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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:
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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)
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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)
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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)
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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)
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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