Brucella melitensis

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Go to brucellosis for general information about the Brucella species with reference to wildlife diseases



Brucella melitensis infection in wildlife is generally considered not to be of great importance and its confirmed occurrence is currently limited to a number of species, such as ibex[1], chamois[2], one-humped camels, and llamas and other small camelids, usually in association with contact with B. melitensis-infected domesticated small ruminants[3]. In Saudi Arabia Neuman's gazelles (Gazella erlangeri) are infected[4]. In African wildlife, it has been isolated from impalas (Aepyceros melampus)[5] and a sable antelope [6], and there is serological evidence that it occurs in elands in Zambia [7]. Interrestingly, B. melitensis biovar 3 has been isolated from naturally-infected Nile catfish (Clarias gariepinus)[8]. In South Africa, there was a recent outbreak in a group of 30 sable antelopes (Hippotragus niger) of Brucella melitensis biovar 3 infection characterised by the occurrence of hygromas (sometimes bi-lateral) (Fig. 1) over the carpal and tarsal joints. Unpublished outbreaks in sable antelope also occurred in the Eastern Cape Province of South Africa where the infection seems to be much more prevalent.

The role of wildlife as a reservoir of Brucella for humans and domestic livestock remains to be properly established. In areas in which there is a known Brucella infection such as in wild boar in the Spanish Iberian Peninsula, they represent an important threat for domestic pigs. By contrast, wild ruminants there, though infected, are not a significant brucellosis reservoir for livestock.[9]. What now seems to be a sustainable B. melitensis infection in a semi-domestic free-ranging Alpine ibex population, spilled over to domestic ruminants[10] though there are indications that the infection is not that easily transmitted between infected wildlife and livestock[11]. Livestock at a livestock–wildlife interface are at risk, as is reflected by a negative binomial regression model that identified geographical area, contact with wildlife, and herd size as having significant effects on counts of seropositive cattle in a herd in Zambia at the interface.[12]. Limited, but unknown infections that may be relevant, as has been shown by the isolation of Brucella melitensis biovar 1 from bovine milk samples from a herd in central Kenya, in cattle with reproductive problems kept in mixed herds indicating that cross-infection occurs from small ruminants.[13]


Given the limited knowledge about the prevalence of B. melitensis in wildlife, its epidemiology is unknown, except that it is assumed that the infection may be contracted from infected small ruminants. Limited information about the spread of the disease indicate that even when there is relatively close contact between different species, transmission of the infection does not easily take place[14] [15]


The pathogenesis of the disease in wildlife species is unknown but it is assumed that the general trends in the pathogenesis in domesticated species would equally apply to the various species of wildlife.

Brucella infects phagocytic and non-phagocytic cells in the hosts. Brucellae display strong tissue tropism for lymphoreticular and reproductive systems with an intracellular lifestyle that limits exposure to innate and adaptive immune responses, sequesters the organism from the effects of antibiotics, and drives clinical disease manifestations and pathology. Brucella infection is dependent on i) evasion of intracellular destruction by restricting fusion of type IV secretion system-dependent Brucella-containing vacuoles with lysosomal compartments, ii) inhibition of apoptosis of infected mononuclear cells, and iii) prevention of dendritic cell maturation, antigen presentation, and activation of naive T cells[16].

Clinical signs and pathology

In sable antelope the presence of 'hygromas' (Fig. 1) in a number of animals suggested a Brucella spp. infection. These were of different sizes and occurred either uni- or bilaterally. In sable in which no obvious swelling occurred over the joints, a potential space could be detected subcutaneously over the respective joints (Fig. 4). The swellings occurred subcutaneously (Fig. 2), and had no connection with the underlying joint that had no lesions attributable to the infection. The cystic space contained a clear, yellowish serous fluid in association with large amounts of clotted fibrin (Fig. 3). Smears made from the aspirated exudate from the lesion contained fibrin, a mixed cellular reaction consisting of lymphocytes, macrophages, epithelioid cells, multi-nucleated giant cells, and scattered degenerated neutrophils. In Stamp's-stained smears numerous coccoid bacteria can be detected. Histologically similar cellular elements can be detected in the wall of the lesion. The lining of the lesion lacks an epithelial or endothelial layer suggesting the likely origin of the granulomatous lesion to be the potential subcutaneous space detectable in animals without clinical swelling over the respective joint (Fig. 4). The underlying joints were invariably normal (Fig. 4)

