What Is Latent Tuberculosis (TB)?

This bacterial infection can be deadly if left untreated

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Tuberculosis (TB) is an airborne, contagious disease that can be fatal if left untreated. Fortunately, most people who are infected with TB never get sick. They acquire what's called a "latent" form of TB, which means that the infecting bacteria are alive in the body, but are inactive.

People who have latent TB infections do not have TB symptoms and cannot spread the infection to others, but they are at risk of developing an active infection that is both symptomatic and contagious.

About 3 percent to 5 percent of latent TB becomes active TB in the first year and about 5 percent to 15 percent of latent TB evolves into active TB after that. About one in three people in the world have latent TB. 

What Is Tuberculosis?

TB is a disease that starts by infecting the lungs but can spread to other organs, including the kidneys, brain, abdomen, and spine. It is the leading infectious-disease killer worldwide. It kills more people than other deadly diseases like HIV, Malaria, and Ebola. TB is less prevalent in the U.S. than it used to be, but the infection can last a long time and appear unexpectedly.

TB is scary enough by itself, but there is another serious link that it's critical to keep an eye on. Several studies have also found an increased risk of lung cancer among those who have tuberculosis (TB).

Assessing Your Risk

People who are at highest risk for developing active TB include those with HIV/AIDS, the very young and the very old, people who are immunocompromised (including cancer patients who are treated with chemotherapy), injection-drug users, and people whose infections occurred recently (within 2 years).

Should You Be Tested for a Latent TB Infection?

Overall rates of TB are low in the U.S. However, some people are at higher risk. Reasons for getting tested include:

  • Exposure to people with known or suspected active TB, including those in hospitals, homeless shelters, and prisons, or those who have a history of living in or traveling to countries where TB is endemic, such as in Latin America, Africa, Asia, Eastern Europe, and Russia
  • Having a condition where your immune system is weakened (such as HIV/AIDS) or taking immunosuppressive drugs, like those for rheumatoid arthritis
  • Working in a medical facility
  • Having symptoms of active TB, like fever, night sweats, weight loss, a chronic cough (for three weeks), coughing up blood

It's important to note that you may not know whether you've been exposed to TB. The disease is sometimes not diagnosed, as it can seem like other more common diseases.

How to Get Tested

If you suspect that you have a latent TB infection, one way to know for sure is to take a TB skin test. A health care worker will inject a few TB extracts under the skin on your forearm. This is called a PPD test. It is often used when someone is starting work or going to school to make sure that he or she is not at risk for developing TB.

An immune reaction to these extracts will result in swelling that can be detected within two to three days. Sometimes the test may be given too soon after exposure to TB for an immune response to develop. Therefore, a second skin test is usually given eight to 10 weeks after exposure. It's worth noting that there are new ways to test for TB, so talk to your doctor to see whether this test or a different test might be right for you.

How Latent TB Becomes Active TB

TB becomes active when an infected person’s immune system isn't strong enough to keep the infectious bacteria in check. The presence of the tuberculosis bacteria causes an immune response in which many types of white blood cells are recruited to sites where the bacteria are growing. They form a walled-off lesion, which is known as a “tubercle” or “granuloma." The bacteria within the tubercle can survive for decades, and conditions leading to a weakened immune response can allow the bacteria to break out of the lesion and reactivate to develop into active TB. 

The disease has a tendency to pop up at the worst time—when someone is sick with another disease or isn't eating or resting enough.

It can be a pretty cruel disease. 

Treatment for Latent TB and Active TB

Treatment of a latent TB infection significantly reduces the likelihood of developing active TB. Latent TB can be treated with a standard a nine-month course of isoniazid (INH), an antibiotic that kills the bacteria that causes TB. There are also alternative regimens that are growing more popular and can sometimes be shorter.

It may be helpful to have someone help you make sure you take all of your medications every day. You may also want to take vitamin B6 with INH to avoid having any nerve tingling or other side effects. Some doctors will recommend prophylactic (preventive) treatment for children or HIV patients who have had recent exposure to people with contagious TB since they are at higher risk for developing active TB.

Active TB is treated by taking a variety of drugs for six to nine months. However, some TB can become resistant to drugs, which makes it difficult to treat.

Sources:

Centers for Disease Control. Division of Tuberculosis Elimination. http://www.cdc.gov/tb/?404;http://www.cdc.gov:80/tb/default

World Health Organization. Tuberculosis. http://www.who.int/tb/en/

Centers for Disease Control and Prevention. Treatment for TB Disease. https://www.cdc.gov/tb/topic/treatment/tbdisease.htm

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