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It’s estimated that about two billion people worldwide are infected with mycobacterium
tuberculosis, often just shortened to tuberculosis or simply ‘TB’.
Two billion is a ton of people, but even though they’re infected, that doesn’t mean all
those people have symptoms, the vast majority, about 90-95%, aren’t even aware they’re
infected.
And this is because usually the immune system can contain it such that it isn’t able to
multiply, and often remains latent, or dormant, as opposed to active, which usually causes
symptoms and can be spread to others.
If the host’s immune system becomes debilitated at some point down the road, like with AIDS
or some other illness, or as a person grows older, it can be allowed to reactivate, or
basically wake up and become very serious, especially if it spreads through the body.
Mycobacteria are an interesting bunch, they’re slender, rod-shaped, and need oxygen to survive,
in other words, they’re “strict aerobes”.
They’ve got an unusually waxy cell wall, which is mainly a result of the production
of mycolic acid.
Because of this waxy cell wall, they’re “acid-fast”, meaning that it can hold
on to a dye in spite of being exposed to alcohol, leaving it bright red colored when a Ziehl–Neelsen
stain is used.
The wall also makes them incredibly hardy, and allows them to resist weak disinfectants
and survive on dry surfaces for months at a time.
Now Mycobacterium tuberculosis is usually transmitted via inhalation, which is how they
gain entry into the lungs.
Now, we breathe in all sorts of virus and bacteria all the time, but we’ve got defenses
that take care of most of them.
For one, air that we breathe in is turbulent in the upper airways, and drives most bacteria
against mucus which is then cleared pretty quickly.
Ultimately, though, TB can avoid the mucus traps and make its way to the deep airways
and alveoli where we have macrophages which eat up foreign cells, digest, and destroy
them.
With TB, they recognize foreign proteins on their cell surface, and phagocytize them,
or essentially package them into a space called a phagosome.
With most cases, the macrophage then fuses the phagosome with a lysosome, which has hydrolytic
enzymes that can pretty much break down any biochemical molecule.
TB’s tricky, though, and once inside the macrophage, they produce a protein that inhibits
this fusion, which allows the mycobacterium to survive.
It doesn’t just survive, though, it proliferates, and creates a localized infection.
At this point somebody has developed primary tuberculosis, which means that they have signs
of infection soon after being exposed to TB.
Even though it sounds bad, most people at this stage are actually asymptomatic or maybe
have a mild flu-like illness.
About 3 weeks after initial infection, cell-mediated immunity kicks in, and immune cells surround
the site of TB infection, creating a granuloma, essentially an attempt to wall off the bacteria
and prevent it from spreading.
The tissue inside the middle dies as a result, a process referred to as caseous necrosis,
which means “cheese-like” necrosis, since the dead tissue is soft, white, and looks
a bit like cheese.
This area is known as a “Ghon focus”.
TB also gets to hilar lymph nodes, either carried over by immune cells through the lymph
or by direct extension of the Ghon focus infection and causes caseation there as well, and together,
this caseating tissue and associated lymph node make up the characteristic “Ghon complex”.
Ghon complexes are usually subpleural and occur in the lower lobes of the lungs.
The tissue that’s encapsulated by the granuloma undergoes fibrosis, and often calcification,
producing scar tissue that can be seen on x-ray, this calcified ghon complex is called
a “Ranke complex”.
In some cases, although a scar is leftover, the mycobacteria is killed off by the immune
system, and that’s the end of that.
In other cases, even though they were walled off, they remain viable, and are therefore
still alive, but they’re just dormant.
If and when a person’s immune system becomes compromised, like with AIDS or with aging,
the Ghon focus can become reactivated, and the infection can spread to either one or
both upper lobes of the lungs, it’s thought that this is because oxygenation is greatest
in these areas, and TB being an aerobe, prefers areas of greater oxygenation.
Since they were previously exposed, the immune system’s memory T cells quickly release
cytokines to try and control the new outbreak, which forms more areas of caseous necrosis,
this time, though, it tends to cavitate, or form cavities, which can allow the bacteria
to disseminate, or spread through airways and lymphatic channels to other parts of the
lungs, which can cause bronchopneumonia; but it can also spread via the vascular system
and infect almost every other tissues in the body, called systemic miliary TB.
When TB spreads to other tissues, it causes complications related to the organ affected.
Kidneys are commonly affected, resulting in sterile pyuria, or high levels of white blood
cells in the urine.
It might also spread to the meninges of the brain, causing meningitis, the lumbar vertebrae,
causing Pott disease, the adrenal glands causing addison’s disease, the liver causing hepatitis,
and the cervical lymph nodes causing lymphadenitis in the neck, also known as scrofula.
