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Geographic Distribution

Brucellosis occurs throughout the world, but is particularly common in the tropics. It is a disease of farmers, herdsmen, meat packers, dairymen, veterinarians, and laboratory workers.

Br. melitensis occurs wherever goats are common, usually in warm, dry climates: South, East, and West Africa, Somalia, around the Mediterranean (especially in Israel, Italy, Spain, southern France, and Malta) and South America.

Br. abortus is more common where there are cattle, which means more temperate climates: North America (especially in Texas and New Mexico), Mexico, South America, New Zealand, and other dairy and cattle areas.

Br. suis occurs mainly in North America. There is a Danish strain which does not infect man.

Br. canis occurs where there are dogs, but human infection is usually accidental.

Epidemiology and Pathology

Brucellosis is primarily a disease of animals: all four Brucella organisms are pathogenic in man, but have not been directly transmitted from human to human. There is seasonal variation in incidence in temperate climates, coinciding with animal parturition times. It is a year-round infection in the tropics. Man is infected by ingestion of infected dairy products, meat, or, rarely, water. Infection also occurs through direct contact with animals, their carcasses or excreta, and, rarely, from contaminated soil. Infection can follow inhalation of contaminated aerosols or dusts: the organism can be transmitted by blood transfusion or organ transplant. It may penetrate through damaged skin or, in the laboratory at least, through the eye. Mosquitoes and flies can be infected experimentally, but it is doubtful whether transmission occurs naturally through insect bites.

Each organism has a particular mode of transmission:

Br. melitensis is found naturally in goats, but also in a wide variety of domestic and wild animals, from cows and horses to elands and camels. Man is infected by milk and dairy products (cheese, butter, ice cream), or by contact with the animals, their carcasses, excreta, or by-products of abortion.

Br. abortus is found naturally in cattle, but occurs also in sheep and, rarely, in pigs. Transmission is through drinking raw milk, but not through cheese, butter, or ice cream. Veterinarians may contract the infection by handling infected cattle. Br. abortus is the least infective strain.

Br. suis occurs naturally in sows and hogs, but has been isolated from horses, cows, dogs, and fowl. There are two strains, the American and Danish. Only the former infects man and does so through the ingestion of pork and pork products or by the handling of pig's meat or carcasses, even after refrigeration.

Br. canis infection is almost always the result of a laboratory accident.

Animals and man manifest a variable reaction and susceptibility to Brucella organisms. The clinical disease, even when due to the same species of Brucella, varies considerably in different countries. The histopathological findings and radiological changes reflect this variation and notes have been added at the end of this chapter, based on Spanish and French reports, to illustrate the differences. However, these are not the only patterns that occur, and radiologists must be prepared to find brucellosis presenting in different ways in different countries. This is because animals and humans show considerable individual variation in their resistance to the disease. Also, hypersensitivity can develop, with an acute reaction to Brucella antigens.

The Brucella bacillus, like other infectious organisms, is very invasive and causes necrosis and suppuration. There is a resulting severe toxemia in many patients. Infection which has started through the skin causes a much more severe and rapid reaction than when the bacillus is ingested or inhaled. The primary invasion is through the lymphatics to the regional lymph nodes and then into the bloodstream. Massive enlargement of regional nodes may result, and the organism is disseminated primarily to the spleen, liver, bone marrow, brain, and nose, but may affect any organ or tissue.

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