PHWR Public Health Weekly Report

Syphilis is a sexually transmitted infection (STI) caused by the bacterium Treponema pallidum, primarily transmitted through sexual contact, and is classified as a chronic systemic infectious disease [1]. The manifestation of syphilis varies depending on the stage of progression following infection. The disease is categorized into two main stages: early syphilis (which includes primary syphilis, secondary syphilis, and early latent syphilis) and late syphilis (which encompasses tertiary syphilis and late latent syphilis). Early syphilis is contagious within 1 year of infection, while late syphilis is non-infectious after 1 year [2]. Tertiary syphilis develops in approximately one-third of untreated individuals and can result in severe complications, including neurological and cardiovascular disorders, as well as the formation of gummas. Additionally, vertical transmission to the fetus is possible in late latent syphilis, highlighting the need for effective maternal and child health management [3].

The World Health Organization reported approximately 8 million new syphilis cases globally in 2022 [4]. Following the introduction of penicillin, syphilis incidence saw a significant global decline. However, in recent decades, the infection has resurged, particularly among high-risk populations such as men who have sex with men, individuals living with human immunodeficiency virus (HIV), and pregnant women. The rise in anonymous sexual encounters facilitated by mobile dating applications and similar platforms has been identified as a contributing factor to the epidemic, alongside shifts in sexual behavior [5]. Congenital syphilis remains a major cause of miscarriage and stillbirth in low- and middle-income countries, with notably high incidence rates. Furthermore, the coronavirus disease 2019 pandemic has led to disruptions in the management and surveillance of STI, adversely affecting the timely diagnosis and treatment of these conditions [6].

In the Republic of Korea, incidence rates declined following high prevalence in the 1950s and 1960s owing to national surveillance and treatment initiatives; however, cases have been on the rise since the 2000s [7]. The syphilis surveillance system transitioned from a sentinel surveillance model in 2001 to a mandatory surveillance system in 2010, reverting to sentinel surveillance in 2020, and again to mandatory surveillance in 2024. The scope of reporting has been broadened to include early latent syphilis and tertiary syphilis alongside primary, secondary, and congenital syphilis, enabling refined surveillance by disease stage and facilitating the identification of specific causes. These modifications serve as a foundation for accurately determining nationwide incidence rates as well as regional and clinical characteristics [8].

The Gyeongnam region, which includes Busan, Ulsan, and Gyeongsangnam-do, is notable for its status as a major port area and the presence of extensive industrial complexes. The region is characterized by significant population mobility and exchange, factors that may influence the incidence and spread of STIs. This study aims to analyze the incidence and patient characteristics of syphilis in the Gyeongnam region using data from the first year of mandatory surveillance implemented in 2024. The objective was to identify regional epidemiological characteristics and provide evidence for enhancing the surveillance system and establishing targeted prevention and management strategies tailored to the Gyeongnam region.

1. Research Design and Data Sources

A cross-sectional study was conducted utilizing comprehensive surveillance data for notifiable infectious diseases, collected through the Integrated Disease Control and Prevention Information System of the Korea Disease Control and Prevention Agency (KDCA). The analysis period spanned from January 1, 2024, to December 31, 2024. The data source consisted of patient occurrence reports and epidemiological investigation information reported through the Integrated Disease Control and Prevention Information System of the KDCA. For three cases whose registered addresses were in the Gyeongnam region but whose local government jurisdictions fell outside that region, access permissions were not available. Consequently, demographic characteristics, as well as clinical and epidemiological information, excluding personal details, were obtained from the AIDS Management Division of the KDCA [9]. In addition to the Integrated Disease Control and Prevention Information System, data sources included the 2024 resident-registered population and number of births, as provided by the National Statistics Portal of Ministry of Data and Statistics, to calculate the incidence rate per 100,000 population. To facilitate comparisons with the situation prior to 2024, mandatory surveillance statistics for syphilis from 2011 to 2019, as documented in the “2019 Annual Report on Notified Infectious Diseases,” were reviewed. Furthermore, sentinel surveillance statistics from 2020 to 2023 were sourced from the Infectious Disease Portal (dportal.kdca.go.kr) of the KDCA.

