Tuberculosis due to Environment, Biology, or Both?

Abstract

In most studies on the subject, persons with diabetes mellitus (DM) have a twofold to fourfold higher incidence of active tuberculosis (TB) than nondiabetic persons. 1 Centers for Disease Control and Prevention Targeted tuberculin testing and treatment of latent tuberculosis infection. MMWR Morb Mortal Wkly Rep. 2000; 49: 1-54 PubMed Google Scholar Furthermore, some studies report that diabetics may present with more advanced disease at the time of TB diagnosis and may have an increased mortality rate. 2 Koziel H Koziel MJ Pulmonary complications of diabetes mellitus. Infect Dis Clin North Am. 1995; 9: 65-96 PubMed Google Scholar The predisposition of diabetics to infections that are normally controlled by cell-mediated immunity (eg, infection with Mycobacterium tuberculosis) may result from one or more defects of pulmonary host defense, including conditions that interfere with normal clearance mechanisms or that impair pulmonary immune cell function. The greatest difficulty in studying DM as an independent risk factor for the development of TB is the presence of potential confounding variables. These variables include other coexistent medical conditions (eg, malnutrition, chronic renal disease) and personal behaviors, such as smoking and alcohol abuse, that may further weaken host defenses. 2 Koziel H Koziel MJ Pulmonary complications of diabetes mellitus. Infect Dis Clin North Am. 1995; 9: 65-96 PubMed Google Scholar Alternatively, some of the differences in the risk for TB disease may be more attributable to exposure differences. Hendy and Stableforth 3 Hendy M Stableforth D The effect of established diabetes mellitus on the presentation of infiltrative pulmonary tuberculosis in the immigrant Asian community of an inner city area of the United Kingdom. Br J Dis Chest. 1983; 77: 87-90 Abstract Full Text PDF PubMed Scopus (28) Google Scholar described findings from a group of diabetic TB patients living in the United Kingdom. Asian diabetic TB patients presented with a higher incidence of cavitary disease and sputum-positive status than did a control group of non-Asian diabetic TB patients. This was presumably due to higher dose exposure to M tuberculosis. This higher exposure risk of the Asians appeared to be associated with their living in settings that replicate and concentrate the social behaviors and conditions of their birth communities. Even though DM patients were shown to be powerfully predisposed to TB in the era before insulin therapy, some experts now consider this risk to be rather modest 4 Iseman MD A clinician's guide to tuberculosis. Lippincott, Williams & Wilkins, Philadelphia, PA2000 Google Scholar ; in studies of diabetes as a risk factor for TB, investigators need to consider other risks, such as exposure, when comparing differences between populations. In this context of uncertainty, what might account for a specific type of TB disease presenting in diabetic patients at a much higher rate than in nondiabetic TB patients?