Tuberculosis is caused by Mycobacterium Tuberculosis bacteria. This bacteria is present in the droplet of cough or sneeze by a person who is suffering from sputum positive lung tuberculosis. This tiny droplet nuclei whenever inhaled by healthy person in close vicinity gets TB infection.
This infection is present in one third of world population and in India this is around 40% (50 crores of Indian population). These bacterial infections in person without disease are called latent TB infection (LTBI). They are neither the case to spread TB infection to others nor do they have signs and symptoms. The fate of tubercular infection depends upon the person's immunity.
According to WHO- In 2014, 9.6 million people fell ill with TB, 1.5 million people died and 1 million children became ill and .14 million (1,40000) children died. Over 95% TB deaths occurred in low and middle income countries. TB is the leading killer of HIV positive people in 2015 and 1 in 3 HIV deaths was due to TB.
Globally in 2014, 480,000 people developed MDR-TB. TB incidence has fallen by 1.5% per year since 2000 and death rate dropped 47% between 1990 -2015. An estimated 43 million lives were saved between 2000- 2014. Ending the TB epidemic by 2030 is among the health target gets of newly adopted sustainable Development Goals.
According to WHO, latent tuberculosis infection (LTBI) is defined as, state of persistent immune response to stimulation by Mycobacterium tuberculosis (MTB) antigens, without evidence of clinically manifested active TB. This population is non-infectious to the community and have lifetime 10% risk of developing active TB disease in absence of co-morbid condition.
There are many factors which are responsible for the conversion of latent infection into the active disease. Person's immunity plays vital role. The person's having LTBI pose a major difficulty for global control of TB as they act as reservoir for the development of new cases of tuberculosis.
Majority of individuals who are exposed to Mycobacterium tuberculosis contain the infection using their innate and acquired host mechanisms. T-cell immune defences play the major role. When the person's immune system is intact, there occurs a balance between host and bacteria leading to containment of infection.
Following the exposure of Mycobacterium tuberculosis, depending upon the host immune status, alveolar macrophages ingest the pathogen and destroy them. But certain number of pathogens remains unharmed and persistently present in the lungs in dormant state. They are responsible for the development of latent tubercular infection.
When the host immunity declines, these dormant bacilli reactivate and result in development of active TB disease and become the source of infection to others. Co-morbid conditions if present along with LTBI have many fold increase in developing tuberculosis disease from LTBI, e.g. - HIV, Diabetes, Patients on long term corticosteroid therapy, patients on cancer chemotherapy, Malnutrition, patients receiving dialysis, patients preparing for organ or bone marrow transplantation, Silicosis.
High risk population e.g. Children's contacts with sputum positive cases, Prisoners, Health workers, Immigrants from high TB burden countries, Homeless persons and Illicit drug users also reported increased incidence of active disease. Hence, whenever body's immunity declines due to any reason, the possibility of developing active TB disease increases.
There are two methods for identification of latent tubercular infection, one is tuberculin skin test (TST), and also called as Mantoux test and another is Interferon gamma release assay (IGRA). They are indirect markers of MTB exposure, however their sensitivity decrease in immunocompromised state.
In high TB burden countries like India, approximately 40% of population is estimated to be latently infected with MTB; hence detection of LTBI has not been a priority in country's National TB Control Program. So WHO recommends LTBI testing in high risk populations mentioned above?
If any of the above tests is positive, presence of active disease has to be ruled out. Once it is done, LTBI treatment can be initiated. Identification and treatment of LTBI can significantly reduce the risk of development of active disease and it is the important TB control strategy in low burden TB countries.
As far as preventive therapy in cases of LTBI is concerned, single anti-tubercular drug is sufficient. According to WHO, 6 to 9 months of Isoniazid or 3 month regimen of weekly rifapentine plus isoniazid or 3 to 4 months of isoniazid plus rifampicin or 3 to 4 months of rifampicin alone is recommended.
In countries like India, both TST and IGRA are highly misused and Standards for TB Care in India (STCI) also does not recommend the routine use of these tests in high endemic area.
To conclude, LTBI screening is strictly restricted to specific high risk populations in India because decreased immunity is mainly responsible for the development of latent infection into active disease in these subgroup of populations. Prophylaxis therapy for LTBI should always be started in high risk group only after ruling out active TB disease, so that the development of new TB cases can be reduced.