2. Study Population

The number of syphilis cases reported in the Gyeongnam region for 2024 refers to cases reported through the Integrated Disease Control and Prevention Information System from January 1 to December 31, 2024, among individuals whose registered addresses were located in the Gyeongnam region (Busan, Ulsan, and Gyeongsangnam-do). A total of 383 confirmed cases were documented. The case definition adhered to the established reporting criteria for notifiable infectious diseases [2]. A case was classified as having clinical symptoms consistent with primary, secondary, or tertiary syphilis while meeting at least one of the confirmatory diagnostic test criteria. Early latent syphilis was defined as cases in which an individual was identified as a pathogen carrier, exhibiting no clinical symptoms but confirmed to be infected with the pathogen according to the aforementioned criteria. In this context, late latent syphilis was excluded from reporting requirements based on the diagnostic findings of physicians.

Patient locations were determined based on the registered addresses in the resident registration system, adhering to the criteria for compiling incidence statistics outlined in the Annual Report on Notified Infectious Diseases. During data processing, the jurisdiction and the registered residential address were inconsistent for seven cases. Among these, four cases with addresses in the Gyeongnam region were included in the analysis. One individual had both their registered address and jurisdiction within the Gyeongnam region; however, three individuals had jurisdictions in other regions, specifically Seoul, Gyeonggi-do, and Gyeongsangbuk-do. Epidemiological investigation information for these cases was managed by the Seoul Metropolitan Disease Response Center (covering Seoul and Gyeonggi-do) and the Gyeongbuk Regional Disease Response Center (covering Gyeongsangbuk-do), with data obtained separately through the AIDS Management Division of the KDCA. Conversely, three cases reported and investigated by local governments in the Gyeongnam region had registered addresses in other regions (Gyeonggi-do, Daegu, and Gyeongsangbuk-do) and were excluded from the analysis (Figure 1).

Figure 1. Number of cases included in analysis

3. Analytical Methods

Frequency analysis of the collected case reports and epidemiological survey data aimed to identify demographic characteristics, including the status of cases with syphilis based on stage, gender, age group, and region. The incidence rate per 100,000 population was calculated based on the population sizes of the respective regions and age groups. Trends in syphilis incidence within the Gyeongnam region throughout the surveillance period were illustrated in graphs, accompanied by descriptive statistics depicting the distribution of diagnostic stages based on gender and age group. A frequency analysis of the clinical and epidemiological characteristics was conducted to elucidate regional incidence patterns and transmission pathways. Microsoft Excel 2016 (Microsoft) was employed as the analytical tool.

1. Demographic Characteristics

In 2024, a total of 383 syphilis cases were reported across the three metropolitan areas and provinces of the Gyeongnam region: Busan, Ulsan, and Gyeongsangnam-do. From 2011 to 2016, the annual average number of reported cases over the six-year period remained at 106.7 for primary syphilis, 43.2 for secondary syphilis, and 4.5 for congenital syphilis, which were under surveillance at the time. Subsequently, from 2017 to 2019, the number of cases continued to increase, reaching a maximum of 206 cases for primary syphilis, 84 cases for secondary syphilis, and 5 cases for congenital syphilis (Figure 2). Regionally, 244 individuals (63.7%) resided in Busan, 106 individuals (27.7%) in Gyeongsangnam-do, and 33 individuals (8.6%) in Ulsan. Among these cases, congenital syphilis syndrome was identified in two cases in Gyeongsangnam-do and one case in Ulsan, with no cases reported in Busan (Table 1).

Figure 2. Number of syphilis cases reported through mandatory of sentinel surveillance in Busan Metropolitan City, Ulsan Metropolitan City, and Gyeongsangnam-do, Republic of Korea, 2024

Table 1 Sociodemographic characteristics of syphilis in Busan Metropolitan City, Ulsan Metropolitan City, and Gyeongsangnam-do, Republic of Korea, 2024

