infectious disease of humans and domestic animals (livestock) characterized by an insidious onset of fever, chills, sweats, weakness, pains, and aches, all of which resolve within three to six months. The disease is named after the British army physician David Bruce, who in 1887 first isolated and identified the causative bacteria, Brucella, from the spleen of a soldier who had died from the infection.

Three main species of Brucella bacteria are common causes of human brucellosis, and the bacillus of each of the species has its major reservoir in domestic animals. The causative bacteria are B. melitensis (goats (goat) and sheep), B. suis (swine (suid)), and B. abortus ( cattle). The infection may not be apparent in animals, for the brucellae and animals that they infect have become fairly well adapted to one another. In cattle, for example, the only signs of illness (also known as Bang disease) may be a drop in milk production or a general malaise, although abortion is also common. Brucellosis is, therefore, of considerable economic importance, although it causes no dramatic loss of livestock.

Brucellae are highly invasive microbes, causing infection to spread rapidly from animal to animal. Infection of healthy animals takes place through the ingestion of contaminated food or by direct entrance of brucellae through abrasions of the skin or through the mucous membrane of the eyes. In swine, infection of the genitalia of the boar can cause infection to spread throughout a herd. Infected but healthy-appearing cattle, sheep, and goats can excrete considerable numbers of brucellae in their milk for months or even years.

Humans are not natural hosts for the Brucella bacteria, and they often react violently when infected. Humans contract brucellosis either directly or indirectly from infected animals. For reasons not clearly understood, children are more resistant than adults to brucellosis. The disease is very rarely transmitted from one human being to another. In humans, acute brucellosis lasts for two weeks and then may abate, but the symptoms often return with waves of fever (from which the name undulant fever was derived) in recurring bouts of illness for about six months or a year. The infection then ceases in most people, although it can persist, sometimes for years. Chronic brucellosis is perhaps the most difficult form of the disease to diagnose because the patient's symptoms are vague and may easily be mistaken as psychological in origin. Brucellosis may be complicated by infection of the joints or spine or involvement of the heart, eyes, kidneys, or lungs. Brucella spondylitis is an arthritis of the spine that generally occurs several weeks after initial infection with brucellae and may involve any part of the spine, although the lumbar region is the most commonly affected site. The disease destroys both intervertebral disks and adjacent vertebrae but can be arrested with antibiotics and immobilization of the joints.

Brucellosis in humans is treated with antibiotics (antibiotic). A combination of a sulfonamide drug and streptomycin has proved to be successful, and therapy with tetracyclines (tetracycline) has also given satisfactory results. Therapy in the acute form is fairly satisfactory, but the chronic form is more difficult to treat. There is no dependable or practical form of drug therapy for animal brucellosis. To build up brucellosis-free herds, infected animals must be removed; vaccination (vaccine) of young animals is also a useful measure.