Status of Vector Surveillance for Infectious Diseases in Quarantine Areas at International Points of Entry, 2025

Abstract

Objectives

This study aims to prevent the introduction and establishment of overseas-origin infectious disease vectors associated with climate change and increasing international travel and trade by assessing the current status of vector surveillance at international points of entry in the Republic of Korea (ROK).

Methods

Mosquito surveillance was conducted at 40 major international airports and seaports in the ROK across 16 collection cycles from March to November 2025. Adult mosquitoes were collected using Biogents (BG)-Sentinel traps, light-emitting diode (LED) black-light traps, and Gravid traps. All specimens were morphologically identified to the species level and subsequently screened for six major viral pathogens using multiplex real-time reverse transcription polymerase chain reaction.

Results

A total of 37,544 mosquitoes representing 18 species were collected during the surveillance period. Although the overall mosquito population decreased by 21.6% compared with the previous year, species diversity increased by the detection of three additional species. Culex pipiens was the dominant species, accounting for 64.4% of all collected specimens, with its peak annual density observed in September (28.8%). Regional analysis showed that the highest Trap Index (T.I.) was recorded in the Capital region (104.6), whereas species richness was greatest in Gyeongnam Province (15 species). With respect to trapping efficiency, the BG-Sentinel trap (T.I.=75) demonstrated a significantly higher capture performance the than LED black-light traps. None of the mosquito samples tested positive for any of the six targeted viral pathogens.

Conclusions

The observed increase in mosquito species diversity at airports suggests a higher risk of invasive vector introduction through air travel. Accordingly, differentiated surveillance strategies are required: seaports should prioritize monitoring population density trends, whereas airports should focus on the early of invasive mosquito species at points of entry, thereby strengthening national preparedness against the spread of vector-borne diseases.

Key messages

① What is known previously?

Climate change and expanding international trade increase the risk of introducing exotic vector-borne diseases. Although Aedes albopictus is a native species, it serves as a critical vector for imported viral pathogens, whereas conventional surveillance systems have primarily emphasized population density and pathogen detection rather than the risk of vector invasion.

② What new information is presented?

In 2025, extreme climatic conditions, including higher temperatures and drought, were associated with a 21.6% reduction in overall mosquito abundance. However, species diversity was higher at airports (Shannon Index=1.06) than at seaports (Shannon Index=0.89), with Gyeongnam Province exhibiting the highest species richness (15 species).

③ What are implications?

Vector surveillance systems should integrate climate data into routine monitoring frameworks. Surveillance objectives should be site-specific, prioritizing early detection of invasive species at airports and population density management at seaports. Establishing a proactive “introduction–spread” prevention framework is vital for mitigating emerging vector-borne disease threats.

Introduction

Mosquitoes are distributed worldwide and are the deadliest vectors of infectious diseases that threaten human survival [1]. According to a report by the World Health Organization (WHO), mosquito-borne infectious diseases account for more than 17% of all infectious diseases and cause more than 700,000 deaths annually [2]. In particular, dengue fever has been classified as the most rapidly spreading infectious disease, with the number of reported cases increasing approximately eightfold over the past 20 years [3,4]. Japanese encephalitis in the Republic of Korea (ROK) also has a high case fatality rate and leads to substantial socioeconomic losses, as approximately 30–59% of patients are left with permanent neurological sequelae, such as cognitive impairment or paralysis, even after recovery [5,6].

These threats are being further exacerbated by recent rapid warming within the country due to climate change [7,8]. Data from the Korea Meteorological Administration indicate that the average summer temperature in 2025 increased by 2.0°C compared to the climate normal and that precipitation patterns changed. These high temperatures not only advance the onset of mosquito activity and expand suitable habitats but also accelerate mosquito metabolism, shortening the life cycle and leading to a rapid increase in population size [9,10]. In fact, as of 2025, the issuance of the Japanese encephalitis advisory has advanced by approximately 3 months compared to 1975 [11], and with the confirmed presence of Aedes laniger, a species previously unrecorded in the ROK, and the tropical house mosquito (Culex quinquefasciatus) on Jeju Island, the disruption of the mosquito ecosystem due to subtropicalization is intensifying [12,13].

In addition to these ecological changes, the expansion of airline routes and the increase in port calls by foreign vessels following the coronavirus disease 2019 (COVID-19) pandemic are further increasing the risk of imported infectious diseases. In 2025, the number of air passengers reached 124.79 million, exceeding pre-COVID-19 pandemic levels, and quarantine blind spots are expanding, particularly as new international routes from regional airports have surged. The WHO has already warned of the possibility of the cross-border movement of the vector mosquitoes of infectious diseases via ships and aircraft and has strongly recommended vector surveillance at airports and seaports [14,15].

