Introduction
Tuberculosis(TB) is among the most serious infectious causes of global morbidity and mortality. Nearly one third of the world's population is infected with Mycobacterium tuberculosis. Although in the past 10 years there has been substantial progress in the development and implementation of the strategies necessary for effective tuberculosis
control, the disease remains an enormous and growing health problem. Under the
current conditions, it is expected for this decade that 90 million people will
develop the disease and 30 million will die from TB.
The global concern in the treatment of tuberculosis is the emergence of drug resistant strains. Drug-resistant tuberculosis is TB disease caused by M. tuberculosis organisms that are resistant to at least one first-line anti-TB drug." Although the phenomenon of drug resistance in Mycobacterium tuberculosis was observed as early as 50 yr ago, the current threat is due to the emergence of strains resistant to the at least two most potent anti-TB drugs viz., isoniazid (H) and rifampicin (R) (multidrug resistant-
tuberculosis, MDR-TB).5 MDR-TB treatment requires the use of second-line drugs (SLDs)
that are less effective, more toxic, and costlier than first-line isoniazid- and
rifampin-based regimens.
Top
In the year 2000, the Stop TB Partnership's Green Light Committee was created to increase access to SLDs worldwide while ensuring their proper use to prevent increased drug resistance. While assisting MDR-TB treatment programs worldwide, the committee encountered reports of multiple cases of TB with resistance to even SLDs. Such cases in persons with TB whose isolates were resistant to both first-and second-line drugs were termed as extensively drug-resistant tuberculosis
(XDR-TB) cases.
Drug-resistant tuberculosis is therefore a serious emerging threat to public health. Since, treatment is difficult, and appropriate second-line drugs are not universally available, there is a raising concern that epidemics of MDR-TB and XDR-TB may jeopardize TB control efforts.
Top
Types of Drug Resistance
Drug resistance
may be of two types - Primary and Acquired.
- Primary drug resistance may be defined as drug resistance
in a patient who has not received any anti-tubercular treatment in the past.
- The resistance that develops in a patient who has received prior chemotherapy is
defined as acquired drug resistance.
Continuous surveillance of the primary and acquired drug resistance patterns of M. tuberculosis
is important in assessing the efficacy of chemotherapy regimens used in past
years as well as in detecting problems in past treatments.
Top
Recently the terms "resistance in new cases" and "resistance in previously treated cases" have been proposed for use because of the difficulty to confirm the validity of the patients' past history of treatment. When one is not sure whether the resistance is primary or acquired due to concealed history of previous treatment or unawareness of treatment taken before, it is known as initial
drug resistance.
Thus, initial resistance is primary resistance plus some undisclosed acquired
resistance. Combined resistance is defined as sum of primary and acquired
resistance.