Laboratory Surveillance of Water- and Foodborne Infectious Diseases Imported from Overseas in 2024

Abstract

Objectives

This study aimed to isolate and detect water- and foodborne pathogens imported by overseas travelers entering the country through Incheon International Airport in 2024, based on sex, age, and country. It further aimed to provide fundamental information on the epidemiology and risk factors of water- and foodborne diseases in the Republic of Korea to facilitate effective prevention and response strategies.

Methods

The Capital Regional Center for Disease Control and Prevention participates in the Enteric Pathogens Active Surveillance Network, which is coordinated by the Korea Disease Control and Prevention Agency. From January to December 2024, specimens were collected from overseas travelers exhibiting symptoms of water- and foodborne diseases at the Incheon Airport National Quarantine Station, with their prior consent. Five genera of bacteria and five types of viruses causing water- and foodborne infectious diseases were isolated and detected from these specimens.

Results

Among the 203 collected specimens, 91 (44.8%) pathogenic bacteria causing water- and foodborne diseases, and 28 (13.8%) pathogenic viruses were detected. Among these, the most prevalent pathogens were pathogenic Escherichia coli and norovirus associated with 47 (51.6%) and 25 cases (89.3%), respectively. The Philippines and Indonesia were the major sources of these imported pathogens. Moreover, the isolation and detection rates were the highest during the vacation season (January, February, July, and August).

Conclusions

Education and public awareness campaigns on preventive measures against water- and foodborne infectious diseases should be strengthened for travelers to cholera quarantine areas such as the Philippines and Indonesia. In addition, continuous monitoring of the causative pathogens should be conducted through surveillance systems to prevent the importation of water- and foodborne infectious diseases.

Key messages

① What is known previously?

The risk of domestic introduction of water- and foodborne infectious diseases through Incheon International Airport is increasing.

② What new information is presented?

In 2024, among specimens collected from 203 symptomatic international travelers entering through Incheon International Airport, bacterial and viral pathogens were detected in 44.8% and 13.8% of cases, respectively. The main pathogens isolated were pathogenic Escherichia coli and norovirus.

③ What are implications?

Education and public awareness campaigns for food poisoning prevention should be conducted for travelers to Southeast Asia, particularly those visiting the Philippines and Indonesia. In addition, monitoring symptomatic individuals and maintaining an effective surveillance system are essential for preventing the domestic introduction of water- and foodborne pathogens.

Introduction

Following the World Health Organization’s declaration in May 2023 ending the Public Health Emergency of International Concern for coronavirus disease 2019, the number of outbound travelers from the Republic of Korea (ROK) has returned to pre-pandemic levels. As of 2024, approximately 37.5 million travelers have entered through Incheon International Airport [1]. Waterborne and foodborne diseases are among the most common illnesses affecting travelers. Some reports estimate that 20% to 50% of individuals traveling from developed to developing countries develop such diseases [2,3]. Although most diseases, characterized by symptoms such as diarrhea, abdominal pain, and vomiting, resolve within a few days without specific treatment, they can disrupt travel regardless of severity and show a high incidence in vulnerable populations such as the elderly and children. Furthermore, because travelers can introduce endemic exotoxin-producing pathogens from one community to another, waterborne and foodborne diseases in travelers represent a significant public health concern [4,5].

Recognizing this public health importance, the Korea Disease Control and Prevention Agency (KDCA) has operated the Enteric Pathogens Active Surveillance Network (Enter-Net) since 2003. This laboratory-based surveillance system monitors the causative pathogens of waterborne and foodborne infectious diseases, connecting public health centers, municipal and provincial institutes of health and environment, and KDCA laboratories nationwide. This study aimed to isolate and detect the acute diarrhea-causing bacteria and viruses and to describe their epidemiological characteristics among travelers entering through Incheon International Airport in 2024 who reported symptoms of waterborne and foodborne diseases, such as diarrhea, abdominal pain, and vomiting. These surveillance findings will serve as foundational data for preventing the introduction and spread of overseas-acquired infectious diseases, informing travel destination-specific prevention and outreach campaigns, and strengthening the laboratory surveillance system.

