TB has been found in ancient Egyptian mummies. It infects 1/3 of humans in the world. In the U.S., it’s mainly found among immigrants and in poor urban areas, Alaska, Hawaii, and poorer parts of Texas and Arkansas. In most people with TB, the bacteria remains dormant (“latent”), walled off by the immune system, never to cause harm. But in 10%, the germ will awaken & cause active disease sometime in their life. Only people with Active TB are contagious (& only if it’s in their lungs).
The TB bacteria is breathed in, but travels around the body to almost any organ. By 10 weeks, the body’s immune system controls it, and it remains latent (dormant, “asleep”). In 1 of 10 people, it awakens sometime in life (50% in the first 2 years), causing illness and able to be spread. In the US, most TB is in the lung; in poor countries & among people with weak immune systems, 50% of it occurs in other organs (where it’s not contagious, but still serious).
Latent TB Infection (LTBI) is when the bacteria is dormant. There are no symptoms, it’s not contagious. Diagnosis is by a skin test (the PPD) or a newer blood test; both are equally accurate. Treatment greatly reduces the chance of getting the disease later in life (though not everyone needs treatment for LTBI).
Active TB Disease means the person is sick, and contagious (if TB is in the lung). The most common symptom is a nagging cough. Anyone with a cough lasting over 3 weeks without starting to improve needs a chest x-ray, unless another diagnosis (asthma, etc.) is evident. We x-ray sooner if there are worse symptoms like coughing blood, fevers, drenching sweats, or weight loss. Final diagnosis is made by testing sputum (phlegm). TB outside the lung can be harder to diagnose. A normal LTBI skin or blood test does not rule out active TB.
Without treatment, 50% of people with active TB die in 5 years. About 30% get better on their own, while 20% recover, fall ill again, etc. (& eventually die, longer than 5 years). Treatment requires 4 drugs to begin, then at least 2 drugs for a total of 6 months minimum. It’s best that TB experts at the public health department take charge, because if patients don’t take them correctly, the bacteria will become resistant, & need more complex treatment for years. So the public health department keeps track daily. As such, Active TB is curable.
The best way to prevent TB is 1) treat people with Active TB so they don’t spread it; and 2) treat people with LTBI before the dormant bacteria awaken. There are possible side effects to LTBI treatment, but the risk of harm is much less than the risk of getting Active TB (when then the more complicated meds may have even more side effects).
There are many common misconceptions about TB. For example:
- A positive skin test or blood test does not mean you were “exposed.” It means you are infected — the germ is latent, but alive inside you; it’s asleep (but may awaken some day).
- For someone with a cough, a negative skin or blood test does not prove they don’t have Active TB. The test is false-negative in 20% of people; an x-ray tells for sure.
- A positive PPD skin test is usually not due to BCG vaccine (given in other countries). Most people who got BCG get small PPD reactions, which we read as negative. A reaction large enough to be called positive (over 10 mm) is most likely due to TB. The TB blood test can often help distinguish.
- Active TB may cause fevers, bloody sputum, sweats, or weight loss, but not usually. The most common symptom is simply a persistent cough. I’ve seen clinicians mistakenly not get chest x-rays because the patient wasn’t sick enough.
- On x-ray, you can’t tell the difference between Active TB and most scars from old TB that got better on its own. I’ve seen radiologists make this mistake. You have to repeat the x-ray in 2 months, & get sputum tests in the meantime.
TB is a fascinating disease. There’s a wonderful 1952 book all about its history, & its relationship to poverty, The White Plague by Dubos.