Disease
Brucella species cause brucellosis (undulant fever).
Important Properties
Brucellae are small gram-negative rods without a capsule. The three major human pathogens and their animal reservoirs are Brucella melitensis (goats and sheep), Brucella abortus (cattle), and Brucella suis (pigs).
Pathogenesis & Epidemiology
The organisms enter the body either by ingestion of contaminated milk products especially queso fresco, or through the skin by direct contact in an occupational setting such as an abattoir. They localize in the reticuloendothelial system, namely, the lymph nodes, liver, spleen, and bone marrow. Many organ isms are killed by macrophages, but some survive within these cells, where they are protected from antibody. The host response is granulomatous, with lymphocytes and epithelioid giant cells, which can progress to form focal abscesses. The mechanism of pathogenesis of these organisms is not well defined, except that endotoxin is involved. No exotoxins are produced.
Imported cheese made from unpasteurized goats’ milk produced in either Mexico or the Mediterranean region has been a source of B. melitensis infection in the United States. The disease occurs worldwide but is rare in the United States because pasteurization of milk kills the organism.
Clinical Findings
After an incubation period of 1 to 3 weeks, nonspecific symptoms such as fever, chills, fatigue, malaise, anorexia, and weight loss occur. The onset can be acute or gradual. The undulating (rising-and-falling) fever pattern that gives the disease its name occurs in a minority of patients. Enlarged lymph nodes, liver, and spleen are frequently found. Pancytopenia occurs. B. melitensis infections tend to be more severe and prolonged, whereas those caused by B. abortus are more self-limited. Osteomyelitis is the most frequent complication. Secondary spread from person to person is rare.
Laboratory Diagnosis
Blood cultures or bone marrow cultures typically yield the organism. Recovery of the organism requires the use of enriched culture media and incubation in 10% CO2. The organisms can be presumptively identified by using a slide agglutination test with Brucella antiserum, and the species can be identified by bio chemical tests. If organisms are not isolated, analysis of a serum sample from the patient for a rise in antibody titer to Brucella can be used to make a diagnosis. In the absence of an acute-phase serum specimen, a titer of at least 1:160 in the convalescent-phase serum sample is diagnostic. A PCR test is available.
Treatment & Prevention
The treatment of choice is doxycycline to which either streptomycin or rifampin is added. There is no significant resistance to these drugs. Prevention of brucellosis involves pasteurization of milk, immunization of animals, and slaughtering of infected animals. There is no human vaccine.
