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Zoonoses are human diseases caused by organisms that are acquired from animals. There are bacterial, viral, fungal, and parasitic zoonoses. Some zoonotic organisms are acquired directly from the animal reservoir, whereas others are transmitted by vectors, such as mosquitoes, fleas, or ticks.

There are four medically important gram-negative rods that have significant animal reservoirs: Brucella species, Francisella tularensis, Yersinia pestis, and Pasteurella multocida (Table 20–1).

TABLE 20–1Gram-Negative Rods Associated with Animal Sources

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX, entitled Infectious Diseases.



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 organisms 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 ...

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