	Primary				Secondary				Tertiary				Congenitala)				Early latent				Total
	n	(%)	Rateb)		n	(%)	Rateb)		n	(%)	Rateb)		n	(%)	Rateb)		n	(%)	Rateb)		n	(%)	Rateb)
Totalc)	142	(37.1)	1.9		69	(18.0)	0.9		15	(3.9)	0.2		3	(0.8)	0.0		154	(40.2)	2.0		383	(100)	5.0
Sex
Male	117	(82.4)	3.1		54	(78.3)	1.4		8	(53.3)	0.2		0	-	-		100	(64.9)	2.6		279	(72.8)	7.4
Female	25	(17.6)	0.7		15	(21.7)	0.4		7	(46.7)	0.2		3	(100)	0.1		54	(35.1)	1.4		104	(27.2)	2.7
Age (yr)
≤9	0	-	-		0	-	-		0	-	-		3	(100)	0.6		0	-	-		3	(0.8)	0.6
10–19	8	(5.6)	1.2		5	(7.2)	0.7		0	-	-		0	-	-		9	(5.8)	1.3		22	(5.7)	3.2
20–29	56	(39.4)	7.0		19	(27.5)	2.4		2	(13.3)	0.2		0	-	-		48	(31.2)	6.0		125	(32.6)	15.6
30–39	42	(29.6)	4.8		20	(29)	2.3		3	(20)	0.3		0	-	-		31	(20.1)	3.6		96	(25.1)	11.0
40–49	21	(14.8)	1.8		14	(20.3)	1.2		1	(6.7)	0.1		0	-	-		26	(16.9)	2.3		62	(16.2)	5.4
50–59	11	(7.7)	0.8		7	(10.1)	0.5		2	(13.3)	0.2		0	-	-		15	(9.7)	1.1		35	(9.1)	2.7
60–69	3	(2.1)	0.2		2	(2.9)	0.2		3	(20)	0.2		0	-	-		13	(8.4)	1.0		21	(5.5)	1.7
≥70	1	(0.7)	0.1		2	(2.9)	0.2		4	(26.7)	0.4		0	-	-		12	(7.8)	1.2		19	(5.0)	1.8
Si-Do
Busan	90	(63.4)	2.8		41	(59.4)	1.3		11	(73.3)	0.3		0	-	-		102	(66.2)	3.1		244	(63.7)	7.5
Ulsan	14	(9.9)	1.3		5	(7.2)	0.5		0	-	-		1	(33.3)	0.1		13	(8.4)	1.2		33	(8.6)	3.0
Gyeongsangnam-do	38	(26.8)	1.2		23	(33.3)	0.7		4	(26.7)	0.1		2	(66.7)	0.1		39	(25.3)	1.2		106	(27.7)	3.3
Nationality
National	138	(97.2)	-		68	(98.6)	-		15	(100)	-		2	(66.7)	-		150	(97.4)	-		373	(97.4)	-
Foreigner	4	(2.8)	-		1	(1.4)	-		0	-	-		1	(33.3)	-		4	(2.6)	-		10	(2.6)	-

^a)All congenital syphilis cases were asymptomatic and classified as suspected cases. The incidence rate was calculated based on the provisional number of live births in Republic of Korea in 2024. ^b)Incidence rate per 100,000, denominator: Korea Ministry of Data and Statistics, 「Resident population (by Si-Gun-Gu/Sex/Age, Mid year, 2023~)」, Korean Statistical Information Service. ^c)Percentage by stage were calculated using the total number of syphilis cases in the Gyeonam ragion (Busan Metropolitan City, Ulsan Metropolitan City, and Gyeongsangnam-do) as the denominator..

In terms of demographic characteristics, the majority of the population was composed of Korean nationals, representing 97.4% of the sample. The remaining 2.6% consisted of foreign nationals. The gender distribution revealed a predominance of males; 279 individuals identified as male (72.8%) and 104 as female (27.2%), resulting in a male-to-female ratio of approximately three to one. This ratio closely aligns with national incidence patterns, where males constituted 78.0% of cases and females constituted 22.0%. The most commonly diagnosed stage of syphilis was early latent syphilis, observed in 154 cases (40.2%), followed by primary syphilis in 142 patients (37.1%), secondary syphilis in 69 cases (18.0%), tertiary syphilis in 15 cases (3.9%), and congenital syphilis in three cases (0.8%). Within the male population of 279, the number of primary syphilis cases was 117 (41.9%), representing the highest proportion. Among the 101 cases, early latent syphilis was the most prevalent form, recorded in 54 cases (53.5%) (Table 1). Additionally, the distribution of cases by gender and age group indicated that among cases in their teens and 20s, males accounted for 33.3% (93 out of 279), while females represented a higher proportion at 53.5% (53 out of 101) (Figure 3).

