**2. Transmission and pathogenesis**

TB is spread from person through the air. When a person with pulmonary or laryngeal TB coughs, sneezes, speaks, or sings, droplet nuclei containing *Mycobacterium tuberculosis* are expelled into the air. Depending on the environment, these tiny particles (1-5 microns in diameter) can remain suspended in the air for several hours. If another person inhales air containing droplet nuclei, transmission may occur. The probability that TB will be transmitted depends on these factors 4, 5:


The best way to stop transmission is to isolate patients with infectious TB immediately and start effective TB therapy. Infectiousness declines rapidly after adequate therapy is started, as long as the patient adheres to the prescribed regimen. Persons at the highest risk of becoming infected with *Mycobacterium tuberculosis* are close contacts, the persons who had prolonged, frequent, or intense contact with a person with infectious TB. Close contacts may be family members, roommates, friends, coworkers, or others. Data collected by CDC since 1987 show that infection rates have been relatively stable, ranging form 21-23% for the contacts of infectious TB patients 6-9. Some people with infection develop TB disease. This disease develops when the immune system cannot keep the tubercle bacilli under control and the bacilli begin to multiply rapidly. The risk that TB disease will develop is higher for some people than for others 3, 10-12. Among contacts of persons with drug-resistant TB, infection rates seem to be similar. However, because they may have a poor response to treatment persons with drug-resistant disease are often infectious for longer periods and therefore have the potential to infect more contacts 10-13.

Extra pulmonary TB is rarely contagious; however, transmission from extrapulmonary sites has been reported during aerosol-producing procedures, such as autopsies and tissue irrigation 14-16.

#### **3. Pathogenesis**

The tubercle bacilli that alveoli are ingested by alveolar macrophages, the majority of these bacilli are destroyed or inhibited. A small number multiply intracellulary and are released

immunocompetent people infected with *M. tuberculosis* will fall sick in their lifetimes, among those with HIV infection, one in ten per year will develop active TB. In developing countries, the impact of HIV infection on the TB situation, especially in the 20-35 age groups, is overwhelming. While wealthy industrialized countries with good public health care systems can be expected to keep TB under control, in much of the developing world a catastrophe awaits. In poorly developed countries, TB remains a significant threat to public health, as incidences remain high, even after the introduction of vaccination and drug treatment. The registered number of new cases of TB worldwide roughly correlates with economic conditions: highest incidences are seen in the countries of Africa, Asia, and Latin America with the lowest gross national products. Supervised treatment, including sometimes direct observation of therapy (DOT), was proposed as a means of helping patients to take their drugs regularly and complete treatment, thus achieving cure and

TB is spread from person through the air. When a person with pulmonary or laryngeal TB coughs, sneezes, speaks, or sings, droplet nuclei containing *Mycobacterium tuberculosis* are expelled into the air. Depending on the environment, these tiny particles (1-5 microns in diameter) can remain suspended in the air for several hours. If another person inhales air containing droplet nuclei, transmission may occur. The probability that TB will be

The infectiousness of the person with TB (the number of organisms expelled into the air).

The best way to stop transmission is to isolate patients with infectious TB immediately and start effective TB therapy. Infectiousness declines rapidly after adequate therapy is started, as long as the patient adheres to the prescribed regimen. Persons at the highest risk of becoming infected with *Mycobacterium tuberculosis* are close contacts, the persons who had prolonged, frequent, or intense contact with a person with infectious TB. Close contacts may be family members, roommates, friends, coworkers, or others. Data collected by CDC since 1987 show that infection rates have been relatively stable, ranging form 21-23% for the contacts of infectious TB patients 6-9. Some people with infection develop TB disease. This disease develops when the immune system cannot keep the tubercle bacilli under control and the bacilli begin to multiply rapidly. The risk that TB disease will develop is higher for some people than for others 3, 10-12. Among contacts of persons with drug-resistant TB, infection rates seem to be similar. However, because they may have a poor response to treatment persons with drug-resistant disease are often infectious for longer periods and

Extra pulmonary TB is rarely contagious; however, transmission from extrapulmonary sites has been reported during aerosol-producing procedures, such as autopsies and tissue

The tubercle bacilli that alveoli are ingested by alveolar macrophages, the majority of these bacilli are destroyed or inhibited. A small number multiply intracellulary and are released

preventing the development of drug resistance 5-7.

**2. Transmission and pathogenesis** 

transmitted depends on these factors 4, 5:

The environment in which exposure occurred.

therefore have the potential to infect more contacts 10-13.

irrigation 14-16.

**3. Pathogenesis** 

The duration of exposure and the virulence of the organism.

when the macrophages die. These bacilli can spread through the lymphatic channels to regional lymph nodes and then through the bloodstream to more distant tissues and organs, including areas in which TB disease is most likely to develop: the apices of the lung, the kidneys, the brain, and bone. Extracellular bacilli attract macrophages form the bloodstream. The immune response kills most of the bacilli, leading to the formation of a granuloma. At this point the person has TB infection, which can be detected by using the tuberculin skin test. It may take 2-10 weeks for the infected person to develop a positive reaction to the tuberculin skin test. Immune responses soon develop to kill the bacilli. Within 2-10 weeks after infection, the immune system is usually able to halt the multiplication of the tubercle bacilli, preventing further spread 4, 17-19.

In persons infected with *Mycobacterium tuberculosis* but that don't have TB disease cannot spread the infection to other people. TB infection in persons who does not have TB disease is not considered a case of TB and referred to as ¨latent TB infection¨. In some persons, TB bacilli overcome the defenses or the immune system and begin to multiply, resulting in the progression from TB infection to TB disease. This process may occur soon after or many years after infection. Some study demonstrated that approximately 5% of person who have been infected with *Mycobacterium tuberculosis* will develop TB disease in the first year or two after infection and another 5% will develop disease some time later in life. Recent infection (with the past 2 years) with *Mycobacterium tuberculosis* is therefore an important risk factor for progression to TB disease and in approximately 10% of persons with normal immune system who are infected with *Mycobacterium tuberculosis* , TB disease will develop at some point 5-7.

Some medical conditions increase the risk that TB infection will progress to disease. Some studies suggest that the risk is mayor in inmmunosuppressed patients, for example persons with Diabetes mellitus, prolonged therapy with corticosteroids, immunosuppressive therapy, certain types of cancer, severe kidney disease, injection of illicit drugs, and infection with Human Immunodeficient Virus (HIV) 2,3,12.

TB disease most commomly affects the lung, 73% of TB cases are exclusively pulmonary, and however, TB is a systemic disease and may also commonly occur in the following ways;as pleural effusion in the central nervous, lymphatic, or genitourinary systems, as disseminated disease (military TB). Also the infection for *Mycobacterium tuberculosis* can occur in the other body sites; in the breast, skin, or peritoneum 16,20-23. Extrapulmonary TB is more common in immunosuppressed persons and in young children; meningoencephalitis TB, lymphatic TB and military disease are particularly common in immunosuppressed persons, in some case the extrapulmonary TB is often accompanied by pulmonary TB 3, 8, 16-23**.** 
