Canine brucellosis

Etiology and Epidemiology

Canine brucellosis is caused by Brucella canis (B. canis), a rough, small, gram-negative, intracellular

bacterium. Other Brucella species, i.e., B. abortus, B. suis, have occasionally caused canine infections but

they are not discussed here. The canine brucella was first recognized in 1966 as a cause of abortions and

reproductive failures and it has since been recognized in several countries. It is especially common in Central

and South America, in the southern states of the United States, and has been diagnosed in commercial or

research breeding kennels (beagle) in several other countries, including Japan and, more recently, in The

People's Republic of China. The disease has been reported sporadically in Europe. Humans may be infected;

however, dogs and other canine species are believed to be the only true hosts. Natural infections occur most

commonly after ingestion of contaminated placental materials or aborted fetuses, vaginal discharges from

infected bitches that are in heat or who abort, and during breeding. Following an abortion, organisms may be

shed for several weeks or, intermittently, for months following an abortion. Males also may shed organisms in

the urine, but bacterial numbers are relatively low, except when urine is contaminated with seminal or

prostatic fluids. Prevalence data is meager, but seroprevalence rates appear high (20-30%) in Mexico and

Central/South America. Estimates in the southern United States, Japan have been reported to be 7-8% in stray

dogs. Food-producing animals are highly resistant. True prevalence rates are unknown and other

epidemiological aspects of canine brucellosis are lacking.

Clinical Signs

Clinical signs are associated principally with the reproductive tract. In females, the most prominent sign is

abortion after 45-55 days of gestation in about 75% of the cases. Early embryonic death and resorption, or

abortion 10-20 days after mating, may occur in some cases. These may go unnoticed and the female may

present with the chief complaint of "failure to conceive". In males, the main sign is epididymitis of one or both

testes, and infertility. Testicular atrophy and a moist scrotal dermatitis may be present. Semen from infected

males usually contains large numbers of abnormal sperm and inflammatory cells, especially during the first 3

post-infection months. Chronically infected males may have no sperm, or reduced numbers of immature

sperm. Autoimmune (anti-sperm) antibodies are present and probably contribute to male infertility.

Nonspecific signs in both sexes include lethargy, loss of libido, premature aging and generalized lymph node

enlargement. B. canis has been isolated from field cases of diskospondylitis, a condition that also has been

reproduced experimentally in SPF dogs. Recurrent uveitis has been occasionally reported in infected dogs

after several weeks of infection.

Infected males harbor organisms in the prostate gland and epididymides for several months. Bacteria are

disseminated the seminal fluids and, occasionally, urine. B. canis is short-lived outside the dog and is readily

inactivated by common disinfectants.

Diagnosis

The diagnosis of canine brucellosis requires laboratory confirmation. Blood cultures are strongly

recommended before declaring an animal infected. Serological tests which are presently available to most

veterinarians in the U.S. are imprecise since surface antigens of rough Brucella, such as B. canis, cross-react

strongly with antibodies to several other nonpathogenic bacterial species. The most commonly used tests

include:

Serological Tests

Rapid Slide Agglutination Test (RSAT) - The RSAT requires brief treatment of serum with

2-mercaptoethanol (0.2M), and is available in the U.S. as an office screening test (D-Tec CB; Symbiotics

Corp., Kansas City, Missouri, USA.). The antigen is rose bengal-stained B. ovis, which cross-reacts with B.

canis. A negative slide test is strong evidence that the dog is not infected, but only about 40% of dogs whose

sera agglutinate the slide test antigen are actually positive for canine brucellosis. Thus, dogs positive on the

slide agglutination test should not be considered infected until additional serological tests are done -- blood

cultures are always indicated because of the long and continual periods of bacteremia. More than 50% of

infected dogs have a bacteremia lasting 1 year or longer. More specific antigens (B. canis M-) are now

available (NYS Diagnostic Laboratory, Cornell University), but they have not been made commercially

available (see (3) below).

Tube agglutination (TAT) and Agar Gel Immunodiffusion (AGIDcwa) Tests - These tests utilize cell wall

antigens and are additional serologic methods available through diagnostic laboratories. However, those tests

also are flawed by false-positive reactions and difficulties in interpretation, especially with `early' sera or sera

from chronically infected dogs. Results obtained by the RSAT, TAT, and AGIDcwa tests should be confirmed

by more specific tests (below) and isolation attempts.

