Phase 2 - Week 7 (MILE - Tuberculosis) Flashcards
Define tuberculosis
A bacterial infection caused by the bacterium Mycobacterium Tuberculosis. It results in the formation of ‘tubercules’ (granulomas), predominantly in the lungs.
Describe the staining of M. Tuberculosis
- Cannot be classified as Gram positive or negative - stains as either
- Contains peptidoglycan in its cell wall
- Resistant to many stains but is called an acid-fast bacterium as once stained it resists decolorisation by acids
Describe the structure of M. Tuberculosis and how this increases its virulence
- Typical bacteria structure - except cell wall which gives resistance
- Cell wall contains peptidoglycan and complex lipids composed of mycolic acids, cord factor and wax D
- Mycolic acids are strong hydrophobic alpha-branched lipids which form a lipid shell and prevent cationic proteins, lysosomes and oxygen radicals from phagocytic granules from attacking the mycobacterium
- High concentration of lipids in cell wall give resistance to antibiotic, acidic and alkaline chemicals, lysis by immune cells and phagocytosis
- Also gives ability to survive within macrophages
How is tuberculosis most commonly contracted?
Via inhalation of airborne droplets containing the bacteria
Describe the pathogenesis of primary tuberculosis
- Bacteria inhaled
- Bacteria inhabit alveoli
- Bacteria ingested by alveolar macrophages and transported to lymph nodes where adaptive immune response is triggered
- Body reacts to contain the infection within ‘tubercles’ or granulomas, which consist of epitheloid cells and giant cells
- Tissue within granulomas becomes necrotic and caseous
Describe the features of primary tuberculosis
- The presence of lesions and enlarged local lymph nodes indicates that primary tuberculosis has developed
- Can include bacillaemia and subsequent dessemination of infection to organs
List the common sites for organ dissemination in tuberculosis
- Bone
- Adrenal glands
- Kidneys
- Brain
What is the result of tuberculosis infection in organs?
Can lead to end-organ tuberculosis in which caeseous granulomatous destructive lesions form and can develop into a tumour-like mass of necrosis called tuberculomas.
How does secondary tuberculosis occur?
- Bacilli may stay latent within macrophages in the lung apex for decades
- Can reactivate to cause secondary tuberculosis
Describe secondary tuberculosis
- Necrotic tissue within the granulomas becomes liquified
- If granuloma is connected to an airway, debris will be coughed up, along with M. tuberculosis bacteria, allowing for further spread of disease
- This leaves a cavity in the lung, lined internally by granulomatous inflammation and externally by fibrotic tissue
- Lesion is called fibrocaeseous tuberculosis
- Can be spread of bacilli throughout the body to other organs during secondary tuberculosis, but to a lesser extent than primary tuberculosis
List the risk factors associated with tuberculosis
- Being in close contact with a person who has infectious (secondary) tuberculosis
- Homelessness
- Intravenous drug use
- Being infected with HIV
- Diabetes mellitus
- Severe kidney disease
- Head and neck cancer
- Malnutrition
- Corticosteroid use
- Treatments for rheumatoid arthritis or Crohn’s disease
Describe the symptoms of tuberculosis
Pulmonary symptoms:
- Cough lasting 3 weeks or longer
- Chest pain
- Haemoptysis
- Coughing up sputum
Other symptoms:
- Asthenia/fatigue
- Weight loss
- Loss of appetite
- Chills
- Fever
- Nocturnal hyperhidrosis
Latent tuberculosis is asymptomatic
Describe the diagnosis of tuberculosis
- Presence of clinical symptoms
- Characteristic changes on a chest X-ray
- Mantoux test
- Stained smears of sputum, lung tissue or CSF used to identify acid-fast bacteria under a microscope
- Cultures of M. tuberculosis grown to confirm diagnosis
- Does not differentiate between latent and active tuberculosis - latent tuberculosis is diagnosed if patient has positive tuberculosis test but medical evaluation does not indicate tuberculosis disease
Describe the characteristic changes seen on a chest X-ray in tuberculosis
- Consolidation of areas of the lungs
- Hilar enlargement due to lymphadenopathy
- Fibrotic scarring/calcification in healed primary or post-primary tuberculosis
Describe a Mantoux test
- Purified protein derivative (tuberculin) is injected intradermally
- Result is interpreted 2-3 days later
- Induration of less than 6mm = negative result
- Induration of more than 6mm = positive result
Positive result means patient has had previous exposure to M. tuberculosis, not necessarily that the patient has tuberculosis.