Brucellosis is a genitourinary infection of sheep, cattle, pigs, and other animals. Humans such as farmers, slaughterhouse workers, and veterinarians become infected directly by occupational contact or indirectly by consumption of contami-nated animal products such as milk. In humans, brucellosis is a chronic illness characterized by fever, night sweats, and weight loss lasting weeks to months. Because the infection is localized in reticuloendothelial organs, there are few phys-ical findings unless the liver or spleen become enlarged. When patients develop a cycling pattern of nocturnal fevers, the disease has been called undulant fever.
Brucellosis, a chronic infection that persists for life in animals, is an important cause of abortion, sterility, and decreased milk production in cattle, goats, and hogs. It is spread among animals by direct contact with infected tissues and ingestion of contaminated feed and causes chronic infection of the mammary glands, uterus, placenta, seminal vesicles, and epididymis. Although the associations are not absolute, each species is linked to a different animal: B. abortus tends to infect cattle; B. melitensis,sheep and goats; and B. suis, pigs.
Humans acquire brucellosis by occupational exposure or consumption of unpasteurized dairy products. The bacteria may gain access through cuts in the skin, contact with mucous membranes, inhalation, or ingestion. In the United States, the number of cases has dropped steadily from a maximum of more than 6000 per year in the 1940s to the current level of less than 100 per year. Of these cases, 50 to 60% are in abattoir employees, government meat inspectors, veterinarians, and others who handle livestock or meat products. Consumption of unpasteurized dairy products, which accounts for 8 to 10% of infections, is the leading source in persons who have no connection with the meat processing or livestock industries. Some recent cases of this type have been associated with “health” foods. In the United States, the distribution of human cases of brucellosis includes virtually every state, but is concentrated in those with large livestock industries or proximity to Mexico (California, Texas). An outbreak of B. melitensis in Texas was traced to unpasteurized goat cheese brought in from Mexico.
All Brucella species are facultative intracellular parasites of epithelial cells and profes-sional phagocytes. After they penetrate the skin or mucous membranes, they enter and mul-tiply in macrophages in the liver sinusoids, spleen, bone marrow, and other components of the reticuloendothelial system. Understanding the mechanisms for intracellular survival is incomplete but involves suppression of the myeloperoxidase system, inhibition of phago-some–lysosome fusion, and impairment of monocyte cytokine production. Thus, intracellu-lar events in monocytes determine the outcome of a Brucella infection. In cows, sheep, pigs, and goats, erythritol, a four-carbon alcohol present in chorionic tissue, markedly stim-ulates growth of Brucella. This stimulation probably accounts for the tendency of the or-ganism to locate in these sites. The human placenta does not contain erythritol.
If not controlled locally, infection progresses with the formation of small granulomas in the reticuloendothelial sites of bacterial multiplication and with release of bacteria back into the systemic circulation. These bacteremic episodes are largely responsible for the recurrent chills and fever of the clinical illness. These events resemble the pathogene-sis of typhoid fever .
Although antibodies are formed in the course of brucellosis, there is little evidence they are protective. Control of disease is due to T cell–mediated cellular immune responses. Development of helper T cell–type responses and the production of cytokines (tumor necrosis factor- , tumor necrosis factor- , interleukin-1, interleukin-2) are associated with the elimination of Brucella from macrophages.