The researchers said that three studied conducted by them had shown that type 2 diabetes, especially when it involves chronic high blood sugar, is associated with altered immune response to TB.
The said that their studied also showed that patients with diabetes and TB take longer to respond to anti-TB treatment, and that patients with active tuberculosis and Type 2 diabetes are more likely to have multi-drug resistant TB.
Writing about their findings in the journal Clinical Infectious Diseases, the researchers said that the immune systems of patients with Type 2 diabetes and tuberculosis could respond differently compared with patients with TB alone.
"This immune impairment may be what makes patients with diabetes so susceptible to TB," said Dr. Susan P. Fisher-Hoch, professor of epidemiology.
Dr. Blanca I. Restrepo, assistant professor of epidemiology, said that her team found that innate and type 1 cytokine responses were significantly higher in patients with tuberculosis who had diabetes than in the control group of patients with TB and no diabetes.
The effect was consistently and significantly more marked in diabetic patients with chronic hyperglycemia, or uncontrolled high blood sugar, said the researcher.
Diabetes results in the body's ineffective use of insulin. If left uncontrolled, the chronic high sugar in the bloodstream can affect the critical immune system and damage the body's systems, especially the nerves, the retina of the eyes and blood vessels.
"These findings are the opposite of what we were expecting. These innate and type 1 cytokines are typically associated with TB protection, but in patients with diabetes, it appears the cytokines are not effective. Diabetics may have more advanced TB with more bacteria, and hence, more stimulation for secretion of type 1 cytokines," Restrepo said.
The researchers wrote: "More detailed knowledge of the underlying mechanisms should focus on the effect of chronic hyperglycemia on the immune response to help in understanding the enhanced susceptibility of diabetic patients with tuberculosis."
The team said that their findings also showed that patients with diabetes were more at risk of developing multi-drug resistant tuberculosis.
The researchers said that they had observed that almost six percent of diabetics living along the Texas-Mexico border had TB that was resistant to rifampin and isoniazid, common medications for tuberculosis.
They further said that 30 per cent of those with multi-drug resistant tuberculosis also had Type 2 diabetes.
"It is possible that impaired immunity in Type 2 diabetes increases susceptibility to infection with resistant strains," they write in the online edition of the Scandinavian Journal of Infectious Diseases.
Dr. Joseph B. McCormick, the senior author of the three studies, said that the findings of the research cast new light on a long-known correlation between diabetes and tuberculosis.
"It opens a door to doing something about it. We can educate physicians and offer more TB screenings. We have an opportunity to make sure patients are diagnosed correctly and that there is no delay in diagnosis," said McCormick, the university's James H. Steele Professor.
Fisher-Hoch said the research could help diagnosis TB patients who previously would not be considered at risk for contracting the airborne, contagious disease.
"The classic TB patient in this country is a younger male in an urban setting who may have alcohol and drug abuse problems and be HIV-infected. Our research shows older female patients who have never been in jail and have no history of alcohol and drug abuse or HIV infection are at risk of contracting TB if they have diabetes," she said.
"I think we are illuminating a very important association between TB and diabetes that had pretty well been overlooked. The public health aspect is that we are trying to make sure we can prevent and treat these patients, and when they are TB-infected, treat them better," she added.
She suggested that doctors screen patients for TB if they have diabetes and a chronic cough, and that patients with Type 2 diabetes take precautions.
"If they are visiting an area where there is a lot TB, they need to be careful. TB spreads in crowded places with poor ventilation," she said.