among men without information about sex of sex partners, and 34 (0.1%) were cases reported with unknown sex. Among the 24,176 male cases with information on sex of sex partners, 77.6% occurred among men who have sex with men.

Figure 40 - Among men, the rate of P&S syphilis has increased every year since 2000, and during 2017–2018, the rate among men increased 11.3%. In contrast, the P&S syphilis rate among women fluctuated between 0.8 and 1.7 cases per 100,000 females during 2000–2013, but has increased substantially since 2013. During 2014–2018, the P&S syphilis rate among women more than doubled (172.7% increase). During 2017–2018, the P&S syphilis rate among women increased 30.4%.

Figure 41 - A total of 36 states were able to classify at least 70.0% of reported P&S syphilis cases as MSM, MSW, or women each year during 2014–2018. In these states, during 2017–2018, the number of cases increased 5.3% among MSM, 16.3% among MSW, and 32.9% among women.

P&S Syphilis by Age

As in previous years, in 2018, rates of reported P&S syphilis cases were highest among persons aged 25–29 years (Figure 42, Table 34). In 2018, the highest rates were observed among men aged 25–29 years (55.7 cases per 100,000 males), 30–34 years (45.8 cases per 100,000 males), and 20–24 years (44.6 cases per 100,000 males). The highest rates among women were among those aged 20–24 years (10.0 cases per 100,000 females) and those aged 25–29 years (9.4 cases per 100,000 females).

During 2017–2018, the overall rate of reported P&S syphilis cases increased in all age groups among those aged 15 years or older (Figures 43 and 44, Table 34). Rates increased 14.9% among those aged 15–19 years, 10.3% among those aged 20–24 years, 12.6% among those aged 25–29 years, 21.2% among those aged 30–34 years, 17.2% among those aged 35–39 years, 17.1% among those aged 40–44 years, 7.2% among those aged 45–54 years, 21.1% among those aged 55–64 years, and 28.6% among those aged 65 or older.

In 2018, persons aged 15–44 years accounted for 80.6% of reported P&S syphilis cases with known age.

Figure 42 - As in previous years, in 2018, rates of reported P&S syphilis cases were highest among persons aged 25–29 years. In 2018, the highest rates were observed among men aged 25–29 years (55.7 cases per 100,000 males), 30–34 years (45.8 cases per 100,000 males), and 20–24 years (44.6 cases per 100,000 males). The highest rates among women were among those aged 20–24 years (10.0 cases per 100,000 females) and those aged 25–29 years (9.4 cases per 100,000 females).

Figure 43 - In 2018, persons aged 15–44 years accounted for 80.6% of reported P&S syphilis cases with known age. During 2017–2018, the P&S syphilis rate increased in all age groups among those aged 15–44 years.

Figure 44 - In 2018, persons aged 15–44 years accounted for 80.6% of reported P&S syphilis cases with known age. During 2017–2018, the P&S syphilis rate increased in all age groups among those aged 15–44 years.

P&S Syphilis by Race/Hispanic Ethnicity

In 2018, the rate of reported P&S syphilis cases was highest among Blacks (28.1 cases per 100,000 population) (Table 35B). The P&S syphilis rate among Blacks was 4.7 times the rate among Whites (6.0 cases per 100,000 population), the rate among Native Hawaiians/Other Pacific Islanders (NHOPI) (16.3 cases per 100,000 population) was 2.7 times the rate among Whites, the rate among American Indian/Alaska Natives (AI/AN) (15.5 cases per 100,000 population) was 2.6 times the rate among Whites, the rate among Hispanics (13.0 cases per 100,000 population) was 2.2 times the rate among Whites, and the rate among Asians (4.6 cases per 100,000 population) was 0.8 times the rate among Whites.

During 2014–2018, the P&S syphilis rate increased among all race/Hispanic ethnicity groups (Figure 45). The greatest increases during 2017–2018 were observed among AI/AN (40.9%) and those who identified as Multirace (22.1%), followed by NHOPI (19.0%), Whites (11.1%), Blacks (17.1%), Hispanics (13.0%), and Asians (9.5%).

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P&S Syphilis and HIV Co-infection

Reported cases of P&S syphilis continue to be characterized by a high rate of HIV co-infection, particularly among MSM (Figure 46). Among 2018 P&S syphilis cases with known HIV status, 41.6% of cases among MSM were HIV-positive, compared with 7.9% of cases among MSW, and 4.0% of cases among women.

