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Extensively Drug-Resistant Tuberculosis (Xdr-Tb): The Facts
Article Date: 29 Mar 2007 - 0:00 PDT
What is XDR-TB?
TB can usually be treated with a course of four standard, or first-line, anti-TB drugs. If these are misused or mismanaged, multidrugresistant TB (MDR-T can develop. MDR-TB takes longer to treat with second-line drugs, which are more expensive and have more side-effects. If these drugs are also misused or mismanaged, extensively drug-resistant TB (XDR-T can develop. Because XDR-TB is resistant to first- and second-line drugs, treatment options are seriously limited and so are the chances of cure.
What is the medical definition of MDR-TB and XDR-TB?
MDR-TB is due to bacteria that are resistant to at least isoniazid and rifampicin, the two most powerful first-line anti-TB drugs. XDR-TB is due to bacteria that are resistant to any fluoroquinolone, and at least one of three injectable second-line drugs (capreomycin, kanamycin and amikacin), in addition to isoniazid and rifampicin. This is a revised definition of XDR-TB, on which the WHO GlobalTask Force on XDR-TB agreed in October 2006.
How do people develop XDR-TB?
People who are ill with pulmonary TB (TB of the lungs, the site most commonly affected) are often infectious and can spread the disease by coughing, sneezing or simply talking, as these acts propel TB bacteria into the air. Another person breathing in these bacteria may become infected with TB but without disease; only the TB skin test becomes positive. If the bacteria overcome the body's immune system, the person becomes ill with TB. A person ill with TB develops XDR-TB when first- and second-line anti-TB drugs are misused or mismanaged during the course of treatment and become ineffective (that is, when drugs are taken in the wrong combination, are fewer than those prescribed or taken in insufficient doses or insufficient time). People with XDR-TB can be infectious and pass the drug-resistant bacteria to other people.
How easily is XDR-TB spread?
There is probably no difference between the speed of transmission of XDR-TB and those of any other forms of TB. The spread of TB bacteria depends on factors such as the number and concentration of infectious people in any one place and the time of exposure, along with the presence of people with a higher risk of being infected, such as those with HIV/AIDS.
Can XDR-TB be cured or treated?
Several countries with good TB control programmes have shown that up to 50-60% of affected people can be cured. Nevertheless, successful treatment also depends greatly on the extent of the drug resistance, the severity of the disease and whether the patient's immune system is compromised.
Can vaccination prevent XDR-TB?
The TB vaccine, called the bacille Calmette- Guérin (BCG) vaccine, prevents severe forms of TB in children, such as TB meningitis. BCG would be expected to have the same effect in preventing severe forms of TB in children, even if they were exposed to XDR-TB, but it may be less effective in preventing TB in adults. New vaccines are urgently needed, and WHO and members of the Stop TB Partnership are actively working on them.
How do I know if I have TB or XDR-TB?
Symptoms of XDR-TB are no different from those of ordinary or drug-susceptible TB:
- a cough with thick, cloudy mucus (or sputum), sometimes with blood, for more than 2 weeks;
- fever, chills and night sweats;
- fatigue and muscle weakness;
- weight loss; and
- in some cases, shortness of breath and chest pain.
If you have these symptoms, you do not necessarily have XDR-TB, but you must see a doctor for a check-up. If you are already being treated for TB and at least some of these symptoms are not improving after a few weeks of treatment, you should inform your clinician or nurse.
How quickly can XDR-TB be diagnosed?
This depends on the patient's access to health care services. If TB bacteria are found in the sputum, TB can be diagnosed in a day or two, but this finding will not be able to distinguish between drug-susceptible and drug-resistant forms. To evaluate drug susceptibility, the bacteria need to be cultivated and tested in a suitable laboratory. Such a final diagnosis for TB, and especially XDR-TB, may take 6-16 weeks. To reduce this period, new tools for rapid TB diagnosis are urgently needed.
How can a person with drug-sensitive TB avoid getting XDR-TB?
The most important thing is to continue taking all treatment exactly as prescribed. No doses should be missed and treatment should be taken right through to the end. If patients suffer from side-effects - for example, the tablets make them feel sick - they should inform their clinicians or nurses, because simple solutions are often available. If they need to travel for any reason, patients should make sure they have enough tablets with them for the duration of the trip.
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作者:admin@医学,生命科学 2011-03-14 17:11
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