Tuberculosis (TB) Flashcards
What is tuberculosis and what is the two different types?
A disease caused by infection of a bacterium called myobacterium tuberculosis.
People can be infected with myobacterium tuberculosis but not have the disease so two forms of TB are noted:
latent TB infection (LTBI) and TB disease. If not treated properly, TB disease can be fatal.
Approx what fraction of the worlds population are infected with TB?
Approx 1/3. (note: the majority of these people infected have latent TB and hence dont even know they are infected).
How is TB spread?
TB bacteria are spread through the air from one person to another. The TB bacteria are put into the air when a person with TB disease of the lungs or throat coughs, sneezes, speaks, or sings. People nearby may breathe in these bacteria and become infected.
When a person breathes in TB bacteria, the bacteria can settle in the lungs and begin to grow. From there, they can move through the blood to other parts of the body, such as the kidney, spine, and brain.
TB disease in the lungs or throat can be infectious. This means that the bacteria can be spread to other people. TB in other parts of the body, such as the kidney or spine, is usually not infectious
Once TB is inhaled and goes down into the alveoli into the lungs, what happens next? (up until the formation of a granuloma)
- The immune system reacts to the bacterium by macrophages phagocytosing the bacteria. The bacteris actually wants to be phagocytosed and so LAM on the surface of the TB bacteria binds to receptors on the phagocyte
- Not all of the bacterium will be killed by the macrophages and so the remaining bacteria persists inside the macrohpages, feeding off them and replicating.
- The TB bacteria has an array of mechanisms that allow it to stay int he macrophage and prevent it from signalling to other immune cells for help
- The infected areas will gradually and eventually form a granuloma due to T cell signalling. This is an attempt from the immune system to build a ‘prison’ of macrophages/immune cells around the TB antigen
What is the structure of a granuloma and what effect on lung tissue does it have?
Remember - granuloma formation is Chronic Infection!
A granuloma consists of a centre of necrotic tissue (destroyed macrophages that were infected with TB and the left over TB antigen).
Surrounding this is a collar of macrophages (epitheloid macrophages and giant cells which are a giant fusion of several macrohages)
On the outside are T cells which through interferon gamma control the inner macrophages and stimulate the macrophages to form the collar around the necrotic tissue.
The necrotic tissue is a combination of blood and sputum which is what TB is known for in terms of symptoms of the lung.
This necrosis can cause pockets or cavities that form in the lungs. These damaged areas may cause bleeding in the lungs or may become infected with other bacteria and form pockets of pus (abscesses).
A hole that forms between nearby airways in the lungs.
Difficulty breathing because of blocked airways.
After becoming infected with TB, you can either get persistent infection (disease) or containment of the infection (latent). What are the possible outcomes of each of these?
Persistent:
Spread by blood to other parts of the body eg lymph nodes, spleen, kidney, bone marrow. This can cause disease of these organs.
Or the infection can remain local (in the lungs) and cause cavities, burst into the pleural space, pneumonia, damage to the pericardium.
Latent:
It can remain dormant right up until end of life and you may never know you had it
or it can be re-activated and you can get a secondary infection. This is most common in the elderly, infants, or people with weakened immune systems eg HIV infected people, people on chemotherapy, people with diabetes etc.
What is a common test used to see if someone is infected with TB?
Mantoux test.
This test is where you inject a small amount of TB protein (PPD) .
If within 72 hours a lump shows up on the skin it shows that the bodies immune system recognised the protein (the person is hypersensitive to TB anitgen) then they are potentially infected.
However some people with TB wont react to this and also it isnt very specific. Another disadventage is that it takes 3 days.
How long does TB take to treat and what is used to treat it?
Takes a long time - minimum 6 months!
You cannot treat with normal antibiotic such as peniciliin as penicillin targets transpeptidase enzymes in cell wall but TB dosent reply on transpeptidase for its cell wall integrity.
