Introduction

Brucellosis was first identified in 1884 by David Bruce in the Mediterranean region of Malta and is also known as undulant, Mediterranean, or Malta fever. This zoonotic infection spreads through direct or indirect contact with infected animals or their products. It is one of the most common zoonotic diseases worldwide, with an estimated annual global incidence of 2.1 million cases [1]. The causative pathogen, Brucella spp., exhibits host preferences, meaning that it tends to infect specific types of susceptible animals. These include Brucella abortus (cattle), Brucella melitensis (goats, sheep), Brucella suis (pigs), Brucella canis (dogs), Brucella ovis (cotton sheep), Brucella neotomae (woodland rats), and Brucella maris (marine mammals). To date, a total of 12 species have been identified [2]. Among these, 4 species are pathogenic to humans: B. abortus, B. melitensis, B. suis, and B. canis [3,4]. The bacterium invades the skin, mucous membranes, conjunctiva, respiratory system, and gastrointestinal tract, affecting both phagocytic and non-phagocytic cells and surviving intracellularly [5]. Brucella can potentially affect nearly all organs in the human body and may progress to chronic conditions such as undulant fever, arthritis, myocarditis, and neuropathies if not properly treated [6,7]. B. melitensis is recognized as the most virulent species of Brucella affecting humans [8].

In domestic livestock in Korea, B. abortus has primarily been isolated from cattle and deer. The Brucella pathogen has not yet been detected in other livestock, such as pigs or goats. Consequently, B. abortus, is recognized as the main pathogen affecting domestic livestock in Korea [9,10]. According to the Infectious Disease Prevention and Control Act, brucellosis is a statutory class 3 infectious disease and is subject to comprehensive surveillance. Consequently, Brucella spp. have been regularly isolated from culture tests of human infection cases since 2014, and the pattern of occurrence and infection route have been identified in all brucellosis patients from 2014 to 2023. This data will be used to determine if the dominant species of the causative pathogen in Korea has changed and will serve as reference material when devising measures for the prevention of infectious diseases.

Materials and Methods

The epidemiological and sociodemographic characteristics of 51 patients diagnosed with brucellosis between 2014 and 2023 were analyzed using data from the Korea Disease Control and Prevention Agency’s legal infectious disease surveillance system (http://is.kdca.go.kr). Additionally, the distribution of the isolates and the detailed routes of infection were determined for 26 patients from whom the causative pathogens were isolated. BruMLSA21 analysis, based on the full-length base sequences of 14 isolates of B. melitensis, was matched with infection information from epidemiological investigations. Whole-genome sequencing analysis was conducted on the gDNA extracted from these 14 B. melitensis isolates using the Nextera DNA Flex Library Kit (Illumina), after which the library was constructed and then sequenced using the MiSeq system (Illumina). For the comparative analysis of these genomes, 77 full-length base sequences of B. melitensis were obtained from NCBI GenBank (http://ncbi.nlm.nih.gov/genbank). The comparison strains, isolated from humans between 1957 and 2019, were selected based on having fewer than 50 contigs, indicating high-quality genomes.

The full-length base sequences of the isolates and comparison strains were analyzed using the PubMLST DB to determine the BruMLSA21 Sequence Type (ST) of each genome, and the sequences of the 21 loci composing the ST of each genome were extracted [11,12]. These sequences were then concatenated using MEGA7, and a phylogenetic tree was constructed to examine the evolutionary relationships among them, marking the ST of each genome and the country of isolation [13].

Ethics Approval

The study protocol was approved by the Institutional Review Board (IRB) of Korea Disease Control and Prevention Agency (IRB No: 2023-10-02). Informed consent was waived by the IRB.

Results

Basic Characteristics of Annual Report (2014–2023)

Over a 10-year period, a total of 51 patients were registered, with fewer than 10 cases reported annually. Males (45 patients, 88.2%) were affected 7.5 times more frequently than females (6 patients, 11.8%), reflecting a 76.4% difference in incidence rates. Additionally, the incidence increased with age. By nationality, locals (37 patients, 72.5%) were 2.6 times more likely to be affected than foreigners (14 patients, 27.5%), indicating a 45.0% difference in prevalence. The estimated locations of infection showed that domestic infections (35 cases, 68.6%) were 2.2 times higher than cases imported from abroad (16 cases, 31.4%) (Table 1; Figure 1).

