Etiology and pathogen
The causative agents of the disease are non-motile Gram-negative bacteria of the genus Brucella. They exhibit pronounced polymorphism (appearing as cocci or rods) and grow very slowly on nutrient media (up to 3–4 weeks).
Brucellae are capable of variation (transitioning into S-, R-, and L-forms). Their structure includes a lipopolysaccharide complex (endotoxin), but the main factor of pathogenicity is considered to be resistance to phagocytosis.
Different species of Brucella affect different animals, and their hazard to humans varies:
- Br. melitensis (3 biovars) — affects sheep and goats, as well as cattle. This is the most virulent species for humans.
- Br. abortus (9 biovars) — affects cattle.
- Br. suis (4 biovars) — typical for pigs, reindeer, and hares.
- Br. canis — affects dogs, rarely transmitted to humans.
- Br. ovis and Br. neotomae — non-pathogenic to humans.
Pathogens are stable in the environment, but quickly killed by heat, instantly by boiling, and are sensitive to disinfectants.
Epidemiology
Brucellosis is a classic zoonosis. The main reservoir and source of infection are domestic animals: most commonly goats and sheep, as well as cattle and pigs. Infected animals shed Brucella into the environment with milk, amniotic fluid, urine, and feces.
It is important to remember: an infected person is not contagious to others.
Human infection occurs via several routes:
- Fecal-oral (most frequently, through contaminated food, e.g., unpasteurized milk or meat).
- Contact (the pathogen penetrates through microtraumas of the skin and mucous membranes, which is typical for livestock farmers and veterinarians).
- Airborne.
In Russia, 400–700 cases are registered annually, mostly sporadic. Up to 80% of patients are rural residents, often livestock workers, veterinarians, and slaughterhouse workers. The risk of infection persists everywhere due to the sale of imported meat and dairy products in markets and weakened veterinary control.
Clinical presentation
The infection is prone to a prolonged, undulating course with periods of exacerbations and remissions. Acute, subacute, chronic, and residual brucellosis are distinguished.
A typical picture (especially with chronization) includes:
- Fever (often subfebrile, with episodes of elevation), which is accompanied by severe chills and profuse sweating.
- Musculoskeletal involvement: pronounced pain in joints and lower back, gonitis (inflammation of the knee joint, often with deformation and swelling), restricted mobility.
- Fibrositis: characteristic painful subcutaneous nodules up to the size of a hazelnut (often in the lumbosacral region).
- Hepatosplenomegaly: enlargement of the liver and spleen.
- Nervous system involvement: positive tension signs (Lasegue, Wassermann).
- Polymikrolymphadenopathy.
At the same time, a complete blood count usually shows a normal leukocyte count and ESR.
Diagnosis and management
A key role is played by epidemiological history: identifying contacts with animals or animal-derived raw materials.
Specific laboratory verification utilizes:
- Serological methods (using brucellosis antigens): Wright's agglutination test, Huddleson-Kaitmazova test, indirect hemagglutination assay (IHA), complement fixation test (CFT).
- Antigen detection methods in blood or urine: ELISA, counterimmunoelectrophoresis (CIE).
- Skin allergy tests: Burnet test.
Differential diagnosis of fever and joint syndrome is carried out with rheumatic fever and infectious polyarthritis.
Patients with an exacerbation or newly diagnosed disease are subject to planned hospitalization in an infectious diseases hospital for etiotropic and pathogenetic therapy.
Briefly: answers to topical questions
What is the Burnet test and how is it performed?
The Burnet test is a skin allergy diagnostic method for brucellosis based on detecting delayed-type hypersensitivity (sensitization of the organism to the pathogen).
A specific preparation is used for this procedure:
- Brucellin — most commonly administered intradermally into the middle third of the flexor surface of the forearm, less frequently via the epicutaneous route (scarification test).
The reaction results are evaluated after 48–72 hours. The main criterion for a positive response is the presence and size of the resulting infiltrate (papule) at the allergen injection site.
What antibacterial drugs are used for the etiotropic therapy of brucellosis?
Etiotropic therapy of brucellosis involves the simultaneous prescription of two antibacterial drugs, one of which must be capable of penetrating the cell membrane.
The main regimens of etiotropic therapy include the following combinations:
- Rifampicin and doxycycline — taken orally as a continuous course for at least 6 weeks.
- Doxycycline and streptomycin — the first drug is taken orally for 3–6 weeks, the second is administered intramuscularly during the first 2 weeks.
- Ofloxacin and rifampicin — prescribed orally.
In case of disease relapses, patients are prescribed repeated courses with a change of antibacterial drugs.
What pathomorphological changes and granulomas form in brucellosis?
In brucellosis, the primary pathomorphological changes are localized in the reticuloendothelial system, mesenchyme, and vessels of target organs.
The development of changes depends on the phase of the disease:
- Acute brucellosis — characterized by alteration and serous interstitial inflammation in the organs of the mononuclear phagocyte system, and infectious-allergic vasculitis. Productive thrombovasculitis may form in the microvasculature. Brucellar granulomas are described in the membranes and tissue of the brain, perineurium, myocardium, walls of coronary arteries, and bone marrow.
- Subacute brucellosis — productive inflammation predominates with the formation of typical brucellar granulomas in the organs of the macrophage system: lymph nodes, spleen, liver, and synovial membranes. Dystrophic changes in organs and infectious-allergic vascular changes are also noted.
- Chronic brucellosis — new inflammatory foci of a granulomatous nature appear; previously formed brucellar granulomas may undergo regression: resolution, partial suppuration, or, rarely, fibrosis and sclerosis.
Class and exam questions
Can a person be infected with brucellosis from a sick individual?
No, a sick person does not pose an epidemiological hazard to others.
Which species of Brucella causes the most severe disease in humans?
The most virulent species for humans is *Brucella melitensis*, the main reservoir of which is sheep and goats.
What changes in the complete blood count are characteristic of brucellosis?
Normal white blood cell counts and ESR are typical for brucellosis.
What else to study on this topic
- Pathogenetic mechanisms and granulomatous inflammation in the mononuclear phagocyte system
- Clinical features of acute and subacute brucellosis
- Aetiotropic therapy regimens for various forms of the disease
- Procedure and interpretation of the Burnet test
- Specific prophylaxis of brucellosis and indications for vaccination
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