The Big 3: Tuberculosis Part 2

Here we are for part two of the posts covering the basics of tuberculosis. Part one is over here if you haven't read that yet.

Last time we talked a little about the history of tuberculosis as well as the biology of bacteria. So today we're going to dive back in and talk about how TB spreads.

Transmission and Spread

One of the biggest benefits of living in the 21st century is that we have a much better understanding of the world around us compared to our ancestors. For example, nowadays we know that tuberculosis is caused by a tiny infectious organism. For the majority of our species' existence (which is also probably how long TB has been infecting us), we thought otherwise. Before germ theory got going, it was a common belief that tuberculosis was a hereditary disease that spread among family members.(6)

But now we know better. The tuberculosis bacteria spread from person to person through airborne droplets. We produce these droplets by coughing, sneezing, and talking. Once it's in the air, Mycobacterium tuberculosis can remain suspended for a bit of time. How long is a question of how ventilated the environment is as well as how much sunlight is present because UV light kills TB bacteria.(10)

So remember how the physicians of antiquity thought tuberculosis was hereditary? That observation makes some sense when you consider that families tend to live together in close, unlit, and poorly ventilated spaces. Any location with those three characteristics is a perfect place for tuberculosis to hang out in the air until someone breathes it in.

Tuberculosis has had a large impact on how we design the modern hospital simply because the goal of a hospital is to heal patients, not infect them with deadly organisms. Look carefully in some of the waiting rooms and wards and you may notice UV lighting. It is also becoming more common for hospital rooms to have negative pressure, meaning the air within a patient's room stays in the patient's room and doesn't go anywhere else. Whenever health care workers deal with a possible TB case, they get to put on their individually fitted masks. Tuberculosis is a strong impetus for theses changes and reforms.

So you might be imagining one-third of the world population hacking tuberculosis up into the air. This thought might make you never want to go into crowded spaces ever again. But don't worry, that's not an accurate image because most of those people are not infectious.

Why's that? Well, let's look at what happens inside your body.

What TB does in your body: Latent vs Active

So M. tuberculosis was hanging out in the air and then someone inhaled it. Where does it go?

Your lungs of course. The bacteria arrive and look for a nice patch of lung tissue to set up in. Then your body notices the squatters. A macrophage shows up on the scene and promptly eats M. tuberculosis.

As we explore the multitude of nasty infections which can show up in humans, we will be talking about various components of your immune system as well. So let's get introduced to our first character: the macrophage. A macrophage is an immune cell. It patrols its beat keeping an eye out for anything that doesn't really belong, and when it catches sight of trouble, that trouble get's engulfed. Once inside, the invader gets to experience the chemical funhouse that is a macrophage and dies.

Having acquainted itself with M. tuberculosis, the macrophage gets down to business with a barrage of powerful chemicals. But it has some trouble here because unlike your typical bacteria, tuberculosis is not affected by the chemicals. Remember that nice thick layer of fat I mentioned that surrounds M. tuberculosis? Well, that fat prevents the macrophage from killing the bacterium.

Eventually, the macrophage realizes that it ate something really bad, feels regret, and calls in backup. Your immune system deploys in force and realizes that it can't destroy whatever is inside their buddy the macrophage. So they build a wall encasing the doomed macrophage and the bacterium. Then your immune cells shake hands and congratulate each other for a good day's work. The medical term for this is a granuloma.

So now tuberculosis has been encased within a wall of cells. It sits there patiently, dividing slowly and waiting. But if you have a fully functioning immune system, your cells probably did a good job. TB is stuck. Technically, you are infected but you show no symptoms and don't feel anything. This is called a latent infection. For that one-third of the world infected with tuberculosis, most of them have this. Depending on where they live, those people may never even know they're infected. They could live a full life without tuberculosis ever giving them a problem.

