Undulant fever, also known as brucellosis, is an infectious illness caused by the Brucella bacteria. It is characterized by fluctuating fevers, sweating, weakness, headaches, muscle pain, and fatigue. The name "undulant" comes from the wave-like pattern of the fever. The disease is primarily transmitted through consumption of contaminated, unpasteurized milk and dairy products or direct contact with infected animals such as cattle, sheep, goats, pigs, camels, buffaloes, wild ruminants, and more recently, seals, as well as through handling their carcasses. In the United States, infection is more commonly contracted by consuming unpasteurized milk and dairy products. Those at a higher risk include slaughterhouse workers, meat inspectors, animal handlers, veterinarians, and laboratory personnel.The incubation period for brucellosis typically ranges from one to three weeks but can occasionally extend to several months post-exposure. Symptoms resemble those of many other infectious diseases but prominently affect the musculoskeletal system, causing widespread aches, fatigue, weakness, and mental depression. Some individuals may exhibit symptoms predominantly related to this system. The illness can last from several weeks to many months.Brucellosis manifests in several forms. In its acute stage (less than 8 weeks from onset), it presents with non-specific, flu-like symptoms. During the undulant phase (8 weeks to a year after onset), symptoms can include the characteristic wavy fevers and orchiepididymitis (inflammation of the testis and epididymis) in young males. In the chronic stage (more than a year after onset), symptoms may resemble chronic fatigue syndrome and include depressive episodes.The long-term effects of undulant fever can be quite diverse, potentially resulting in granulomatous hepatitis, arthritis, spondylitis, leukopenia (low white blood cell count), thrombocytopenia (low platelet count), uveitis (eye inflammation), optic neuritis (inflammation of the optic nerve), and endocarditis (inflammation of the heart lining and valves).Millions of people worldwide are vulnerable to this disease, especially in developing regions where animal infection control is lacking, and pasteurization of milk is not consistently practiced. Consuming raw milk and poor hygiene further contribute to human infection. In the United States, the incidence is low, with fewer than 0.5 cases per 100,000 people, primarily reported in California, Florida, Texas, and Virginia.Antibiotics, such as doxycycline and rifampin, are effective against Brucella but must be used in combination over several weeks due to the bacteria's intracellular nature. Prevention focuses on controlling or eradicating the disease in animals and avoiding raw milk products. Proper pasteurization is crucial for preventing human brucellosis.There has been concern about brucellosis as a potential bioterrorism weapon. However, a 1999 report by the General Accounting Office, the investigative body of the US Congress, deemed it "unlikely" to be a biological threat due to the challenges in obtaining the Brucella agent, the moderate difficulty in processing it, the long incubation period, and its "very low" lethality.
