Bovine brucellosis

Bovine brucellosis

J GODFROID, P P BOSMAN,* S HERR* AND G C BISHOP*

Introduction

Bovine brucellosis is a highly contagious disease caused by Brucella abortus, a bacterium which occurs intracellularly in its mammalian host. Apart from causing characteristic mid- to late-term abortion and infertility in cows, B. abortus also occasionally causes orchitis and inflammation of the accessory sex glands in bulls. Other livestock and wild animal species, though of varying susceptibility, are sometimes infected.69 Bovine brucellosis is also an important zoonosis.6 In some countries, particularly in southern Europe and western Asia, where cattle are kept in close association with sheep or goats, infection and abortion can also be caused by Brucella melitensis. 152 Occasionally, Brucella suis may cause an infection in cattle but has not been reported to cause abortion.62

By visiting the OIE (Office International des Epizooties) website122 information on the worldwide brucellosis situation, as well as those animal diseases that have been included in the two official OIE lists of diseases due to their implications for international trade or public health, can be obtained. This information is regularly updated and is based on the emergency of the situation and on monthly and annual reports sent to the Central Bureau of the OIE by national veterinary administrations and other official sources.

In sub-Saharan Africa, brucellosis is an important disease in both humans and livestock. In general, the assessment of the relative occurrence of brucellosis is restricted to few published studies based on serological surveys and it is considered to be the highest in pastoral production systems in arid and semi-arid areas.104 The surveillance and control of brucellosis in sub-Saharan Africa is rarely implemented outside southern Africa. The rate of infection in humans is virtually unknown and public awareness is extremely low. Hence, the impact of brucellosis in terms of public health and social importance is rarely correctly addressed.104

It is suspected that bovine brucellosis was introduced into southern Africa with cattle imported from Europe,77 but there is also a possibility that it was introduced into the subcontinent much earlier during the migration of people and their cattle herds from other African countries.101 The first reliable record of its existence in South Africa was that of Gray in 1906 when he reported a serious outbreak of abortion among cattle near Johannesburg.77 Its presence was finally confirmed by Hall in 1913 when he isolated B. abortus from the stomach of an aborted bovine foetus.77 Outbreaks of abortion thought to be bovine brucellosis were first observed in Zimbabwe in 1906, and the presence of brucellosis was confirmed in that country in 1914. According to an obituary notice in The Veterinary Record of 1957, Bevan, in Zimbabwe, was the first to show that B. abortus-infected cattle could transmit the pathogen to humans and that goats were not the source of the infection.5

The disease has a relatively high prevalence in southern Africa, especially in intensively farmed areas, and it is the most important bacterial cause of abortion on the subcontinent. It has an important economic impact on the beef and dairy cattle industries, especially as in 1990, 14,7 per cent of the herds in South Africa were known to be infected and the losses to cattle farmers exceeded R300 million per annum.10

Aetiology

Brucella abortusis a small, Gram-negative, non-sporulating, non-encapsulated coccus, coccobacillus or short rod, 0,6 to 1,5 µm in length and 0,5 to 0,7 µm in width.4, 42 The organism is not acid-fast but does resist decolorization by weak acids and thus stains red with Stamp’s modification of the Ziehl-Neelsen stain.4, 142

Most wild strains are fastidious and slow-growing, and require carbon dioxide (5 to 10 per cent) supplementation for primary isolation at an optimal growth temperature of 36 to 38 °C. Brucella abortus strain 19 is an attenuated strain of reduced virulence which is used for the production of a live vaccine (see Control, below). It grows well in a normal atmosphere at 37 °C.4 Complex media, containing serum, are required for the growth of B. abortus and, although most strains grow on sheep blood agar, the colonies may not be as distinctive as when grown on serum dextrose agar. Growth, on primary isolation, is seldom clearly visible before 48 hours of incubation at which stage the colonies are usually 0,5 to 1,0 mm in diameter. The use of selective media, such as Farrell’s medium, may substantially enhance the chances of isolation by inhibiting the growth of contaminants.63 The growth rate of B. abortus may, however, be markedly retarded by selective media and for this reason such cultures should be incubated for five days or longer.4 Smooth colonies on a clear growth medium, such as serum-dextrose agar, are convex, entire-edged, have a smooth, shiny surface, and are pale yellowish-brown when viewed under transmitted light.4, 142 Considerable variation in colour and surface texture are found in the rough strains. Smooth forms are often markedly pathogenic whereas the rough variants are usually less so.

There is no single test by which B. abortus may be identified with absolute certainty, but a combination of growth characteristics, colonial...

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