Improved Serodiagnostic Tests - Improved tests include: (1) A RSAT that employs a mutant strain (less

mucoid, "M-") of B. canis that has high specificity (M-RSAT); (2) an agar gel immunodiffusion test

(AGIDcpa) that employs cytoplasmic protein antigens extracted from the bacterial cytoplasm. The protein

antigens are highly specific for the Genus Brucella and are useful in distinguishing between infected and

noninfected dogs who possess antibodies that react in agglutination or AGID (cell wall antigens) tests, noted

above (1 & 2). (3) enzyme-linked immunosorption tests (ELISA) that employ as antigen cell wall LPS extacts

of B. canis M- or cytoplasmic proteins extracted from B. abortus. Published results indicate significant

advantages of the improved tests noted above and the warrant serious consideration for further development

for use in diagnostic laboratories or as veterinary office kits.

Indirect Fluorescent Antibody Test (IFAT) - An IFAT is used by several diagnostic laboratories in the U.S.,

but data on its accuracy have not been published. Results from Cornell University's Diagnostic Laboratory

indicate a high rate of false positive reactions with the IFA test.

Blood Cultures

Despite improvements in serologic diagnostic methods, blood cultures should always be performed when the

disease is suspected. B. canis is readily isolated from the blood on tryptose or trypticase soy media for several

months after infection. Cultures should be incubated aeobically, since CO2 is inhibitory. Detailed methods are

described in the references (Alton et al, 1988). Many laboratories are insufficiently familiar with the

interpretation of canine brucellosis diagnostic procedures, which has frequently resulted in the destruction of

non-infected dogs solely on the basis of agglutination test results that, in fact, were false-positive reactions.

Prevention and Control

Attempts to develop a suitable vaccine which would induce immunity, yet not provoke serological responses

that interfere with the diagnosis, have not been successful. Presently, the development of a vaccine is

considered undesirable since the brucella vaccines which have been studied offered only moderate protection

and vaccinated dogs developed antibodies which would confuse the serodiagnosis. Prevention of infection and

elimination of infected dogs should be the principal control strategy in kennels.

Prevention requires yearly testing of all breeding stock and the testing of all dogs to be introduced into a

kennel. In the United States, the New York State Diagnostic Laboratory at Cornell University is recognized as

the principal, and most reliable, testing lab. Only proved non-infected dogs should be bred. In the United

States, females in breeding kennels are commonly tested by the RSAT before their expected estrus. At least 3

weeks should be allowed to perform further tests to ascertain whether a seropositive test indicates actual

infection or a false-positive result. Two negative tests done at 4-6 week intervals should be required for all

dogs to be introduced into breeding colonies. The two tests will detect dogs who may be incubating the

disease. If a bitch aborts, assume infection until it is proved otherwise. Bitches who abort should be kept

isolated and the premises must be disinfected. If a male dog loses interest in mating, or develops testicular

abnormalities and poor fertility, it should be examined for brucellosis.

Treatment

Treatment is not recommended for dogs in breeding kennels, and where dogs cannot be isolated and

accurately monitored following antibiotic therapy. Treatment is expensive and cures are difficult to achieve,

especially in chronically infected males. Repeated blood cultures and serologic monitoring is required for at

least 3 post-treatment months before a dog can be declared negative. Recrudescence of the infection after the

cessation of antibiotic treatment is common. Even if the organism can be successfully eliminated, males

frequently remain sterile because of irreversible damage to the testes and epididymides. Spaying or castration

is believed to reduce the risk of transmission from infected dogs; however, this hypothesis has not been tested

experimentally and neutering does not eliminate organisms from the body. All neutered dogs should receive a

course of antibiotics. The most successful and practical treatment results have been obtained with a

combination of a tetracycline drug, e.g., tetracycline hydrochloride, doxycycline, minocycline, and

streptomycin administered during the first 3 months of infection. More than 80% cure rates have been

achieved in kennels, where dogs initially diagnosed as infected were euthanized and additional ('early') cases

were treated. Unfortunately, dihydrostreptomycin is unavailable for treatment of dogs in the United States. A

cure is more difficult to achieve in chronic infections.

