Latent tuberculosis (LTB), also called latent tuberculosis infection (LTBI), is a condition that occurs when a person is infected with the bacterium Mycobacterium tuberculosis but does not have active tuberculosis (TB). The active form of the disease can be contagious, while the latent form is not; therefore, it is impossible to catch TB from someone with LTB. For LTB, various treatment regimens are used, generally requiring several months for completion.
Epidemiology As of 2023, it is estimated that one-quarter of the world's population has latent or active TB. TB is estimated to have newly infected 10.8 million people per year. The spread of TB is uneven throughout the world: approximately 80% of the populations in many Asian and African countries test positive on tuberculin tests, while only 5–10% of the US population tests positive.
Transmission
Latent disease It is impossible to catch TB from someone with LTB. In people who develop active TB of the lungs—also called pulmonary TB—the Mantoux test is often positive. Such people are contagious and can pass the bacteria to others. TB can be transmitted by exhalation. Tiny droplets exhaled by a person with active TB can remain suspended in the air for several hours, depending on environmental conditions. Another person could then inhale these droplets and potentially become infected with TB. However, such infection is unlikely to occur, because infection generally requires longer exposure. This reduced probability includes brief exposures that occur when a person is exposed to TB during brief social contact; prolonged exposure (over two months or so, e.g., by living with a relative with TB) is generally required for TB infection. Statistics show that approximately one-third of people exposed to pulmonary TB become infected with the bacteria, but only one in ten of these infected people develop active TB disease during their lifetime. People with latent TB are not contagious and do not feel sick. However, latent infections can become active TB infections, which are contagious, at a later time. In countries such as Canada and the United States, people have medical privacy or confidentiality, so they are not required to reveal their active TB diagnosis to family, friends, or coworkers. A person knows that they have been exposed to TB only through required testing for certain jobs; developing symptoms of active TB; or testing by a medical doctor. Because TB is uncommon in the US, doctors may not initially suspect TB; therefore, they may not test for potential infection. If a patient shows symptoms of TB, it is prudent for them to be tested. Diabetes is associated with a significantly increased risk of progression from latent to active TB, and the mortality rate from TB may be higher in patients with diabetes. People with both human immunodeficiency virus (HIV) and LTB have a 10% chance of developing active TB each year. "HIV infection is the greatest known risk factor for the progression of latent M. tuberculosis infection to active TB. In many African countries, 30–60% of all new TB cases occur in people with HIV, and TB is the leading cause of death globally for HIV-infected people."
Reactivation A diagnosis of LTBI, with confirmation that no TB disease is active, requires ongoing vigilance for symptoms of active TB. A full course of treatment does not guarantee that all M. tuberculosis bacteria have been eliminated. When a person develops active TB, symptoms may be mild for many months. Such symptoms can lead to delays in a person's seeking medical care, as well as result in the transmission of the bacteria to others. TB bacteria may exist in latent form both inside and outside the lungs. In many cases, the bacteria may be latent in another organ, with no remaining bacteria in the lungs. If reactivation of TB occurs in the brain, organs, kidneys, joints, or other areas, symptoms are often slow to progress, frequently being ignored by patients until they become severe. Extrapulmonary tuberculosis (TB outside of the lungs) can affect virtually any organ, with distinct manifestations depending on the location: TB in the spine causes back pain and potential paralysis; lymph node involvement presents as painless or occasionally painful swelling in the neck or armpits; meningeal infection triggers persistent headaches and altered consciousness; and urogenital tuberculosis leads to painful urination, blood in urine, and infertility. All of these manifestations may feature generalized symptoms such as fever, night sweats, and weight loss; these symptoms, combined with their nonspecific presentations and lower amounts of bacteria, make the underlying conditions more difficult to diagnose than the classic pulmonary disease.
Risk factors Situations in which TB may become reactivated are the following:
The onset of a disease affecting the immune system (such as acquired immunodeficiency syndrome [AIDS]) or a disease whose treatment affects the immune system (such as chemotherapy for cancer; systemic steroids for asthma; or Enbrel, Humira, or Orencia for rheumatoid arthritis) Malnutrition, which may result from illness or injury affecting the digestive system; a prolonged period of not eating; or a disturbance in food availability, such as during a famine or while residing in a refugee or concentration camp Degradation of the immune system because of aging Certain systemic diseases, such as diabetes Other conditions, such as debilitating disease (especially hematological and some solid cancers), the use of long-term steroid medication, end-stage renal disease, silicosis, gastrectomy, and jejuno-ileal bypass Old age Young age
Concomitant immunity Compared to uninfected individuals, people with LTB appear to have some (35–80%) protection against developing active TB after exposure to M. tuberculosis in the environment. This type of protection is called concomitant immunity. Such immunity seems to be due to the presence of tissue-resident memory T cells.
Diagnosis Two classes of tests are commonly used to identify patients with LTB: tuberculin skin tests and Interferon-gamma (IFN-γ) tests. The two tuberculin skin tests currently include the following:
Mantoux test Heaf test The three IFN-γ tests include the following:
T-SPOT.TB QuantiFERON-TB Gold QuantiFERON-TB Gold In-Tube
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