Testing for TB often starts with a purified protein derivative or PPD intradermal skin
test, sometimes known as a tuberculin skin test, Mantoux test, or simply TB test.
With this test, tuberculin is injected between layers of the dermis, tuberculin is a component
of the bacteria, and if a person has previously been exposed to TB, the immune system reacts
to the tuberculin and produces a small, localized reaction within 48 to 72 hours; if the reaction
creates a large enough area of induration (rather than just redness), it’s considered
to be a positive test.
Positive tuberculin tests simply mean the patient’s been exposed at some point to
TB—it doesn’t differentiate between active and latent disease.
As an alternative to tuberculin skin tests, there are also interferon gamma release assays
(or IGRAs) which look for evidence in the blood of previous exposure to TB proteins.
Since this one’s a blood test, you don’t need to show up again to have the test read
like you do with the PPD.
Also, the IGRA is more specific to TB rather than other types of mycobacterial infections
and is unlikely to be positive as a result of having BCG vaccine in the past, a vaccine
that protects against TB.
And this is a pretty useful feature of IGRAs, since BCG vaccine is given to many children
around the world to prevent disseminated TB.
After doing a screening test with PPD or IGRA, anyone with a positive result typically gets
a chest Xray to look for signs of active TB disease.
In patients with symptoms like as fevers, night sweats, weight loss, and coughing up
blood, or hemoptysis.
it’s important to collect samples from either the sputum, or via a bronchoalveolar lavage,
which is where a bronchoscope is inserted through the mouth or nose into the lungs,
fluid is squirted, and then the fluid is collected.
These samples can get sent to the lab for staining, culture, and PCR to look evidence
of mycobacterium tuberculosis.
Treatment of latent TB infection typically involves using a single drug for a prolonged
period of time—the most common approach is Isoniazid for 9 months.
Treatment of active TB disease is typically done with different combinations of anti-TB
antibiotics, like isoniazid, rifampin, ethambutol, and pyrazinamide,which results in patients
being non-infectious to others usually within a few weeks.
Until that point, though, patients can spread TB to others and it’s typically adults with
reactivated TB that are the most infectious.
As a result, patients are typically kept in negative pressure rooms and visitors are asked
to wear protective N-95 masks that can’t filter out oil aerosols (N for “not resistant
to oil”) but can filter out at least 95% of other aerosols (in this case TB).
Even after patients are no longer contagious, they’re typically kept on multiple medications
for many months to be sure the bacteria are destroyed usually with directly observed therapy
or DOT where somebody watches and confirms that you’re taking the medication.
Additionally, there’s an enormous worry about new drug-resistant strains of TB that
are causing infections in various parts of the world.
You may hear of MDR-TB or Multi-drug resistant TB or even XDR-TB which is Extremely drug
resistant TB which is incredibly hard to treat because they don’t die in the presence of
our usual antibiotics.
The bottom line is that to get an effective treatment, it’s super important to make
sure that the drugs being used will work against the specific strain of TB, that multiple medications
are used together to prevent drug resistance from developing, and that medications are
used for the entire course of therapy so that all of the mycobacterium tuberculosis is killed
off.
Recently, the US Food and Drug Administration has approved a breakthrough medication for
XDR-TB, called pretomanid.
According to recent studies, a combination of pretomanid, bedaquiline and linezolid has
been effective for almost 90% of XDR-TB cases.
As an added bonus, this combination therapy can be taken orally, and is also very well
tolerated compared with other XDR-TB drug regimens.
Alright, as a quick recap.
Tuberculosis, or TB, is an infection caused by Mycobacterium tuberculosis, that mainly
affects the lungs.
3 weeks after the primary infection, the immune system tries to contain the infection by creating
a granuloma, and the tissue in the middle undergoes caseous necrosis, resulting in an
area called Ghon focus.
TB also extends to nearby hilar lymph nodes, causing caseous necrosis there as well.
Ghon focus plus the affected lymph node make up the characteristic Ghon complex.
If the immune system becomes compromised, like with AIDS, or older age, Ghon focus can
reactivate, and the infection may spread to one or both upper lobes of the lungs, where
it forms cavities.
From cavities, bacteria can disseminate to other parts of the lungs, causing bronchopneumonia,
but it can also spread through the vascular system and infect almost every tissue in the
body, which is called systemic miliary TB.
Testing for TB starts with a PPD intradermal skin test, or with an IGRA blood test, and
a chest X-ray can identify signs of active TB disease.
In patients with fevers, night sweats, weight loss, and hemoptysis, a sputum sample can
be stained, cultured and PCR can be done to look for Mycobacterium tuberculosis.
Treatment of active TB includes a combination of antibiotics, given for several months,
and special regimens are used for MDR-TB and XDR-TB.