Figure 3. Syphilis cases by sex, age, stage in Busan Metropolitan City, Ulsan Metropolitan city, and Gyeongsangnam-do, Republic of Korea, 2024
(A) Male, (B) Female.

2. Clinical and Epidemiological Characteristics

The preliminary clinical symptoms identified through the epidemiological investigation are summarized as follows. In cases of primary syphilis, ulceration was the most prevalent symptom, reported by 87 cases (50.6%), followed by rash reported in 48 cases (27.9%). In secondary syphilis, rash emerged as the predominant symptom, affecting 61 cases (58.1%). Conversely, in tertiary syphilis, nonspecific symptoms were the most prevalent, occurring in nine cases (50.0%). The primary method of diagnosis was medical consultation owing to symptoms, accounting for 285 cases (75.0%). Additionally, there were 45 cases (11.8%) diagnosed during health screenings, seven cases (1.8%) identified during blood donation, five cases (1.3%) recognized through prenatal testing and testing following partner’s recommendation in four cases (1.1%), and two cases (0.5%) diagnosed via regular health screenings for individuals undergoing examinations in accordance with the “Regulations on Health Examinations for Sexually Transmitted Infections and Acquired Immunodeficiency Syndrome.” A total of 25 respondents (6.6%) provided information regarding the means by which they encountered their sexual partners; among these, 20 respondents (5.3%) reported meeting them offline (Table 2).

Table 2 Major symptoms, diagnostic routes and by stage of syphilis in Busan Metropolitan City, Ulsan Metropolitan City, and Gyeongsangnam-do, Republic of Korea, 2024

	Total	Primary	Secondary	Tertiary	Early latent
Major symptomsa),b)	295 (100)	172 (100)	105 (100)	18 (100)	-
Rash	111 (37.6)	48 (27.9)	61 (58.1)	2 (11.1)	-
Ulcer	95 (32.2)	87 (50.6)	8 (7.6)	0 (0.0)	-
Fatigue	20 (6.8)	7 (4.1)	8 (7.6)	5 (27.8)	-
Headache	10 (3.4)	5 (2.9)	4 (3.8)	1 (5.6)	-
Fever	9 (3.1)	2 (1.2)	7 (6.7)	0 (0.0)	-
Myalgia	7 (2.4)	2 (1.2)	4 (3.8)	1 (5.6)	-
Chills	6 (2.0)	2 (1.2)	4 (3.8)	0 (0.0)	-
Lymphadenopathy	5 (1.7)	4 (2.3)	1 (1.0)	0 (0.0)	-
Othersc)	32 (10.8)	15 (8.7)	8 (7.6)	9 (50.0)	-
Diagnostic routed)	380 (100)	142 (100)	69 (100)	15 (100)	154 (100)
Symptome)	285 (75.0)	127 (89.4)	62 (89.9)	14 (93.3)	82 (53.2)
Health checkup	45 (11.8)	8 (5.6)	2 (2.9)	1 (6.7)	34 (22.1)
Pre-admission screening	14 (3.7)	1 (0.7)	2 (2.9)	0 (0.0)	11 (7.1)
Others	14 (3.7)	3 (2.1)	0 (0.0)	0 (0.0)	11 (7.1)
Blood donation	7 (1.8)	1 (0.7)	1 (1.4)	0 (0.0)	5 (3.2)
Prenatal screening	5 (1.3)	0 (0.0)	1 (1.4)	0 (0.0)	4 (2.6)
Partner’s recommendation	4 (1.1)	0 (0.0)	1 (1.4)	0 (0.0)	3 (1.9)
Sexually transmitted infections screening (mandate)	2 (0.5)	0 (0.0)	0 (0.0)	0 (0.0)	2 (1.3)
Unknown (investigation failed)	4 (1.1)	2 (1.4)	0 (0.0)	0 (0.0)	2 (1.3)
Sexual contact historyd) (last 12 months)	380 (100)	142 (100)	69 (100)	15 (100)	154 (100)
Yes	174 (45.8)	83 (58.5)	32 (46.4)	4 (26.7)	55 (35.7)
No	126 (33.2)	29 (20.4)	21 (30.4)	10 (66.7)	66 (42.9)
Unknown (refused to answer)	80 (21.1)	30 (21.1)	16 (23.2)	1 (6.7)	33 (21.4)
Partner contact routed)	380 (100)	142 (100)	69 (100)	15 (100)	154 (100)
Offline	20 (5.3)	9 (6.3)	4 (5.8)	1 (6.7)	6 (3.9)
Online	5 (1.3)	1 (0.7)	1 (1.4)	0 (0.0)	3 (1.9)
Unknown (refused to answer)	355 (93.4)	132 (93)	64 (92.8)	14 (93.3)	145 (94.2)

