PHWR Public Health Weekly Report

Japanese encephalitis (JE) is a mosquito-borne disease that is endemic to Asia and the Western Pacific, including Austria, Bangladesh, Brunei, Myanmar, Japan, and China. Since its initial emergence in Japan in 1871, it has annually affected more than 68,000 people, primarily in Asia [1]. In the Republic of Korea (ROK), the average number of patients with JE per year during the last 5-year period from 2020 to 2024 was 15.8 [2]. JE is a viral infectious disease transmitted to humans through the bite of infected mosquitoes [3]. JE virus is a single-stranded RNA virus that belongs to the family Flaviviridae, exhibiting a close genetic relationship with other notable viruses such as dengue, yellow fever, and West Nile virus [1]. JE has five genotypes and develops after an incubation period of approximately 5 to 15 days following infection. Most individuals infected with JE are asymptomatic [3].

Globally, 14 mosquito species have been reported as vector of JE, including Culex tritaeniorhynchus, Culex bitaeniorhynchus, Armigeres subalbatus, and the Culex pipiens complex [4]. Among them, Cx. tritaeniorhynchus has been recognized as the principal vector of JE. This species has been identified in 41 countries, including 22 in Asia, with major distributions in China, India, Japan, Thailand, and ROK [5].

The Division of Vectors and Parasitic Diseases at the Korea Disease Control and Prevention Agency (KDCA) employs JE vector surveillance. This information is used to issue for JE alerts and warnings and recommend vaccination. A JE alert is issued when Cx. tritaeniorhynchus is first collected in a particular collection area. A JE warning is issued when the Cx. tritaeniorhynchus population exceeds 500 individuals and constitutes more than 50% of the total mosquito population, when JE virus is isolated or detected in collected mosquitoes, and when a confirmed case of JE is documented [2]. Weekly surveillance data on vector density are shared via the KDCA website. Since 1975, surveillance programs have been implemented to effectively control the incidence of JE and contribute to the improvement of public health.

Given the continued incidence of JE cases in ROK and the presence of the vector mosquito, continuous surveillance is necessary. Therefore, this study aimed to investigate the occurrence of vector mosquito in 2024 and analyze their relationship with meteorological factors.

1. Vector Surveillance

Vector surveillance for JE was conducted in collaboration with the Regional Centers for Disease Control (13 site), the Ministry of National Defense (4 site), Institute of Health and Environment in local government (13 site), and the Regional Centers for Vector Surveillance against Climate Change (32 site). Mosquito collection was carried out from April 1 (week 14) to October 31 (week 44) of 2024. In the southern provinces (Busan, Jeollanam-do, Gyeongsangnam-do, and Jeju Island), where Cx. tritaeniorhynchus is more likely to occur, collection began one week earlier, on March 25 (week 13). Mosquitoes were captured using black light trap placed in cattle shed, urban areas, military bases and migratory bird habitats. Collected specimens were identified to the species level at each institution, and the identification results were subsequently compiled through VectorNet system of KDCA. For comparative analysis of mosquito densities, the data were standardized using a trap index (TI, number of mosquitoes/number of trap/day). In this study, comparative analysis of vector occurrence was conducted using data from 10 long-term surveillance sites.

2. Data Analysis

To analyze the JE vectors density, the average, minimum and maximum temperatures and rainfall were obtained from the Korea Meteorological Administration’s Open Data Portal (https://data.kma.go.kr/cmmn/main.do). Vector distribution maps were created using the inverse distance weighted method and the Spatial Analyst tool in ArcGIS 9.0 (Environmental Systems Research Institute), based on the TI.

1. Vector Surveillance

A total of 18 mosquito species from 7 genera were collected at 10 sites in 2024, with an overall TI of 266.5 (Table 1, Figure 1). The most dominant species was Aedes vexans (37.3%), followed by Cx. tritaeniorhynchus (33.7%) and Anopheles spp. (23.5%). While the total mosquito index had been decreasing since 2021, the index for Cx. tritaeniorhynchus continued to rise, reaching its highest level of 89.8 in 2024. Cx. tritaeniorhynchus was primarily observed from June to October, with the peak density recorded in August (Figure 2).

Figure 1. Compare the tatal mosquito and Culex tritaeniorhynchus TI by year
TI=trap index.

Figure 2. Climate factors (temperature and rainfull) and Culex tritaeniorhynchus TI in 2024
TI=trap index.

Table 1 Mosquitoes collected during monitoring in 2024

Species	Mosquito index (%)
	2022	2023	2024
Culex tritaeniorhynchus	62.0	66.6	89.8 (33.7)
Aedes vexans	214.5	153.8	99.5 (37.3)
Anopheles spp.	65.7	93.8	62.5 (23.5)
Culex pipiens	8.1	9.3	6.2 (2.3)
Ochlerotatus koreicus	1.9	4.6	3.0 (1.1)
Armigeres subalbatus	3.6	2.9	2.3 (0.9)
Mansonia uniformis	0.4	0.4	0.5 (0.2)
Culex orientalis	0.7	4.2	1.2 (0.5)
Ochlerotatus togoi	0.2	0.9	0.5 (0.2)
Aedes albopictus	0.2	0.3	0.3 (0.1)
Culex bitaeniorhynchus	0.1	0.1	0.2 (>0.1)
Ochlerotatus nipponicus	0.2	0.3	0.2 (>0.1)
Ochlerotatus hatorii	0.1	0.1	0.1 (>0.1)
Ochlerotatus dorsalis	0.0	0.0	0.1 (>0.1)
Aedes alboscutellatus	0.0	0.0	0.0 (>0.1)
Culex vagans	0.0	0.0	0.0 (>0.1)
Aedes lineatopennis	0.0	0.0	0.0 (>0.1)
Coquillettidia ochracea	0.0	0.0	0.0 (>0.1)
Total mosquito index	357.7	337.5	266.5 (100.0)

2. Change in Cx. tritaeniorhynchus Index by Month/Week

The presence of Cx. tritaeniorhynchus was documented in Jeju-si, Jeju Island in March and in the southern coastal regions during April and May (Figure 3). Its occurrence gradually expanded to inland areas and nationwide from June to October, with peak density recorded in August. Compared to 2023, the onset of occurrence advanced by 1 to 12 weeks depending on the region (Table 2).

