Tuberculosis is an infectious disease caused mainly by Mycobacterium tuberculosis, less commonly – Mycobacterium bovis. M.tuberculosis primarily affects lungs but may also disseminate throughout the body affecting various organs.
Mycobacteria are typically transmitted from a person who has pulmonary tuberculosis to a healthy one by droplet nuclei which are released into the air when an ill person coughs, sneezes or speaks. About 3000 of infection-borne particles get into the air per one cough, almost the same amount of infectious droplets gets into the air during a 5 minute talk or when a person sneezes for several times. The bacilli-containing nuclei reach the terminal parts of the lungs where bacteria are being kept under control/or killed by the person’s defense system and the infection may remain dormant for decades or even lifelong, although it is considered that the majority of individuals who still develop active tuberculosis do so within the two years after they were infected.
Primary tuberculosis typically affects the middle and lower lung lobes because the inspired air that carries the bacteria is typically distributed to these areas. Usually, the lesion known as the Ghon focus is seen peripherally whilst the hilar and paratracheal lymph nodes also appear enlarged due to the dissemination of the bacteria via lymphatic vessels to the nearest lymph nodes, forming the primary complex.
Secondary tuberculosis occurs due to the reactivation of the latent infection. It typically affects the apical and posterior segments of the upper lobes of the lungs. The superior segments of the lower lobes may also be involved. However, the process is usually asymmetrical. Caseation (formation of the cheesy necrosis), fibrosis and cavity formation frequently develop.
Symptoms of pulmonary tuberculosis
Primary tuberculosis infection is asymptomatic. Later on when the infection spreads and grows various nonspecific symptoms arise and cough eventually develops.
- Persistent cough;
- Bloody sputum (bloody sputum);
- Chest wall pain with breathing or coughing;
- Loss of weight;
- Low-grade persistent fever and sweating;
- General malaise;
- Loss of appetite (anorexia);
- Enlarged cervical lymph nodes;
A cough lasting longer than for 3 weeks with the discharge of sputum is suggestive of tuberculosis. Initially, the cough may be dry/nonproductive and typically occurs in the morning, but later it becomes productive with the discharge of sputum. The sputum may have different appearance: it may be clear, green or yellowish (purulent sputum), it may contain various amounts of blood – from a few spots to sudden episodes of cough with large amounts of blood (hemoptysis) due to the erosion of of a pulmonary blood vessel.
Subpleural lesions (meaning that these lesions are located close to the surface of the lung, right under the pleura) and involvement of the pleura may cause chest wall pain which worsens on breathing and coughing.
If the large areas of the lungs are affected breathlessness (dyspnea) develops which significantly worsens the course of the disease and person’s well-being.
Absence of fever does not exclude tuberculosis, especially in the elderly.
In some ill individuals’ rales and/or rhonchi over the affected areas may be heard during inspiration. The large cavities in the lungs produce amphoric breath sounds.