Unit: n (%). ^a)Congenital syphilis and early latent syphilis were excluded. ^b)Multiple symptoms could be reported per case. Percentages were calculated using the total number of cases by stage as the denominator (number of cases with the symptom/total number of cases in the stage×100). ^c)Others: pruritus, pain, ocular symptoms, respiratory symptoms, alopecia, neurological symptoms, genital discharge, ear symptoms, urinary symptoms, nausea, vomiting, and abdominal pain. ^d)Excluded 3 cases of congenital syphilis. ^e)Included patients who visited a medical institution due to syphilis-related or other symptoms..

Regarding the epidemiological survey items, the response patterns related to exposure to risk factors revealed that, among the total of 380 individuals (excluding three with congenital syphilis), 174 (45.8%) affirmed having had single or ongoing sexual contact within the 12 months preceding the diagnosis. Furthermore, 126 (33.2%) indicated “no,” while the remaining 80 (21.1%) either declined to respond or could not be classified (Table 2). In terms of gender, among 211 male cases aged 20–40 years, only 96 (45.5%) reported a history of sexual contact, while 67 (31.8%) reported no such history, and 48 (22.7%) declined to respond. Among the 72 female cases within the same age group, 39 (54.2%) reported having a history of sexual contact, 19 (26.4%) indicated no history, and 14 (19.4%) declined to respond. A notable finding from the analysis of adolescent data was the high level of reluctance to provide verbal responses. Specifically, 27.3% of male cases and 45.5% of female cases were refused to respond, indicating a substantial overall rate of refusal to respond (Table 3).

Table 3 Responses on history of sexual contact among syphilis cases by sex and age group in Busan Metropolitan City, Ulsan Metropolitan City, and Gyeongsangnam-do, Republic of Korea, 2024

	Total	10–19	20–29	30–39	40–49	50–59	60–69	≥70
Total	380 (100)	22 (100)	125 (100)	96 (100)	62 (100)	35 (100)	21 (100)	19 (100)
Malea)	279 (73.4)	11 (50.0)	82 (65.6)	81 (84.4)	48 (77.4)	31 (88.6)	16 (76.2)	10 (52.6)
Yes	126 (45.2)	7 (63.6)	44 (53.7)	32 (39.5)	20 (41.7)	16 (51.6)	5 (31.3)	2 (20.0)
No	95 (34.1)	1 (9.1)	19 (23.2)	34 (42.0)	14 (29.2)	11 (35.5)	9 (56.3)	7 (70.0)
Refused to answer	58 (20.8)	3 (27.3)	19 (23.2)	15 (18.5)	14 (29.2)	4 (12.9)	2 (12.5)	1 (10.0)
Femalea),b)	101 (26.6)	11 (50.0)	43 (34.4)	15 (15.6)	14 (22.6)	4 (11.4)	5 (23.8)	9 (47.4)
Yes	48 (47.5)	6 (54.5)	24 (55.8)	10 (66.7)	5 (35.7)	2 (50.0)	1 (20.0)	0 (0.0)
No	31 (30.7)	0 (0.0)	11 (25.6)	4 (26.7)	4 (28.6)	1 (25.0)	3 (60.0)	8 (88.9)
Refused to answer	22 (21.8)	5 (45.5)	8 (18.6)	1 (6.7)	5 (35.7)	1 (25.0)	1 (20.0)	1 (11.1)

Unit: n (%). ^a)Percentages for sex (%)=(sex sub-total for age group/total cases for age group)×100. ^b)Excluded 3 cases of congenital syphilis..

