This has been a good decade in the fight against tuberculosis (TB). We are likely to achieve the UN Millennium Development Goal of cutting TB prevalence and mortality in half by 2015 from 1990 levels. At least a dozen new vaccines and drug candidates are in clinical trials, and the WHO has endorsed a new diagnostic test called the GeneXpert.
This progress is all the more important given the complacency that led to a complete standstill in research and development for new TB interventions toward the end of the 20th century. The TB drugs currently in use were developed between 1950 and 1970: The Bacille Calmette–Guerin vaccine is almost 100 years old, while microscopic detection of bacilli in sputum — the most widely used diagnostic test for TB — was developed 130 years ago.
It is no wonder that the efficacy of these tools has weakened. The current vaccine prevents severe TB in infants, but not the most prevalent pulmonary TB in all age groups and the microscopic test gives false results in nearly half of all cases.
We like to think of TB as a malady of the past, but 9 million people develop the active disease every year and one in five will die as a result. This places the tubercle bacillus second only to human immunodeficiency virus (HIV) on the list of top microbial killers.
One-third of the world’s population is infected with the causative agent, though only one in 10 develops the active disease. The bad news is that infected individuals carry the agent with them for their entire lives. When one’s immune system weakens, the disease may erupt. With the emergence in the 1980s of HIV, which compromises the immune system, TB re-emerged and became the No. 1 killer of HIV-infected individuals. About 15 million people suffer from co-infection of HIV and Mycobacterium tuberculosis, the primary causative agent in most TB cases.
Moreover, the tubercle bacillus engages in trench warfare, gaining resistance to conventional antibiotics and canonical vaccination approaches, as well as delaying diagnosis and determination of drug sensitivity. Whereas pandemics appear suddenly, spread rapidly and raise enormous fears of an impending threat, TB has been spreading slowly, but consistently, for tens of thousands of years, patiently waiting for new opportunities.
Treatment of TB requires a minimum of three drugs administered over at least six months. Compare this with antibiotic treatment of, say, a urogenital infection, which lasts a few weeks at most. As a result, compliance with TB drug treatment is particularly weak, paving the way for the emergence of multidrug-resistant TB, which can no longer be treated by conventional therapeutic regimes. About 50 million people are infected with tubercle bacilli that are multidrug-resistant.
While treating multidrug-resistant TB remains possible, doing so is arduous, with treatment time being approximately two years and requiring drugs that are neither as efficient nor as benign as canonical drugs — and at a cost that rises by a factor of 10 to 100. While this additional cost can be borne by health care systems in the rich world, it is excessive for poor countries, implying no, or insufficient, treatment for those infected with this strain of TB.
Moreover, in 85 countries, extensively drug-resistant TB, which is virtually untreatable, has been diagnosed. Surgical resection of affected lungs has become the treatment of choice in many extensively drug-resistant-TB hotspots — welcome back to the pre-antibiotic era.