According to infectious disease statistics from the Korea Disease Control and Prevention Agency (KDCA), the occurrence of major mosquito-borne infectious diseases has been a continuing threat over the recent 6-year period (2020–2025) [16,17]. In particular, in 2025, an expanding trend of Chikungunya fever centered in Southeast Asia (including Vietnam) and Guangdong Province, China, was confirmed, with 109 cases of dengue fever reported. Dengue fever and Chikungunya fever share the same vector, Aedes albopictus, a domestically established mosquito species; therefore, the risk of domestic spread of introduced viruses is closely linked [17].

Therefore, under the leadership of the Gyeongnam Regional Center for Disease Control and Prevention, the five Regional Centers for Disease Control and Prevention and thirteen National Quarantine Stations are collaborating to conduct a vector surveillance project within quarantine areas through vector surveillance and pathogen analysis to enable the early detection of the introduction and spread of imported infectious diseases and to prevent the local establishment of invasive alien species. The 2025 surveillance results are presented herein.

Methods

1. Surveillance Area and Period

Surveillance was conducted at major airports and seaports nationwide in collaboration with the 5 Regional Centers for Disease Control and Prevention and 13 National Quarantine Stations. The survey was conducted over a total of 9 months from March to November 2025 across 40 sites (Figure 1), including 12 sampling points at 6 airports (Incheon, Cheongju, Muan, Daegu, Gimhae, and Jeju) and 28 sampling points at 15 seaports (Incheon, Daesan, Donghae, Samcheok, Pyeongtaek, Gunsan, Janghang, Mokpo, Yeosu New Port, Jeju, Pohang, Ulsan, Busan, Masan, and Gamcheon). Collections were conducted a total of 16 times, once per month in March and November and twice per month during the remaining period (April–October).

Figure 1. Mosquito sampling sites at airports and seaports.

2. Mosquito Collection and Classification

The surveillance primarily surveyed major vector species of imported infectious diseases, including Ae. albopictus, Aedes aegypti, Culex tritaeniorhynchus, and Culex pipiens. Considering the collection environment and ecological characteristics, Biogents (BG) traps, light-emitting diode (LED) traps (BioTrap), and Gravid traps (John W. Hock Company) were used in combination [18,19]. The BG traps and LED traps used dry ice as an attractant, while the Gravid traps used stagnant plant-based infusion water as an attractant. All traps were installed at 10:00 A.M. on the collection day and operated for 24 hours until 10:00 A.M. the following day.

To analyze the optimal temperature for survival and reproduction by mosquito species, the concept of temperature of maximum abundance (TMA) was introduced. This concept refers to the statistically optimal temperature range at which a specific mosquito species is observed at the highest density in the environment [20].

Collected mosquitoes were kept in an ultra-low temperature freezer (−70°C) for at least 30 minutes together with the retrieved collection trap. Then, they were retrieved, classified, and identified based on morphological characteristics under a stereomicroscope (Olympus SZ61; Olympus) [21].

3. Viral Gene Detection Test in Mosquitoes

1) Sample pretreatment and nucleic acid extraction

The sample pretreatment and homogenization process of the classified mosquitoes was performed in accordance with the recommendations of the Division of Vectors and Parasitic Diseases, KDCA, and previous research standards [22]. Up to 30 individuals were combined into a single pool, placed in a lysis tube (Reinforced Bead Tube, Zirconia 3 mm, Clear Tube; MP Biomedicals) containing 5–6 ceramic beads, and homogenized for 30 seconds at 4.5 m/s (or 7,500 rpm) using a tissue homogenizer (FastPrep-24; MP Biomedicals), after which RNA was extracted using the Clear-STM Total RNA Extraction Kit (Invirustech).

2) Real-time RT-PCR reaction composition

The conditions and analytical methods used for the detection of six mosquito-borne pathogens (dengue, yellow fever, West Nile, Japanese encephalitis, Zika, and Chikungunya viruses) with the Clear-MD Multiplex Real-Time RT-PCR Detection Kit (Invirustech) also followed the recommendations of the Division of Vectors and Parasitic Diseases, KDCA [22]. The reaction mixture was prepared by mixing 11 µl of 2X RT-qPCR Mix, 2 µl of Detection Mix, and 7 µl of RNA template for a total volume of 20 µl, and positive and negative controls were included in each reaction.