If available, dihydrostreptomycin (10 mg/kg IM bid) is given for the initial 7 days of treatment together with a

tetracycline antibiotic (25 mg/kg orally tid), which is continued for 4 weeks. During the last 7 days of

tetracycline therapy, streptomycin is again given. In some instances where the first course failed, a second

course of treatment has been successful. Since streptomycin is no longer available in the USA for treatment of

dogs, gentamicin has been recommended by some clinicians as a substitute antibiotic. However, there is

insufficient data to recommend gentamicin, and preliminary studies indicate that its efficacy is unsatisfactory,

except in very early cases - i.e, in dogs infected for less than 1-2 months. Treatment is not recommended for

breeding dogs, or when long-term (3 months) follow-up is unlikely. Treatment failures are especially common

in infected males where organisms are commonly sequestered in the prostate gland and epididymides.

Testing and elimination of infected dogs is the only proven method of eradication of B. canis from an infected

kennel. An attempt should be made to identify the source of the infection - unfortunately, this has rarely been

accomplished since breeders are reluctant to admit culpability.

Management of infected kennels/dogs is time consuming and expensive. Veterinarians must be prepared to

address owners' concerns and render judicious advice, which may vary according to circumstances. Prevention

is essential to avoid the ordeal of infection in a breeding kennel. As soon as canine brucellosis is diagnosed in

a kennel, vigorous measures must be implemented until the disease has been eradicated. Infected kennels

should be quarantined, even though most states/countries do not have formal regulations. Lack of such

measures has lead to widespread, even international, spread of B. canis infection.

Control Strategies

KENNEL DOGS

Positive dogs - euthanasia. Isolate dogs

as much as possible.

Test sera from all dogs: Agglut/AGID

tests (blood cultures of all suspicious

animals).

Euthanasia - all infected dogs.

Test dogs monthly for 3 months until

colony is negative on 2 successive tests.

PET DOGS - CHOICES DIFFICULT

Isolate dogs.

Spay or castrate + treatment.

Treatment uncertain; chances greater for

success in early infections.

Follow-up serology for 3 months

post-treatment.

Euthanasia should be considered:

Uncertainty of treatment; cost great;

disappointment common.

Public Health Significance

Humans are susceptible to B. canis, but infections are uncommon and they are usually mild. Approximately

40 cases of human infection have been reported in several countries, however the actual number is unknown

since cases are rarely diagnosed, or reported. Symptoms are usually vague - prolonged febrile illness with

lymph node enlargement. Most natural infections have been acquired through close contact with infected

dogs. Laboratory infections also have been reported. Unlike the dog, infected humans usually respond rapidly

to antibiotics (tetracyclines or tetracyclines + streptomycin).

References

Alton GG, Jones LM, Angus RD and Verger JM. Techniques for the brucellosis laboratory. Paris: Institut

National de la Recherche Agronomique 1988;169-174.

Baldi PC, Wanke MM, Loza ME, Fossati CA. Brucella abortus cytoplasmic proteins used as antigens in an

ELISA potentially useful for the diagnosis of canine brucellosis. Vet Microbiol 1994; 41:127-134. - PubMed -

Carmichael LE, Greene CE. Canine brucellosis. In: Greene CE, ed. Infectious Diseases of the Dog and Cat.

Philadelphia, WB Saunders, Co, 1990; 573-584.

Carmichael LE. Canine Brucellosis: A diagnostician's dilemma. In: Seminars in Veterinary Medicine and

Surgery (Small Animals), Jacobson R, ed. 1996; 11:161-165.

Carmichael LE, Joubert JC, Jones L. Characterization of Brucella canis protein antigens and polypeptide

antibody responses of infected dogs. Vet Microbiol 1989; 19:373-387. - PubMed -

Jian H. Identification and characterization of 200 strains of Brucella canis under test from China. Wei Sheng

We Hseuh Pao 1992; 32:370-375. - PubMed -

Johnson CA, Walker RD. Clinical signs and diagnosis of Brucella canis infection. Compend Cont Educ Pract

Vet 1992; 14:763-772.

Mateu de Antonio EM, Martin M, Soler M. Use of indirect enzyme-linked immunosorbent assay with hot

saline extracts of a variant (M-) strain of Brucella canis for diagnosis of brucellosis in dogs. Am J Vet Res

1993; 54:1043-1046. - PubMed -

Nicoletti P, Chase A. An evaluation of methods to diagnose Brucella canis infection in dogs. Compend Cont

Ed Pract Vet 1987; 9:1071-1077.

