PHWR Public Health Weekly Report

Malaria is a major vector-borne disease in the Republic of Korea (ROK), where it is endemic and transmitted by Anopheles mosquitoes [1]. Eight Anopheles species present in the ROK (Anopheles sinensis, An. kleini, An. pullus, An. lesteri, An. belenrae, An. sineroides, An. koreicus, and An. lindesayi), of which seven, excluding An. koreicus, are known as malaria vectors [2].

In 2024, 659 domestic malaria cases were confirmed, with the majority reported from the regions adjacent to the demilitarized zone near the Democratic People’s Republic of Korea (DPRK) [3]. The regions of Seoul, Incheon, Gyeonggi, and Gangwon province accounted for 94.1% (620 cases), and 13 districts in Seoul were newly designated as malaria-risk areas in 2024 [1,3].

The distribution of Anopheles mosquitoes varies with ecological characteristics and is influenced by environmental factors such as rainfall, temperature, and humidity [4]. Therefore, to effectively interrupt malaria transmission, it is essential to continuously monitor the distribution of vectors and implement control strategies accordingly [5].

Since 2009, the Division of Vectors and Parasitic Diseases (DVPD) at the Korea Disease Control and Prevention Agency (KDCA) has conducted a surveillance program to monitor the density and distribution of malaria vectors in malaria-risk areas of the ROK. This program, conducted in collaboration with local governments and the Ministry of National Defense, periodically monitors the seasonal abundance of vector mosquitoes and the infection rate of P. vivax. The surveillance results are published weekly on the KDCA website and support the issuance of public health advisory and warnings for malaria risk (Table 1).

Table 1 Malaria advisory and warning issuance criteria

Category	Issuance criteria	Scope	Authority
Advisory	When the average daily number of vector mosquitoesa) is 0.5 or higher in three or more cities/counties/districts	Nationwide	KDCA(KDCA DVPD)
Warning	Issued after an advisory when at least one of the following conditions is met:. ① The first cluster case occurs. ② The average daily number of vector mosquitoesa) remains 5.0 or higher for two consecutive weeks in the same city/county/district.	Relevant city/county/district	Relevant city/province
	When malaria parasites are detected in collected mosquitoes	Nationwide	KDCA(KDCA DVPD)

KDCA=Korea Disease Control and Prevention Agency; DVPD=Division of Vectors and Parasitic Diseases. ^a)The average daily number of vector mosquitoes (trap index): mosquito/trap/day..

This report aims to contribute to the malaria control policies by reporting the results of mosquito vector surveillance conducted in malaria-risk areas of the ROK in 2024, highlighting the importance of vector monitoring and control.

1. Mosquito Collection and Identification

In 2024, malaria vectors mosquitoes were collected from 62 residential-area sites and 14 military base located within malaria-risk areas in the Seoul, Incheon, Gyeonggi, and Gangwon provinces (Supplementary Figure 1; available online). Weekly collection was conducted weekly from April to October, the active mosquito season, in collaboration with local public health centers, military base, military preventive medicine units, and the Institute of Health & Environment.

Collection methods varied by region: mosquitoes were collected one day a week using black light traps in Seoul; seven days a week using black light LED traps in civilian areas of Incheon, Gyeonggi, and Gangwon; and two days a week at military base.

All collected female mosquitoes were identified under the stereomicroscope. These data were compiled in the VectorNet system (https://eid.kdca.go.kr). The DVPD subsequently calculated the data into a trap index (TI; defined as the number of female mosquitoes collected per trap per day) to analyze temporal and regional variations.

2. Plasmodium vivax Infection Rates

Infection rates of P. vivax were assessed by the Institutes of Health & Environment in Seoul, Incheon, northern Gyeonggi, and Gangwon, and by military preventive medicine units. Each week, up to 200 female Anopheles mosquitoes per site were tested for DNA extraction, pooled with a maximum of 10 individuals per tube.

The extracted genomic DNA was analyzed for the P. vivax small subunit rRNA gene using a nested polymerase chain reaction assay [6]. Suspected positive pools were sent to the KDCA for reproducibility via further testing and gene sequencing. The minimum infection rate (MIR), which assumes at least one infected mosquito per positive pool, was calculated as (number of positive pools/total mosquitoes tested)×1,000.

3. Provide and Share the Information on Vector Surveillance

Surveillance data on vector density and P. vivax infection rates were updated weekly and made public available on the KDCA website (https://www.kdca.go.kr/index.es?sid=a3) in the “Archives” - “National Infectious Diseases Statistics” section.

In 2024, the cumulative TI for malaria vectors collected over 31 weeks at 76 sites reached 120.6, representing a 66.6% increase from 2023 (72.4) and a 59.3% increase compared to the average year (2020–2022) (75.7). The proportion of malaria vectors among all collected mosquitoes was 32.6% in 2024, an increase of 1.1%p from the average year (31.5%) but a decrease of 6.4%p from 2023 (37.9%) (Table 2).

Table 2 Comparison of TI of collected mosquitoes in 2024

Division (yr)	No. of survey site	Total mosquito				Malaria vector mosquito			Proportion of malaria vector mosquito
		Accumulate of weekly TI	Proportion of change			Accumulate of weekly TI	Proportion of change
Normal year (2020–2022)	50	240.0	-			75.7	-		31.5%
2023	50	191.2	-			72.4	-		37.9%
2024	76	370.5	Normal year	54.4%		120.6	Normal year	59.3%	32.6%
			Previous year	93.8%			Previous year	66.6%

TI=trap index..