In the year following the reinstatement of mandatory syphilis surveillance in 2024, the Gyeongnam region reported the second-highest number of syphilis cases nationwide, following Seoul [9]. Regionally, Busan exhibited the highest proportion of syphilis cases overall and by stage, followed by Gyeongsangnam-do and Ulsan. The Gyeongnam region is characterized by high population density and a significant presence of highly mobile groups, including port and manufacturing workers, as well as foreign workers on short-term stays. Studies indicate that the incidence of STIs tends to rise with increasing city size and population density [10]. An analysis of patient demographics revealed a distribution by stage that mirrors the national trend: early latent syphilis accounted for 1,220 cases (43.7%), primary syphilis for 983 cases (35.2%), secondary syphilis for 524 cases (18.8%), tertiary syphilis for 51 cases (1.8%), and congenital syphilis for 12 cases (0.4%). Additionally, domestic research has shown a persistent increase in syphilis among HIV-infected males in Busan [11]. International studies have further demonstrated that seafarers, fishery workers, and port workers are at a heightened risk of STIs [10,12-14], suggesting that highly mobile populations with increased exposure risks serve as potential transmission chains.

According to the “2023 Survey on Immigrants’ Living Conditions and Labor Force” conducted by Ministry of Data and Statistics, non-professional workers (E-9 visa holders) represented 30.6% of all foreign residents in the Gyeongnam region, significantly exceeding the national average of 18.8% [15]. The non-professional employment (E-9) visa facilitates temporary employment in sectors experiencing labor shortages, such as manufacturing, construction, agriculture, and services. This demographic composition implies that the Gyeongnam region is relatively reliant on foreign labor in manufacturing, construction, and other industries due to its specific regional characteristics. Moreover, the region exhibits structural features characterized by the coexistence of large labor populations, particularly in ports and industrial complexes, alongside areas with a high concentration of entertainment establishments. According to the Ministry of the Interior and Safety’s “Status of Entertainment Establishments (2023)” data, the total number of entertainment establishments in the Gyeongnam region (4,235), Busan (2,329), and Ulsan (995) amounted to 7,559, representing approximately 30% of all entertainment establishments nationwide, second only to Seoul. Utilizing mid-year resident registration population data from Ministry of Data and Statistics, the number of entertainment establishments per 100,000 people by region was calculated to account for population size differences. The Gyeongnam region had the highest number of entertainment establishments per 100,000 people, at 100 establishments, followed by the Honam region (68), the Gyeongbuk region (62), the Chungcheong region (39), and capital region (34). This confirms the high density of entertainment establishments relative to the population in the Gyeongnam region [16]. Given these characteristics, the syphilis outbreak in the Gyeongnam region may warrant an interpretation that considers the interplay between its industrial, port-centered urban structure and the entertainment industry.

The number of syphilis cases and their stage distribution in the Gyeongnam region in 2024 were comparable to those recorded during the mandatory surveillance period from 2011 to 2019. In contrast, from 2017 to 2019, over a three-year period, primary syphilis peaked at 206 cases, secondary syphilis at 84 cases, and congenital syphilis at five cases. The slightly lower incidence observed in 2024 compared to 2019, just prior to the transition to sentinel surveillance in 2020, suggests a need for further observation and analysis. This is particularly pertinent given that 2024 marked the return to mandatory surveillance following the sentinel surveillance period from 2020 to 2023.