Types of Occupation and Routes of Infection

Out of 51 total participants, 25 patients (49.0%) were involved in the livestock industry, while the other 26 (51.0%) held various other occupations, including students studying abroad, self-employed individuals, and unskilled laborers. Within the livestock sector, 16 were involved in general livestock, 3 were animal health officials, and 2 were veterinarians. The proportion of locals employed in the livestock sector was 62.2%, significantly exceeding the 37.8% in other occupations. In contrast, 85.7% of foreigners worked in non-livestock-related jobs, with only 14.3 involved in the livestock industry. Excluding 9 cases with unknown infection routes, an analysis of the remaining 42 cases revealed that the infection route in the livestock sector was exclusively through contact (25/25 cases), whereas in other occupations, it was almost evenly split between contact (9/26 cases, 34.6%) and ingestion (8/26 cases, 30.8%) (Table 2).

Laboratory Tests

Laboratory tests consisted of culture tests (31 cases, 60.8%) and antibody detection tests (Indirect Immunofluorescence Antibody Assay, IFA) (20 cases, 39.2%). The culture tests identified 11 cases of B. abortus, 1 case of B. canis, 14 cases of B. melitensis, and 5 cases involving species other than the primary brucellosis species associated with human infection. In the 20 cases that underwent antibody titer testing, the diagnostic criteria included an antibody titer in the recovery period that was more than 4 times that of the acute phase (confirmed) or an antibody titer in the acute phase of 1:160 or higher (probable). For the 20 cases that underwent antibody detection tests, culture tests were not conducted, because the types of samples submitted differed from those typically used in such tests.

All cases involving the isolated pathogens, B. abortus and B. canis, were linked to domestic infections (12 cases), whereas all B. melitensis infections originated from foreign sources (14 cases) (Table 3).

The presumed countries of infection encompassed 7 countries, with B. abortus (11 cases) and B. canis (1 case) being domestic infections and B. melitensis (14 cases) being imported cases from China, Saudi Arabia, UAE, Iraq, India, and Kuwait. The majority of the cases of foreign origin were from China (9 cases), with the remaining cases distributed evenly, 1 per country, among the other 5 countries.

Occupation and infection routes for B. abortus cases were primarily associated with the livestock sector (8/11 cases, 72.7%) and contact (9/11 cases, 81.1%), respectively. In contrast, B. melitensis occurred predominantly among individuals in non-livestock occupations, such as laborers, construction workers, seamen, students studying abroad, and caregivers, with contact (7/14 cases, 50.0%) and ingestion (5/14 cases, 35.7%), both representing the main routes of transmission (Table 4).

Whole-Genome Sequencing Analysis of B. melitensis

Among the B. melitensis isolates from foreign import cases, 13 out of 14 belonged to Sequence Type 8 (ST8), and 1 belonged to ST38. The principal regions of isolation for ST8, as analyzed in BruMLSA21, were predominantly in Asia, including China, India, Iraq, Kuwait, and Saudi Arabia, while ST38 was found in Iraq, Iran, Syria, and Turkey, aligning with the presumed areas of infection according to epidemiological investigations (Table 5; Figure 2) [11–13].

Discussion

In 2000, brucellosis was designated as a legally notifiable communicable disease in the Republic of Korea. The first patient, diagnosed in 2002 due to the consumption of unpasteurized milk, marked the beginning of a yearly rise in cases, reaching a peak of 215 cases in 2006, followed by a decline to fewer than 10 cases reported annually since 2014.

