TB, an airborne infectious disease caused by bacteria known as Mycobacterium tuberculosis, has afflicted man for millennia. It has claimed over a billion lives in the last 200 years, more than any other infectious disease in history. The World Health Organization (WHO) declared TB a global emergency in 1993. Twenty –five years on, TB is reported by the WHO as the leading cause of death globally from a single infectious agent, with an estimated 1.6 million deaths and 10 million persons afflicted in 2017.
Associate Professor Cynthia Chee and Professor
Leo Yee Sin
Why is TB still a major global health threat today?
TB mostly affects the lungs. Persons with active TB disease may have chronic, low grade and intermittent symptoms such as chronic cough and weight loss which may not be brought to medical attention. They may remain undiagnosed for several months or even years during which they transmit the TB germs via aerosols generated especially by coughing to those with whom they share the same air space on a prolonged basis. Among their close contacts who acquire the infection, one in 10 will progress from latent TB infection (a state in which the bacteria are kept under control by the body's immune system and the person is well) to active TB disease, in which the germs are actively multiplying and the patient has symptoms. Half of those who develop active disease do so within the first two to five years after infection.
The highest priority in TB control is the early detection and successful treatment of persons with active, infectious TB to cut the chain of transmission in the community. Treatment of active TB disease however is difficult for many patients as it requires strict adherence to multiple drugs for 6 to 9 months, failing which patients risk remaining infectious, developing drug resistance and future relapse. Many countries where TB is an endemic struggle with inadequate healthcare infrastructure and systems , making the prompt diagnosis and successful treatment of active TB disease is difficult. Even in developed countries, lack of political will in implementing programmatic public health measures impede progress in TB control. The next priority in TB control is contact investigation to trace close contacts of infectious TB patients to detect and treat latent TB infection or active TB in these persons. This requires the cooperation of the TB patient in accurately identifying his or her close contacts, and that of the identified contacts to undergo testing and treatment if indicated. This resource intensive activity is accorded high priority in low TB incidence, high income countries but is a challenging undertaking for the overburdened, underfunded TB programmes of developing countries.
Over the last 20 years, Singapore's national TB programme has implemented three key interventions, namely Directly Observed Therapy (DOT) for the majority of active TB patients, surveillance of the treatment progress and outcome of all active TB patients, and screening of close contacts who are given preventive treatment if found to have latent TB infection. These measures served to decline our TB rate from 58/100,000 population in 1998 to a historical low of 35/100,000 population in 2007. This trend reversed in 2008 and our rate has remained at ~40 / 100,000 population since. This is ten times higher than that of the United States, Canada, Australia and New Zealand.
The obvious question is why our TB rate have not further declined in the past decade. We believe multiple factors account for this. Recent changes in our population demographic i.e. our rapidly aging population who acquired latent TB infection in the 1950s and 60s when Singapore's TB rate was very high (~300/100,000 population) and the influx of permanent and transient migrants from high TB incidence countries now form an increased pool of persons with latent TB from which active TB cases arise. A survey by the National University of Singapore reported latent TB prevalence of almost 30% in persons older than 70 years old, and in citizens and PRs originating from nearby high TB prevalence countries, as compared to ~5-10% among younger local-born Singapore residents.
We also believe that the stagnation in our TB rate is due to on-going community transmission resulting from delay in TB diagnosis amidst a backdrop of urban, high density living. Data from the national TB registry show that almost one in five infectious TB cases had more than 3 months of cough before they were diagnosed. It is also the experience of the TB Control Unit that patients with infectious TB are reluctant to identify their close contacts who are not their household or family members. For the TB rate to be further brought down in Singapore, a concerted effort to address these issues is required.
How can the community help in the fight against TB? TB disproportionately affects the poor and disadvantaged in society and too often, the diagnosis of TB carries with it fear of stigmatization, loss of livelihood, and rejection by family, colleagues and friends. Financial and non-financial barriers to early diagnosis of TB must be removed. The community can show its support for the TB patients by encouraging and facilitating adherence to DOT. The community should also appreciate TB patients for identifying their close contacts. Ultimately, this will be in the interest of the local and global community.
Associate Professor Cynthia Chee is Senior Consultant and Director of the Tuberculosis Control Unit; and Director, Singapore TB Elimination Programme at the National Centre for Infectious Diseases.
Professor Leo Yee Sin is the Executive Director of the National Centre for Infectious Diseases.