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P&S Syphilis by Reporting Source

In 2018, 16.3% of P&S syphilis cases were reported from STD clinics, 71.2% were reported from venues outside of STD clinics, and 12.5% of cases had an unknown reporting source (Table A2). During 2017–2018, the number of P&S syphilis cases reported by STD clinics and by non-STD clinic settings increased (Figure 47). However, the proportion of P&S syphilis cases that were reported by STD clinics has declined by nearly half over the last decade from 31.1% of cases in 2008 to 16.3% of cases in 2018. In 2018, private physicians/health maintenance organizations (HMOs) and STD clinics were the most common reporting sources for all reported P&S syphilis cases, regardless of sex of sex partners. Of cases reported among MSM, private physicians/HMOs and STD clinics reported 30.4% and 22.4% of cases, respectively. Of cases reported among MSW, they reported 22.6% and 18.5%, respectively, and of cases reported among women, they reported 25.8% and 12.2%, respectively (Figure 48).

Figure 47 - During 2017–2018, the number of P&S syphilis cases reported by STD clinics and by non-STD clinic settings increased (Figure 47). However, the proportion of P&S syphilis cases that were reported by STD clinics has declined by nearly half over the last decade from 31.1% of cases in 2008 to 16.3% of cases in 2018. In 2018, private physicians/health maintenance organizations and STD clinics were the most common reporting sources for all reported P&S syphilis cases, regardless of sex of sex partner.

Figure 48 - Of cases reported among MSM, private physicians/health maintenance organizations (HMO) and STD clinics reported 30.4% and 22.4% of cases, respectively. Among MSW, 22.6% and 18.5%, respectively, were reported by private physicians/HMOs and STD clinics. Among women, private physicians/HMOs and STD clinics reported 25.8% and 12.2% of cases, respectively.

Congenital Syphilis

After decreasing from 10.5 to 8.4 reported congenital syphilis cases per 100,000 live births during 2008–2012, the rate of reported congenital syphilis has subsequently increased each year since 2012 (Table 1). In 2018, there were a total of 1,306 reported cases of congenital syphilis, including 78 syphilitic stillbirths and 16 infant deaths, and a national rate of 33.1 cases per 100,000 live births. This rate represents a 39.7% increase relative to 2017 (23.7 cases per 100,000 live births) and a 185.3% increase relative to 2014 (11.6 cases per 100,000 live births). As has been observed historically, this increase in the congenital syphilis rate has paralleled increases in P&S syphilis among all women and reproductive-aged women during 2014–2018 (172.7% and 165.4% increases, respectively) (Figure 49, Table 28).

During 2014–2018, the increase in reported congenital syphilis cases was primarily attributable to increases in the West and South. During this period, the congenital syphilis rate increased 278.9% in the West, 190.3% in the South, 84.8% in the Northeast, and 45.9% in the Midwest (Table 41). During 2017–2018, the congenital syphilis rate increased 49.5% in the South, 44.1% in the Northeast, 30.5% in the Midwest, and 29.3% in the West. In 2018, the highest congenital syphilis rates were reported from the West (48.5 cases per 100,000 live births), followed by the South (44.7 cases per 100,000 live births), Midwest (12.4 cases per 100,000 live births), and the Northeast (8.5 cases per 100,000 live births). In addition, rates were highest among Blacks (86.6 cases per 100,000 live births) and AI/AN (79.2 cases per 100,000 live births), followed by Hispanics (44.7 cases per 100,000 live births), Whites (13.5 cases per 100,000 live births), and Asians/Pacific Islanders (9.2 cases per 100,000 live births) (Table 42).

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Syphilis among Special Populations

More information about syphilis and congenital syphilis among race/Hispanic ethnicity groups, women of reproductive age, adolescents, and MSM can be found in the Special Focus Profiles.

Syphilis Summary

The national rate of reported P&S syphilis cases reached an historic low in 2000 and 2001, but has increased almost every year since then. This increase was largely attributable to an increase among men, and in particular among MSM. However, in the last five years, rates have increased among both men and women, and the P&S syphilis rate among women has almost tripled. Rates of reported congenital syphilis cases also increased substantially during 2014–2018 and during 2017–2018. MSM continued to account for the majority of reported P&S syphilis cases in 2018. Nationally, the highest rates of P&S syphilis in 2018 were observed among men aged 20–34 years, among men in the West, and among Black men.

References

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