Brucella infection in humans could vary from an acute febrile disease to a chronic, low–grade, ill–defined disease, lasting several days, months or occasionally, even years. The acute phase is characterized by a sudden or insidious onset of illness with:

• Swinging pyrexia (up to 40–41°C), rigors and sweating.
• Arthralgia/arthritis (usually monoauricular) involving larger joints such as the hip, knee, shoulder and ankle.
• Low back pain.
• Headache insomnia.
• Small firm splenomegaly and hepatomegaly.
• Leucopenia with relative lymphocytosis.

The most striking aspect of the clinical picture is the severity of the illness, and the absence of clinical signs. The acute phase subsides within two to three weeks. If the patient is treated with tetracycline, the symptoms may persist giving rise to sub–acute or relapse of the disease. In a few patients, (up to 20%), symptoms recur for prolonged periods. Diagnosis is established by isolation of the organism from cultures of blood, bone marrow, exudates and biopsy specimens during the acute phase of the disease, and by serological tests.