Tuberculosis

Etiology and transmission routes

In the vast majority of cases, the disease is caused by Mycobacterium tuberculosis (MTB), and much less frequently by bovine (M. bovis) and African (M. africanus) species. Mycobacteria possess colossal viability: they survive in the external environment for months, but are sensitive to insolation, ultraviolet light, and standard disinfectants.

An extremely dangerous property of MTB is the ability to lose their cell wall under unfavorable conditions, transforming into L-forms. Such bacteria cannot be detected by standard laboratory methods, and they acquire resistance to specific drugs.

The primary reservoir of infection is an infected human with bactericidal shedding (most often with the pulmonary form), with infected animals and birds serving as an additional reservoir. The contagiousness of mycobacteria is considered low, and infection depends on the macroorganism's immune status.

Routes of transmission:

• Aerogenic (leading route);
• Alimentary (consumption of contaminated products, e.g., milk);
• Additional routes: contact and transplacental (intrauterine).

The spread of infection is influenced by socio-economic factors (decline in living standards, migration), organizational defects, and biomedical factors (HIV epidemic, pathogen mutations).

Pathogenesis and morphological picture

The primary affect is most often formed in the lungs and intrathoracic lymph nodes, and in the case of alimentary infection, in the intestine. The introduction of MTB provokes focal tissue necrosis, around which productive inflammation develops with the formation of specific tubercles (granulomas).

Manifestation of the disease occurs in only 5–15% of infected individuals. The remaining (85–95%) develop specific non-sterile immunity: the pathogen persists in the body under the strict control of the immune system. In a favorable outcome, the granuloma resolves or calcifies (forming a calcified nodule). However, bacteria can persist within calcifications, threatening endogenous reactivation if resistance drops.

Unlike many infections, tuberculosis runs an acyclic course (without strict staging). In the early stages, the neuroendocrine system is involved in the process. Further on, the disease may regress, become chronic, or generalize (dissemination of MTB with the risk of death).

Tuberculous intoxication

This is the most common form in pediatrics. Its essence lies in the presence of an active infectious process without instrumentally detectable local foci.

Diagnosis is based on the evaluation of immunological tests: a "virage" (transition from a negative to a positive reaction) is recorded, along with an increasing reaction or hyperergic response.

Clinical manifestations:

• Low-grade fever, poor appetite, and weight deficit.
• Neurovegetative disorders and frequent intercurrent infections.
• Micropolyadenia (moderate enlargement of peripheral lymph nodes).
• Skin syndrome — erythema nodosum (erythema nodosum). These are purple, painful nodules (1–5 cm) on the extensor surfaces of the shins, thighs, and shoulders.

When suspecting this form, the physician is obligated to rule out abdominal tuberculosis, parasitoses, and chronic inflammation of the ENT organs.

Primary tuberculosis complex (PTC) and lymph node involvement

For the development of PTC, a combination of a massive infecting dose of MTB and a decrease in the patient's reactivity is required.

Classical morphological triad of PTC:

1. Primary affect (focus of specific pneumonic inflammation in the lung).
2. Regional lymphangitis (involvement of draining lymphatic vessels).
3. Specific lymphadenitis (involvement of bronchopulmonary nodes).

The disease may be asymptomatic or accompanied by intoxication and a rare dry cough. Percussion reveals shortened sound, auscultation reveals weakened vesicular breathing, prolonged expiration, and moist fine rales. Blood tests show moderate leukocytosis and an accelerated ESR.

The most common clinical form in children is tuberculosis of the intrathoracic lymph nodes (TITLN) — an isolated lesion of the lymphatic apparatus of the lung root and mediastinum.

Class and exam questions

What else to study on this topic

• Extrapulmonary forms of tuberculosis (involvement of the central nervous system, bones, joints, and urogenital system)
• Interpretation of tuberculin diagnostic results (differences between a virage and post-vaccination allergy)
• Mechanisms of endogenous reactivation and generalization of tuberculosis
• Scheme of differential diagnosis of tuberculous intoxication and chronic inflammation

Other topics from the section «Chapter 12. Infectious and parasitic diseases»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.