illness has been effectively eliminated from the United States, Scandinavia,
the UK and other countries by campaigns to eradicate the organism in the
national dairy herds through a programme of testing, immunization of
young calves and compulsory slaughter of infected cattle.
7.3.2 The Organism and its Characteristics
Brucella are Gram-negative, catalase-positive, oxidase-positive, short
oval rods (0.3mm0.4mm) which are non-motile and usually occur
singly, in pairs, or, rarely, in short chains. It grows optimally around
371 C and is killed by heating at 63 1 C for 7–10 min. When shed in the
milk of an infected animal it can survive for many days provided the
acidity remains low (o0.5% as lactate).
7.3.3 Pathogenesis and Clinical Features
Brucellosis is a protracted and debilitating illness characterized by an
incubation period of from one to six weeks followed by a chronic,
relapsing fever with accompanying lassitude, sweats, headache, consti-
pation, anorexia, pains in the limbs and back, and weight loss. After the
temperature has returned to normal for a few days, another bout of fever
may ensue and such episodes recur a number of times over several
months. Treatment is commonly with a mixture of tetracycline and
streptomycin.
It is a facultative parasite and can live intracellularly or in extracellular
body fluids. During the febrile stage, caused by circulating endotoxin, the
organism may be isolated from the bloodstream but in the majority of
laboratory-confirmed cases diagnosis is based on serological tests rather
than cultural techniques.
7.3.4 Isolation and Identification
Brucellaare quite fastidious organisms and do not grow in conventional
laboratory media. Liver infusions or calf serum are normally added. The
Table 7.3 Differential characteristics ofBrucellaspecies
B. abortus B. canis B. melitensis B. suis5% CO 2 required þ
H 2 S produced þþ
Urease weak strong weak strong
Growth on dye medium:
Basic fuchsin (1:10^5 ) þþ
Thionin (1:10^5 ) þþþ
Chapter 7 191