Methods

1. Study Population

From January to December 2024, a total of 67,584 individuals who arrived at Incheon International Airport after visiting designated quarantine areas, including those for cholera, reported symptoms of waterborne or foodborne diseases on their Health Declaration Forms. Following investigation by quarantine officers, specimens were collected from 203 individuals who had a confirmed epidemiological link to a waterborne or foodborne infectious disease and consented to testing. These specimens were analyzed for five bacterial genera (Vibrio spp., Salmonella spp., Shigella spp., pathogenic Escherichia coli, and Campylobacter spp.) and five viruses (group A rotavirus, norovirus, enteric adenovirus, astrovirus, and sapovirus).

2. Analytical Methods

Stool specimens were collected via rectal swabs from travelers who met the case definition for a suspected quarantinable waterborne or foodborne disease, such as the presence of diarrhea or abdominal pain. Pathogen diagnosis and identification were conducted in accordance with the Integrated Guidelines for the Diagnosis of Reportable Infectious Diseases [6]. Pretreated samples were inoculated into tryptic soy broth (Oxoid) and enriched by incubation at 37°C for 24 hours, after which deoxyribonucleic acid (DNA) was extracted. The extracted DNA was used as a template to detect bacteria by amplifying species-specific genes. For positive results, single bacterial colonies were isolated using selective media as specified in the Integrated Guidelines for the Diagnosis of Reportable Infectious Diseases. Isolated colonies were identified through biochemical tests using the VITEK 2 system (bioMérieux) [6]. For viral pathogens, rectal swabs were diluted in phosphate-buffered saline, and nucleic acids were extracted using a QIAGEN RNA Mini Kit (QIAGEN) and a Maxwell nucleic acid extraction instrument (Promega) according to the manufacturers’ instructions. The extracted nucleic acids served as templates for detecting viruses through the amplification of virus-specific genes.

Results

1. Specimen Collection and Characteristics

In 2024, pathogen identification tests were performed on 203 specimens. The monthly distribution of tests showed 22 (10.8%), 25 (12.3%), 51 (25.1%), and 34 (16.7%) cases in May, June, July, and August, respectively. This 4-month period accounted for 132 (65.0%) of the 203 total cases (Figure 1A).

Figure 1. Travel country distribution of suspected water- and foodborne infection cases.

The sex distribution was 112 male individuals (55.2%) and 91 female individuals (44.8%). With regard to age group, individuals in their 20s had the highest representation, with 75 cases (36.9%), followed by those in their 30s (66 cases, 32.5%) and individuals in their teens (23 cases, 11.3%). Together, these age groups accounted for over 80% of all subjects (Figure 1B).

2. Isolation and Detection of Causative Pathogens

The isolation rate of bacterial pathogens and the detection rate of viral pathogens both peaked in the warmer months of July and August (Figure 1C, D). No graphical correlation was observed between the number of outbound travelers and the pathogen positivity rates (Figure 1E). Correlation analysis confirmed no significant relationship, with the coefficients being –0.025 (p=0.94) for bacteria and 0.360 (p=0.25) for viruses.

Overall, 91 causative bacterial pathogens were isolated, including co-infecting pathogens, and the overall bacterial isolation rate was 44.8%. A total of 28 viral pathogens were detected (viral detection rate: 13.8%). The overall positivity rate for bacteria and viruses combined was 58.6%.

Among the isolated bacteria, pathogenic E. coli was the most frequent (47 cases, 51.6%), followed by Campylobacter spp. (17 cases, 18.7%), Salmonella spp. (15 cases, 16.5%), Shigella spp. (7 cases, 7.7%), and Vibrio spp. (5 cases, 5.5%) (Figure 2A). Among pathogenic E. coli isolates, enterotoxigenic E. coli (ETEC) was the most common (28 cases, 30.8%), followed by enteropathogenic E. coli (17 cases, 18.7%), and enteroinvasive E. coli (2 cases, 2.2%). Class 2 reportable diseases, including infection with enterohemorrhagic E. coli (EHEC), typhoid fever, and paratyphoid fever, were not identified.