3) Amplification conditions and analysis

Real-time polymerase chain reaction (PCR) was performed using a 7500 Fast Real-Time PCR System (Applied Biosystems; Thermo Fisher Scientific) at 50°C (15 minutes) and 95°C (2 minutes), followed by 40 cycles of incubation at 95°C (15 seconds) and 58°C (1 minute). The fluorescence channels for each pathogen were set to FAM (dengue virus, Japanese encephalitis virus), HEX (yellow fever virus, West Nile virus), and ROX (Zika virus, Chikungunya virus) for simultaneous detection.

4) Result determination criteria

The threshold for result analysis was fixed at 30,000. Cases with a cycle threshold (Ct) value of 38 or less were determined to be positive (+), and cases where the Ct value was absent or exceeded 38 were considered negative (−). If the control group results were invalid, the corresponding experimental results were excluded from the analysis, and a retest was performed.

4. Species Diversity Analysis

In this study, to quantitatively compare the species diversity of mosquito communities by site (airport and seaport) within the quarantine areas, the following indices were calculated and analyzed.

1) Species diversity index (Shannon–Wiener Diversity Index [H’])

The Shannon Index was calculated to comprehensively evaluate species richness and the evenness of population size distribution at each survey site [23]. This index reflects not only the number of species but also the degree of dominance of specific species, and a higher value indicates greater community diversity. The formula is as follows (here, p_i is the proportion of the i-th species out of the total population size).

$$H\text{'}={\displaystyle \sum _{i=1}^R{p}_i\text{In}\left(p_i\right)}$$ (Equation 1)

2) Species evenness index (Simpson’s Diversity Index [1−D])

Simpson’s Index was used to confirm, in a complementary manner, the proportion of specific dominant species and species diversity within the community [24]. In this study, the “1−D” value was used to interpret values closer to 1 as indicating higher species diversity and lower ecological dominance by a specific species (where n_i is the number of individuals of a specific species, and N is the total number of individuals).

$$1-D=1-\frac{{\displaystyle \sum n_i\left(n_i-1\right)}}{N\left(N-1\right)}$$ (Equation 2)

Results

1. Mosquito Collection Status and Analysis of Meteorological Factors

A total of 37,544 mosquitoes from 18 species were collected within the quarantine area during the 2025 project period; compared to the previous year (47,879 individuals), the number of individuals decreased by 10,335 (21.6%), but the number of collected species increased by 3 (Table 1).

Table 1. Species composition and abundance of collected mosquitoes compared with the previous year (2025).

Mosquito species	2025		2024		Comparison
	No. mosquitoes (%)		No. mosquitoes (%)		Change in abundance (n)
Culex pipiens	24,185 (64.4)		25,068 (52.4)		▼883
Aedes vexans	6,375 (17.0)		8,379 (17.5)		▼2,044
Aedes albopictus	3,734 (9.9)		3,518 (7.3)		▲216
Aedes dorsalis	929 (2.5)		1,568 (3.3)		▼639
Armigeres subalbatus	569 (1.5)		1,831 (3.8)		▼1,265
Culex tritaeniorhynchus	523 (1.4)		2,572 (5.4)		▼2,049
Anopheles spp.	459 (1.2)		171 (0.4)		▲288
Ochlerotatus koreicus	440 (1.2)		2,447 (5.1)		▼2,007
Ochlerotatus togoi	143 (0.4)		116 (0.2)		▲27
Culex inatomii	72 (0.2)		2,086 (4.4)		▼2,014
Culex rubensis	31 (0.1)		0 (0.0)		-
Culex bitaeniorhynchus	29 (<0.1)		66 (0.1)		▼37
Mansonia uniformis	26 (<0.1)		31 (0.1)		▼5
Aedes lineatopennis	16 (<0.1)		15 (<0.1)		▲1
Coquillettidia ochracea	5 (<0.1)		4 (<0.1)		▲1
Culex orientalis	5 (<0.1)		7 (<0.1)		▼2
Ochlerotatus oreophilus	2 (<0.1)		0 (0.0)		-
Tripteroides bambusa	1 (<0.1)		0 (0.0)		-
Total	37,544 (100.0)		47,879 (100.0)		▼10,335

▼=decrease; ▲=increase.

The analysis indicated that meteorological patterns during the summer (July–August), including a heat wave with an average temperature 2.0°C above normal, below-normal precipitation remaining at 85.1% of the normal level, and dried-up water puddles that serve as mosquito larval habitats, thereby significantly suppressing the adult emergence rate (Supplementary Figure 1; available online).

