Brucellosis is a bacterial infectious disease caused by species of the genus Brucella, transmitted from animals to humans through contact with infected animals or their products. The most common routes of exposure include consumption of unpasteurized dairy products and direct contact with infected livestock or their secretions. Animals most frequently associated with transmission include sheep, cattle, goats, pigs, and dogs. The condition is also known by historical names including Malta fever, undulant fever, Mediterranean fever, Gibraltar fever, and Cyprus fever, reflecting regions where the disease has been historically prevalent. Recognized subtypes of brucellosis include infections caused by distinct Brucella species with differing primary animal reservoirs. Individuals working in agriculture, veterinary medicine, abattoirs, or laboratory settings are at heightened occupational risk. Brucellosis is not transmitted between humans under ordinary circumstances. This summary reflects clinical data available as of the date of packet generation.

The clinical presentation of brucellosis encompasses a broad range of manifestations that may appear acutely or follow a more protracted course. Initial symptoms commonly include fever, night sweats, malaise, loss of appetite, headache, fatigue, and pain in the muscles, joints, or back. The characteristic undulant fever, in which body temperature rises and falls in a wave-like pattern, gives the condition one of its historical names. Symptoms that may persist or recur include arthritis affecting one or multiple joints, swelling and tenderness of the testicle and scrotum in males, inflammation of the heart valves (endocarditis), and neurologic symptoms including headache, confusion, and peripheral nerve involvement, which occur in a minority of cases. Swelling of the liver and spleen is also reported. Chronic fatigue and depression may persist after the acute phase. The range and severity of symptoms vary depending on the infecting Brucella species and the individual's immune response.

Brucellosis is caused by infection with bacteria belonging to the genus Brucella. The most common species responsible for human disease include Brucella melitensis, which is primarily associated with sheep and goats and tends to produce more severe illness, Brucella abortus, associated with cattle, and Brucella suis, associated with pigs. Brucella canis, associated with dogs, can also infect humans. Transmission to humans occurs through ingestion of contaminated food products such as raw milk, unpasteurized cheese, or undercooked meat; through inhalation of infectious aerosols in occupational settings; or through direct contact with infected animal tissues via skin abrasions. The bacteria are capable of surviving intracellularly within host immune cells, which contributes to the chronic and relapsing nature of infection. Person-to-person transmission is rare.

Diagnosis of brucellosis depends on a combination of clinical presentation, epidemiologic history, and laboratory confirmation. A history of exposure to livestock, unpasteurized dairy products, or occupational risk factors is an important diagnostic clue. Blood cultures, though the gold standard when positive, have variable sensitivity depending on disease phase and laboratory technique. Serologic tests detecting antibodies against Brucella antigens are widely used and include the serum agglutination test and enzyme-linked immunosorbent assay. Bone marrow cultures may increase diagnostic yield in some cases. Molecular testing using polymerase chain reaction can aid diagnosis, particularly in culture-negative cases. The differential diagnosis includes other febrile illnesses such as typhoid fever, malaria, tuberculosis, Q fever, and other zoonotic infections. Localized complications such as spondylitis or endocarditis require additional imaging or echocardiographic evaluation.

Treatment of brucellosis requires antibiotic therapy, and combination regimens are preferred over single-agent treatment to reduce the risk of relapse. Standard antibiotic-based combination therapy is the foundation of management, with prolonged courses typically spanning several weeks to months depending on disease severity and the presence of complications. Cases involving focal complications such as spondylitis, endocarditis, or neurobrucellosis may require extended treatment durations and, in some instances, surgical intervention. Supportive care including adequate rest, hydration, and analgesics for pain management accompanies antibiotic treatment. Relapses following treatment completion are recognized and may require retreatment. Prevention through pasteurization of dairy products, proper handling of animal products, and vaccination of livestock represents the primary public health strategy for reducing disease burden.

The outlook for most individuals with brucellosis is favorable with appropriate antibiotic treatment, though recovery may take several weeks to months and relapses occur in a subset of patients. Death from brucellosis is uncommon with treatment, though complications such as endocarditis carry a higher risk of serious outcomes if not identified and managed promptly. Persistent symptoms including fatigue, musculoskeletal pain, and mood disturbance can continue in some individuals even after microbiological cure, a syndrome sometimes referred to as chronic brucellosis. Early diagnosis and initiation of appropriate treatment are associated with better outcomes and a lower risk of complications.

Clinical research related to brucellosis and related zoonotic febrile illnesses includes observational and epidemiologic studies examining disease characteristics, long-term outcomes, and optimal treatment strategies. Several ongoing studies are investigating aspects of diagnosis, treatment efficacy in different patient populations, and the burden of chronic post-treatment symptoms. Vaccine development for human brucellosis remains an area of scientific interest given the limitations of current prevention approaches. Research into the host-pathogen interaction and intracellular survival mechanisms of Brucella species continues to inform understanding of disease chronicity and relapse.