1. Identify the primary routes of transmission for Brucella spp. 2. Name the populations at risk for developing brucellosis. 3. Identify the signs and symptoms associated with brucellosis. 4. State two reasons the microbiology laboratory should be notified when Brucella infection is suspected. 5. Describe the principle and procedure for the rapid test for presumptive identification of Brucella spp. 6. Describe the differential characteristics of Brucella abortus, Brucella melitensis, Brucella suis, and Brucella canis. Each of the four Brucella spp. that are pathogenic for humans has a limited number of preferred animal hosts (Table 36-1). In the host, Brucella tend to localize in tissues rich in erythritol (e.g., placental tissue), a four-carbon alcohol that enhances their growth. Humans become infected by four primary routes: TABLE 36-1 Brucella spp. and Their Respective Natural Animal Hosts • Ingestion of infected unpasteurized animal milk products (most common means of transmission) • Inhalation of infected aerosolized particles (laboratory-acquired infection is the most important source of transmission) • Direct contact with infected animal parts through ruptures of skin and mucous membranes • Accidental inoculation of mucous membranes by aerosolization Survival and multiplication of Brucella organisms in phagocytic cells are features essential to the establishment, development, and chronicity of the disease. The mechanisms by which brucellae avoid intracellular killing are not completely understood. Brucella spp. can change from a smooth to a rough colonial morphology based on the composition of their cell wall lipopolysaccharide O-side chain (LPS); those with a smooth LPS are more resistant to intracellular killing by neutrophils than those with a rough LPS. The smooth phenotype has been identified in B. abortus and B. melitensis. Brucellae ensure intracellular survival by interfering with the phagosome-lysosome fusion in macrophages and epithelial cells. In addition, as do Legionella spp. (see Chapter 35), brucellae use a type IV secretion system, VirB, for intracellular survival and replication. Unlike Legionella spp., however, brucellae modulate phagosome transport to avoid being delivered to lysosomes. Essentially, VirB is involved in controlling the maturation of the Brucella vacuole into an organelle that allows replication. In the mouse model, if mutations occur in this gene region, B. abortus is unable to establish chronic infections. In addition, Brucella spp. produce urease, which provides protection during passage through the digestive system when the organism is ingested in food products. Urease breaks down urea, producing ammonia, and neutralizes the gastric pH. Despite our current knowledge, many questions remain about the pathogenesis of disease caused by Brucella spp. • To ensure that specimens are cultivated in an appropriate manner for optimum recovery from clinical specimens • To avoid accidental exposure of laboratory personnel handling the specimens, because Brucella spp. are considered class III pathogens (specimen labels should indicate that Brucella spp. are a potential pathogen) Blood for culture can be collected routinely (see Chapter 68) into most commercially available blood culture bottles and the lysis-centrifugation system (Isolator; Alere, Waltham, MA). For other clinical specimens, no special requirements must be met for collection, transport, or processing.
Brucella
Epidemiology and Pathogenesis
Organism
Preferred Animal Host
B. abortus
Cattle
B. melitensis
Sheep or goats
B. suis
Swine
B. canis
Dogs
B. ovis
Rams (not associated with human infection)
B. neotomae
Desert and wood rats (not associated with human infection)
Laboratory Diagnosis
Specimen Collection, Transport, and Processing
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