Introduction

Bacteria of the genus Brucella cause disease primarily in domestic, feral, and some wild animals, and most are also pathogenic to humans. In animals, brucellae usually affect the reproductive organs, and abortion is usually the only sign of the disorder. Human brucellosis is an acute febrile illness or a persistent illness with a wide variety of symptoms. It is a true zoonosis in which virtually all human infections are acquired from animals. The disease is controlled through the routine practice of pasteurizing milk and milk products, as well as comprehensive campaigns to eradicate the disease by destroying domestic animals with positive serological reactions to brucella. There are vaccines that provide some protection for cattle, sheep and goats.

Clinical manifestations

Brucellosis is a severe acute febrile illness caused by bacteria of the genus Brucella. Relapses are not uncommon; Focal lesions may occur in bones, joints, the genitourinary tract, and other sites. Hypersensitivity reactions may follow occupational exposure. The infection may be subclinical. Chronic infections can occur.

Structure

Brucellae are gram-negative coccobacillus; non-spore-forming and non-motile; aerobic, but may need added CO2.

Classification and antigenic types

Three species (B melitensis, B abortus, and B suis) are important human pathogens; B canis is of minor importance. Species are differentiated by urease and H2S production, sensitivity to dyes, cell wall antigens, and sensitivity to phage. The main species are divided into multiple biovars.

Pathogenesis

Portals of entry are the mouth, conjunctivae, respiratory tract, and broken skin. The organisms spread, possibly in mononuclear phagocytes, to reticuloendothelial sites. Small granulomas reveal a mononuclear response; hypersensitivity is an important factor.

Host defences

Effective host defence depends primarily on cell-mediated immunity.

Epidemiology

Brucellosis is a zoonosis, acquired by handling infected animals or by consuming contaminated milk or milk products. Exposure is frequently occupational. The disease is now rare in the United States and Great Britain but is common in the Mediterranean and Arabian Gulf regions, Latin America, Africa, and parts of Asia.

Diagnosis

The diagnosis can be made clinically if there is a history of exposure. Blood cultures may be positive in the early stages of the disease, but serology is the basis of diagnosis. Interpretation is complicated by subclinical infections and persistent antibody levels.

Control

Brucellosis is prevented by pasteurizing milk, eradicating the infection from herds and flocks, and observing safety precautions (protective clothing and laboratory containment). The disease is treated with doxycycline, streptomycin, and rifampicin.