Study Finds that Nearly One in Four Are Infected with Latent Tuberculosis

The recent Global Tuberculosis Report 2016 released by the World Health Organization (WHO) acknowledged that the worldwide tuberculosis (TB) epidemic is even larger than previous estimates suggested. In a recent study that highlights the scope of TB, researchers from the United Kingdom found that nearly one-quarter of the world’s population is infected with latent TB.

According to WHO, TB infections impacted 10.4 million people in 2015, killing 1.8 million of those infected. Approximately 170,000 of those who died were children. India, Indonesia, China, Nigeria, Pakistan, and South Africa carry the greatest burden of TB and make up 60% of all cases globally. As deadly as TB infections caused by Mycobacterium tuberculosis (Mtb) can be, the majority of individuals who become infected never go on to have symptoms and instead experience what is known as latent TB (LTBI). Individuals with latent TB are likely to not be aware of the fact that they carry the bacteria, as they do not suffer from a bad cough, chest pain, fatigue, weight loss, or other symptoms that mark TB disease, and they cannot transmit TB to others. Latent infections can be diagnosed through a skin or blood test, and despite the lack of symptoms, people with latent TB still need to undergo an antibiotic treatment regimen that can last between three and nine months in order to prevent the onset of active disease.

Of those with latent TB, 5% to 10% will go on to develop TB disease at some point in their lives, most often within the first two years of Mtb infection. Researchers from the London School of Hygiene and Tropical Medicine and the University of Sheffield in the United Kingdom recently studied worldwide rates of LTBI, and their findings were published in the open-access journal, PLOS Medicine. They conducted their study in support of WHO’s End TB Strategy, the agency’s global plan to cut TB-related deaths by 95% and TB cases by 90%, all by the year 2035.

Using a combination of country trends and historical demographic data, the researchers estimated infection risk for 168 countries. They assessed the size and worldwide distribution of LTBI, studying scenarios that seek to answer questions such as: what number of active TB cases would arise from the current pool of people with latent infections if all TB transmission stopped immediately? With data on skin test surveys and WHO estimates on TB prevalence, the study found that 1.7 billion people worldwide had LTBI in 2014, making up 23% of the global population. With the rate of latent infections that progress to active disease, currently 1% of the world population, or 56 million individuals, were infected in the last two years and are at high risk of developing TB disease, and 11% of those cases are believed to be with an isoniazid-resistant strain.

“Treatment for LTBI still relies heavily on isoniazid, either as monotherapy or as part of a combination regimen,” wrote the authors in the study. “We found that just under 11% of all recent Mtb infections are likely to be isoniazid-resistant, with much higher rates in some regions, and this proportion is likely to increase. While less common, rifampicin resistance also has the potential to threaten the usefulness of rifampicin-containing prophylactic regimens. New treatments that bypass the rising resistance to isoniazid and rifampicin are needed to fully operationalize interventions to test and treat LTBI.”

In the study, the researchers found LTBI was most prevalent in Southeast Asia, the Western Pacific, and African regions, which together accounted for 80% of all latent TB cases. The Americas, Europe, and Eastern Mediterranean regions had a low rate of latent TB of less than 17%.

Based on their findings, the authors noted that current latent infection rates coupled with the active TB disease cases likely to arise pose a serious challenge for the goals set by the End TB Strategy. They call for updated policies, improved diagnostic and testing, a vaccine to prevent the progression of latent infection to active TB, and better social and economic conditions to make significant reductions in TB disease worldwide.