A notable aspect of the diagnostic stage distribution was the higher proportion of tertiary syphilis in the Gyeongnam region compared to the 2024 national incidence reported by the AIDS Management Division of the KDCA in July 2025. While the national rate of tertiary syphilis was 1.8%, with an incidence rate of 0.1 cases per 100,000 people, the Gyeongnam region exhibited a tertiary syphilis rate of 3.9%, nearly double the national figure, and an incidence rate of 0.2 cases per 100,000 people, also twice as high [9]. Since tertiary syphilis manifests after a prolonged latent period that can span several years to decades following infection, these statistics may reflect the progression of historical infections rather than a short-term rise in incidence. Do et al. [17] found that syphilis cases in the United States from 2017 to 2024 were predominantly concentrated among young adults aged 18–34 years. Other studies indicated that primary and secondary syphilis were most prevalent among individuals aged 15–49 years, whereas tertiary and latent syphilis were more common in those aged ≥50 years [18]. In the Gyeongnam region, early-stage syphilis was predominantly observed among young adults in their 20s and 30s, while late-stage syphilis was primarily noted in older adults, reflecting trends similar to those reported in international studies. The progression to tertiary syphilis appears to be influenced by a combination of factors, including delayed diagnosis following infection and limited access to medical care. Therefore, it is crucial for health authorities to identify the specific factors involved to facilitate early intervention.

Among the 25 respondents queried about their encounters with sexual partners, 20 indicated that they had met their partners offline. However, the notably low response rate of 6.6% complicates the consideration of this sample as representative of risk factors across all cases. The prevalence of respondents reporting one-time partners, offline encounters, and interactions in entertainment establishments, as opposed to online encounters, may reflect regional characteristics, as previously noted in the Introduction. Nevertheless, the low response rate concerning risk factors among younger individuals in their 20s and 30s, who are typically active online communicators, raises concerns about the potential underestimation of online encounter incidence. Recent studies, both domestically and internationally, have highlighted changes in transmission routes, including increased anonymity and diversified contact pathways resulting from the rise of online platforms such as dating applications. Concurrently, traditional forms of prostitution have declined, contributing to a resurgence of syphilis [9]. Research indicates that users of dating applications face a higher risk of contracting STIs compared to non-users [6,19]. Simulation studies based on real-world data have also demonstrated that increased utilization of dating applications complicates infection spread pathways when not properly managed [20]. These shifts may lead to a higher number of sexual contacts for infected individuals, as well as increased opportunities for such encounters. Consequently, early detection of infected individuals and contact tracing becomes increasingly challenging. Accurate identification of transmission routes will thus pose a critical challenge. Furthermore, the possibility that some individuals who reported “no” sexual contact history may actually be non-respondents suggests that the meaningful response rate could be even lower. This underscores the necessity for measures aimed at enhancing response rates to facilitate more accurate identification of infection sources.

In light of the findings of this study, future syphilis management in the Gyeongnam region should prioritize enhancing preventive education, as well as improving accessibility to testing and treatment for high-risk groups, particularly younger age cohorts. Additionally, it is essential to establish a survey environment grounded in trust to improve response rates during epidemiological investigations. Furthermore, the protection of personal information must be strengthened to facilitate the identification of transmission routes and the management of contacts. These initiatives are anticipated to promote early detection and treatment while establishing effective syphilis management strategies tailored to local characteristics.

Nevertheless, this study has certain limitations. First, due to the nature of the data collected during the initial year of mandatory surveillance implementation, there is a potential for temporary underreporting. Second, the analysis was based solely on data from the year 2024, which restricts the assessment of temporal changes in syphilis incidence and the progression through various disease stages. The nature of the data also precluded an in-depth comparative analysis of incidence patterns by province. Third, the low response rate for detailed epidemiological information, such as transmission routes, sexual contact patterns, foreign nationality, and residency status, limited the quantitative analysis of risk factors. In the future, the accumulation of comprehensive syphilis surveillance data and improvements in the quality of epidemiological investigation data through enhanced response rates are expected to enable more detailed analyses that reflect regional and population group characteristics. This, in turn, will facilitate the development of syphilis prevention and management strategies specifically tailored to the Gyeongnam region.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: None.

Conflict of Interest: The authors have no conflicts of interest to declare.

Author Contributions: Conceptualization: JHH, JEA. Data curation: JHH, JEA. Formal analysis: JHH, JEA, KSK. Project administration: HJL. Supervision: HJL, SEL. Visualization: JEA, KSK. Writing – original draft: JHH, JEA. Writing – review & editing: HJL, SEL.