What causes brucellosis?

Causes of brucellosis

Brucellosis is caused by representatives of the genus Brucella of the family Brucellaceae. Human brucellosis can be caused by four species of Brucella: B. melitensis, B. abortus, B. suis and B. canis. The most common cause of the disease is Brucella melitensis, which is divided into three biotypes. The main hosts are sheep and goats. Brucella abortus is somewhat less common, represented by nine biotypes; the main host is cattle. The third species of Brucella, Brucella suis, has 4 biotypes. The main hosts are pigs (types 1-3), hares (type 2) and reindeer (biotype 4). Relatively rarely, the disease is diagnosed due to Brucella canis. The main host of this microorganism is dogs.

Brucella are characterized by pronounced polymorphism; they can be spherical, oval, or rod-shaped. Their size is 0.3-0.6 µm for coccal forms and 0.6-2.5 µm for rod-shaped forms. They are immobile, do not form spores, have no flagella, and are gram-negative. They grow slowly on complex nutrient media. Brucella are intracellular parasites; they are antigenically homogeneous and contain endotoxin. They are characterized by significant variability and pass from the S-form to the K- and L-forms. Brucella are resistant in the environment. They survive in water for over 2 months, in milk for 40 days, in feta cheese for 2 months, in raw meat for 3 months, in salted meat for up to 30 days, and in wool for up to 4 months. They die instantly when boiled and are sensitive to disinfectants, tetracycline antibiotics, aminoglycosides, rifampicin, and erythromycin.

[ 1 ], [ 2 ], [ 3 ], [ 4 ]

Pathogenesis of brucellosis

The entry point of infection is microtrauma of the skin, mucous membranes of the digestive organs and respiratory tract. No changes are observed at the site of introduction of the pathogen. Brucella reach regional lymph nodes through the lymphatic pathways, but there are no pronounced changes here either. Reproduction and accumulation of brucellae occurs mainly in the lymph nodes, from which they periodically enter the blood, and death is accompanied by the release of endotoxin, which causes fever, damage to the autonomic nervous system. With the bloodstream, the pathogen is carried throughout the body, concentrating in organs and tissues rich in macrophages (liver, spleen, muscles, fascia, joint bags, tendons), where, due to incomplete phagocytosis, it persists for a long time, causing an inflammatory reaction with the formation of specific granulomas.

Brucellosis is characterized by a pronounced allergic restructuring of the body, a sharply expressed DTH, which persists for a long time even after the body is freed from the pathogen. Allergy plays a major role in the formation of secondary foci of infection. Brucellosis is characterized by a tendency to a chronic course, which is associated with the long-term persistence of brucellae in the body. Before the introduction of antibiotics into medical practice, brucellae persisted in the body for up to two years, a longer course of the disease is associated with the influence of antibiotics: some brucellae can pass into L-forms and persist intracellularly for a long time.

Classification

A distinction is made between acute and chronic brucellosis. According to the severity of clinical symptoms, mild, moderate and severe forms are distinguished. Along with clinically expressed forms, latent and asymptomatic forms are possible. In children, especially at an early age, the acute form of the disease predominates. Chronic brucellosis is more common in adults and school-age children.

[ 5 ], [ 6 ], [ 7 ], [ 8 ], [ 9 ], [ 10 ], [ 11 ], [ 12 ]