Clinical Perspective Tuberculosis (TB) is caused by Mycobacterium tuberculosis (M. TB. ) and is the leading cause of death in the world from a single infectious disease. Although very little is known of the mechanisms of it pathogenesis and how to protect a person from the disease. There was a decline in TB in the United States in the last century but the disease is now increasing and there are now multiple drug resistant strains that have emerged.
This increase has multiple causes some of which are changes in social structure in cities, HIV epidemic, and a failure by some public entities to improve their public health programs. In 2002 M. TB. is responsible for more morbidity in humans than any other bacterial disease. M.
TB. infects 1. 7 billion people per year which is equal to 33% of the entire world population and is responsible for over 3 million deaths / year . (Todar’s 2002) In the United States since 1985 there has been an increase in cases of M. TB.
this is due largely to the increase of Human Immunodeficiency virus (HIV) infections which occurred during the same period of time. More recently there has also been an increase in the number of cases of multi drug resistant (MDR) strains of M. TB. due to patient non compliance with medication. Mycobacterium tuberculosis is the cause of TB in humans. Humans are the only known reservoir for the bacterium.
Mycobacterium bovis is the etiologic agent of TB in cows and rarely in humans, but both cows and humans can serve as reservoirs. Humans can also be infected by consuming unpasteurized milk. This method of transmission can cause the development of extra pulmonary TB, as seen in history by bone infections that caused hunched backs. (Todar’s 2002).
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Mycobacterium tuberculosis is a large n nonmotile rod shaped bacterium. The rods are 2-4 microns in length and 0. 2-0. 5 microns in width. Many non pathogenic mycobacterium compose the normal flora of humans most commonly found in dry or oily locales. M.
TB. is not classified as gram negative or gram positive because it does not have the chemical characteristics, although if a gram stain is performed it is considered weakly gram positive or not at all. M. TB. is classified as acid fast bacilli due to the impermeability of certain dyes and stains (Todar’s 2002) Tuberculosis (TB) has two forms latent infection and disease. In the infection form of TB the immune system is able to keep the bacteria under control.
The body accomplishes this by producing macrophages the surround the tubercle bacilli and forming a hard shell, thereby isolating the M. TB. and keeping it under control. There are TB bacilli in the body, with the number of organisms from 103-104, the TB skin test is usually positive, but the chest x-ray is normal; and a sputum smear for acid fast bacilli (AFB) is negative, the patient has no signs or symptoms of being infected TB and is not infectious.
In a person with the latent infection form of M. TB. there will be a positive TB skin test with no other symptoms present. The chest X-ray will be normal and a sputum test for acid fast bacilli will be negative. A person with this form cannot spread M. TB.
to others. A person with M. TB. disease will have a cough that lasts longer than 2 weeks, pain in the chest, cough up blood or sputum. They will also experience fatigue, anorexia, weight loss, chills, fever and night sweats. They are also contagious and will have a positive skin test and an abnormal chest x-ray and sputum smear will be positive for acid fast bacilli.
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Skin testing is performed as the tuberculin or Mantoux test. Purified protein derivative (PPD) is used at the test antigen in the Mantoux test. PPD is generated by boiling a culture of M.
TB. 5 Tuberculin units (TU), is injected subcutaneously into the forearm. The test is read within 48-72 hours. The test is considered positive if the is a lesion of 10 mm or greater.
The lesion is characterized by erythema (redness), swelling and indurated (raised and hard).
90% of people that have a lesion of 10 mm or greater are currently infected with M. TB. or have a previous exposure.
100% of people that have a lesion of 15 mm or greater are currently infected with M. TB. or have been previously exposed to M. TB. A false positive test will usually have a lesser reaction. This lesser reaction could indicate prior exposure or infection with other Mycobacterium or vaccination with BCG.
In places the vaccine is not used a lesser reaction should be highly suspicious. (Todor’s 2002).
False negatives are rarer than false positives but are common in acquired immune deficiency syndrome (AIDS) patients because of an impaired cell mediated response. There are other conditions such as malnutrition and steroids that can result in a false negative reaction although rare. Treatment of M. TB.
is generally treated with four different antimicrobial drugs. This is because the administration of only a single drug contributes to the development of bacterial resistant organisms. When two or more drugs are used for treatment simultaneously each helps prevent the emergence of the tubercle bacilli resistant to the others. At the beginning of drug therapy when it is unknown which drugs the patient’s bacilli is susceptible to then choosing the wrong drug can lead to further development of drug resistant M. TB. Therefore M.
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TB. is treated with four different antimicrobial agents. (CDC 2001).
The drug therapy usually lasts from 6-9 months.
The most commonly used drugs are rifampin, isoniazid, pyrazinamide, and ethambutol or streptomycin. When a patient adheres to this drug regimen it is highly effective. Based on the prev elance and characteristics of the drug resistant organisms present 95% of patients will receive an adequate regimen of at least two drugs to which the bacilli is susceptible, if this four drug regimen is used at the beginning of therapy. A patient that is treated with a four drug regimen but who is non compliant with the treatment is more likely to be cured and not relapse when compared with a patient treated for the same length of time with a three drug regimen. (NIH 2001).
There is also a vaccine available for M. TB. it is called Bacillus of Calmette and Guerin (BCG), after the two Frenchmen that developed it. It consists of a live attenuated strain derived from mycobacterium bovis. The vaccine is not 100% effective and there are studies that suggest the rate of effectiveness in children is 60-80% (Todor’s 2002).
The vaccine is not administered in the United States for several reasons.
The vaccine does not prevent infection only the disease. The vaccine cannot prevent disease reactivation in previously exposed individuals. The vaccine may also complicate the way the TB skin test is read in this country.