**1.1. Epidemiology**

John Last has defined epidemiology as "The study of the distribution and determinants of health-related states or events in specified populations, and the application of this

© 2013 Magana-Arachchi; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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‐ tribution while analytical studies are used to study determinants.

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

'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‐

Epidemiology of Multidrug Resistant Tuberculosis (MDR-TB)

http://dx.doi.org/10.5772/54882

185

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

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

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,

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

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

**tuberculosis**

*2.2.1. New case*

*2.2.2. Previously treated case*

*2.2.3. Primary resistance*

*2.2.4. Acquired resistance*

there is evidence of such history.

transmission of a drug-resistant strain.

subsequently failed therapy or relapsed.

tic units that regularly address high-risk cases.

refer the document WHO/HTM/TB/2009.420].

among new cases is vital in the assessment of recent transmission.

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) manual on Basic Epidemiology].

#### **1.2. Epidemiology of tuberculosis**

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