Molecular characterization of Brucella ceti from a bottlenose dolphin (Tursiops truncatus) with osteomyelitis in the western Pacific.

Although the presence of Brucella spp. in the western Pacific has been suggested by epidemiological studies on cetaceans, it has not been confirmed by bacterial isolation. Here, for the first time, we report that a marine Brucella strain was isolated in the western Pacific from a bottlenose dolphin with osteomyelitis. The isolate from the lesion was confirmed to be B. ceti of sequence type 27 by multilocus sequence typing and Bruce-ladder PCR. Infrequent-restriction-site PCR and omp2 gene sequencing revealed that molecular characteristics of this isolate were similar to those of Brucella DNA previously detected from minke whales in the western North Pacific. These results suggest that genetically related Brucella strains circulate in cetacean species in this region.

Brucellosis is a worldwide zoonotic disease caused by Brucella spp. that affects terrestrial and marine mammals, although human cases caused by marine Brucella spp. are rare. [7, 8, 18, 19]. Among Brucella spp., B. ceti and B. pinnipedialis have been preferentially isolated from marine mammals, cetaceans, and pinnipeds, respectively [5, 7, 8, 19]. The microbiology and molecular biology of these two marine Brucella species differ from those of terrestrial species. Two molecular techniques, infrequent-restriction-site PCR (IRS-PCR) targeting marine Brucella-specific DNA fragments [2] and Bruce-ladder multiplex PCR [11] are useful for distinguishing between the marine and terrestrial Brucella spp. Furthermore, a recently developed multilocus sequence typing (MLST) scheme targeting 9 or 21 housekeeping genes has systematically classified Brucella strains and discriminated between B. ceti and B. pinnipedialis [18, 19] (PubMLST database: https://pubmlst.org/brucella/). Compared with brucellosis in terrestrial mammals, relatively little pathological evidence has been reported in marine mammals, despite a substantial number of pathological changes and abortions being witnessed in cetaceans [7, 8].

Most B. ceti strains have been isolated from cetaceans stranded on coastlines in European and North American waters. In previous epidemiological studies, common minke whales (Balaenoptera acutorostrata) in the western North Pacific showed high positive rate of antibodies to Brucella spp. [14, 16]. Those sero-positive whales often showed granular testes frequently seen in Brucella-infected ruminants [14, 16], and marine Brucella-specific DNA was detected from the testicles presenting granulomas by PCR [15]. Though these results strongly suggest that Brucella spp. are present in the western North Pacific, the pathogen has not been isolated. In this study, we report the first isolation of B. ceti in the western Pacific from a cetacean.

In October 2009, a male bottlenose dolphin (Tursiops truncatus) estimated to be 3 years old was captured off the Pacific coast of Japan (33° 36′ N, 135° 57′ E) under the permission of the Fisheries Agency in Japan. It was transferred to Ocean Expo Park in Okinawa prefecture, where it has remained. During the last decade, leukocyte elevation was detected three times, and medical treatments were conducted each time, although the leukocyte number has remained normal for the past 3 years. Details of these medical treatments and clinical data would be reported in future. A slight swelling appeared on the left side of the waist in May 2018, though the animal was apparently healthy. X-ray and computed tomography examination revealed clear osteolysis with evidence of sequestrum formation. A marked lesion was observed on the second-from-last lumbar vertebra, and some minor lesions were seen on caudal vertebrae (Fig. 1). Biopsy samples from the connective tissue around the lesion were subjected to histopathological examination and bacterial isolation. Histopathological analysis showed inflammatory cell infiltration mainly comprising lymphocytes and macrophages (Fig. 2). According to the image exams and histopathological results, the animal was diagnosed with osteomyelitis. Anti-Brucella spp. antibodies in the dolphin serum samples were examined via indirect-ELISA according to a modified method reported previously [1]. The specific antibodies were detected in the dolphin sera collected both on the day of arrival to the park and on the day of the biopsy, indicating that marine Brucella spp. infected the dolphin in the western Pacific and remained latent in the dolphin’s body. None of the other dolphins living with this dolphin were sero-positive. Captive medical and diagnostic treatments were conducted in accordance with the treatment plans determined by veterinarians with ample experience and approved by Aquarium Business Department of Okinawa Churashima Foundation.

Fig. 1.

Computed tomography image of the lesion site (abdominal). Major osteolysis with sequestrum formation (blue arrow) was observed on the transverse process and diapophysis of the second-from-last lumbar vertebra. Minor osteolysis areas were observed around the swelling of the peduncle lesion (red arrows).

Fig. 2.

Histopathological examination of biopsy samples from connective tissue around the lesion. Hematoxylin and eosin staining. Infiltration of inflammatory cell mainly comprising lymphocytes and macrophages was observed.

