**2. Global epidemiology of MDR-TB**

#### **2.1. Global epidemiology of MDR-TB (global tuberculosis control: WHO report 2011)**

Globally, around 50 000 cases of MDR-TB were notified to WHO in 2010, mostly by Eu‐ ropean countries and South Africa. This represented 18% of the 290 000 (range, 210 000– 380 000) cases of MDR-TB estimated to exist among patients with pulmonary TB who were notified in 2010. The proportion of TB patients estimated to have MDR-TB that were actually diagnosed was under 10% in all of the 27 high MDR-TB countries outside the European Region, with the notable exception of South Africa where 81% of estimated cases were diagnosed. In, 15 high MDR-TB burden countries in the European Region, the proportion of estimated cases that were diagnosed ranged from 24% (in Tajikistan) to over 90% of cases (in Belarus and Kazakhstan); no data were reported from Lithuania. In Russian Federation, which ranks third in terms of estimated numbers of cases of MDR-TB at the global level, the proportion of estimated cases that were diagnosed was 44% in 2010. The numbers of patients diagnosed with MDR-TB and started on treatment with recommended second-line drug regimens in the high MDR-TB burden countries in 2010, at just under 40 000, was less than the number of cases notified [6]

#### **2.2. Regimen surveys and definitions of patients registration groups for treatment of tuberculosis**

'Regimen surveys' measure first-line and/or second-line drug resistance among a group of selected patients that cannot be considered representative of a patient population [7]. These surveys help to determine the predominant patterns of drug resistance, and are useful in providing guidance on appropriate regimens for MDR-TB treatment for particular patient groups. These include return cases after treatment failure, chronic cases and symptomatic contacts of MDR-TB cases. According to WHO, Regimen surveys should be conducted in the process of developing MDR-TB treatment programmes, or within selected centres or diagnos‐ tic units that regularly address high-risk cases.

The fourth edition of WHO *Guidelines for treatment of tuberculosis* defines patient registration groups by history of previous treatment [8]. [For a comprehensive study on definitions, please refer the document WHO/HTM/TB/2009.420].

### *2.2.1. New case*

study to the control of health problems" [4]. Epidemiologists are concerned not only with death, illness and disability, but also with more positive health states and, most impor‐ tantly, with the means to improve health [5]. Epidemiological studies are classified as ei‐ ther observational or experimental. Various methods can be used to carry out epidemiological investigations: surveillance and descriptive studies are used to study dis‐

The two mostly common terms used in epidemiology are the 'prevalence' and the 'incidence'. The incidence of disease represents the rate of occurrence of new cases arising in a given period in a specified population, while prevalence is the frequency of existing cases in a defined population at a given point in time [5]. These are fundamentally different ways of measuring occurrence, and the relation between incidence and prevalence varies among diseases [5]. [For a comprehensive study on epidemiology, please refer the World Health Organization (WHO)

Despite the availability of highly efficacious treatment for decades, TB remains a major global health problem. In 1993, WHO declared TB a global public health emergency, at a time when an estimated 7–8 million cases and 1.3–1.6 million deaths occurred each year. In 2010, there were an estimated 8.5–9.2 million cases and 1.2–1.5 million deaths from TB [6]. According to the newest report, has observed a gradual decline in the absolute number of TB cases since

**2.1. Global epidemiology of MDR-TB (global tuberculosis control: WHO report 2011)**

at just under 40 000, was less than the number of cases notified [6]

Globally, around 50 000 cases of MDR-TB were notified to WHO in 2010, mostly by Eu‐ ropean countries and South Africa. This represented 18% of the 290 000 (range, 210 000– 380 000) cases of MDR-TB estimated to exist among patients with pulmonary TB who were notified in 2010. The proportion of TB patients estimated to have MDR-TB that were actually diagnosed was under 10% in all of the 27 high MDR-TB countries outside the European Region, with the notable exception of South Africa where 81% of estimated cases were diagnosed. In, 15 high MDR-TB burden countries in the European Region, the proportion of estimated cases that were diagnosed ranged from 24% (in Tajikistan) to over 90% of cases (in Belarus and Kazakhstan); no data were reported from Lithuania. In Russian Federation, which ranks third in terms of estimated numbers of cases of MDR-TB at the global level, the proportion of estimated cases that were diagnosed was 44% in 2010. The numbers of patients diagnosed with MDR-TB and started on treatment with recommended second-line drug regimens in the high MDR-TB burden countries in 2010,

tribution while analytical studies are used to study determinants.

manual on Basic Epidemiology].

184 Tuberculosis - Current Issues in Diagnosis and Management

**1.2. Epidemiology of tuberculosis**

2006 and also in the incidence rates of TB since 2002 [6].

**2. Global epidemiology of MDR-TB**

For the purpose of surveillance, a 'new case' is defined as a newly registered episode of TB in a patient who, in response to direct questioning denies having had any prior anti-tuberculosis treatment (for up to one month), and in countries where adequate documentation is available, for whom there is no evidence of such history. Determining the proportion of drug resistance among new cases is vital in the assessment of recent transmission.

#### *2.2.2. Previously treated case*

For the purpose of surveillance, a 'previously treated case' is defined as a newly registered episode of TB in a patient who, in response to direct questioning admits having been treated for TB for one month or more, or, in countries where adequate documentation is available, there is evidence of such history.

#### *2.2.3. Primary resistance*

Patients with TB resistant to one or more anti-tuberculosis drugs, but who have never been previously treated for TB, are said to have "primary resistance" (or "initial resistance") due to transmission of a drug-resistant strain.

### *2.2.4. Acquired resistance*

Patients diagnosed with TB who start anti-tuberculosis treatment and subsequently acquire resistance to one or more of the drugs used during the treatment, are said to have developed "acquired resistance". In the past, resistance among previously treated cases (defined as cases with ≥ one month history of treatment) was used as a proxy for acquired resistance; however, this patient category is now known to also be comprised of patients who have been re-infected with a resistant strain, and patients who were primarily infected with a resistant strain and subsequently failed therapy or relapsed.