Of all collected species, Cx. pipiens was predominant, accounting for 24,185 individuals (64.4%), followed by Aedes vexans (17.0%), Ae. albopictus (9.9%), and Aedes dorsalis (2.5%). No vector species introduced from overseas, such as Ae. aegypti, were collected.

2. Monthly Occurrence Trends and TMA Correlation

The monthly occurrence trend showed a bimodal pattern, with a first peak in June (21.1%), followed by a lull due to high temperatures during the summer season (July–August), before recording the peak annual density in September (28.8%). Particularly in September, as the temperature (27.0°C) aligned with the TMA of 26.4°C for Ae. vexans, an explosive increase in the density of this species was confirmed, with the species accounting for 45.9% of the total catch in September (Supplementary Table 1; available online) [10,25,26].

3. Comparison of Regional Occurrence Characteristics and Species Composition

To exclude differences in the number of surveillance sites by region and compare standardized mosquito population density, the Trap Index (T.I.) was calculated, and species richness (number of identified species) by region was analyzed to understand the ecological characteristics of the communities (Table 2).

Table 2. Regional distribution of mosquito collections, 2025.

Category	Total	Capital	Chungcheong	Honam	Gyeongbuk	Gyeongnam
No. of mosquitoes	37,544	14,421	8,393	2,879	7,528	4,323
Rate (%)	100.0	38.4	22.4	7.7	20.1	11.5
No. of sites	40	11	5	6	5	7
Trap Index	62.8	104.6	58.3	87.9	94.1	33.8
No. of species	18	13	9	9	10	15

The analysis showed that the Seoul Metropolitan Area (site=11), which had the highest density of surveillance sites, recorded a T.I. of 104.6, exhibiting the highest population density per site, followed by the Gyeongbuk region (T.I.=94.1), the Honam region (T.I.=87.9), and the Chungcheong region (T.I.=58.3), in descending order of density.

In contrast, with respect to species richness, a pattern opposite to the trend in population density was observed. In the Gyeongnam region (site=7, T.I.=33.8), which had the lowest total catch and T.I., a total of 15 mosquito species were collected, showing the highest species richness and diversity. This number was significantly higher compared to the corresponding values from the Chungcheong region (nine species) and the Honam region (nine species).

In particular, a comparative analysis of the mosquito community structures at airports and seaports confirmed that airports maintained higher species diversity and evenness than seaports for two consecutive years (Table 3). The species diversity index (Shannon–Wiener Diversity Index, H’) was calculated to be higher for airports (1.06) than for seaports (0.89) in 2025. This result was consistent with the 2024 survey, wherein the airport (1.62) also recorded a higher value than the seaport (1.29), indicating that the relative ecological diversity advantage of the airport area has been maintained. Furthermore, the species evenness index (Simpson’s Diversity Index, 1−D), which evaluates ecological dominance due to specific dominant species, was also higher at airports (0.46) than at seaports (0.38) in 2025 and was also calculated to be higher at airports (0.73) than at seaports (0.64) in 2024, demonstrating that mosquito communities at airports were not abnormally biased toward specific species such as Cx. pipiens and that various species were present in relatively even and stable proportions [23,24].

Table 3. Comparative analysis of mosquito community structure at airports and seaports.

Year	Category	No. of mosquitoes	Trap Index	No. of species	Shannon Index (H’)	Simpson Index (1–D)
2025	Airports	8,205	51.3	13	1.06	0.46
	Seaports	29,620	66.1	17	0.89	0.38
2024	Airports	6,127	54.7	15	1.62	0.73
	Seaports	41,752	106.5	13	1.29	0.64

Airports: Incheon, Cheongju, Muan, Daegu, Gimhae, Jeju. Seaports: Incheon, Daesan, Donghae, Samcheok, Pyeongtaek, Gunsan, Janghang, Mokpo, Yeosu, Jeju, Pohang, Ulsan, Busan, Masan, Gamcheon.

4. Comparison of Collection Equipment Efficiency

To evaluate the efficiency of three collection equipment types (BG trap, LED trap, and Gravid trap) used in a wide range of environments within the quarantine areas, the T.I. for each trap was calculated and compared (Supplementary Table 2; available online). The analysis showed that the BG trap recorded a T.I. of 75, demonstrating significantly higher collection efficiency than the LED trap (T.I.=43). A total of 28,827 mosquitoes were collected using the BG trap, accounting for approximately 76.8% of the total catch, confirming it as the most practical equipment for vector surveillance within the quarantine areas. In contrast, Gravid traps using only stagnant plant-based infusion water accounted for just 1.2% (342 individuals) of the total catch.