Figure 2. Distribution rate of bacterial and viral pathogens.

(A) Distribution rate of bacterial pathogens. (B) Distribution rate of viral pathogens.

Among the detected viruses, norovirus was the most prevalent (25 cases, 89.3%), followed by astrovirus (2 cases, 7.1%) and group A rotavirus (1 case, 3.6%) (Figure 2B).

3. Analysis by Country of Origin

Symptomatic travelers in 2024 reported visiting 19 countries across Asia, Europe, Africa, the Americas, and Oceania. By country, the Philippines was the origin for the most specimens (109 cases, 53.7%), followed by Indonesia (34 cases, 16.7%), Vietnam (10 cases, 4.9%), and India and Thailand (9 cases each, 4.4%) (Figure 3). These countries have been designated as quarantine areas; therefore, travelers visiting or transiting through these countries must submit a Health Declaration Form upon arrival in the ROK in accordance with the Quarantine Act. Consequently, these nations accounted for a high proportion of specimens collected from travelers.

Figure 3. Travel country distribution of suspected water- and foodborne infection cases.

Regarding pathogenic E. coli, 30 of the 47 total cases were isolated from travelers from the Philippines, followed by 5 from Indonesia and 3 from India. For Salmonella spp., 9 of the 15 total cases were identified in travelers from the Philippines, marking the highest isolation rate for any single country. Regarding all 7 cases of infection with Shigella spp., a Class 2 reportable disease, the causative pathogen was identified as Shigella sonnei, and the case distribution was as follows: 4 cases from the Philippines, 2 from India, and 1 from Indonesia. With regard to Campylobacter spp., 16 isolates were of Campylobacter jejuni and 1 was of Campylobacter coli. The country-wise distribution of the cases was as follows: 7 cases were from the Philippines, 6 from Indonesia, and 2 from India. All 5 Vibrio isolates were identified as Vibrio parahaemolyticus, with 4 cases originating from travelers from the Philippines. Norovirus was detected in 13 travelers from the Philippines and 3 from Indonesia. Astrovirus was detected in one traveler each from the Philippines and Mongolia (Table 1).

Table 1. Distribution of pathogens isolated and detected from traveler-associated cases by imported country.

	Laos	Malaysia	Morocco	Mongolia	Vietnam	Brazil	Sri Lanka	Singapore	Ethiopia	India	Indonesia	Japan	China	Qatar	Cambodia	Thailand	France	Philippines	Hong Kong
No. of suspected cases of infection	4	3	1	4	10	1	1	1	2	9	34	6	2	1	3	9	1	109	2
Bacteria
Salmonella	1	-	-	-	1	1	-	-	-	-	-	1	-	-	-	1	-	9	1
Pathogenic Escherichia coli
EHEC	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
ETEC	1	-	-	-	-	1	-	1	-	3	3	-	1	-	-	1	-	17	-
EPEC	-	-	-	2	1	-	-	-	-	-	2	-	-	-	1	-	-	11	-
EIEC	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	2	-
Shigella	-	-	-	-	-	-	-	-	-	2	1	-	-	-	-	-	-	4	-
Vibrio	-	-	-	-	1	-	-	-	-	-	-	-	-	-	-	-	-	4	-
Campylobacter	-	-	-	-	1	-	-	-	-	2	6	-	-	-	-	1	-	7	-
Total of isolation	2	-	-	2	4	2	-	1	-	7	12	1	1	-	1	3	-	54	1
Virus
Norovirus	1	-	-	2	1	-	1	-	1	1	3	-	2	-	-	-	-	13	-
Group A rotavirus	-	-	-	-	-	-	-	-	-	-	1	-	-	-	-	-	-	-	-
Adenovirus	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
Astrovirus	-	-	-	1	-	-	-	-	-	-	-	-	-	-	-	-	-	1	-
Sapovirus	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
Total of detection	1	-	-	3	1	-	1	-	1	1	4	-	2	-	-	-	-	14	-

Values are presented as number. EHEC=enterohemorrhagic E. coli; ETEC=enterotoxigenic E. coli; EPEC=enteropathogenic E. coli; EIEC=enteroinvasive E. coli; –=not available.