The malaria vector TI peaked at 18.2 in week 27 of 2024, representing a 97.8% increase from the 2023 peak of 9.2, which occurred in week 26 (Figure 1A). This sharp early-summer increase in TI was found to be primarily driven by high vector densities in Josan-ri, Paju-si. However, even excluding data from Josan-ri, vector density during the early summer remained more than double that of 2023 and the average year (Figure 1B). This rise is likely attributable to average temperatures reaching levels suitable for mosquito proliferation from mid-May 2024, combined with May rainfall that created favorable larval habitats (Figures 1, 2).

Figure 1. Weekly malaria vector TI in 2024
(A) Weekly vector TI of all collection sites (76 sites), (B) weekly vector index of collection sites excluding Josan-ri, Paju-si (75 sites). TI=trap index.

On the basis of collection results from week 23, a national malaria advisory was issued on June 18, 2024. This was approximately 25 days earlier than 2023 (July 12). This change was due to an adjustment in the advisory criteria from “a TI ≥2 for two consecutive weeks” in 2023 to “a TI ≥0.5 in three or more municipalities (cities, counties, or districts)” in 2024. Earlier issuance of the advisory is estimated to have contributed to reducing malaria risk by enabling early vector control that interrupts patient–mosquito transmission (Table 1).

The heavy rainfall in malaria-risk areas likely washed away larvae and eggs, leading to a subsequent decline in mosquito density (Figure 2).

Figure 2. Weekly malaria vector TI and number of domestic malaria patients
TI=trap index.

Following the rainy season, an expansion of larval habitats due to puddle formation led to a resurgence in mosquito density, with secondary peaks in weeks 33 and 35. This increase in vector density was followed by a rise in human malaria cases approximately five to six weeks later (Figure 3).

Figure 3. Average temperature, daily precipitation, and malaria vector TI in 2024
TI=trap index.

In the 2024 P. vivax survey, three mosquito pools were positive for P. vivax DNA; they were collected from military base in Paju-si during weeks 31, 32, and 34 (Supplementary Figure 1; available online). A total of 20,392 malaria vector mosquitoes were tested for malaria infection in 2024, yielding an MIR of 0.15. Although the number of positive pools was the same as in 2023 (n=3), the MIR decreased 0.06 from 0.21 because a larger number of Anopheles mosquitoes were tested in 2024 as a result of higher vector density. Following the detection of the first positive mosquito pool, a nationwide malaria warning was issued on August 7, 2024. Moreover, the 2024 criteria were revised to enable density-based warnings at the municipal level, allowing for early regional warning in Cheorwon-gun (July 11), Ganghwa-gun (July 17), and Yanggu-gun (July 30) prior to the issuance of a nationwide warning.

In 2024, the population density of malaria vectors was markedly higher than 2023 and the average year (2020–2022). This increase is likely the result of a combination of favorable meteorological conditions, including elevated early-summer temperatures and a prolonged period of tropical nights, which are conducive to vector development. Indeed, the national average yearly temperature in 2024 was 14.5℃, the highest since record-keeping began and 0.8℃ warmer than the previous record set in 2023 (13.7℃). The number of tropical nights also reached a record of 24.5, approximately 3.7 times climatological normal (1991–2020) of 6.6. While the annual precipitation in 2024 (1,414.6 mm) was comparable to the climatological normal (1991–2020), 78.8% of the summer rainfall was concentrated during the rainy season (late June to late July) [7], which appears to have temporarily suppressed vector density.

The sharp increase in malaria vectors in early summer 2024 was largely driven by malaria vector population in Josan-ri, Paju city. Josan-ri, a controlled-access area near the DPRK, is a region where malaria vectors constituted over 51.2% of the total mosquito catch in 2024. According to meteorological data from nearby Panmunjom, numerous days with average maximum temperatures exceeding 30℃ were recorded before the rainy season, presumably creating favorable conditions for mosquito proliferation [8]. Due to its mostly rice paddy and rural setting, Josan-ri appears to have experienced a greater increase in vector mosquito density compared to other regions. Additionally, the replacement of conventional black light traps with black light LED traps at 36 civilian sites in 2024, including in Josan-ri, may have also contributed to the increased mosquito catch.

During the 29th week of the rainy season, mosquito density decreased sharply in Paju-si and Yeoncheon-gun due to heavy rain over a short period of time [9]. In the ROK, vector mosquito density typically increases with rising temperatures but is temporarily suppressed by adverse weather conditions such as the rainy season. However, following the rainy season, the formation of puddles creates favorable habitats for larvae, leading to a subsequent resurgence in mosquito populations. Notably, post-rainy season mosquitoes are more likely to contribute to human infections, highlighting the importance of active larval control strategies such as eliminating standing water.

Anopheles mosquitoes predominantly inhabit rural rather than urban areas [10]. However, as larvae can develop in various aquatic habitats including streams, wetlands near mountains, and not only rice paddies [10,11], rigorous management of all potential breeding sites, including peri-urban environments, is essential for malaria prevention.

In 2024, revisions to the surveillance-based criteria led to the national advisory was issued approximately 25 days earlier than in 2023. Regional warning for Cheorwon-gun, Ganghwa-gun, and Yanggu-gun were also issued before the nationwide warning based on P. vivax mosquito detection. This early vector control approach in high-risk areas enhanced public awareness and intensified local vector control efforts, which likely contributed to a reduction in malaria incidence.

Strengthening an integrated management system that connects vector surveillance remains a key strategy for achieving the goal of malaria re-elimination. This requires the sustained operation of a meticulous and accurate surveillance system, coupled with rapid control activities.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: None.

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

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

Supplementary data are available online.