TB Most Commonly Affects The Lungs Biology Essay


Tuberculosis (TB) is an infectious disease caused by bacteria whose scientific name is Mycobacteruim tuberculosis.The importance of the disease has been compounded by the emergence of AIDS and the appearance of multiple drug-resistant (MDR) strains of M. Tuberculosis. Mycobacteria are obligate aerobic, acid-fast rods.

TB most commonly affects the lungs but also can involve almost any organ of the body. Many years ago, this disease was referred to as "consumption" because without effective treatment, these patients often would waste away. Today, of course, tuberculosis usually can be treated successfully with antibiotics.

There is a vaccine against Tb called Bacille Calmette Guérin, also known as BCG, which is a vaccine given throughout many parts of the world.

-TB is one of the most serious infectious disease of the resource poor world. It is persistent wherever poverty, malnutrition and poor housing prevail. It affects the apparently healthy as well as being a serious disease of the immunocompromised, as it is particularly with patients with HIV/AIDS. TB is primarily a disease of the lungs, but may spread to other sites . ( Mims page 245)

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ii. Transmission

M. tuberculosis causes disease in healthy individuals and is transmitted man-man in airborne droplets. A person can become infected with tuberculosis bacteria when he or she inhales minute particles of infected sputum from the air. The bacteria get into the air when someone who has a tuberculosis lung infection coughs, sneezes, shouts, or spits (which is common in some cultures). People who are nearby can then possibly breathe the bacteria into their lungs.

-TB is acquired by inhalation of Mycobacterium tuberculosis in aerosols and dust. Air-borne transmission of TB is every effective because infected people cough up enormous numbers of mycobacterial into the environment, where the waxy outer coat allows them to withstand drying and therefore, survive for long periods of time in air and house dust. (Mims page 245)

iii. Pathogenesis

The organism primarily infects the lung, but is distributed systemically within macrophages and survives intra-cellularly by inhibition of phagosome-lysosome fusion. Cell mediated immunity develops which causes infiltration of macrophages and lymphocytes with development of granulomas (tubercles)..

Other pathogenesis factors (of considerably less importance than delayed hypersensitivity) include mycobactin (a siderophore) and cord factor that damages mitochondria. When the inhaled tuberculosis bacteria enter the lungs, they can multiply and cause a local lung infection (pneumonia). The local lymph nodes associated with the lungs may also become involved with the infection and usually become enlarged. The hilar lymph nodes (the lymph nodes adjacent to the heart in the central part of the chest) are often involved.

In addition, TB can spread to other parts of the body. The body's immune (defense) system, however, can fight off the infection and stop the bacteria from spreading. The immune system does so ultimately by forming scar tissue around the TB bacteria and isolating it from the rest of the body. Tuberculosis that occurs after initial exposure to the bacteria is often referred to as primary TB. If the body is able to form scar tissue (fibrosis) around the TB bacteria, then the infection is contained in an inactive state. Such an individual typically has no symptoms and cannot spread TB to other people. The scar tissue and lymph nodes may eventually harden, like stone, due to the process of calcification of the scars (deposition of calcium from the bloodstream in the scar tissue). These scars often appear on X-rays and imaging studies like round marbles and are referred to as a granuloma. If these scars do not show any evidence of calcium on X-ray, they can be difficult to distinguish from cancer Sometimes, however, the body's immune system becomes weakened, and the TB bacteria break through the scar tissue and can cause active disease, referred to as reactivation tuberculosis or secondary TB. For example, the immune system can be weakened by old age, the development of another infection or a cancer, or certain medications such as cortisone, anticancer drugs, or certain other medications. The breakthrough of bacteria can result in a recurrence of the pneumonia and a spread of TB to other locations in the body. The kidneys, bone, and lining of the brain and spinal cord (meninges) are the most common sites affected by the spread of TB beyond the lungs.

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It may take many months from the time the infection initially gets into the lungs until symptoms develop. The usual symptoms that occur with an active TB infection are a generalized tiredness or fever, weakness, weight loss, and night sweats. If the infection in the lung worsens, then further symptoms can include coughing, chest pain, coughing up of sputum (material from the lungs) and/or blood, and shortness of breath. If the infection spreads beyond the lungs, the symptoms will depend upon the organs involved.

in TB primary infection which is infection in individuals encountering M. tuberculosis for the first time, the organisms are engulfed by the alveolar macrophages in which they can both survive and multiply. Primary TB is often mild and asymptomatic and in 90% of cases does not proceed further. But clinical manifestations develop in the remaining 10%. (Mims page 245)


iv. Laboratory diagnosis

The disease can be diagnosed by skin testing for delayed hypersensitivity with tuberculin (protein purified derivative, PPD). A positive test does not indicate active disease; merely exposure to the organism

The presence of acid-fast bacteria in sputum is a rapid test for tuberculosis. Subsequently, when cultured, M. tuberculosis will grow very slowly producing distinct non-pigmented colonies (2 weeks or more). M. tuberculosis can be differentiated from most other mycobacteria by the production of niacin. A rapid alternative to culture is polymerase chain amplification

Examination of the sputum on a slide (smear) under the microscope can show the presence of the tuberculosis-like bacteria. Bacteria of the Mycobacteriumfamily, including atypical mycobacteria, stain positive with special dyes and are referred to as acid-fast bacteria (AFB). A sample of the sputum also is usually taken and grown (cultured) in special incubators so that the tuberculosis bacteria can subsequently be identified as tuberculosis or atypical tuberculosis.

Several types of skin tests are used to screen for TB infection. These so-called tuberculin skin tests include the Tine test and the Mantoux test, also known as the PPD (purified protein derivative) test. In each of these tests, a small amount of purified extract from dead tuberculosis bacteria is injected under the skin. If a person is not infected with TB, then no reaction will occur at the site of the injection (a negative skin test). If a person is infected with tuberculosis, however, a raised and reddened area will occur around the site of the test injection. This reaction, a positive skin test, occurs about 48-72 hours after the injection. When only the skin test is positive, or evidence of prior TB is present on chest X-rays, the disease is referred to as "latent tuberculosis." This contrasts with active TB as described above, under symptoms.

A special test to diagnose TB called the PCR(polymerase chain reaction) detects the genetic material of the bacteria. This test is extremely sensitive (it detects minute amounts of the bacteria) and specific (it detects only the TB bacteria). One can usually get results from the PCR test within a few days