Tuberculosis (TB) is one of the biggest but silent killers among the infectious diseases. Its common characteristics include dry, productive cough, night sweats, weakness, sudden weight loss etc.
Tuberculosis is mainly caused by Mycobacterium tuberculosis. According to the World Health Organization (WHO), one-third of the world’s population, i.e. around 2 billion people, are infected with Mycobacterium tuberculosis (MTB). 95% of infection occurs in the under-developed world, where diagnostic and treatment facilities are scarce or absent.
Mycobacterium tuberculosis is a rod-shaped, acid-fast bacteria. For revealing its structural details, it is stained with Ziehl-Nelson stain. Auramine-phenol fluorescent staining can also be used. Mycobacterium tuberculosis shows red/pink rode like structure with a blue background.
Other Mycobacteria that may contribute to the infection include,
- Mycobacterium bovis
- Mycobacterium africana
- Mycobacterium microti
Types of TB
TB basically has two types. These are as follow,
- Pulmonary tuberculosis
- Extra-Pulmonary tuberculosis
If the Mycobacterium inhabits and damages the lungs only, it is called pulmonary tuberculosis. However, if it invades and damages other organs too, the resulting condition is called “Extrapulmonary tuberculosis.” Along with pulmonary tuberculosis, extrapulmonary tuberculosis (EPTB) is also an important clinical problem. Pulmonary tuberculosis involves the lungs only while extra-pulmonary tuberculosis (EPTB) accounts damage to the other organs. It includes,
- TB Lymphadenitis
- Pleural TB
- Genitourinary TB
- Neurological TB
- Abdominal TB
- Intestinal TB
- Hepatobiliary TB
- Peritoneal TB
- Splenic TB
- Pancreatic TB
- Musculoskeletal TB
- Pericardial TB
- Ocular TB
- Cutaneous TB
Risk of Infection
The risk of infection depends upon multiple factors including how close the patient is with an individual. As compared to most viral infections, tuberculosis is only moderately infectious in ordinary circumstances. However, smear-positive cases are highly infectious.
A person shedding tubercle bacilli in sputum is termed as “open or infectious”. Closed environments like army barracks, jails, and nursing homes play an important role in the spread of the infection. Personal protective measures should be adopted while dealing with patients of AIDS having pulmonary tuberculosis as they are highly contagious in the absence of cavitations and even with normal chest radiographs.
Mode of Spread
The major mode of transmission of almost all of the infectious diseases is due to inhalation of infectious particles in the form of droplets. These droplets are generated during sneezing, coughing, or talking. The small size of the droplets allows them to travel through the whole respiratory tract and settle in the terminal air passages.
The room air can also carry infectious particles even during the temporary absence of the patient.
In order to acquire the infection, prolonged and multiple exposures are usually required. Gastrointestinal tuberculosis was common in the past due to ingestion of milk carrying Mycobacterium bovis. However, it has become rare due to the effective sterilization of milk.
Mechanism of Spread
Mycobacterium spreads within the body by two mechanisms. These are as follow,
- A tubercle can erode into a bronchus, empty its caseous contents and thereby spread the organism to other parts of the lungs. It can transfer to the gastrointestinal tract if swallowed and to other persons, if expectorated.
- It can circulate via the bloodstream to many internal organs. Spreading of the bacteria can occur at an early stage if the cell-mediated immunity fails to contain the initial infection or at a late stage if a person becomes immune-compromised.
The body resists the Mycobacterium by cellular immunity provided by T-cells and macrophages. Antibodies are also formed but they play no special role in immunity and thus are not used for diagnostic purpose.
Early diagnosis of tuberculosis makes effective treatment possible and increases the probability of clinical outcome owing to quite effective anti-tuberculosis therapy. However, the tuberculosis diagnosis has certain difficulties.
According to International standards, tuberculosis diagnosis must be confirmed either by bacteriology or by histology of the organism. However, the bacteriological methods don’t always allow detecting Mycobacterium tuberculosis in people affected with pulmonary tuberculosis and especially with extrapulmonary tuberculosis. Approximately 20 to 50% of patients with pulmonary tuberculosis are smear negative, and 10% of these patients are culture negative.
The definite diagnosis of tuberculosis is the detection of acid-fast bacilli (AFB) from clinical specimens, by microscopy and culture. Mycobacterium tuberculosis culture technique is accepted worldwide and it is more specific than other tuberculosis detection tests. The specimen analyze for detecting the disease include,
- Bronchial washings
- Laryngeal swabs
- Extra Pulmonary Specimen
- Other body fluids
After positive results from the diagnostic tests, first-line anti-mycobacterial drugs (SIRE) are used which includes,
The drugs used for the treatment of tuberculosis are divided into three main groups.
- Bactericidal drugs that sterilize the lesions effectively.
- Bactericidal drugs that kill the tubercle bacilli only in certain situations. For example, streptomycin is ineffective against bacilli within macrophages and Isoniazid is effective only against multiplying bacilli.
- Bacteriostatic drugs which are not included in the standard drug regimens as they are of limited use.
For a successful therapy, it is important that at least two drugs should be used at a time to which the organism is sensitive. It also prevents the rise of multidrug-resistant strains.
According to WHO treatment guidelines, the earlier phase consists of 2 months therapy followed by a continuation phase of 4-6 months.
WHO treatment guidelines are shown in the table below,
|Treatment Category||Definition||Initial phase||Continuation phase|
|1||New sputum smear positive; smear negative with extensive lung involvement; severe non-pulmonary disease||H,R,Z,E||4 months H,R or
4 months H3E3 or
6 months H,E
|II||Sputum smear positive after relapse, treatment failure||H,R,Z,E or H,R,Z,E,S||5 months H,R,E
5 months H3R3E3
|III||New smear negative and less severe non pulmonary disease (other than Category-I)||H,R,Z||4 months H,R or
4 months H3E3 or
6 months H,E
|IV||Chronic cases||Advice on use of second-line drugs from specialized centers.|
H-Isoniazid, R-Rifampin, Z-Pyrazinamide, E-Ethambutol, S-Streptomycin.
Epidemiology and Prevalence
Although the incidence of tuberculosis was highest in the African region, however; in 2004 the largest number of TB cases occurred in South-East Asia Region. It accounted for 33% of incident cases globally. The incidence of a disease is the highest among the young individuals between 20-45 age groups thereby, affecting both the social and economic development of the affected regions (WHO, 2005).
The global prevalence of tuberculosis was 14.6 million cases (229/100,000) in the year 2004. 80% of these cases were found in South-East Asia, sub-Saharan Africa, & Western Pacific Regions. Highest burden of tuberculosis (TB) cases occurred in South East Asian Region. There were an estimated 5.7 million prevalent cases of TB or 351 cases per 100,000 in this region in 2005.
TB is a contagious disease. People must avoid excessive contact with the infected individuals. In case of contact, don’t forget to wear safety measures like masks, gloves etc. TB patients are suggested to take proper treatments and maintain a healthy lifestyle for a speedy recovery.