Preplanned Studies: Global Species/Biovars and Genotype Diversity Atlas of Brucella spp. — 102 Countries, 1923−2020

Brucellosis represents a globally prevalent zoonotic disease that poses significant public health challenges and causes substantial economic losses in livestock populations (1). The past few decades has witnessed continuous expansion in recognized diversity within the Brucella genus, with novel strains isolated from marine mammals to ocean fish revealing previously unknown ecological niches (2). These developments present new challenges for both regional and global surveillance and control of brucellosis. However, global species/biovars and genotype diversity atlas of Brucella spp. remain unclear. Therefore, this study aims to elucidate the global distribution patterns of species/biovars and genetic diversity among Brucella strains to enhance understanding of epidemiological changes and facilitate tailored surveillance and control strategies worldwide.

In this study, 7,212 Brucella strains collected from multiple locus variable-number tandem repeat analysis (MLVA) databases (https://microbesgenotyping.i2bc.paris-saclay.fr/databases) through June 30, 2024, representing isolates collected in 102 countries from 1923 to 2020. Data extraction included species/biovar, isolation location, quantity, host spectrum, panel 1 profiles, MLVA-11 patterns, lineage information, and isolation dates. Data analysis was performed using Excel 2021 software (Microsoft, Redmond, WA, USA). The minimum spanning tree (MST) was constructed using PHYLOVIZ 2.0 (3) online software (https://online2.phyloviz.net/index) to elucidate genetic relationships among strains.

Among the 7,212 Brucella strains analyzed, at least 12 species, 19 biovars, and several atypical Brucella species were identified (Figure 1). B. abortus strains distributed across 59 countries (regions) spanning six continents, while B. melitensis exhibited widespread presence in 64 countries throughout Asia, Europe, and North Africa (Figure 1). Notably, the distribution pattern of B. melitensis correlates strongly with regions reporting high incidence rates of brucellosis in both humans and animals. B. suis strains were documented in 34 countries across Europe, North America, and Latin America. B. canis demonstrated a more limited geographic range, primarily concentrated in East Asia and Latin America (Figure 1). Other species showed distinct regional patterns: B. neotomae in North America; B. ovis in Europe, North Africa, Oceania, and Latin America; B. ceti predominantly in West Europe and North America; B. microti concentrated in Middle Europe; B. papionis restricted to Tanzania and USA; B. vulpis exclusively in Austria; and B. pinnip primarily in Europe (Figure 1). These distribution patterns indicate the global predominance of B. abortus and B. melitensis, while other species exhibit distinct geographic specificities.

Figure 1. 

Territory distribution of Brucella spp. Strains.

Note: The number of strains marked with different color scales; green: refers to the continent with no strains found; red: indicates the continent with the most strains.

The predominant species was B. melitensis (n=4,042, 56.05%), followed by B. abortus (n=2,600), B. suis (n=695), and B. canis (n=52) (Supplementary Table S1). The geographic distribution of B. abortus biovars showed distinct patterns, for example: biovar 1 was predominantly found in Portugal, South Korea, and Brazil; and biovar 9 in Xinjiang, China. Within the B. melitensis population, biovar 3 (n=1,909) was dominated species. B. melitensis biovars 1 and 3 showed widespread distribution across Asia and Europe, particularly in Asian countries with high brucellosis prevalence.

The B. suis population exhibited distinct biovar distributions, such as B. suis biovar 1 showed broad geographic distribution across China, the USA, Mexico, France, Zimbabwe, Egypt, and Australia. B. ovis (n=53) was distributed across France, Spain, Brazil, Greece, the USA, Australia, New Zealand, Croatia, and Argentina. B. neotomae (n=13) was confined to Costa Rica and the USA. Among non-classical species, there were 173 B. ceti isolates, 61 B. pinnipedialis isolates, 13 B. microti isolates, 2 B. papionis isolates, and 1 B. vulpis isolate. B. ceti was predominantly found in Scotland, Italy, Spain, Costa Rica, Germany, France, and the UK. B. microti was isolated from the Czech Republic and Austria, B. papionis from Tanzania and the USA, and B. pinnipedialis from Scotland, Norway, United Kingdom, and USA.

Asia exhibits the lowest species diversity, with B. abortus and B. melitensis being the only classical species distributed across all Asian countries (Figure 2). Sporadic cases of B. suis have been documented in China, India, Nepal, Palestine, and the United Arab Emirates, while B. canis has been reported exclusively in China, Republic of Korea, and Japan. This species distribution profile aligns precisely with the regions reporting the highest human brucellosis burden in Asia. Notably, substantial isolations of B. melitensis have been recorded in China, Kazakhstan, Kyrgyzstan, Palestine, Qatar, and Turkey - countries that consistently report among the highest global incidence rates of brucellosis.

Figure 2. 

Distribution of areas and composition patterns of Brucella species populations.

Note: Brucella species are coded with color, and dots on the x-axis indicate the borders of different areas.

Europe demonstrates higher species diversity than other continents, with 9 of the 12 known species documented (Figure 2). Three species (B. abortus, B. melitensis, and B. suis) are widely distributed across the continent, particularly in historically high-burden regions. B. suis exhibits a unique continental distribution pattern, with significant presence in Hungary, Germany, Belgium, France, Croatia, Spain, and Portugal, predominantly isolated from swine and wild boar populations. These findings suggest that despite successful control of brucellosis in Europe’s historically endemic areas, continued surveillance remains essential.