All rights reserved. This document is available on-line at www.ivis.org. Document No. A0101.1199

Brucella melitensis

: A nasty bug with hidden credentials for virulence

1. Edgardo Moreno*,† and
2. Ignacio Moriyón‡

+Author Affiliations

1. ^*Tropical Disease Research Program, Veterinary School, National University, Apartado 304-3000, Heredia, Costa Rica; and ^‡Department of Microbiology, University of Navarra, Apartado 177, 3208, Pamplona, Spain

On September 23, 1905, a cargo carrying 60 goats from Malta arrived in New York. The herd was kept in quarantine because of several deaths that occurred during the journey. Crewmen, an agent from the U.S. Bureau of Animal Industry, which was responsible for the shipment, and a woman who drank milk that “escaped” from the quarantine station displayed the characteristic symptoms of “Mediterranean fever.” Lieutenant Colonel David Bruce, a physician of the Royal Army, who discovered “Micrococcus melitensis ” in 1887 in infected British soldiers residing in Malta, had forewarned the U.S. sanitary authorities about the risk of “Mediterranean fever” by importing goats from Malta. In November 1906, after isolation of “M. melitensis,” the goats were destroyed. Almost 100 years after this episode, the genome sequence of Brucella melitensis (renamed after David Bruce) has been resolved by DelVecchio et al. (1), bringing new light to the understanding of the biology of this pathogen. The disease, known as brucellosis, is found in all continents, affecting mainly low-income countries; in addition, it constitutes a contemporary concern because Brucella strains are potential agents of biological warfare.

The six recognized Brucella species, named according to their host preference, affect economically important livestock, and several undesignated strains infect marine mammals. Abortion is the main outcome of the infection in pregnant animals, resulting from complex, not well understood interactions between the placental tissues, the intracellular brucellae, and the fetus. Brucella invades professional and nonprofessional phagocytes and replicates within compartments resembling the endoplasmic reticulum after evading fusion with lysosomes (2). The brucellae are exceedingly well adapted to this niche (see Fig. 1, which is published as supporting information on the PNAS web site, www.pnas.org) and do not survive for protracted periods of time outside the host. Their textbook description, “facultative intracellular parasites,” does not give credit to their true behavior, which is better described as that of a facultatively extracellular intracellular parasite. Therefore, understanding the pathogenicity of brucellae is relevant not only because this pathogenicity represents a major infectious disease but also because it will shed light on basic aspects of intracellular parasites and of cellular immunity. One of the striking features that distinguishes Brucella organisms is that they do not display obvious virulence factors such as capsules, fimbriae, flagella, exotoxins, exoproteases, or other exoenzymes, cytolysins, resistance forms, antigenic variation, plasmids, or lysogenic phages. Thus identification of classical virulence factors has been elusive. It is in this context that genomics and comparative phylogenetic analyses are yielding data that improve our understanding of Brucella pathobiology (1, 3–5) and are leading us to a refinement of classical concepts about virulence.

The genome analyses of three Brucella species have confirmed the absence of functional sequences for most of the “classical” virulence factors.

The brucellae are α-Proteobacteria, phylogenetically related to other cell-associated parasites of plants and animals as well as to free living bacteria (3). Their closest relatives (Ochrobactrum sp.) are bacteria of the rhizosphera that behave as opportunistic pathogens of humans. Chromosomal sequences of a number of α-Proteobacteria have been released, facilitating phylogenetic, biochemical, and biological comparisons (see Table 1, which is published as supporting information on the PNAS web site). The genome analyses of three Brucella species have confirmed the absence of functional sequences for most of the “classical” virulence factors, pathogenic islands, as well as the lack of a complete set of genes to mount, types I, II, and III secretion systems. On the other hand, some potential sequences for virulence were discovered. For instance, Brucella recruits actin and activates small GTPases during its internalization to cells (6), but the molecules involved in these events remain unknown. The revelation of putative genes coding for adhesins, invasins, and virG-like genes for attachment and actin recruitment calls for the generation of null mutants in these sequences. Whether some of the presumed hemolysins and proteases could be produced during intracellular parasitism and transferred by alternative secretion systems such as type IV or V, incomplete type III, or flagellar type secretion systems, remains speculative. Among these, the type IV secretion system plays a relevant role during Brucella intracellular trafficking (7) (see Table 2, which is published as supporting information on the PNAS web site), presuming by this the translocation of bacterial factors inside cells. Other proteins such as the putative outer membrane TolC, which is required for hemolysin secretion in enteric bacteria, may also serve for parasitism (8). Legionella hemolysins form pores in the vacuolar and cellular membranes soon after bacterial replication ceases (9), suggesting that a similar phenomenon could take place with other intravacuolar parasites, including Brucella. Concomitantly to this, Brucella inhibits apoptosis (10) and replicates within cells without interfering with mitosis (Fig. 1).

