By Dr Tan Seow Yen, Head, Training and Education Office, Infectious Disease Research and Training Office, National Centre for Infectious Diseases
While tuberculosis (TB) and humans have coexisted for thousands of years, the understanding of the pathophysiology of TB continues to evolve. While the classical model of distinct latent and active forms of TB disease are well known to most, it is increasingly understood that the complex bacterial and host dynamics result in the pathology of TB disease falling on a spectrum.1
Nonetheless, the notion that certain cases of untreated latent TB can progress to active TB still holds true. Overall, without treatment, about 5% to 10% of infected persons will develop TB disease at some time in their lives.2 About half of those people who develop TB will do so within the first two years of infection. For persons whose immune systems are weak, especially those with untreated human immunodeficiency virus (HIV) infection, the risk of developing TB disease is considerably higher than for persons with normal immune systems. When the TB bacteria overcome the defences of the immune system and begin to multiply, it results in the progression from latent TB to active TB. Some people develop active disease soon after infection, while others progress to active disease later when their immune system becomes weak.
There are considerable differences between the two conditions. In terms of symptoms, persons with latent TB do not feel sick and do not have any symptoms, while most persons with active TB experience symptoms, which generally include unexplained weight loss, loss of appetite, night sweats, fever, fatigue and chills. Depending on site of infection, the symptoms experienced by the person could vary, such as prolonged cough, or coughing up blood in persons with pulmonary TB; and back pain in persons with spinal TB.
Persons with latent TB infection are not infectious and cannot spread TB infection to others. However, for active TB, only infections of the lungs and larynx (voice box) is contagious and capable of spread to others.
A latent TB infection is often diagnosed in an asymptomatic individual, who has either a skin test or blood test result indicating infection. It is normal that the chest X-ray would not show indication of infection, and the sputum tests for TB are expected to be negative. In short, aside from the positive blood or skin test, there are no symptoms and signs of active disease. The diagnosis of active TB is made, when there are symptoms and signs of active disease, and a positive test results which can be a sputum sample showing growth of the bacteria, or a positive nucleic acid amplification test.
There are treatments available for both active and latent TB, although the treatment regimen used are slightly different. Treatment of latent TB aims to prevent the progression to active TB, and usually involves a single drug, taken for a duration of a few months. Treatment for active TB is generally more intense, requiring the use of multiple drugs (mostly consisting of four drugs), and the treatment course is generally longer.
1 Tuberculosis, Jennifer Furin et al, Lancet 2019; 393: 1642–56
The article was also published in Volume 1 of Infectious Disease Intelligence here.