Despite Africa being a historically endemic region for brucellosis, comprehensive data on Brucella species and genotypes remain limited due to insufficient surveillance in recent decades. B. abortus strains are particularly prevalent throughout South and West Africa (Figure 2). In Africa presence of multiple Brucella species, underscoring the need for expanded bacteriological surveillance.

While Oceania maintains brucellosis-free status with only sporadic isolations of B. abortus, B. suis, and B. ovis (Figure 2), potential public health risks persist through mammalian reservoirs. The Americas exhibit the highest Brucella species diversity globally, with at least 10 documented species, and B. abortus showing the widest geographic distribution (Figure 2). B. melitensis has been documented in regions with substantial human brucellosis burden, including the USA, Mexico, Peru, and Argentina. The continent harbors the highest concentrations of both B. neotomae (11 isolates in the USA) and B. ovis (115 isolates in Argentina), while Costa Rica reports the majority of B. ceti cases.

The B. abortus population demonstrates remarkable host diversity, with isolates from at least 20 different species. Cattle represent the primary host (n=1,567), followed by bison (n=97) and humans (n=80). B. melitensis exhibits even greater host diversity, with isolations from 24 distinct species, predominantly humans (n=2,719), followed by ovines (n=617) and cattle (n=280). The B. suis population spans 15 host species, B. neotomae has been isolated from both rodents and humans. Host specificity is observed in several species, such as B. papionis in baboons. This extensive host diversity plays a crucial role in maintaining and facilitating the transmission of Brucella strains.

Within the B. abortus population, 83 multiple locus variable-number tandem repeat analysis-11 genotypes (GTs) were identified, with three GTs (82, 72, and 79) emerging as predominant, representing 28.4% (494/1,735), 22.4% (390/1,735), and 10.7% (187/1,735) of the population, respectively (Figure 3A). GT72 exhibited broad geographic distribution across 17 countries, GT82 was detected in 10 countries across 3 continents, GT79 was identified in 9 countries across 4 continents.

Figure 3. 

MLVA-11 genotype diversity (A) and MLVA-16 genetic relationship (B) of global B. abortus strains.

Note: (A) Color marks the MLVA-11 genotypes; numbers in the figure indicate the dominant genotypes; (B) Color-coded countries in which strains isolated from all B. abortus strains were divided into two lineages (abortus A and abortus B), and abortus B were further sorted into C I and C II subgroups.

Minimum spanning tree (MST) analysis based on MLVA-16 data revealed that the B. abortus population segregated into two distinct groups (B and C) (Figure 3B), with African strains clustering in abortus B, while strains from Asia, the Americas, and Europe were predominantly found in abortus C (Figure 3B). Within C I, identical MLVA-16 genotypes were shared among strains from the USA, Costa Rica, Kazakhstan, Italy, and Portugal (Figure 3B). In C II, the majority of shared MLVA-16 genotypes were observed between strains from the USA and Portugal, USA and Brazil, and Bangladesh and Brazil (Figure 3B).

Within the B. melitensis, 216 MLVA-11 genotypes were identified, with five predominant genotypes (GTs): 116, 96, 125, 111, and 87, accounting for 54.4% (2,733/5,019), 7.1% (360/5,019), 6.1% (307/5,019), 3.6% (182/5,019), and 2.2% (112/5,019), respectively (Figure 4A). GT116 was distributed across at least 18 countries, GT96 was identified in nine countries, GT125 was present in 20 countries.

Figure 4. 

MLVA-11 genotypes diversity (A) and MLVA-16 genetic relationships (B) of global B. melitensis strains.

Note: (A) Color marks the MLVA-11 genotypes; numbers in the figure indicate the dominant genotypes. (B) Color-coded countries in which strains were isolated; all B. melitensis strains were divided into three lineages: East Mediterranean, West Mediterranean, and Americas.

Minimum spanning tree analysis revealed that the B. melitensis clustered into three distinct lineages: Eastern Mediterranean, Western Mediterranean, and Americas (Figure 4B). The Eastern Mediterranean strains predominated in Asia and Europe; the Western Mediterranean lineage comprised strains from Italy, France, Egypt, and Algeria; The American lineage encompassed strains from the USA, Peru, Spain, and Portugal (Figure 4B). Among the three lineages, the Eastern Mediterranean exhibited the highest frequency of shared MLVA-16 genotypes (Figure 4B).

In the B. suis population, analysis revealed 67 distinct MLVA-11 genotypes, with five dominant circulating genotypes: GT33 (22.2%, 126/592), GT58 (12.6%, 75/592), GT57 (9.2%, 55/592), GT60 (7.9%, 47/592), and GT44 (7.7%, 46/592) (Figure 5A).

Figure 5. 

MLVA-11 genotype diversity (A) and MLVA-16 genetic relationships (B) of global B. suis strains.

Note: (A) Color marks the MLVA-11 genotypes; numbers in the figure indicate the dominant genotypes; (B) Color-coded countries in which strains were isolated; all B. suis strains were divided into two lineages (S I and S II), and further S I was further sorted into three subgroups (a-c).

Minimum spanning tree analysis demonstrated that B. suis strains clustered into two major lineages (SI and SII), with SI further subdividing into three distinct sub-clades (a−c) (Figure 5B). Notably, shared MLVA-16 genotypes were observed in SI sub-clade b (Figure 5B), while in SII, a single shared MLVA-16 genotype was identified (Figure 5B).