GENERAL INTRODUCTION: GRAM-NEGATIVE AEROBIC OR CAPNOPHILIC RODS AND COCCI Brucella spp. infections

Introduction

Brucella melitensis is the main causative agent of caprine and ovine brucellosis. It is also pathogenic for several other mammal species including humans.^{8, 12} Infected cows may abort and shed B. melitensis in their milk.^{74, 75} Brucellosis in small ruminants is characterized by one or more of the following: abortion, reduced milk yield and retained placenta (particularly in goats) in females; orchitis and epididymitis in males; and, rarely, arthritis in both sexes.^{8, 12} It is worth noting that brucellosis caused by Brucella abortus infection in small ruminantsseems to be an important problem in West Africa whereas Brucella suis infection is rare in small ruminants.

The organism responsible for Malta fever in humans was first discovered by Sir David Bruce on the island of Malta in 1887.²³ It had become evident to him and his co-workers that the disease was not transmitted from person to person. Zammit, a member of the Mediterranean Fever Commission, determined in 1905 that B. melitensis was the cause of contagious abortion in goats on the island and that humans contracted the disease by the consumption of infected fresh goats’ milk and milk products.⁸⁴

Aetiology and epidemiology

Brucella melitensis is morphologically and tinctorially indistinguishable from B. abortus and B. suis. Species identification can be based on lysis by phages, on biochemical tests (such as oxidase, urease and catalase) or  molecular tests. (see Bovine brucellosis and Brucella suis infection).^{10, 14} Unlike B. abortus, growth of B. melitensis is not dependent on an atmosphere of 5 to 10 per cent CO₂, although there might be some exceptions.^{10, 14} The identification of a Brucella species to the biovar level is currently performed by four main tests, namely CO₂ dependence, production of hydrogen sulphide, dye (thionin and basic fuchsin) sensitivity, and agglutination with monospecific A and M anti-sera.^{10, 25, 27} The three biovars (1, 2 and 3) recognized for B. melitensis show no difference in pathogenicity. Brucella melitensis biovar 3 appears to be the biovar most frequently isolated in Mediterranean countries while biovar 1 seems to predominate in Latin America.^{8, 26, 55}

In a Multi Locus Sequence Analysis (MLSA) study,⁸¹  B. melitensis isolates clustered into three distinct lineages that correspond to the “Americas,” “West Mediterranean,” and “East Mediterranean” lineages described earlier using Multiple Locus Variable (number of tandem repeats) Analysis (MLVA) based studies.² Although a few isolates from the USA and Argentina were included in the “Americas” clade, more than 60 per cent of these isolates clustered with strains of African origin. African isolates included in the study originated widely across the continent (Ethiopia, Somalia, Nigeria, Tanzania, Sudan, Zimbabwe) and, as few African isolates fit into the “West Mediterranean” and “East Mediterranean” lineages, they may represent lineages that are endemic in Africa. In agreement with the abovementioned MLVA study, strains falling in the “West Mediterranean” lineage were isolated exclusively in Italy, France, the former Yugoslavia and Germany. In contrast, the “East Mediterranean” lineage, while including isolates from Greece, Turkey, Cyprus, and the Balkan States, extends geographically more widely into the Middle East and Asia (Thailand, India, Pakistan, Mongolia, and Afghanistan). While most isolates in the “Americas” and “West Mediterranean” lineages corresponded to biovar 1 (81 per cent) and biovar 3 (64 per cent) respectively, both lineages included isolates of all three biovars. This MLSA study concluded that the B. melitensis biovar concept is reliant solely on the serological reaction observed with the monospecific anti A and anti M sera and is of no epidemiological significance.⁸¹ Smooth (S) B. melitensis cultures have a tendency to undergo variation during growth, especially on subculturing, and dissociate to rough forms. These changes in colonial morphology are generally associated with changes in virulence and antigenic and immunogenic properties. Such changes may also occur during the production of the live attenuated S vaccines, particularly in the case of B. melitensis Rev. 1 vaccine,^{14, 15, 67} which may have serious consequences (see Control, below). Brucella melitensis wild type strains are usually resistant to benzyl penicillin and sensitive to streptomycin, whereas the Rev. 1 vaccine strain is sensitive to benzyl penicillin and resistant to streptomycin.^{10, 13, 14} This microbiological test and several PCR procedures (see below) can be used to differentiate field B. melitensis biovar 1 strains from Rev. 1.¹⁴

Brucella melitensis