Session 9: Tuberculosis Flashcards by Don J

What is TB caused by?

Mycobacterium tuberculosis

How well did you know this?

Not at all

Perfectly

Virulence factors of Mycobacterium tuberculosis.

Non-motile rod-shaped bacteria

Obligate aerobe

Long-chain fatty acids

Glycolipids in cell wall

This offers structural rigidity, staining characteristics, and acid alcohol fast.

How well did you know this?

Not at all

Perfectly

Generation time of TB (Division)

15-20 hrs

How well did you know this?

Not at all

Perfectly

Transmission of TB

Person to person via infected droplets

(Cough, sneeze)

How well did you know this?

Not at all

Perfectly

Risk factors of TB.

Non-UK born/recent migrants

HIV

Immunocompromised patients

Homeless

Drug users

Close contact

Young adults

How well did you know this?

Not at all

Perfectly

TB is contagious but not easily acquired. How is transmission risk increased?

By prolonged exposure (8 hours a day for up to 6 months)

How well did you know this?

Not at all

Perfectly

Incubation time for TB.

Can take up to 6 weeks.

How well did you know this?

Not at all

Perfectly

Explain the pathogenesis of TB.

Inhaled aerosols reach the Ghon’s focus (close to a fissure). Here macrophages phagocytose the bacterium. However the macrophages are unable to digest and kill the bacterium.

The bacterium can also reach lymphatics and reach lymph nodes. This is what happens in primary infection.

After the initial contact and the primary infection there are two outcomes.

How well did you know this?

Not at all

Perfectly

What are the outcome of the primary infection?

Latent infection (95%)

Progression to active disease (5%)

How well did you know this?

Not at all

Perfectly

What is the outcome of latent infection?

95% heals/self cure

5% get reactivation of their latent infection.

How well did you know this?

Not at all

Perfectly

What is the Ghon’s focus and associated lymph node infection collectively called?

Ghon’s complex or more recently primary complex.

How well did you know this?

Not at all

Perfectly

What happens to the macrophages as they engulf the Mycobacterium tuberculosis?

There is a granulamatous reaction and a granuloma is formed.

How well did you know this?

Not at all

Perfectly

Explain the characteristics of a granuloma due to tuberculosis.

Spherical with a caseating core (caseous necrosis)

Also Langerhans giant cells can be found

Epithelioid macrophages

Lymphocytes

How well did you know this?

Not at all

Perfectly

How well did you know this?

Not at all

Perfectly

What is extrapulmonary TB?

Reactivation of TB in other sites than the lungs.

How well did you know this?

Not at all

Perfectly

Give locations of extrapulmonary TB.

Larynx

Lymph nodes

Pleura

Brain

Kidneys

Bones and joints

How well did you know this?

Not at all

Perfectly

In which patients might you see extrapulmonary TB?

More often found in HIV-infected

Immunocompromised

Young children

What is the most common cause of TB?

Reactivation of latent TB.

This occurs most commonly in the lungs.

What is post-primary TB?

Reactivation of a latent primary TB infection

Where is post-primary pulmonary TB most commonly found?

In the upper lung zones.

Give complications of post-primary TB.

Cavity formation

Haemorrhage

Spread to involve rest of the lung

Pleural effusion

Miliary TB

Explain cavity formation in post-primary TB.

Softening and liquefaction of caseous material leads to cavity formation.

Fibrous tissue forms around the periphery of the lesions (cavity formation)

Explain why haemorrhage might occur in post-primary TB.

Extension of the caseous process into vessels in the cavity wall.

This leads to haemoptysis.

Explain how post-primary TB can spread to rest of the lung.

Through the bronchial tree to other lung sites.

Explain how post-primary TB can cause pleural effusion.

Seeding of TB bacilli in pleura or hypersensitivity.

What is miliary TB?

Formation of multiple miliary (small seed-like) tuberculous foci in the entire body.

How can post-primary TB cause miliary TB?

By rupture of a caseous pulmonary focus into a blood vessel and then dissemination into systemic circulation.

Explain the differences of latent TB and active TB.

Latent TB has an **inactive tubercle bacilli** in the body. Active TB has an **active multiplying tubercle**. No symptoms in latent. Symptoms in active. Latent is not infections. Active is infectious.

Clinical features of pulmonary TB.

Gradual onset over several weeks or even months. Malaise, tiredness, weight loss, fever, sweats and cough. Cough may be productive or not. Haemoptysis may occur. Also pleural effusion can occur.

Typical CXR finding on pulmonary TB.

**Shadowing** due to patchy solid lesions/cavitated solid lesions/streaky fibrosis/flecks of calcification. Most commonly found in the **upper lobe usually in the apex.**

How is active TB diagnosed?

By identification of the tubercle bacillus. Culture (bodily fluid) (5000 or more organisms) Direct smear CXR NAAT

Treatment of TB.

A **combination** of antibiotics over **several months**. Also Vitamin B6

What combination of antibodies are used to treat TB?

Rifampicin Isoniazid Pyrazinamide Ethambutol

Why is vitamin B6 given in TB treatment?

It's needed to be given along side with isoniazid to prevent **peripheral nerve damage.**

Why is a combination of 4 antibiotics used?

The mycobacterium tuberculosis strain contain a small number of **drug resistant** organisms that will arise spontaneously by mutations. If there is only one antibiotic used the **resistant strain** might emerge. The likelihood of the bacilli to be resistant to all 4 given drugs is very unlikely. This prevents emergence of **resistant strains**.

What is used to test for a latent infection of TB.

Interferon gamma release assay (IGRA) aka QuantiFERON test. Tuberculin skin test.

Explain IGRA.

Test the ability of mycobacterium tuberculosis antigens to stimulate a host production of interferon gamma. **Lymphocytes from the patient's blood are cultured with these antigens. If patient has been exposed to TB before then T cells will produce interferon gamma in response.**

Can IGRA differentiate between latent TB and active TB?

Can IGRA differentiate between latent TB and other atypical mycobacterium or previous BCG vaccination?

Yes

How does IGRA differentiate between atypical mycobacterium/BCG and latent TB?

The antigen that is used in the IGRA test is not present in Non-TB mycobacteria or the bacilli used in BCG.

Explain the tuberculin skin test.

A protein derived from the mycobacteria is injected **intra-dermally**. There is a skin reaction 48-72 hours **if** there has been previous exposure to TB. This is due to type IV hypersensitivity reaction from the protein.