The bacteria isolated from the biopsy specimens formed small, translucent, non-motile, and smooth colonies on tryptic soy agar supplemented with 1% glucose and 5% horse serum after 4 days incubation at 37°C under ambient and 10% CO2-containing air conditions (Fig. 3A). Bacterial growth was promoted under the CO2-containing conditions compared with that under ambient conditions. Colonies comprised gram-negative coccobacilli (Fig. 3B) and were judged as Brucella species from a biochemical examination using VITEK® 2 System (GN; bioMerieux, Marcy I’Etoile, France). The isolate was designated as BD1442, and its DNA was extracted by InstaGene Matrix (Bio-Rad Laboratories, Hercules, CA, USA). When the 16S rRNA gene sequence of BD1442 determined according to a previous study [9] (accession no. LC496781) was compared with those of other bacterial species using EzBioCloud server (https://www.ezbiocloud.net/) [21], it was found to be identical to those of marine and terrestrial Brucella spp., including the type strains of B. melitensis, B. ceti, and B. pinnipedialis (accession nos. AE008918, AM158982, and AM158981, respectively). The isolate BD1442 was analyzed by MLST, targeting 21 housekeeping genes as described in PubMLST database [19], and was identified as sequence type (ST) 27 (Fig. 4); therefore, we identified BD1442 as B. ceti. Information on the host animal and type of B. ceti BD1442 is summarized in Table 1, together with data on Brucella DNA from a minke whale in the western North Pacific, other ST27 strains and representative strains of other ST complexes.

Fig. 3.

Bacterial colony (A) and Gram staining (B) of strain BD1442. The strain BD1442 was cultured on tryptic soy agar supplemented with 1% glucose and 5% horse serum for 4 days at 37°C under 10% CO2-containing air conditions.

Fig. 4.

Phylogenetic relationships of sequence types in marine Brucella based on concatenated sequence data of multilocus sequence typing 21. A Neighbor-Joining phylogenetic tree was constructed with concatenated 21 housekeeping gene sequences of marine Brucella isolates and the type strain of B. melitensis. The position of BD1442 in the tree is shown in red letters. Representative Brucella strains are shown in parentheses. The bootstrap values (1,000 replicates) are shown next to the branches. All nucleotide positions containing gaps and missing data were eliminated. Evolutionary analyses were conducted using MEGA7: Molecular Evolutionary Genetics Analysis software, ver. 7.0 [10]. The bar represents substitutions per nucleotide position.

Table 1.

Comparison of isolate BD1442 with other sequence type (ST) 27 strains and representative Brucella strains

Straina)	Species	Country	Sea area	Host	MLST 9	MLST 21	Omp2 sequenceb)		IRS-PCRc)	Bruce-ladder PCRd)
							Omp2a	Omp2b	I/II/III/IV	IS711 post bp26	794 bp fragment
BD1442	B. ceti	Japan	Pacific	Bottlenose dolphin	ST27	ST27	100 (LC484043)	100 (LC484044)	+/−/−/−	+	−
JM13/00e)		Japan	Pacific	Minke whale	NA	NA	100 (AB126348)	100 (AB126348)	+/−/−/−	+	NA
F5/99	B. ceti	USA	Pacific	Bottlenose dolphin	ST27	ST27	100 (DQ865282)	100 (DQ865283)	NA	+	−
F8/08–1	B. ceti	USA	Pacific	Bottlenose dolphin	ST27	ST27	NA	NA	+/−/−/−	+	−
F8/08–24	B. ceti	USA	Pacific	California sea lion	ST27	ST27	NA	NA	+/−/−/−	+	−
85A05748	B. ceti	Peru	Pacific	Human	ST27	ST27	NAf)	NA	NA	+	−
01A09163	B. ceti	Peru	Pacific	Human	ST27	ST27	NAf)	NA	NA	+	−
02/611	Brucella sp.	New Zealand	Pacific	Human	ST27	ST27	100 (DQ865280)	100 (DQ865281)	+/−/−/−	+	−
350/1	B. ceti?	Croatia	Adriatic sea	Bottlenose dolphin	ST27	NA	NA	NA	NA	+	NA
B1/94 (T)	B. ceti	UK, Scotland	Atlantic	Harbor porpoise	ST23	ST23	92.6 (AF300817)	94.9 (AF300816)	−/+/+ /−	+	+
B14/94	B. ceti	UK, Scotland	Atlantic	Common dolphin	ST26	ST26	90 (AF300815)	93.1 (AF300814)	−/−/−/+	+	+
B2/94 (T)	B. pinnipedialis	UK, Scotland	Atlantic	Common seal	ST25	ST25	100 (AF300819)	94.5 (AF300818)	+ /−/−/−	+	+
16M (T)	B. melitensis	USA		Caprine	ST7	ST73	98.6 (AE008917)	96.6 (AE008917)	−/−/−/−	−	+

