Brucella species cause brucellosis. Clinical manifestations ofbrucellosis are protean in nature, and the course of disease is variable. The clinical manifestations of brucellosis depend on the infecting Brucella species. Of the four Brucella species caus-ing human infections, B. melitensis is the most virulent species. It causes the most severe and acute cases of brucellosis. It is also the most common cause of brucellosis. Because B. melitensis can survive in phagocytic cells and multiply in them, it produces a large number of bacteria.
B. suis is associated with a prolonged course of illness oftenwith suppurative, destructive lesions. B. abortus causes mild to moderate sporadic disease that is rarely associated with com-plications. B. canis infection is associated with an insidious onset, more relapses. This species usually do not cause chronic disease.
The incubation period may range from 3 days to several weeks. Human infections may be of three types, as follows: (a) acute brucellosis, (b) chronic brucellosis, and (c) localized infection.
Acute brucellosis: Acute brucellosis is seen in approximately50% of patients infected with Brucella. Patients usually complain of nonspecific symptoms, such as anorexia, fatigue, weakness, malaise, or joint pain. Fever is an important symptom and is seen in almost all patients. The fever is intermittent and undu-lant (hence known as undulant fever) and can be associated with a relative bradycardia. In untreated patients with adverse disease, the patients can show respiratory tract symptoms (20%), bone and joint symptoms (20–60%), neuropsychiatric symptoms, and gastrointestinal tract symptoms.
Chronic brucellosis: This condition develops in incom-pletely treated patients. This condition is associated with a low-grade nonbacteremic infection with periodic exacerbations.Symptoms often last for 3–6 months and occasionally for a year or more.
Localized infection: B. suisis more likely to cause localizedand suppurative infection. Complications include infections of the heart, central nervous system (CNS), and the skin. Brucella endocarditis is the most dangerous complication and is responsible for 80% of deaths in brucellosis. Chronic meningoencephalitis is the usual manifestation of CNS infection.