Most features related to virulence seem to be concentrated or to act at the Brucella surface (Table 2). The Brucella LPS gathers a remarkable set of properties. Some are ancestral, such as its very low biological activity, a favorable attribute for not activating intracellular killing mechanisms through cytokine networks (3). Others are idiosyncratic (the resistance to bactericidal peptides). A few may have been acquired horizontally (the O-chain). Most other factors depict ancestral systems present in plant and animal cell-associated relatives, with departures reflecting adaptation to the new environment (Table 2, www.pnas.org). Comparison with the various α-Proteobacteria chromosomes reveals that most genes known to be critical for cycle progression, translation machinery, stress responses, membrane lipids, basic heterotrophic metabolism, and energy conversion have been retained in Brucella organisms. Some of these genes seem to be in the interface with virulence (Table 2), stressing the fine adjustments between essential functions and parasitism. In contrast, genetic cassettes for autothrophy, antibiotic resistance, or for mounting the required structures for living outside host cells (e.g., flagella) are absent, cryptical, or truncated. Similarly, the absence of plasmids and lysogenic phages in the intracellular α-Proteobacteria of animals corresponds to their confined environment, as these bacteria do not require additional genetic systems to confront variable external conditions, in contrast to their free-living and plant-associated relatives (11).

Commensurate with these features are the intermediate values regarding the genome size and the G + C content of Brucella in comparison with its free-living/plant-associated and obligate intracellular α-Proteobacteria relatives (Table 1, www.pnas.org). The presence of two chromosomes with the same G + C content and almost identical proportion of potential coding regions (1,028 and 1,035, respectively) in relation to the chromosomal sizes, as well as the equilibrated distribution of housekeeping genes, reveal that both replicons have a long coexistence. Indeed, the closest Brucella relative, the free-living and opportunistic Ochrobactrum intermedium, possesses two chromosomes (12), suggesting that the ancestor of these two genera already exhibited two megareplicons (11). Thus, it is tempting to speculate that the smaller chromosome of the Brucella/Ochrobactrum ancestor evolved from a megaplasmid. Indeed, certain clusters, such as the arginine and ornithine cyclodeamidase genes and the virB operon, all located in chromosome II, are homologous to genes located in the same order in the Ti plasmid of Agrobacterium tumefaciens. Translocation of housekeeping genes to the ancestral megaplasmid, promoted by an extensive number of insertion elements and transposases, could have transformed this megareplicon into a chromosome. Although this explanation takes into account the ancestor/descendant rules, an alternative hypothesis has been offered (13).

Because Brucella is so well adapted to intracellular life, it is expected that this behavior would be reflected at all levels of its biology. It may come as a surprise that a bacterium generally described as nutritionally fastidious is endowed, with exceptions, with all major biosynthetic pathways. However, it has been known for a long time that the growth requirements of smooth Brucella are not excessive because, in chemically defined media containing mineral salts and glutamate or glucose, many strains require only niacin and thiamin (14). This property is largely consistent with the genome analysis of B. melitensis (1). Niacin dependence is the phenotype of nadA-C mutants of prototrophic bacteria and, therefore, the absence of quinolinate synthetase (nadA), and nicotinate-nucleotide pyrophosphorylase (nadC) genes was not unexpected. On the other hand, the presence of the genes predicted to be necessary for thiamin synthesis contrasts with the requirement of this vitamin, a point that needs reexamination. The conflict may lie in the fact that some steps en route to the thiazole unit and the regulation of the pathways remain to be elucidated (15). Thus, comparative analyses of the B. melitensis genome may help in understanding basic aspects of thiamin metabolism. Strain 16 M (1) has been reported to require also cysteine or methionine (16) and differs from other strains in this and possibly other minor requirements. Consistent with the ability of most brucellae to grow with sulfate or thiosulfate as the only sulfur source (14, 16), the reductive assimilatory pathway and related permease are predicted to be in B. melitensis 16 M, and the difference between this and other strains may lie in the activity of the O-acetylserine sulfhydrylase. This and other minor strain differences are now amenable to investigation.

