Brucella spp. infections

J. Godfroid -, Faculty of Biosciences, Fisheries and Economics, UiT - The Arctic University of Norway, Hansine Hansens veg 18, Tromsø 9019, Norway and Faculty of Veterinary Science, University of Pretoria, Private, Bag X04, Onderstepoort, Gauteng, South Africa, 0081

Brucellosis is caused by facultative intracellular α-Proteobacteria of the genus Brucella, which is related phylogenetically to other bacterial genera such as Agrobacterium, Phyllobacterium, Rhizobium and Ochrobactrum.

Bacteria of the genus Brucella are Gram-negative cocci, coccobacilli or short rods with straight or slightly convex sides and rounded ends that do not ferment carbohydrates in conventional media.⁴ They are transmissible to a wide range of animal species. Early bacteraemia is followed by localization of the infection in the genital organs and cells of the monocyte-macrophage series. Brucellosis is an important disease of humans and domestic and wild animals worldwide. In humans the disease caused by Brucella spp. (mainly Brucella melitensis, Brucella abortus and Brucella suis) is also known as Malta fever, Mediterranean fever or Bang’s disease, or as undulant fever.^{4, 8, 14} Among domestic animals, cattle, sheep, goats and pigs are mainly affected (Table 1). A number of wildlife species, such as African buffalo, antelope, bison, camels, elk, reindeer, caribou and other wild ruminants and carnivores, are also susceptible to infection.⁸⁰ (see Brucella infections in terrestrial wildlife).

The genus Brucella contains 12 validly published species that include B. abortus, B. melitensis, B. suis, B. ovis, B. canis and B. neotomae. Seven are recognized for B. abortus, three for B. melitensis and five for B. suis. However, the degree of genetic relatedness, as shown by DNA hybridization studies and whole genome sequencing techniques, is consistent with the existence of a single species within the genus Brucella.^{14, 73}

Brucella abortus, B. melitensis, B. suis and B. neotomae generally occur in the smooth form, while B. ovis and B. canis are invariably rough species. Differentiation of the biovars of the species of the genus Brucella is shown in Table 2.

Recently  new Brucella species have been described besides the six classical species: Brucella ceti and Brucella pinnipedialis in cetaceans and pinnipeds, respectively; Brucella microti in vole and soil; Brucella vulpis in red foxes; Brucella inopinata in frogs and from a woman’s breast implant, whose origin has not been determined; and Brucella papionis in baboon.¹⁴

In animals, the most important clinical manifestations are abortion, the birth of weak offspring, orchitis and epididymitis. Signs of arthritis may be evident in chronic infections. The primary sources of contamination of the environment are foetal membranes and fluids and vaginal discharges expelled by infected females when they abort or at parturition. Brucellae are also commonly shed in milk and semen.

The rare occurrence of brucellosis in domestic animals in many developed countries is directly linked to the success of brucellosis control/eradication programmes.¹⁴

Table 1Brucella species and preferential livestock species

In cattle, biovars of B. abortus are usually the cause of brucellosis but in some countries, particularly in southern Europe, central Asia and in the Middle East, B. melitensis has also been implicated as a cause of abortion in cattle where they are kept in close association with infected sheep or goats. Occasionally Brucella suis may infect the mammary gland of cattle but it has not been reported to cause abortion in this species.¹⁴

Caprine and ovine brucellosis (excluding Brucella ovis infections) are most commonly caused by B. melitensis (see Brucella melitensis infection) but sporadic cases are caused by B. abortus. In sheep and goats the disease caused by B. melitensis and its epidemiology are very similar to those of B. abortus infection in cattle. Importantly, in West Africa there is no report of the isolation of B. melitensis in sheep and goats, and infection in small ruminants seem to be due to B. abortus.¹⁴

Brucellosis in pigs (see Brucella suis infection) is caused by B. suis and is characterized by an initial bacteraemia followed by the production of chronic lesions in the bones and reproductive organs of both sexes. Brucella suis consists of five biovars. Pigs are infected by B. suis biovars 1, 2 or 3: disease caused by biovars 1 and 3 is similar, while that caused by biovar 2 differs in its pathology, host range, and geographical distribution (mainly in Europe). Brucella suis biovar 2 is rarely pathogenic to humans, whereas biovars 1 and 3 are highly pathogenic and cause severe disease in humans.¹⁴

Brucella ovis is one of the most common causes of epididymitis in rams but is a rare cause of abortion in ewes and neonatal mortality in lambs (see Brucella ovis infection). Low reproductive rates may occur in affected flocks. It is not a zoonosis.

Brucella canis causes epididymitis and orchitis in male dogs and metritis in bitches, and is a rare infection in humans. It does not infect other animal species.

Brucella neotomae has been isolated from a rodent species...