Ask the Doctors

Dear Doctor: So many people are required to undergo tuberculosis screening even though they don't know anyone with TB and have no symptoms. Just how contagious is tuberculosis?

Dear Reader: Tuberculosis, or TB, is an extremely infectious and potentially life-threatening disease. One-third of the world's population is infected with TB, which causes about 1.7 million deaths each year. Infection rates in the United States are quite low by comparison, with 9,287 new cases reported in 2016.

The causative agent of TB is a rod-shaped bacterium called Mycobacterium tuberculosis. It's released into the air when an infected person sneezes, coughs, or even speaks or breathes. By breathing in these microscopic droplets, which can remain airborne for several hours, a person who comes into contact with an individual with an active TB infection can contract the disease.

Transmission of TB after brief exposure to an infected individual is rare. According to the Centers for Disease Control and Prevention, TB cannot be spread by contact with clothes, a handshake, the toilet or other surfaces.

Once the TB bacterium is inhaled, it can move through the bloodstream to other parts of the body, like the kidneys, spine or brain. However, in most cases, the bacterium affects the lungs. There, if left untreated, it can cause pockets and cavities to form. These can bleed, get infected and form abscesses, cause holes or block airways. Left untreated, TB can kill you.

You asked why even those without TB symptoms are tested. The reason is that, in most healthy people, the body's immune system is good at neutralizing the TB bacterium. It does this by forming a protective capsule, known as a granuloma, around the bacillus. Although this isolates the bacterium, it doesn't kill it. That means that the person is now a TB carrier, otherwise known as latent TB. The danger here is that the immune system can lose the upper hand at any time, and the latent infection can become active.

People with latent TB don't have symptoms and are not contagious. However, they must receive treatment to get rid of the disease. Without anti-TB drugs, active disease can develop. That risk is highest during the first few years following infection. For some people, it continues throughout their lives.

So what are the symptoms of TB?

They include a bad cough that lasts for three weeks or more; a cough that brings up blood or sputum; general weakness or fatigue; unexplained weight loss, fever, chills and night sweats.

If you have had contact with someone who has TB, or if you have several of these symptoms, particularly those involving the lungs, you should get tested. A blood or skin test will show whether the TB bacillus is present. A chest X-ray or a sputum culture will reveal active disease.

Treatment will depend on whether the disease is active or latent. Active disease is treated with several drugs taken over the course of up to six to nine months. A further complication is that some TB bacterium is drug-resistant. This requires very specific treatment by infectious disease specialists.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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