Discussion

The Capital Regional Center for Disease Control and Prevention participates in the Enter-Net program for the rapid detection of imported infectious diseases among travelers arriving at Incheon International Airport and minimization of community transmission. In 2024, stool specimens were collected from symptomatic travelers who had visited designated quarantine areas and consented to be tested for five major bacterial genera and five viral species. The age distribution of the travelers showed that teenagers to those in their 30s accounted for over 80% of the subjects. The seasonal distribution of the cases revealed that 58.1% of cases occurred during January–February and July–August. This suggests that infections occurred predominantly in younger age groups during peak travel times, such as school breaks and holiday seasons.

Ninety-one bacterial strains were isolated from 85 of the 203 specimens (44.8% isolation rate), and 28 viral pathogens were detected in 28 specimens (13.8% detection rate). The bacterial isolation rate in this study was considerably higher than the national average of 16.9% reported by the Enter-Net program over the past 3 years [7]. The viral detection rate was comparable to the national average of 11.8% from the Enter-Net program over the past 5 years [8].

The Philippines was the source of the highest number of specimens (109) and yielded the highest number of positive results, with 54 bacterial isolates and 14 detected viruses; it was followed by Indonesia and India. Group travel with family and relatives is common in Southeast Asian countries such as the Philippines and Indonesia. Waterborne and foodborne diseases in these regions are highly transmissible through various routes, including contact with carriers or contaminated food and environments, posing a risk of outbreaks. Therefore, travelers should exercise particular caution in terms of personal hygiene.

Analysis of overseas-acquired cases in the ROK revealed that the primary bacterial pathogen was pathogenic E. coli, specifically ETEC, whereas the primary viral pathogen was norovirus. Although Class 2 reportable diseases such as EHEC infection, typhoid, and paratyphoid fever were not identified, the high isolation rate of pathogenic E. coli among travelers from Southeast Asia warrants special caution. Therefore, to confirm epidemiological links, further analysis of the isolated strains for genetic homology with domestic isolates and antibiotic resistance is necessary.

In 2024, 32 cases of overseas-acquired Class 2 reportable waterborne and foodborne diseases were documented in national reports from medical institutions. The disease distribution was as follows: cholera, 0 cases; typhoid fever, 10 cases; paratyphoid fever, 3 cases; shigellosis, 15 cases; and EHEC infections, 4 cases. In terms of origin, 29 cases were from Asia (primarily Indonesia, the Philippines, and India), 2 from Africa, and 1 from the Americas [9]. The total number of overseas-acquired Class 2 reportable bacterial foodborne diseases reported in 2024 exceeded the number of bacterial pathogens isolated in this study. This discrepancy may be owing to cases imported through other airports or may include cases of travelers who entered via Incheon but were asymptomatic during the incubation period, did not self-report, or declined to provide a specimen.

Although the Incheon Airport National Quarantine Station conducts on-site investigations of symptomatic travelers, analysis of infection routes and exposure risks for waterborne and foodborne diseases remains limited. The strains isolated in this study can be used to determine genetic homology and molecular phylogenetic relationships with domestically circulating strains, which can aid in identifying infection sources and transmission routes. Furthermore, analyzing the antibiotic susceptibility and resistance genes of these isolates can assist in assessing the risk of imported multidrug-resistant bacteria and provide evidence for strengthening surveillance to prevent their domestic spread. However, this study has certain limitations. The subjects represent only a small subset of symptomatic travelers who voluntarily consented to testing and may not be representative of all such travelers. Therefore, the data should be interpreted with caution. Continued surveillance and active quarantine investigations are essential for identifying and managing imported waterborne and foodborne diseases. The accumulated data must be consistently analyzed and utilized to provide critical information for disease prevention.