Figure 3. Monthly occurrence of Culex tritaeniorhynchus in 2024

Table 2 The appearance of the Culex tritaeniorhynchus by year

Province		2021	2022	2023	2024
Gangwon state	Hoengseong-gun	-	32	24	26
	Gangneung-si	-	29	31	24
	Chuncheon-si	35	31	28	24
Chungcheongbuk-do	Cheongju-si	30	30	28	25
Chungcheongnam-do	Yesan-gun	34	32	30	28
Jeonbuk State	Jeonju-si	23	30	29	26
Jeollanam-do	Hampyeong-gun	23	26	28	16
Gyeongsangbuk-do	Gyeongsan-si	32	27	27	25
Daegu Metropolitan City	Dong-gu	-	-	-	24
Gyeongsangnam-do	JinJu-si	25	17	16	16
Ulsan Metropolitan City	Ulju-gun	-	-	-	24
Busan Metropolitan City	Gijang-gun	14	15	13	14
Jeju Island	Jeju-si	14	16	14	13

Unit : week.

3. Issuance of a JE Warning/Alert

In 2024, a nationwide JE warning was issued on March 30, following the first collection of Cx. tritaeniorhynchus on March 27 (week 13), with 16 individuals collected in Jeju-si, Jeju Island, and one in Wando-gun, Jeollanam-do. The JE alert was issued on July 25 (week 30) after more than 500 Cx. tritaeniorhynchus individuals, accounting for 63.2% and 58.4% of all mosquitoes, were collected in Jinju-si, Gyeongsangnam-do, and Hampyeong-gun, Jeollanam-do, respectively.

4. Reporting of JE Vector Surveillance Results

The results of Cx. tritaeniorhynchus surveillance were updated weekly during weeks 14–44 on the KDCA website (https://www.kdca.go.kr) under Archives → National Infectious Diseases Statistics → Public Health Weekly Report → Surveillance Statistics of Infectious Diseases.

In 2024, the total mosquito TI across the 10 collection sites was 266.5, representing a decrease of approximately 21.0% compared to the previous year. The TI for Cx. tritaeniorhynchus, the primary JE vector species, was 89.8, reflecting an increase of approximately 34.8% from 2023. The TI of Cx. tritaeniorhynchus increased in all regions except Gangneung-si, Gangwon Special Self-Governing Province. Increasing of Cx. tritaeniorhynchus TI is attributed to the combined effect of elevated mean temperatures and reduced rainfall. According to the Korea Meteorological Administration, the average summer (June–August) temperature in 2024 was 25.6℃, the highest recorded average temperature since 1973 [6]. Temperature increases lead to increase fecundity, accelerate hatching rates of eggs, and enhance larval development [7]. In 2024, the TI for Cx. tritaeniorhynchus in August increased by 59.9% compared to the 2023 value, consistent with the previous year.

Summer rainfall is a significant climatic factor associated with mosquito density and habitat distribution. Several studies have examined the correlation between rainfall levels and the density of Culex mosquitoes [7,8]. This analysis revealed that the population of Culex mosquitoes peaks in density when the average monthly rainfall is approximately 580 mm. However, rainfall levels exceeding this threshold have been observed to directly affect adult survival and result in the loss of larvae within the habitat [8]. The rainy season in ROK, which typically extends from June to July, contributes to more than 50% of the country’s annual rainfall. Rainfall during the 2024 rainy season was 602.7 mm, approximately 10.3% lower than the climatological average (1991–2020). This reduced rainfall, compared to the previous year, may have created more oviposition sites while minimizing the loss of mosquito eggs and larvae. These conditions likely contributed to the increase in the Cx. tritaeniorhynchus TI in August [5].

Other climatic factors, such as minimum and maximum temperatures and humidity, also strongly influence mosquito ecology, activity, and dispersal [9,10]. However, the scope of this study was limited to the analysis of average temperature and rainfall. Further investigation is needed to evaluate the effects of other climatic factors. This study did not include statistical analysis; instead, it examined variations in vector density during the surveillance period in relation to climatic conditions. A more accurate assessment would require conducting correlation analysis between climatic factors and vector populations.

Cx. tritaeniorhynchus TI has shown an annual increase, and the issuance of JE alerts and warnings has been shifting earlier each year. The density of Cx. tritaeniorhynchus increased by approximately 59.2% compared to 2007 and 34.8% compared to the previous year. In 2024, the JE warning was issued approximately 20 days earlier than in 2007, coinciding with the start of the surveillance period. These changes indicate a shift in vector activity patterns, likely due to rising average temperatures. Consequently, there is an increasing need to extend the mosquito surveillance period in the JE vector surveillance program. To improve vector surveillance, it is necessary to monitor the seasonal emergence of key mosquito species more closely. In addition, expanding surveillance sites would enable a more detailed analysis of regional and zonal mosquito density patterns.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: None.

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

Author Contributions: Conceptualization: HIL. Data curation: HWK, GHK. Formal analysis: HWK, GHK. Investigation: HWK, GHK. Project administration: HIL, JWJ. Resources: HWK, GHK. Supervision: HIL. Visualization: HWK, GHK. Writing – original draft: GHK. Writing – review & editing: HIL.