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Latent Tuberculosis And Its Pathogenesis

Posted By HealthcareOnTime Team Posted on 2022-07-18 Latent Tuberculosis And Its Pathogenesis

Tuberculosis has ailed mankind from ages with ancient Babylonian and Chinese writings as the witness of its archaic presence. From the advice "Just sleep and eat nutritious food" for the affected individuals to the announcement by Dr. Robert Koch of discovering a bacterium responsible for tuberculosis; by the time, this bacterium has already created its impact globally.

Primarily affecting the lungs, tuberculosis also invades other sites of the body including the brain, kidney and spine. It spreads when an individual with active pulmonary infection sneezes, talks, or coughs, releasing small droplets called droplet nuclei that contain the live bacteria. The silver lining to this deadly cloud is that only 5 - 10% of all people exposed to this bacterium develop active tuberculosis. In the rest of the individuals who are exposed, the bacterium is succumbed by the immune system. Such individuals are said to have Latent Tuberculosis infection - where the infection lies dormant in the body, and resurfaces only in certain situations.

Latent Tuberculosis and its Pathogenesis
When the bacteria contained in the droplet nuclei of a person suffering from active TB enter through the nose of a healthy individual, they traverse the upper respiratory tract, descend the bronchi and finally enter the lung alveoli, sustaining through all the natural defence machanism of the body, due to their small size. In the alveoli, locally present macrophages (or dendritic cells) either inhibit or destroy a majority of the invading pathogens. Complete eradication of the bacilli, however, occurs in approximately 10% of all cases of infection while around 90% of the time, these bacilli survive by blunting the immune responses. The few that remain alive, multiply within the macrophages, which either leads to systemic dissemination, or are released on dying. These live bacilli then enter the lymphatic system or blood circulation to reach the distant organs and tissues like kidneys, larynx, lymph nodes, spine and brain. The systemic spread of the bacillus primes an immune response that spans the whole body, which often develops after 2-8 weeks. As a part of this response, macrophages ingest and sequester the tubercle bacilli within barriers called granulomas. This form of infection that is contained and kept under control is called Latent Tuberculosis Infection (LTBI).

What is Latent Tuberculosis Infection?
Individuals suffering from Latent Tuberculosis Infection may be asymptomatic and non contagious. The pathogen, however, is alive in their body, but in a dormant state. Most of the people who develop active tuberculosis infection from Latent Tuberculosis Infection do so within approximately two years of exposure. Due to the absence of symptom, identification of this condition is possible only through blood tests and skin test. These individuals have normal chest X-ray and negative sputum test results.

What are the threat of resurgence of active tuberculosis from Latent Tuberculosis Infection?
In certain situations, the dormant tubercle bacilli overcome the host defenses and progress to develop the active form of the disease. The most common factor for this is an HIV infection, which weakens the immune system
Other factors that contribute towards development of active disease include

  • Poorly controlled diabetes mellitus
  • Old age
  • Malnutrition
  • Use of immunosuppressive drugs
  • Kidney failure
  • Children younger than 5 years of age
  • Those who have previously (and inadequately) been treated for active TB
  • Cancer of head, neck or lungs
  • Cigarette smokers and drug/alcohol abusers
  • Living in endemic regions for TB

When to get tested?
Being symptomless, the key to identify Latent Tuberculosis Infection is being aware of the predisposing risk factors of contracting the infection. These include work places such as long term care facilities, hospitals, clinics, and living in environments like homeless shelters, prisons, crowded areas with poor ventilation, and regions with low medical care facilities. Since the risk of active TB infection is highest immediately following exposure, it is important that those individuals who are at a higher risk of contracting the disease are screened for Latent Tuberculosis Infection.

What are the tests done for Screening Latent Tuberculosis Infection?
One of the tests done for Latent Tuberculosis Infection screening is the targeted tuberculin skin test (TST), which is also called the Mantoux test. In this test, an intradermal injection of heat-killed purified protein derivative (PPD) from M. tuberculosis is given. A positive result is indicated by the formation of a small bump, called the wheal, at the site of the injection having swelling of more than 5 mm when measured. False negative results may also arise if the person was infected many years ago or the immune response against PPD had not developed. These individuals are given a second TST test, which will be positive as the immune response against the first PPD infection will produce a reaction against the subsequent dose. False positives can also arise from a recent BCG vaccination.

An additional test for detection of Latent Tuberculosis Infection is the whole blood interferon gamma test. Two types of interferon gamma tests are available commercially,

  • The ELISA based QuantiFERON-TB Gold assay
  • TSPOT - TB, based on the enzyme linked immunospot (ELISPOT) technology

Once the tests have been confirmed positive, chest radiography is done to rule out the probability of active TB infection.

Astonishingly despite having so many diagnostic advancements, this organism has been a continuous threat to the mankind since 1800s giving stress to the global leaders. Recently with initiatives and measures taken by the government for a TB-free India, and indeed a TB-free world, several programmes are making people more aware about its threat. However this dream will not be complete unless, the people of the country educate themselves about this disease and its various forms, in an effort to take necessary preventive measures for early detection and treatment.

Testing @ HealthcareOnTime Thyrocare offers testing for latent TB using state of the art technologies based on Interferon Gamma Release Assay (IGRA) at affordable rates.


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