Mycobacteria and Tuberculosis Flashcards
Why is tuberculosis important?
It is highly infectious, has a high morbidity and mortality
It can affect people of all ages.
What proportion of the World’s population have been infected with TB?
What % will contract the disease at some point?
1/3rd of the population are infected with TB
5-10% will contract the disease at some point
What is the link between TB and HIV/AIDS?
HIV/AIDS is the biggest infectious killer worldwide, TB is in second place.
TB causes 1/4 of all HIV deaths
What causes tuberculosis?
mycobacterium tuberculosis.
It is 2-4 micrometres by 0.2-0.5 micrometres
It is an obligate aerobe - it prefers specific conditions, such as our well-aerated upper lobe
it is a facultative intracellular parasite (so can live in macrophages)
It has a slow generation time (15-20hrs) and v slow growth
M.bovis from cattle
Its wall contains a v high lipid content
In the UK, what does TB most commonly affect?
Where else can it cause disease?
Most commonly the lungs (pulmonary TB)
Can affect anywhere in the body - e.g. lymph nodes, bones, joints, kidneys and can cause meningitis
How do people catch TB?
Who is it more likely to affect?
Most commonly in droplets (coughing or sneezing)
We need frequent or close prolonged contact with somebody to acquire the organism.
It is more likely to affect people with weakened immune systems (not a true primary pathogen)
What are the at risk groups for TB?
- HIV infection
- steroids, chemotherapy, transplants, elderly (all have weakened immune systems)
- unhealthy, overcrowded conditions
- those exposed to TB in youth
- children of parents from high-rate countries (S.E.Asia, sub-Saharan Africa)
- prisoners, drug addicts, alcoholics
- malnourished
How do you get infected by primary TB?
Where do the droplet nuclei travel to to begin the infection?
What forms in this area?
What makes up the primary complex?
Droplet nuclei are inhaled (they’re so small that they dont drop due to gravity)
They’re taken up by alveolar macrophages - not activated (lipids). Because of the lipids, the macrophages aren’t activated, so the TB can live in them.
Droplet nuclei reach the alveoli where the infection begins (most commonly in the base of lungs)
Body’s immune system then reacts to form a granuloma in the lung, called a ghon focus.
Even though it is walled off, it is still alive and dormant.
Englarged lymph nodes and ghon focus form the primary complex. This may result in flu-like symptoms.
What happens to cause secondary TB? (Post-primary TB)
When and where does this happen in relation to primary TB?
Reactivation of dormant mycobacteria = impaired immune function
Reinfection in a person previously sensitised to mycobacterial antigens.
Occurs months/years/decades after primary infection. Reactivation most commonly occurs at the apex of lungs (highly oxygenated area)
What happens during secondary TB?
Caseous (cheese-like) centres of tubercles liquefy.
Organisms grow very rapidly.
Large Ag load
- Bronchi walls become necrotic and rupture (leaving the cavity behind). There is then a massive influx of O2 which the organism likes, allowing it to grow rapidly.
- cavity formation
- organism spills into airways and spreads to other areas of lung (it is highly infectious)
Primary lesions heal (Ghon complex and Simon foci) so can be seen on X-ray
What is miliary tuberculosis?
Sometimes the immune system can be overwhelmed = organism spills into the bloodstream and spreads throughout the body.
X-ray appearance looks like millet seeds
What is the difference between TB infection and TB lung disease?
INFECTION:
- organism present
- tuberculin skin test positive
- chest X-ray normal
- sputum smears and culture negative
- no symptoms and not infectious
- not defined as a case of TB
DISEASE:
- organism present
- tuberculin skin test positive
- chest x-ray = lesion present
- sputum smears and culture positive
- symptoms and infectious
- defined as a case of TB
What % of those infected develop active disease upon initial infection?
3-4%
5-10% within one year
What are the most common symptoms of TB? What are they caused by?
Mostly caused by cytokine activity (TNF, IL-3, GM-CSF)
- persistant cough (+/- sputum)
- anorexia
- weight loss
- swollen glands (usually in neck)
- fever
- night sweats
- sense of tiredness and being unwell
- coughing up blood (if organism breaks into bronchi)
What is the standard treatment of a patient with TB? (4 drugs)
isoniazid, rifampicin, pyrazinamide and ethambutol - for two months followed by isoniazid and rifampicin for 4 months
6 month standard treatment.
