What is XDR-TB?

Frequently asked questions - XDR-TB

Source: WHO


1. What is XDR-TB?

XDR-TB is the abbreviation for extensively drug-resistant tuberculosis (TB). One in three people in the world is infected with dormant TB germs (i.e. TB bacteria). Only when the bacteria become active do people become ill with TB. Bacteria become active as a result of anything that can reduce the person’s immunity, such as HIV, advancing age, or some medical conditions. TB can usually be treated with a course of four standard, or first-line, anti-TB drugs. If these drugs are misused or mismanaged, multidrug-resistant TB (MDR-TB) can develop. MDR-TB takes longer to treat with second-line drugs, which are more expensive and have more side-effects. XDR-TB can develop when these second-line drugs are also misused or mismanaged and therefore also become ineffective. Because XDR-TB is resistant to first- and second-line drugs, treatment options are seriously limited. It is therefore vital that TB control is managed properly.



2. What is the medical definition of MDR-TB and XDR-TB?
MDR-TB, or multidrug-resistant TB, is a specific form of drug-resistant TB. It occurs when the TB bacteria are resistant to at least isoniazid and rifampicin, the two most powerful anti-TB drugs.
XDR-TB is TB that is resistant to any fluoroquinolone, and at least one of three injectable second-line drugs (capreomycin, kanamycin, and amikacin), in addition to MDR-TB. This definition of XDR-TB was agreed by the WHO Global Task Force on XDR-TB in October 2006.


3. How do people become infected with XDR-TB?
People who are ill with pulmonary TB (i.e. TB of the lungs, the site most commonly affected) are often infectious and can spread the disease by coughing, or sneezing, or simply talking, as this propels TB bacteria into the air. A person needs only to breathe in a small number of these germs to become infected (although only a small proportion of people will become infected with TB disease). Sometimes the bacteria are already drug resistant if they come from a person who already has drug-resistant TB. A second way of developing MDR-TB or XDR-TB is when a patient’s own TB develops resistance. This can occur when anti-TB drugs are misused or mismanaged. This happens when TB control programmes are poorly managed, for example when patients are not properly supported to complete their full course of treatment; when health-care providers prescribe the wrong treatment, or the wrong dose, or for too short a period of time; when the supply of drugs to the clinics dispensing drugs is erratic; or when the drugs are of poor quality.


4. How easily is XDR-TB spread?
There is probably no difference between the speed of transmission of XDR-TB and 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 together with the presence of people with a higher risk of being infected (such as those with HIV/AIDS). The risk of becoming infected increases the longer the time that a previously uninfected person spends in the same room as the infectious case. The risk of spread increases where there is a high concentration of TB bacteria, such as can occur in closed environments like overcrowded houses, hospitals or prisons. The risk will be further increased if ventilation is poor. The risk of spread will be reduced and eventually eliminated if infectious patients receive proper treatment.

5. Can XDR-TB be cured or treated?
Yes, in some cases. Several countries with good TB control programmes have shown that cure is possible for up to 30% of affected people. But successful outcomes also depend greatly on the extent of the drug resistance, the severity of the disease and whether the patient’s immune system is compromised. It is vital that clinicians caring for TB patients are aware of the possibility of drug resistance and have access to laboratories that can provide early and accurate diagnosis so that effective treatment is provided as soon as possible. Effective treatment requires that all six classes of second-line drugs are available to clinicians who have special expertise in treating such cases.

6. How common is XDR-TB?
We do not know at the moment, but XDR-TB is rare. However, WHO estimates that there were almost half a million cases of MDR-TB worldwide in 2004, and MDR-TB usually has to occur before XDR-TB arises. We also know that findings from the only global study carried out so far showed that in some places perhaps as many as 19% of MDR-TB cases were in fact XDR-TB, but this is likely to be uncommon. Wherever second-line drugs to treat MDR-TB are being misused, the possibility of XDR-TB exists. Research is being carried out urgently to find out more.

7. How can a person becoming infected with XDR-TB?
The majority of healthy people with normal immunity may never become ill with TB, unless they are heavily exposed to infectious cases who are not treated or who have been on treatment for less than about one week. Even then, 90% of people infected with TB bacteria never develop TB disease. This applies to XDR-TB as well as to “ordinary” TB. People with HIV infection, however, in close contact with a TB patient, are more likely to catch TB and fall ill. The TB patients whom they meet should be encouraged to follow good cough hygiene, for example, covering their mouths with a handkerchief when they cough, or even, in the early stages of treatment, using a surgical mask, especially in closed environments with poor ventilation. The risk of becoming infected with TB is very low outdoors in the open air. Overall, the chances of being infected with XDR-TB are even lower than with ordinary TB because cases of XDR-TB are still very rare.


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