Traditionally, the swelling over the joints seen clinically, were referred to as hygromas. Given the nature of the reaction seen histologically in the sable, this is a chronic cystic granulomatous inflammation and the use of hygroma to describe it, is inappropriate. The pathogenesis of this lesion is unknown but it may be the consequence of a localised granulomatous lymphangitis - time will tell…

Fig 1. Bilateral hygromas in a sable antelope infected with Brucella melitensis
Fig 2. Skin removed from the swollen carpal area
Fig 3. Content of the hygroma. Note the large amount of fibrin and the clear serous fluid content
Fig. 4. Potential subcutaneous space
Fig 5. Joint underlying a large hygroma showing normal structures

Brucella melitensis-infected water buffalo cows showed different forms of endometritis: ulcerative (28.12%), granulomatous (6.25%), haemorrhagic(3.12%), and chronic (62.5%). Immunohistochemical examination of uterine tissue revealed aggregations of extra-cellular and periglandular, Brucella melitensis antigen. The spleen showed lymphoid depletion and mild immuno-reactive staining of Brucella antigen was detected extracellularly among the lymphocytes of the splenic white pulp[17].


The vast majority of published brucellosis studies in the developing world rely solely on serology. An important shortcoming of brucellosis serology is the impossibility to infer which (smooth) Brucellas pp. induced antibodies in the host. In this respect, mixed farming and especially raising small ruminants along with cattle, a common practice in the developing world, is reported to be a risk factor and a central question that has to be answered is whether cattle are infected with B. melitensis or with B. abortus or with both Brucella species. Therefore the isolation, identification and molecular characterization of Brucella spp. in human and the different livestock species need to be undertaken to define a sound conceptual framework, identify the source of infection and plan appropriate control measures.[18]

Measuring the kinetics of antibody production after Brucella spp. infection is essential for analyzing serological results correctly and may help to predict abortion. Indirect ELISAs help to discriminate 1) between false positive serological reactions and true brucellosis, and 2) between vaccination and infection. Biotyping of Brucella spp. provides valuable epidemiological information that allows tracing an infection back to the sources in instances where several biotypes of a given Brucella species are circulating. Polymerase chain reaction and new molecular methods are likely to be used as routine typing and fingerprinting methods in the coming years. The diagnosis of brucellosis in livestock and wildlife is complex and serological results need to be carefully analyzed[19].

There are many factors besides suboptimal diagnosis that impede the complete and sustained eradication of animal brucellosis. Poor test specificity when resolving positive serology, due to infection with cross-reactive bacteria and vaccination with smooth Brucella strains, is also an impediment to efficient disease eradication. Despite new developments and innovations the classical serological tests remain relevant but there is potential for significant improvement and supplementation. [20]

The problem with false-positive reactions, in particular its implications for surveillance programs in low prevalence or officially brucellosis-free countries, is relevant within the context of interpretation of serological results.[21]

Immunohistochemistry in formalin-fixed, paraffin-embedded tissues is a useful tool for the diagnosis of spontaneous ovine abortion caused by B. melitensis. Microscopic studies demonstrated that Brucella antigens are mainly located in the cytoplasm of macrophages and neutrophils in the lung, and in the cytoplasm of macrophages in the portal infiltrates and Kupffer cells of the liver.[22]


The current lack of information makes it difficult to control the disease in wildlife. It appears that alpine ibex known to be infected with B. melitensis, could be the source of the re-emergence of bovine brucellosis in France. There is a need to maintain an active/reactive surveillance system for livestock and wildlife in known infected areas to provide sufficient information on which to base recommendations for the control and management of the disease.[23]

Vaccination is the cornerstone of control programs in livestock and, although the S19, RB51 (both in cattle) and Rev 1 (in sheep and goats) vaccines have been successfully used worldwide, they have drawbacks and thus the ideal brucellosis vaccine is still awaited. There is no vaccine available for pigs or wildlife[24].

Animal brucellosis control strategies differ in the developed and the developing world. Most emphasis is put on eradication and on risk analysis to avoid the re-introduction of Brucella in the developed world[24].

At the animal/ecosystem/human interface it is critical to reduce opportunities for Brucella to jump host species as already seen in livestock, wildlife and humans. This task is a challenge for the future in terms of veterinary public health, as for wildlife and ecosystem managers, and will need a “One Health” approach to be successful[24].

[25] [26] [27] [28] [29] [30] [31]


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