In the early days, tuberculosis have been considered as one of the most important infections of humans. Due to the devastating morbidity and massive mortality the disease was often called as ‘white plague’ and ’the captain of all the men of death’. The large majority of cases of death are being reported from poor countries.
The disease is caused by a fungus like bacteria called Mycobacterium. They are aerobic, non-motile, non-spore forming, slow growing obligate parasite which are saprophytes or opportunistic pathogens. Tuberculosis in humans is mainly caused by two types of bacillus; Mycobacterium tuberculosis and Mycobacterium bovis. Certain saprophytic Mycobacteria like Mycobacterium butyricum, Mycobacterium phlei and Mycobacterium stecoris were also isolated from sources like butter, grass, dung, etc.
Tubercle bacilli are termed as ‘acid fast bacilli’ as they cannot be stained by staining agent. This is because of the presence of ‘mycolic acid’ which is an unsaponifiable wax in the semi-permeable membrane of the cell. The bacilli are highly susceptible to even traces of toxic substances like fatty acids. They are killed at 600C in 15-20 minutes. But they can survive exposure to 5% phenol, 15% sulphuric acid, 3% nitric acid, 5% oxalic acid and 4% sodium hydroxide.
The source of infection is usually an open case of pulmonary tuberculosis. Mode of infection is by direct inhalation of droplet nuclei of expectorated sputum containing aerosolized bacilli. Coughing, sneezing and speaking releases numerous droplets. Usually the spread is common among household or prolonged contact with the infected person. Infection also spreads through ingestion of contaminated milk.
Tubercle bacillus does not contain or secrete any toxin. The main pathogenesis of tuberculosis is the production of a characteristic lesion; the tubercle, in infected tissues. These are of two types; exudative lesions and productive lesions. Exudative lesions occur when the bacilli are many and virulent. Productive lesions are cellular in nature.
Depending on the time of illness, tuberculosis may be classified as ‘primary’ and ‘post-primary’. Primary tuberculosis is the initial infection by the bacteria in the host. A primary complex is formed about 3-8 weeks from the time of infection. The lesions heal in 2-6 months, leaving behind a calcified nodule. A few bacilli may survive in the healed lesions and remain latent.
In cases of impaired immunity, the lesions may enlarge and cause meningeal or disseminated tuberculosis. The post primary type occurs due to the reactivation of latent infection. It mainly affects the upper lobes of the lungs, the lesions undergoing necrosis and tissue destruction, leading to cavitation. The necrotic materials often enter into the airways and leading to expectoration of sputum containing large amounts of bacilli in them.
Poverty and TB often go together. With improvements in the standard of living, the disease slowly declines. A close relation exists between HIV infection and TB. TB hastens the development of HIV into an active infection.
The diagnosis of TB is mainly by demonstrating the bacilli microscopically from lesions, demonstrating hypersensitivity to tuberculoprotein, etc. for pulmonary TB the sputum sample is tested. Ziehl- Neelsen staining technique is employed. Fluorescent microscopy is also helpful at times. Rifampicin and pyrazinamide are drugs which remove bacilli from lesions. But since drug resistant strains are evolving, DOTS therapy gains importance. This strategy prevents the deterioration of the resistance problem ensuring the patient’s obedience.