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Unit 3 – Overview of TB Disease Botswana National Tuberculosis Programme Manual Training for Medical Officers Learning Objectives At the end of this unit, participants will be able to: • Describe the causes of TB • Describe how TB is transmitted • Describe the difference between TB infection and TB disease • Identify high risk populations and high risk behaviours • Describe the pathogenesis of TB Unit 3: Overview of TB Disease Slide 3-2 What is Tuberculosis? • TB is an illness caused by one of four (4) types of bacteria: • • • • Mycobacterium tuberculosis (M. tuberculosis) Mycobacterium bovis (M. bovis) Mycobacterium africanum (M. africanum) Mycobacterium microti (M. microti) • The majority of TB cases are caused by Mycobacterium tuberculosis • M. tuberculosis complex consists of all four bacteria Unit 3: Overview of TB Disease Slide 3-3 Characteristics of M. tuberculosis (1) • Slightly curved, rod shaped bacilli • 0.2 - 0.5 microns in diameter and 2 - 4 microns in length • Acid fast - resists decolourisation with acid/alcohol TB bacilli as seen under the microscope Source: Kubica GP, CDC, 1979 Unit 3: Overview of TB Disease Slide 3-4 Characteristics of M. tuberculosis (2) • • • • • Thick lipid cell wall Multiplies every 18 - 24 hours Can remain dormant for decades Aerobic Non-motile Unit 3: Overview of TB Disease Slide 3-5 Pathogenesis of TB • Infection (implantation) • Multiplication • Dissemination (spread to other parts of the body) • Cell-mediated immune response (healing) • Reactivation Unit 3: Overview of TB Disease Slide 3-6 Definitions • Alveoli • The small air sac at the end of the airways in the lungs • Macrophage • A type of white blood cell that eats bacteria or foreign organisms, found in the alveoli of the lungs • Granuloma • Nodular inflammatory lesions that contain compactly grouped mononuclear phagocytes (macrophages) • Tuberculous granulomas are often caseating (necrotic at their center) and contain Langerhans giant cells • AFB can often be seen on staining Unit 3: Overview of TB Disease Slide 3-7 Infection (Implantation Stage) Source: CDC, 2001 Unit 3: Overview of TB Disease Slide 3-8 Primary Infection A person with primary infection may have: • No symptoms • Limited symptoms • Progressive symptoms Unit 3: Overview of TB Disease Slide 3-9 How TB is Spread (1) • Person-to-person • Through the air by a person with TB disease of the lungs • Less frequently transmitted by ingestion of Mycobacterium bovis found in unpasteurised milk products • Rarely transmitted by inhalation of aerosolized infected fluids (e.g., TB abscess) Unit 3: Overview of TB Disease Slide 3-10 How TB is Spread (2) • A person with infectious pulmonary TB (PTB) who coughs, sneezes, or speaks • Tiny particles of water (droplet nuclei) containing the TB bacteria enter the air and can remain suspended in the air for several hours • The bacteria can then be inhaled by others sharing © ITECH, 2006 the same air space Source: CDC, 2001 Unit 3: Overview of TB Disease Slide 3-11 Probability of Transmission 1. Environment in which exposure occurred 2. Infectiousness of person with TB 3. Immunologic status of exposed person 4. Duration of exposure 5. Virulence of the organism Unit 3: Overview of TB Disease Slide 3-12 Spread of TB to Other Parts of the Body (Dissemination Stage) Source: CDC, 2001 © ITECH, 2006 Unit 3: Overview of TB Disease 1. 2. 3. 4. 5. 6. 7. Pulmonary Lymph nodes Pleural Effusion Miliary Kidney Spine Meningitis Slide 3-13 Healing Stage: Cell-Mediated Immune Response © University of Alabama at Birmingham, Department of Pathology • Process of halting the multiplication of the TB bacilli and preventing further spread • This same response causes the skin test to be positive Granuloma Unit 3: Overview of TB Disease Slide 3-14 Latent Infection • Period of time during which the person experiences no symptoms but is still infected with the bacteria • The bacteria lives inside macrophages or within a granuloma where the bacteria remains dormant • A person with latent TB infection cannot spread the bacteria to other people Unit 3: Overview of TB Disease Slide 3-15 Reactivation of TB (1) Dormant bacteria can become active again Source: CDC, 2001 Unit 3: Overview of TB Disease Escape granuloma and enter the airway Slide 3-16 Reactivation of TB (2) • Latent infection can reactivate, causing active TB disease • Reactivation occurs when the immune system weakens and the TB bacteria multiplies • TB bacteria and dead cells in the airway will cause a person to cough • Higher proportion of smear-negative PTB in PLWHA Unit 3: Overview of TB Disease Slide 3-17 Question What is the difference between TB infection and disease? Unit 3: Overview of TB Disease Slide 3-18 Infection • The bacteria lives inside a person without that person having any symptoms because the immune system is able to control the infection • Not all infected people develop TB disease • In HIV negative individuals, 10% lifetime risk of developing disease if TB infected • In HIV positive individuals, 10% ANNUAL risk of developing disease if TB infected Unit 3: Overview of TB Disease Slide 3-19 Disease • If the immune system cannot control the infection the bacteria multiply and cause disease • TB disease can develop soon after infection, many years after infection or it may never develop What is one factor, besides HIV, that might increase the chance that TB infection progresses to disease? Unit 3: Overview of TB Disease Slide 3-20 Risk Factors for Progression from TB Infection to TB Disease (1) • • • • HIV/AIDS Malnutrition Recent TB infection Extremes of age (children under 5 years of age and the very old) • Alcoholism and other substance abuse Unit 3: Overview of TB Disease Slide 3-21 Risk Factors for Progression from TB Infection to TB Disease (2) • • • • • Silicosis of lung Renal failure Diabetes Mellitus Lymphatic malignancies Immunosuppressive medications and treatments including prolonged corticosteroid therapy • Gastrectomy Unit 3: Overview of TB Disease Source: Merck, 2005 Slide 3-22 Key Points (1) • • • • • Evidence of TB has been seen to be affecting humans for centuries TB transmission occurs from persons with active pulmonary TB TB droplets remain suspended in the air for hours The bacteria can be killed by direct sunlight Ventilation is important Unit 3: Overview of TB Disease Slide 3-23 Key Points (2) • When TB is first acquired, it causes primary infection • Persons with poor immunity, especially very young children and persons with AIDS, are more likely to have primary progressive disease • Latent infection-- period of time when person experiences no symptoms but is still infected with the bacteria Unit 3: Overview of TB Disease Slide 3-24