Yet, tuberculosis is a patient disease. It's not in a hurry. It can wait for the perfect moment when your body's defenses are worn down and distracted by other problems. Now TB strikes, and you have an active infection. The bacteria break out of the granuloma and start giving you problems. When patients have this form of TB, they are infectious. The more out of control the bacteria divide, the more infectious the patient becomes. Of all the people who have a latent infection, 10% of them will transition to the active infection at some point in their life.(2)

Symptoms and Advanced Infection

When you have an active tuberculosis infection, it can manifest with a number of symptoms. Most of the time, the symptoms of an infection are just your body trying to get rid of it. When your body realizes, "Hey, there's bloody gunk in my lungs," it tries to cough the bad stuff up. But remember, that's what tuberculosis wants your body to do so it can spread. It is a parasite using your body's innate functions and reactions to survive and propagate itself.

In addition to coughing, a tuberculosis patient may experience chest pain and bloody phlegm. The other two big symptoms are night sweats and weight loss. After that, you have the more general symptoms of fever and chills.(2) The weight loss and wasting away is a consequence of your immune system kicking into overdrive.(10)

So now we are talking about an active TB infection that the immune system is struggling to keep under control. The bacteria can stay in your lungs, or they can go on a bit of an adventure. Remember, your macrophages and the bacteria are duking it out in your lungs, specifically around your alveoli. Your alveoli are the part of your lungs where oxygen is transferred to red blood cells. Well, if tuberculosis gets loose it can hop straight into your bloodstream. Now it goes on a wild rollercoaster ride through your circulatory system, and where it'll hop off, nobody knows.

Pictured: Your bloodstream.

This form of TB is known as miliary tuberculosis. It's called this because people thought the lumps it left in tissue bore a resemblance to millet seeds.(6) And it is very deadly without treatment. In some ways, this stage of disease progression isn't so different from a cancer metastasis because it can go anywhere in your body. It can lead to meningitis and other nasty conditions. One such ailment is called scrofula, which arises when tuberculosis gets into your lymph nodes. This can lead to visible swelling and ulceration.

However, these conditions only happen when a tuberculosis infection leaves your lungs. TB is perfectly capable of killing you by staying in your wind pipes. These severe forms of infection are more common in those who are more susceptible to TB, so let's talk about who is most at risk for tuberculosis.

Risk factors

At risk groups for tuberculosis infection can be divided into two groups: those who spend time around people with tuberculosis, and those who have impaired immune systems. When we talk about the first group, many people who fall into this category do some form of institutional work. If you work or live in a hospital, correctional facility, nursing home, or homeless shelter, you are at risk. You are also at risk if you live in close contact with people with active TB, are homeless, or a drug user.(2)

Then there are those whose immune systems aren't quite up to snuff. Remember, most people infected with TB aren't actually spreading the disease because their immune systems wall the bacteria away. If your immune system isn't working, you're more susceptible to infection by TB as well as more likely to spread the disease. First obvious group: AIDS patients. I'll talk about that in the next post though. Two other big groups are the very young and the very old. Infants and the elderly are almost always an at risk group when we talk about infectious disease, and TB is no exception. Typically this is because the very young don't have much of an immune system yet, and in geriatric populations the immune system is breaking down.

Substance abusers are another at risk group, whether the substance in question is legal or otherwise. Too much alcohol weakens your immune system. Smoking nicotine or marijuana irritate your lungs, and remember, TB targets your lungs. Other drugs like meth or cocaine dehydrate your body, and a body without enough fluid will not have a top notch immune system. Having a low body weight also puts you at risk.(2)

If you have received an organ transplant, you are taking medication to suppress your immune system. Therefore, you are at risk for tuberculosis. The same goes if you have an autoimmune disease such as arthritis or Crohn's disease and are being treated with immune suppressants. Diabetics and kidney disease patients are at risk. Those with head and neck cancer are at risk, and so are those with a lung disease called silicosis. People who fall in these groups are the ones who tend to see the really nasty parts of TB infection.

So how do we get rid of tuberculosis?

Check out the next post when it comes out in a couple of days.

It's up.

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