The usefulness of these alternative electron acceptors in the biological niche of Brucella is intriguing.

Critical events take place in the Brucella cell envelope (Table 2), and it is noteworthy that, in contrast to its closest phylogenetic neighbors, the Brucella outer membrane is permeable to hydrophobic compounds (5). Usually, impermeability to hydrophobic molecules is complemented by efflux pumps, whose presence and/or efficiency in Brucella could thus theoretically be questioned. The predicted presence of efflux pumps and outer membrane export channels that, like the AcrAB/TolC system, are characteristically active on a wide range of bulky hydrophobic compounds (17), illustrates how the genetic data raise intriguing questions. For example, it may be asked whether the Brucella efflux pump genes are expressed in vitro, become activated only in the host, or are just “fossil” sequences. These pumps have been detected in various Brucella phylogenetic relatives, and some of them control virulence factors. Also, these pumps may serve to export moderately hydrophobic metabolites, such as the autoinducers of quorum-sensing systems (18). Similar intriguing questions are raised by the conservation of genes predicted to code for heavy-metal pumps, as these are characteristic of soil microorganisms or microorganisms that cycle between animal hosts and the environment. Phagocytes control the level of iron within phagosomes and endosomes. Keeping iron under control is necessary for intracellular parasites, not only because it is an essential nutrient and a component of critical detoxifying enzymes, but also because free iron catalyzes production of harmful hydroxyl free radical. In this regard, the report that B. melitensis 16 M carries enterobactin (an iron chelator derived from 2,3-dihydroxybenzoyl-serine) synthetase gene is also striking. B. melitensis has been shown to release 2,3-dihydroxybenzoate but no complex catechols under conditions fully inducing enterobactin synthesis in control bacteria. In vitro, 2,3-dihydroxybenzoate promotes iron uptake by Brucellis abortus, and its addition to macrophage cultures prevents killing of this bacterium, but its role in infection is unclear (19). Also interesting is the presence in B. melitensis of all of the genes that putatively code for the Entner–Doudoroff pathway enzymes. This pathway occurs in other α-Proteobacteria but has not been detected in Brucella (3). Although 6-phospho-2-keto-3-deoxy-gluconate aldolase activity exists at least in B. abortus US19 vaccine, 6-phospho-gluconate dehydratase activity has not been found, and cell-free extracts yield the same amount of pyruvate from glucose as from ribose-5-phosphate (20). This observation suggests that 6-phospho-gluconate dehydrogenase coupled to the pentose shunt is in fact the major route of pyruvate generation. Are these strain or species peculiarities or more general features of Brucella species? US19 is unable to use erythritol and may be atypical in other pathways, but its pattern of ¹⁴CO₂ release from glucose labeled at different positions is closely similar to that of B. melitensis 16 M (21). US19 is relatively attenuated but still infectious in humans. Anecdotal, but illustrative of a daily problem in Brucella research, the authors of the classical metabolic studies stated that they chose to work with US19, “as a model to minimize the hazard of infection” (20). It seems, therefore, that genomics will bring not only a wealth of information but also safer research! A British relative of US19 was used to elucidate the erythritol catabolic pathway (22) (fully confirmed by the genetic studies), the glucose uptake systems (23), and the components of the electron transport chain (24). The latter studies showed several primary dehydrogenases (including lactate and erythritol-1-phosphate dehydrogenases), a “branched” terminal section suggestive of the ability to adapt to low oxygen tension, and a functional nitrate reductase that should allow anoxybiontic growth. These observations are extended by genomic analyses that also suggest a dissimilatory sulfate system. Obviously, the usefulness of these alternative electron acceptors in the biological niche of Brucella is intriguing.

Many aspects of Brucella biology remain to be understood, and their investigation will provide both basic knowledge and new approaches to cure and prevent brucellosis. Indeed, the fact that Brucella is a monophyletic genus, the various species still display distinct virulence and host preference. The report by DelVecchio et al. (1) is expected to be followed shortly by similar data on other Brucella, which will further expand the possibility of performing comparative analyses. These data will also help us understand how these pathogens emerged during evolution by bringing into light their long-hidden virulence credentials