8 Respiratory Tract Infections: Tuberculosis Flashcards
(23 cards)
How does TB spread?
Through aerosol transmission
Name the organism that causes the tuberculosis condition
Mycobacterium tuberculosis
Describe the mycobacterium tuberculosis bacteria (in terms of structure and staining)
- It has a modified peptidoglycan layer (outmost) - primarily consisted of proteins
- Covalently attached to arabinogalactan polymer
- Mycolic acid waxy coat - lips rich
- Poor gram stain (high lipid content = less permeable to gram staining)
- Acid fast (Ziehl-Neelsen stain) - needed for staining
Describe the mycobacterium tuberculosis bacteria (in terms of features, transmission etc.)
- Obligate aerobes (need oxygen - obligated for oxygen - TB pass into the lung via air)
- TB causes more deaths worldwide than any other single infectious agent
- Facultative intracellular bacteria (can be fine outside host) - usually invading macrophages, dendritic cells
- Slow growing (generation time of 12 to 18 hours; 20-30 mins for E. coli)
- Disease course has insidious nature
Describe the primary pathogenesis of TB (active)
- Generally, affects upper lobes (lower upper or upper lower lobes) of lung
- Ghon focus (caseous necrosis)
- Ghon complex (caseous necrosis) in hilar lymph nodes
Usually resolved but can produce a calcified granuloma or area of scar tissue AND may be a nidus for a secondary TB (reactivated TB)
Describe the secondary pathogenesis of TB (reactivated)
Secondary (reactivation) TB due to reactivation of a previous primary TB site
Common in:
- Immunocompromised patients
- And patients receiving biological therapy (mAb)
Describe what is is meant by TB disease can be active or latent
Latent disease is defined as a state of persistent immune response to stimulation by Mycobacterium tuberculosis antigens, with no evidence of clinically active TB:
- i.e. person is asymptomatic + not infectious
There is a 5–10% risk of progression to active (symptomatic) disease:
- e.g. if the patient is immunocompromised or has intercurrent illness
Multiple longitudinal epidemiological studies indicate that majority of TB disease occurs soon after initial infections, with disease rarely occurring more than 2 years after infection
Describe the clinical course of TB
TB is an example of Type 4 HSR
- In most individuals, cell-mediated immunity (CMI) develops 2-8 weeks after infection (associate with the development of a positive tuberculin skin test)
- Activated T cells and macrophages form granulomas that limit further replication and spread
- Bacterial cells remain in centre of necrotic ‘caveating’ granulomas
- Most individuals are symptomatic (latent infection) and never develop active disease (unless a subsequent in CMI occurs) = +ve skin prick test
Describe some respiratory clinical findings in TB
- Cough
- Shortness of breath
- Haemoptysis
- Chest pain
Describe some general clinical findings in TB
- Fever
- Drenching night sweats
- Weight loss
Explain how a diagnosis of TB can be made
- Early stages of disease can be difficult to detect, leading to diagnostic delays/misdiagnosis
- Suspect active TB in any person who:
> is at high risk of developing TB
AND
> has general symptoms of weight loss, fever, night sweats, anorexia or malaise
Consider pulmonary involvement if the person has a persistent productive cough, which may be associated with breathlessness and haemoptysis - but exclude other causes also
- Extra-pulmonary involvement - involves target organ - and gives relevant symptoms
List some Pulmonary TB complications
Post-TB bronchiectasis, COPD and aspergillosis
- (fungus ball in lung - TB made space in which they exist in - residual lung cavities)
Post-TB corpulmonale (R-sided heart failure - pulmonary hypertension - fibrosis of lung) OR Respiratory failure (low O2 and low CO2)
DEATH
- in 2006, 5.5% people notified in England were reported to have died at the last recorded outcome, and TB is known to have contributed to 35.2% of deaths
Give some active extra-pulmonary complications of TB
- Miliary spread in lungs - invasion into:
> (Bronchus) > (lymphatics) - Miliary spread to extra-pulmonary sites (spread via pulmonary veins):
> Lymph nodes are a common site (firm, discrete, painless lymph nodes) - kidney
> Adrenal improvement may result in Addison disease
> Granulomatous hepatitis
> Spread to vertebra (Pott Disease)
Some some serious complications of TB
Most serious form is Central Nervous System disease:
- TB meningitis (especially children < 5 and HIV+)
- Space occupying lesions (tuberculomas)
Name two screening techniques done for TB
- Mantoux test
- Interferon Gamma Release Assay Test (IGRA)
Describe the Mantoux test as a screening tool for TB
Mantoux test
- Tuberculin (cell envelope protein) is injected intradermally
- Gives a firm red bump (local skin reaction)
- Test considered positive at induration of 5mm or more
Describe the Interferon Gamma Release Assay (IGRA) test as a screening tool for TB
IGRA test
- Blood test based on detecting the response of white blood cells to TB antigens
- Less likely to give a false positive result compared to Mantoux test
- Rapid result
Describe how a chest radiograph may be used in the testing for TB
The chest radiograph can be suggestive of TB, but not diagnostic:
Active: consolidation = opaque (most likely upper of mid zone)
> cavitating lesions, with or without calcifications
> Latent: nodules and fibrotic changes
Describe the steps are/management available after a test for latent TB is positive
If a test for latent TB infection is positive:
- The person should be assessed for active TB,
- and if there is no evidence of active infection on the basis of symptoms and chest X-ray,
- the person should be treated for latent infection by the local MDT TB specialist team to prevent progression to active disease
Drug regimens are usually either:
- 3 months of isoniazid (with pyridoxine) and rifampicin
OR - 6 months of isoniazid (with pyridoxine)
Describe what investigations can be carried out when testing for TB
- Bronchoalveolar lavage best for staining and culture (tissue can be cultured, urine, CSF)
- Sputum cultures (cat 3 pathogen must be handled in cat 3 lab)
- Culture is slow growing (2-8 weeks)
Describe what antibody sensitivity testing is and how it helps in testing for TB
Performed in reference lab
Agents include (class = antimycobacterial)
- Rifampicin (other indications)
- Isoniazid (just TB - on cell wall and inhibition of synthesis of mycelia acids)
- Pyrazinamide (Just TB, mechanism N/A)
- Ethambutamol (just TB, works on cell wall)
Important to detect resistance - especially multi-drug resistant (MDR TB)
- resistance to rifampicin and isoniazid
Who gets the BCG vaccine?
Bacillus Calmette-Guerin (BCG) -
- live attenuated M. Bovis strain
- Anyone who works or lives in an area with a high rate of TB (40/100,000 or more)
- A baby under 12 months OR previously unvaccinated should under 5
- Any child under 16 with specific risk factors for TB
- Anyone who works in close contact with TB bacteria
- Previously unvaccinated people going to live or work (in countries with high rate of TB)
Also:
- Co-morbid conditions
> HIV, diabetes M, history or organ transplant
- Immunosuppressive drugs
- Under-serves groups - more deprived etc
- History of excessive alcohol, injecting drug users and smokers
Describe the epidemiology of TB
- 1.7 billion people estimated to have latent TB in the world
(23% of the world)