5. Mosquito-borne Pathogen Detection Results

A total of 37,544 collected mosquitoes were grouped into 2,133 pools of up to 30 mosquitoes each based on classification and identification criteria (species, collection site, and collection period) and analyzed. The analysis showed that genes of the six pathogens causing dengue fever, Zika virus infection, Japanese encephalitis, West Nile fever, yellow fever, and Chikungunya fever were not detected.

Discussion

This study analyzes the results of a 2025 surveillance program conducted at 40 major airport and seaport quarantine areas nationwide to prevent the introduction and establishment of imported infectious disease vector mosquitoes and to detect their initial introduction. According to the surveillance results, a total of 37,544 mosquitoes representing 18 species were collected, and all six mosquito-borne viral pathogens tested negative, indicating that the risk of infectious disease transmission within the quarantine areas during the study year remained at a stable level.

However, a close causal relationship with climatic factors underlies the 21.6% decrease in population size compared to the previous year (a decrease of 10,335 individuals). A temperature increase of 2.0°C or higher and a precipitation deficit, with rainfall at 85.1% of the normal year’s level, during the summer season (July–August) are believed to have suppressed the emergence of early-season adults by drying up small water puddles, which are essential oviposition sites for mosquito larvae [26,27].

In particular, the explosive surge of Ae. vexans in September (45.9% share), observed in the monthly analysis, clearly demonstrates the mechanism of the species-specific TMA [10]. Unlike Cx. pipiens (TMA 23.5°C), Ae. vexans shows a relatively higher TMA (26.4°C). Therefore, the unusually high-temperature conditions in September 2025 (mean temperature, 27°C) are considered to have promoted the growth of the latter species [20].

A comparative analysis of airports and seaports also has significant implications. Seaports showed a skewed ecological dominance toward Cx. pipiens (Simpson Index 0.38), whereas airports, reflecting the characteristics of anthropogenic pathways, showed high values for both the Shannon Index (1.06) and the Simpson Index (0.46), indicating a mosquito community not biased toward any particular species [23,24]. This finding suggested that airport areas provide a relatively favorable environment for exotic introduced vector mosquito species to overcome the “introduction–establishment” barrier during invasion [27-29].

In terms of collection equipment efficiency, the BG trap recorded a T.I. of 75, more than twofold higher than the LED trap, demonstrating its superior collection efficiency [20,21]. In a comparison of traps using the same attractant (carbon dioxide), the BG trap was considered to have maximized surveillance efficiency because it could be installed at desired locations, unlike LED traps, which have limited flexibility in site selection due to power supply requirements. For a more accurate comparison between traps, the development of battery-operated LED traps should be prioritized, and additional comparative analyses will be required under conditions wherein they can be freely installed at desired locations, as with the BG trap.

The non-detection of all six major viruses in pathogen testing of 37,544 mosquitoes (2,133 pools) suggests that the risk of infectious disease transmission due to vector activity within the quarantine areas was low during the study year. However, the current non-detection does not guarantee long-term safety, considering that climate change is extending the mosquito occurrence period into November and increasing species diversity [30].

The 2025 surveillance program achieved surveillance efficiency (T.I.=62.8) exceeding that of previous years through a surveillance system that had returned to normal following the COVID-19 pandemic. In the future, surveillance methods for introduced exotic mosquitoes within quarantine areas should be differentiated from those used in general areas. Repeated survey methods for observing changes over time are inherently limited for monitoring rare mosquitoes that occur sporadically within a confined area, such as quarantine areas. The invasion process of invasive alien species is generally described as a stepwise process of introduction, establishment, and spread, and each stage must overcome different ecological barriers to proceed successfully. Most alien species die out during the establishment stage following introduction, but only some species reach the spread stage and affect ecosystems and public health [27-29]. Therefore, vector surveillance conducted in quarantine areas can be interpreted as a key strategy for preventing subsequent spread by detecting the introduction and establishment stages, the earliest stages of the invasion process, at an early stage. In conclusion, to detect the subtle signals of introduced mosquitoes entering through quarantine areas, reorganizing surveillance sites to reflect the ecological characteristics of introduced mosquitoes (introduction–establishment–spread) is considered essential for the sustained implementation of this program.

Supplementary Materials

Supplementary data are available online.