The authors point out the importance of tuberculosis throughout the world: "Tuberculosis remains the leading cause of infectious disease death among adults worldwide," they say. "In recent years, drug-resistant TB has emerged as an expanding threat, with an estimated 489,000 new cases in 2006. Treatment of multidrug-resistant TB (MDR-TB) is more than 100 times as costly as treatment of drug-susceptible TB, requiring intensive case management for its prolonged (18-24 months) and more toxic treatment course." As might be predicted, treatment success is lower for patients with MDR-TB in comparison to those whose TB infections are sensitive to drugs. In 2005, extensively drug-resistant TB (XDR-TB) was defined to categorize for straings with even greater drug resistance that had emerged. This was associated with the setting of high Human Immunodeficiency Virus (HIV) prevalence.
To describe the epidemiology of recent TB infection, J. Peter Cegielski, M.D., M.P.H., from the Centers for Disease Control and Prevention, Atlanta, and colleagues, investigated the last 15 years of U.S. national surveillance data, focusing on all culture-confirmed cases of TB reported in all 50 states and the District of Columbia between 1993 and 2007. They identified the unique characteristics of XDR-TB And MDR-TB in contrast with sensitive TB. XDR-TB was defined as strains resistant to isoniazid, a rifamycin, a fluoroquinolone, and at least one of amikacin, kanamycin, or capreomycin in drug susceptibility tests.
The authors summarize the results: "A total of 83 cases of XDR-TB were reported in the United States from 1993 to 2007," the authors report. "The number of XDR-TB cases declined from 18 (0.07 percent of 25,107 TB cases) in 1993 to 2 (0.02 percent of 13,293 TB cases) in 2007..." They continue, noting that "40 XDR-TB cases reported during 1993-1997, 25 (62 percent) were known to be HIV-infected. During 1998-2007, only 6 (14 percent) of 43 XDR-TB cases were known to be HIV-infected
In a total 83 XDR-TB cases, most were between the ages of 25 and 44 years; 64% were males; 53% were born in the US and unemployed; 40% were Hispanic, and 4% were in health care workers. More subjects presenting with XDR-TB tended to be Hispanic or live in correctional facilities. The authors discuss the patients' outcomes: "Twenty-six XDR-TB cases (35 percent) died during treatment, of whom 21 (81 percent) were known to be HIV-infected. ... Death rates were nearly two times greater than among MDR-TB cases and more than six-times greater than among drug-susceptible TB cases. Infection with HIV played an important role in both the occurrence and outcomes of XDR-TB cases."
The authors conclude that TB control should be a major point of focus in the future: "Preventing the further emergence of drug resistance is paramount and must include not only TB program strengthening to ensure that patients complete their treatment regimen but also general health system interventions to improve infection control. Greater vigilance regarding drug resistance must include systematic second-line drug susceptibility testing according to published guidelines. Lessons gained from MDR-TB in the 1990s should be applied: Patients must be identified early, treated effectively, and assisted to complete treatment, and infection control precautions must be in place to prevent further emergence and transmission of XDR-TB."
Extensively Drug-Resistant Tuberculosis in the United States, 1993-2007
N. Sarita Shah, MD, MPH; Robert Pratt, BS; Lori Armstrong, PhD; Valerie Robison, DDS, MPH, PhD; Kenneth G. Castro, MD; J. Peter Cegielski, MD, MPH
Written by Anna Sophia McKenney