Tuberculosis Flashcards
What is tuberculosis
Chronic communicable disease caused by Mycobacterium tuberculosis (MBT)
Describe MBT and how it is spread
MBT is a non-motile, rod-shaped oligate aerobe
Have long-chain fatty acids, complex waves and glycolipids in cell walls giving structural rigidity. Also means MBT not stained using gram stain
Transmitted by infected respiratory droplets
Describe the pathogenesis of TB
Avleolar macrophages phagocytose MTB in alveoli but are unable to kill them so macrophages initiate cell mediated immunity and activation of macrophages with enhanced ability to kill MTB
Ingestion of MTB by macrophages causes granulomatous reaction - spherical granuloma with central caseation/tubercle
Primary infection occurs on first exposure to TB with deposition of TB bacilli in alveoli followed by development of sub-pleural focus of tubercles - primary/Ghon’s focus
What is the pirmary complex
Primary complex consists of primary focus plus the draining hilar lymph nodes
(TB bacilli drain from primary focus into hilar lymph nodes)
After primary complex is formed, what happens and what factors determine the progression
After primary complex, most infections heal +/- calcification of primary complex
Some TB bacilli enter blood stream -> haematogenous spread occurs
Bacterial load and host immunity determine how the infection progresses
Describe latent tuberculosis
Where TB bacilli persist in host without causing disease
Patient remains well but potential for reactivation is present - depends on host factors
Characterised by +ve QuantiFERON test or +ve tuberculin skin test - show patient is infected
Name some reasons why latent TB may reactivate
Usually occurs when patients immune systems fail or wane
Diabetes
HIV
Immunosuppressive therapy
Prolonged corticosteroid therapy
Severe kidney disease/haemodialysis
Malnutrition
Old age
What are two tests used to determine whether a patient has ever been infected with TB and briefly describe the two
Interferon gamma release assay - based on ability of MTB antigens to stimulate host production of interferon gamma. These antigens are specific to MTB -> only get +ve result if infected with MTB
Skin test/tuberculin test - tuberculin/PPD is injected intradermally -> patient checked for hypersensitivity reaction 2-3 days. Induration indicates previous exposure to TB
What are the types of TB
Primary TB
Post primary TB - pulmonary and extra-pulmonary
Miliary TB
Describe primary TB
TB occuring when primary complex doesn’t heal but progresses to cause active TB
Usually asymptomatic as MTB does not cause enough damage to the lungs
Describe post-primary pulmonary TB
Caused by reactivation of latent TB. Often in upper lung zones. Can have:
- Cavity formation - softening and liquefaction of caseous material dischrges into the bronchus resulting in cavity formation.
- Haemorrhage - extension of caseous process into vessels in cavity wall
- Spread to involve rest of lung
- Pleural effusion
- Miliary TB
Describe miliary TB
Miliary TB occurs when there is rupture of the caseous pulmonary focus into blood vessels resulting in widespread dissemination of bacilli throughout body
Have formation of multiple miliary tuberculosis foci in lungs and other organs
Describe extra-pulmonary TB
Reactivation of latent TB in other sites other than the lungs
Includes: lymph nodes, bones, joints, CNS, GI tract, urinary tract, larynx, kidneys
More often in HIV-infected/immunosuppressed and young children
What are the symptoms of TB
Symptoms have a gradual onset over weeks or months
Tiredness, fever and malaise
Weight loss and/or anorexia
Night sweats
Cough - dry or productive of mucoid sputum
Breathlessness - if pleural effusion
What signs are found in TB
May have no signs even when CXR is abrnomal
May have crackles
Signs of cavitatoin, fibrosis or pleural effusion may be present
CXR - pulmonary shadowing. May be patchy solid lesions, cavitated solid lesions, streaky fibrosis flecks of calcification
