India has the highest number of people in the world with active tuberculosis. The incidence of TB is 2.2 million and prevalence is 3.1 million. Similarly, the prevalence of diabetes is 65 million in the country.
But the number of TB cases would see a further rise or decline may not be seen despite the best efforts to detect and treat TB. The reason — diabetes almost triples the risk of developing active TB and is also a risk factor for adverse TB treatment outcomes. Even hyperglycaemia associated with prediabetes can increase a person’s risk of developing active TB.
“Although not completely understood for TB specifically, it is known that diabetes impairs both the innate and the adaptive immune system and therefore the body’s capability to fight microorganisms. This increases the risk of progression to active TB disease in people who have become infected with Mycobacterium tuberculosis,” Knut Lonnroth of the Global TB Programme, WHO, Geneva told this Correspondent in an email. He is the first author of one of the three papers published today (September 4) in the journal Lancet Diabetes & Endocrinology.
Conversely, TB has an effect on diabetes. It can not only worsen the control of blood sugar but also complicate clinical management of diabetes. “TB can, like most infectious diseases, worsen glycaemic control through several pathogenic mechanisms related to the stress on the body caused by the infection, which can result in increased insulin resistance,” he explained.
There is a body of epidemiological evidence showing a causal link between the two diseases. Globally, 15 per cent of TB cases are estimated to be caused by diabetes, accounting for about one million cases of diabetes-associated TB per year. India and China alone account for 40 per cent of diabetes-associated TB cases. The number of adult TB cases associated with diabetes in India is 302,000; it is 156,000 in the case of China.
The chances of TB treatment failure, death and relapse after cure are high in patients who are also diabetic. “The mechanism is largely the same as the mechanisms that increasethe risk of developing TB. The negative impact of diabetes on the immune system can contribute to poor response to TB treatment, and lead to relapse after TB has been cured,” noted Dr. Lonnroth.
According to one of the papers, a study of 163 countries between 1990 and 2004 showed that the increase in TB incidence was nine times higher in countries that also witnessed an increase in diabetes prevalence. The prevalence of diabetes has been increasing in India.
According to an analysis, the increase in diabetes prevalence from three per cent in 1998 to 3.7 per cent in 2008 resulted in 900,000 additional TB cases globally. “These diabetes-associated cases might have contributed to the absence of a decrease in tuberculosis incidence during 1998–2008, despite substantial improvements in TB diagnosis and treatment,” notes one of the three papers.
The incidence of diabetes-associated TB is only bound to grow bigger as the International Diabetes Federation has forecast that the diabetes prevalence would grow from eight per cent in 2013 to 10 per cent in 2035. Lack of diagnosis and poorly controlled diabetes “might be” a dominant factor causing diabetes-associated TB.
“Places where diabetes prevalence is high or is increasing quickly have the most to lose or gain from failures and successes in diabetes strategies,” notes a paper by Dr. Lonnroth.
Though the causal link is known, there is very little known on the “optimum treatment strategy for concurrent TB and diabetes.” Also, whether glucose control can “partly or fully mitigate” TB treatment failure, relapse and death is not known. Whether TB treatment should be different in people with diabetes is unknown.
The WHO’s 2011 framework requires bidirectional screening — patients with TB be screened for diabetes and vice versa. Two large studies conducted using the same methodology in India and China are expected to provide insights into the best clinical approaches to conduct bidirectional screening and management of the diseases.
“Although the scientific evidence on the link between TB and diabetes has been around for a long time, it has been rigorously assessed and consolidated only recently. Now we have compiled solid evidence, and I hope there will be no further delay in pursuing better coordination of diabetes and TB care and prevention,” Dr. Lonnroth noted.