Unit 3 – Overview of TB DiseaseUnit 3 – Overview of TB Disease

Botswana National Tuberculosis Programme Manual Training for Medical Officers

Slide 3-2Unit 3: Overview of TB Disease

Learning ObjectivesLearning 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

Slide 3-3Unit 3: Overview of TB Disease

What is Tuberculosis?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

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Characteristics of Characteristics of M. tuberculosis M. tuberculosis (1)(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

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Characteristics of Characteristics of M. tuberculosisM. tuberculosis (2) (2)

• Thick lipid cell wall

• Multiplies every 18 - 24 hours

• Can remain dormant for decades

• Aerobic

• Non-motile

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Pathogenesis of TBPathogenesis of TB

• Infection (implantation)

• Multiplication

• Dissemination (spread to other parts of the body)

• Cell-mediated immune response (healing)

• Reactivation

Slide 3-7Unit 3: Overview of TB Disease

DefinitionsDefinitions

• 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

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Infection (Implantation Stage)Infection (Implantation Stage)

Source: CDC, 2001

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Primary InfectionPrimary Infection

A person with primary infection may have:

• No symptoms

• Limited symptoms

• Progressive symptoms

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How TB is Spread (1)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)

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How TB is Spread (2)How TB is Spread (2)

© ITECH, 2006

• 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 the same air spaceSource: CDC, 2001

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Probability of TransmissionProbability 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

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Spread of TB to Other Parts of the Spread of TB to Other Parts of the Body (Dissemination Stage)Body (Dissemination Stage)

1. Pulmonary

2. Lymph nodes

3. Pleural Effusion

4. Miliary

5. Kidney

6. Spine

7. Meningitis© ITECH, 2006

Source: CDC, 2001

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Healing Stage:Healing Stage:Cell-Mediated Immune ResponseCell-Mediated Immune Response

• Process of halting the multiplication of the TB bacilli and preventing further spread

• This same response causes the skin test to be positive

© University of Alabama at Birmingham,

Department of Pathology

Granuloma

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Latent InfectionLatent 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

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Reactivation of TB (1)Reactivation of TB (1)

Source: CDC, 2001

Escape granuloma and enter the airway

Dormant bacteria can become active again

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Reactivation of TB (2)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

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QuestionQuestion

What is the difference between

TB infection and disease?

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InfectionInfection

• 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

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DiseaseDisease

• 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?

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Risk Factors for ProgressionRisk Factors for Progressionfrom TB Infection to TB Disease (1)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

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Risk Factors for ProgressionRisk Factors for Progressionfrom TB Infection to TB Disease (2)from TB Infection to TB Disease (2)

• Silicosis of lung

• Renal failure

• Diabetes Mellitus

• Lymphatic malignancies

• Immunosuppressive medications and treatments including prolonged corticosteroid therapy

• GastrectomySource: Merck, 2005

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Key Points (1)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

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Key Points (2)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