Brucella melitensis infection

Brucella melitensis infection

J GODFROID, B GARIN-BASTUJI, J M BLASCO, J THOMSON AND C O THOEN

Introduction

Brucella melitensis (biovar 1, 2 or 3) is the main causative agent of caprine and ovine brucellosis. It is also pathogenic for several other mammal species including humans.8, 10 Infected pregnant cows may abort and shed B. melitensis in their milk.65, 66 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, 10 Brucellosis caused by Brucella abortus and Brucella suis 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 1887.19 It had become evident to him and his co-workers that the disease was not transmitted person to person. Zammit, a member of the Mediterranean Fever Commission, determined in 1905 that this organism 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.71

Brucella melitensis infections are widespread in the world,8, 10 and occur particularly in sheep in the Mediterranean countries, the Middle East and some regions of Asia, and in goats in Latin America.8, 18, 24, 39 Infections have also been encountered in dromedary camels (Camelus dromedarius) and cattle in contact with infected sheep and goats.1, 65, 66 Northern Europe, South-East Asia, Australia, New Zealand and Canada are believed to be free of the disease.8, 10 In the USA,12 B. melitensis was detected in 1999 in cattle and goats in Texas after an absence since the early 1970s, but the source of infection was not determined.45 The disease is present in Mexico45 and certain African countries. 58

Brucella melitensis is very pathogenic for humans, causing the disease known as Malta fever (also called Mediterranean or undulant fever)8, 10 which is rated by the World Health Organization as one of the most important zoonoses.11 Brucellosis in humans is also caused by B.abortus and B. suis infections, the resultant clinical disease being indistinguishable (although less severe) from the disease caused by B. melitensis.8, 10 Brucellosis in humans is transmitted by ingesting infected unpasteurized milk and milk products and by direct contact with infected animals, animal carcasses and aborted material, and the organisms themselves in laboratories, as well as by accidental inoculation with the B. melitensis Rev. 1 vaccine stain.3, 4 Worldwide, millions of humans are at risk, especially those in developing countries where the infection in animals has not been brought under control, heat treatment procedures of milk (e.g. pasteurization) are not routinely applied, and food habits, such as the consumption of raw milk, and poor hygienic conditions favour human infection. 8, 10, 11

The survival of Brucella in milk and dairy products depends on a variety of factors. However, it is generally assumed that industrial pasteurization or prolonged boiling of milk readily kills Brucella. The organism does not persist for periods exceeding three months in ripened fermented cheese. Milk or cream used in the preparation of cheese should be pasteurized in order to ensure its safety for human consumption.

It is worth noting that humans are often the first to be reported as being infected in an area into which the disease has been newly introduced.65, 66, 68 Epidemic human infections caused by B. melitensis have occurred in people frequently in contact with infected goat herds or goat manure, as has recently been reported in Argentina.69 Localization of B. melitensis in the udder of infected camels and cattle is also a major public health problem.1, 65, 66 In laboratories handling Brucella-infected or potentially infected material it is mandatory that conditions of high level biohazard containment are used.7, 10

Aetiology and epidemiology

Brucella melitensis is morphologically and tinctorially indistinguishable from B. abortus and B. suis. Species identification is based on lysis by phages and on biochemical tests (see Bovine brucellosis and Brucella suis infection) (such as oxidase, urease and catalase). 7 Contrary to 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.7 The identification of a Brucella sp. to the biovar level is currently performed by four main tests, i.e., CO2 dependence, production of hydrogen sulphide, dye (thionin and basic fuchsin) sensitivity, and agglutination with monospecific A and Manti-sera.7, 22, 23 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, 24 Smooth 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, antigenic and immunogenic properties. Such changes may also occur during the production of the B. melitensis Rev. 1...

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