A decade-long analysis indicates a decline in domestic infections; however, cases of foreign introduction persist. The incidence rate was more than 7 times higher in men than in women, with particularly high rates observed among individuals in high-risk occupations, such as livestock farming. This suggests occupational exposure as a significant risk factor. In livestock, Brucella bacteria chronically colonize the milk glands and reproductive organs [14], presenting a substantial risk of infection through exposure to blood, placenta, fetuses, and uterine secretions [15].

Of the total infection cases, 31% (16 cases) were imported from overseas, specifically from the Middle East and Asia, regions with a high global incidence of brucellosis [16,17]. Among these, China accounted for the highest number of cases (9 cases), with the disease predominantly originating in the northwest, including Mongolia. The incidence in that region remains high [18] and is spreading from the northwest to the south with the development of large-scale ruminant farms in southern Shaanxi Province [19].

Infections have also been identified in students and others who are not involved in livestock farming, occurring through the consumption of raw goat milk and dairy products, as well as through international travel. Furthermore, the number of infection cases has been rising globally, including in China and Germany [20,21]. Therefore, it is crucial to monitor disease outbreak information for the destination before traveling abroad and to exercise caution regarding the consumption of raw animal products and unpasteurized dairy in endemic areas.

The predominant species of brucellosis affecting livestock in Korea is B. abortus, primarily infecting cattle [8,9]. In human infections, where in addition to B. abortus, other species may be encountered, the route of infection is determined through detailed epidemiological investigation and whole-genome sequencing. Analysis of this data shows that while the incidence of B. melitensis, transmitted through sheep or goats, has increased, all confirmed cases have been identified as imported.

In Korea, the surveillance of brucellosis in livestock, particularly cattle, involves a stringent culling policy for infected animals. In humans, brucellosis was designated as a statutory infectious disease in 2000 and is closely monitored, resulting in fewer than 10 reported cases per year. When a human infection is linked to a livestock farm, a joint epidemiological investigation system is established in collaboration with the animal quarantine agency to address the situation. However, given the global rise in the prevalence of B. melitensis, changes in dietary habits such as increased consumption of lamb, and the growing number of foreign workers and Chinese immigrants, a multi-ministerial One Health response is essential.

The causative bacteria of brucellosis exhibit host preferences, meaning the specific pathogen involved varies depending on the susceptible animal, and cross-transmission between different hosts is possible [22]. This phenomenon is relevant to infections in both livestock and humans, and underscores the necessity for regular monitoring of the causative pathogen. Moreover, in human infections, while there are commercial treatments available, the effectiveness of these treatments can be compromised due to the pathogen's resistance to certain antibiotics [23,24]. Therefore, accurate diagnosis and appropriate treatment require identification of the specific causative pathogen.

One limitation of this study is that the prevalence of B. melitensis may have been overestimated or underestimated because it was based on the analysis of only 31 out of 51 patients (61%) who underwent culture testing. However, this study is significant as it is the first to analyze Brucella species among all patients registered in the national infectious disease surveillance system. It is anticipated that further research addressing the aforementioned limitation will be necessary in the future.

HIGHLIGHTS

• Identification of the causative pathogen of brucellosis is necessary for rapid source containment; thus, regular monitoring is required to recognize changes in dominant species

• Given the global rise in Brucella melitensis infections and changing risk factors, a multi-sectoral One Health approach is needed for effective prevention.

Article information

Ethics Approval

The study protocol was approved by the Institutional Review Board of Korea Disease Control and Prevention Agency (IRB No: 2023-10-02). Informed consent was waived by the IRB.

Conflicts of Interest

The authors have no conflicts of interest to declare.

Funding

None.

Availability of Data

The datasets are not publicly available but can be obtained from the corresponding author upon reasonable request.

Authors’ Contributions

Conceptualization: ISS, KWH; Data curation: ISS; Formal analysis: ISS, SGR; Investigation: YSK, BCG; Writing–original draft: KWH, ISS, SGR; Writing–review & editing: KWH, YSK, BCG. All authors read and approved the final manuscript.

Figure 1.

Cases by infection location (2014–2023).

Figure 2.

Maximum likelihood phylogenetic tree of BruMLSA21.

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References

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