Tuberculosis

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Revision as of 04:38, 11 August 2008

Tuberculosis, or TB, is a contagious bacterial disease caused by mycobacteria mainly Mycobacterium tuberculosis and Mycobacterium bovis which most commonly affects the lungs.

Mycobacterium tuberculosis, along with M. bovis, M. africanum, and M. microti all cause the disease and are members of the tuberculosis species complex. Each member of the TB complex is pathogenic, but M. tuberculosis is pathogenic for humans while M. bovis is usually pathogenic for animals.

Contents 1 History 2 Physiology 3 Diagnosis 3.1 Symptoms of tuberculosis 3.2 Causes 3.3 Risk factors for TB 4 Treatment 5 Prevention

History

M tuberculosis continues to kill millions of people yearly worldwide.

• In 1995, 3 million deaths from TB occurred.
• Up to 8 million new cases of TB develop each year.
• More than 90% of these cases occur in developing nations that have poor resources and high numbers of people infected with HIV.
• In the United States, incidence of TB began to decline around 1900, because of improved living conditions.

TB cases have increased since 1985, most likely due to the increase in HIV. Tuberculosis continues to be a major health problem worldwide. In 1997, the World Health Organization (WHO) estimated that 32% of the global population was infected with TB bacteria.

• 7.9 million new cases of TB developed.
• 1.87 million people died of this disease.

With the spread of AIDS, tuberculosis continues to lay waste to large populations. The emergence of drug-resistant organisms threatens to make this disease once again incurable.

In 1993, the WHO declared tuberculosis a global emergency.

Physiology

Mycobacteria are Gram-positive (no outer cell membrane), non-motile, pleomorphic rods, related to the Actinomyces. Most Mycobacteria are found in habitats such as water or soil. However, a few are intracellular pathogens of animals and humans.

Tuberculosis complex organisms are:

• Obligate aerobes growing most successfully in tissues with a high oxygen content, such as the lungs.
• Facultative intracellular pathogens usually infecting mononuclear phagocytes (e.g. macrophages).
• Slow-growing with a generation time of 12 to 18 hours (c.f. 20-30 minutes for Escherichia coli).
• Hydrophobic with a high lipid content in the cell wall. Because the cells are hydrophobic and tend to clump together, they are impermeable to the usual stains, e.g. Gram's stain.
• Known as "acid-fast bacilli" because of their lipid-rich cell walls, which are relatively impermeable to various basic dyes unless the dyes are combined with phenol. Once stained, the cells resist decolorization with acidified organic solvents and are therefore called "acid-fast". (Other bacteria which also contain mycolic acids, such as Nocardia, can also exhibit this feature.)

Diagnosis

Symptoms of tuberculosis

• Fever
• Night-time sweating
• Loss of weight
• Persistent cough
• Constant tiredness
• Loss of appetite

About 15% of people may develop tuberculosis in an organ other than their lungs. About 25% of these people usually had known TB with inadequate treatment. The most common sites include the following:

• Lymph nodes
• Genitourinary tract
• Bone and joint sites
• Meninges
• The lining covering the outside of the gastrointestinal tract

Causes

All cases of TB are passed from person to person via droplets. When someone with TB infection coughs, sneezes, or talks, tiny droplets of saliva or mucus are expelled into the air, which could be inhaled by another person.

Once infectious particles reach the alveoli, small sacs in your lungs, another cell called the macrophage engulfs the TB bacteria.

Then the bacteria are transmitted to your lymph system and bloodstream and spread to other organs.

The bacteria further multiply in organs that have high oxygen pressures, such as the upper lobes of your lungs, your kidneys, bone marrow, and meninges—the membranelike coverings of your brain and spinal cord.

When the bacteria cause clinically detectable disease, you have TB.

People who have inhaled the TB bacteria, but in whom the disease is controlled are referred to as infected. They have no symptoms, frequently have a positive skin test, yet cannot transmit the disease to others.

Risk factors for TB

• HIV infection
• Low socioeconomic status
• Alcoholism
• Homelessness
• Crowded living conditions
• Diseases that weaken the immune system
• Migration from a country with a high number of cases
• Health care workers

Treatment

Standard therapy for active TB consists of a 6-month regimen. Treatment takes that long because the disease organisms grow very slowly and, unfortunately, also die very slowly. Doctors use multiple drugs to reduce the likelihood of resistant organisms emerging. Often the drugs will be changed or chosen based on the laboratory results. If doctors doubt that you are taking your medicine, they may have you come to the office for doses. Prescribing doses twice a week helps assure compliance.

The most common cause of treatment failure is people's failure to comply with the medical regimen. This may lead to the emergence of drug-resistant organisms. Another important aspect of tuberculosis treatment is public health.

Prevention

TB prevention and control takes two parallel approaches. In the first, people with TB and their contacts are identified and then treated. Identification of infections often involves testing high-risk groups for TB. In the second approach, children are vaccinated to protect them from TB. Unfortunately, no vaccine is available that provides reliable protection for adults. However, in tropical areas where the levels of other species of mycobacteria are high, exposure to nontuberculous mycobacteria gives some protection against TB.