MLST, multilocus sequence typing; IRS-PCR, infrequent-restriction-site PCR +, PCR positive; −, PCR negative; NA, not available. a) ST27 strains, representatives of each cluster and 16M (T) strain as an outgroup in Fig. 4 are listed. b) The sequence identity (%) to omp2 sequences of BD1442. Accession numbers are shown in parentheses. c) IRS-PCR targeting four fragments (fragment I–IV) reported by Clockaert et al. [2]. d) Bruce-ladder multiplex PCR reported by López-Goñi et al. [11]. e) Brucella DNA detected in a minke whale [15]. f) Partially determined sequence (519-bp) [17] is identical to that of BD1442.

B. ceti strains are composed of three distinct paraphyletic clusters, ST26 complex (ST26, ST55, ST56, and ST103), ST23 complex (ST23, ST50, ST98–ST100, and ST102), and ST27 (Fig. 4). Strains belonging to the ST26 and ST23 complexes appeared to prefer dolphins and porpoises, respectively. The remaining ST27 has some unique features (Table 1): (i) It comprises strains from multiple animal species; cetaceans, pinnipeds, and humans. (ii) The strains have often been associated with marked diseases. In marine mammals, the first ST27 strain was isolated from an aborted fetus of a bottlenose dolphin in California, USA [4, 18]. ST27 has also been found in the strains from human cases with neurobrucellosis [12, 17, 19]. (iii) The strains are geographically associated with Pacific waters, although there is an exceptional case from a dolphin in Croatia [3].

To investigate the relationship between BD1442 and previously detected Brucella DNA from minke whales in the western North Pacific [15], BD1442 was subjected to outer membrane protein gene sequencing [15], IRS-PCR [15] and Bruce-ladder multiplex PCR [11] according to the previous studies. IRS-PCR is a method to amplify four marine Brucella-specific DNA fragments (fragments I–IV) using four sets of PCR primers [2], whereas Bruce-ladder multiplex PCR can estimate Brucella species on the basis of the amplified fragment patterns [11]. PCR conditions and primers used in these molecular analyses are shown in Supplementary Table 1, and the results are summarized in Table 1.

Sequences of the outer membrane protein genes omp2a and omp2b of BD1442 (accession nos. LC484043 and LC484044, respectively) were identical to those of Brucella DNA obtained from Pacific minke whales [15] and some of the other ST27 strains (Table 1). IRS-PCR resulted in a specific product of approximately 300-bp from BD1442 only using primers for fragment I (Fig. 5A), and this was consistent with the results for Brucella DNA from Pacific minke whales [15] (Table 1). Using Bruce-ladder multiplex PCR analysis, five DNA fragments, including a 1,320-bp fragment, were amplified from BD1442 (Fig. 5B). The 1,320-bp fragment is known to originate from the marine Brucella-specific insertion of IS711 post bp26 [11], and the insertion has also been reported in minke whale Brucella DNA [15] (Table 1). Furthermore, BD1442 lacked the 794-bp fragment (Fig. 5B), a characteristic feature of ST27 strains [20]. These results reinforced the MLST analysis results (Table 1) and strongly suggest that BD1442 is genetically close to Brucella DNA from the Pacific minke whale and other reported ST27 strains.

Fig. 5.

Agarose gel electrophoresis images of infrequent-restriction-site PCR (A) and Bruce-ladder multiplex PCR (B) products of strain BD1442. (A) PCR products of fragments I–IV are shown. (B) PCR products of BD1442 are compared with the reported fragment patterns of other Brucella spp. shown in the diagram [11, 20].

In this study, for the first time, we reported on the isolation of B. ceti in the western Pacific from a dolphin with osteomyelitis. We cannot conclude that the isolate directly caused osteomyelitis to the dolphin. However, similar osteomyelitis has been reported in another bottlenose dolphin infected with Brucella spp. [6]. It is important to further examine the relationship between the clinical signs and Brucella infection. DNA analysis showed that molecular characteristics of the isolate were closely related to Brucella DNA from minke whales in the western North Pacific. Although isolation from minke whales has not been successful [15], genetically related Brucella strains may circulate among species of cetacean in the western North Pacific. Virulence of these strains in cetaceans is not fully understood, however, reproductive disorders and neurologic diseases in cetaceans caused by B. ceti may significantly affect the ecology and population dynamics of cetaceans [7, 8]. Although there have been no reports of zoonosis in the western Pacific [13, 16], continuous monitoring is important for understanding the threat brucellosis poses to both marine mammals and humans.