A new Danish study concludes that a quarter of the people on earth have the tuberculosis bacillus within their bodies. This disease preys on over 10 million people annually, and kills 2 million a year. It is Number 1 on the list of killer diseases, far above AIDS and other infections.
Apart from active symptomatic tuberculosis, many more carry the causative agent Mycobacterium tuberculosis without any symptoms. This condition is called latent tuberculosis. Here the infected individual’s immune system is activated by the microbe, keeping it from producing any signs or symptoms of the infection. Such individuals cannot spread the disease to others. However, 5% to 10% of these people may develop the active disease at any time, about 50% within 2 years of acquiring the infection, and the rest later.
Secondary tuberculosis in lungs and close-up view of Mycobacterium tuberculosis bacteria, 3D illustration Credit: Kateryna Kon / Shutterstock
The number of people with latent disease is unknown, but estimates have been made on the basis of the likely number of people infected by a person with active disease. Until now, there was no actual measurement of the real number of people with latent tuberculosis in the world. However, most tuberculosis experts estimated that anywhere between a third and a fourth of the world’s population have this condition. This was based on doubtful assumptions as well as on the use of tuberculin skin testing (TST) to detect latent tuberculosis.
The current study carried out by Danish and Swedish researchers makes use of both TST and another much more specific technique to find latent disease, called interferon-gamma release assays (IGRAs). The utility of this method is accepted to be higher than the TST, and it is used widely to detect the latent form of the infection. However, it is being used here for the first time to estimate the worldwide prevalence of this condition.
The researchers looked at almost 90 studies generated from 36 countries, focusing on epidemiological factors which deal with the type of factors that affect the spread of a disease. The number of people covered by the study was over 350 000. They classified the countries by the incidence of tuberculosis as having high, low and intermediate incidence. Among these, 67 had prevalence estimates using TST and 41 using IGRAs.
Using this data they were able to arrive at an estimated prevalence of the disease in other countries which so far lack such data. The prevalence is the number of people per 100 000 population who have the disease at any point of time. In addition, the study also provides an estimated global prevalence. They found that using IGRAs the global prevalence was about 25% but only 21% using TST.
The importance of this study is the finding that both these tests can be of use in identifying those who need chemopreventive agents to keep them from developing the active disease. Chemoprevention is the treatment of individuals who are at risk of developing the active disease, using anti-tuberculous medications like isoniazid. If taken properly and for the designated period, these medications are potent in preventing the activation of the mycobacteria at any point. In other words, the person will never have the disease.
The effort to identify and target people with latent disease will need much more intensive effort to keep the number of those with active disease low, as the former represents a readily available source of potential active tuberculosis. Until now, chemoprevention targets were those children under 5 years who had a history of contact with tuberculous patients, those at high-risk such as those on dialysis, preparing for organ or blood-related transplant therapy, those on immunosuppressive biologics, and those living with HIV.
Though the current study provides an actual estimate which is slightly less than the 1999 WHO estimate of one-third, the conclusion is the shocker: the researchers state that tuberculosis eradication will be virtually impossible by the WHO target year 2035, with about one in four to one in five people worldwide harboring this microbe. This is due to the need to treat this immense pool of individuals with latent tuberculosis, who are all at risk of developing active disease at some point, whether earlier or later.
The study is to be published in the European Respiratory Journal shortly, and has been published prior to publication.
The global prevalence of latent tuberculosis: a systematic review and meta-analysis, Adam Cohen, Victor Dahl Mathiasen, Thomas Schön, Christian Wejse, European Respiratory Journal 2019; DOI: 10.1183/13993003.00655-2019, https://erj.ersjournals.com/content/early/2019/06/12/13993003.00655-2019