The prevalence of isoniazid and rifampicin resistance of Mycobacterium tuberculosis

Abstract

The World Health Organization (WHO) estimated that eight million new cases of tuberculosis (TB) occur every year and that one-third of the world’s population is infected with Mycobacterium tuberculosis (M. tuberculosis). With the increase in HIV/AIDS in the 1980’s, an increase in transmission of TB led to an increase in TB incidence. A study showed that South African adults (ages 15 to 49) will suffer 278 154 deaths between 2008 and 2017 if current control measures are continued. A M. tuberculosis strain that is resistant to isoniazid (INH) and rifampicin (RIF) used in the treatment of TB is known as a multi-drug resistant (MDR-TB) strain. In extensively drugresistant tuberculosis (XDR-TB) the M. tuberculosis strains are not only resistant to INH, RIF and any one of the fluoroquinolones but to at least one of the three injectable second-line drugs such as amikacin or kanamycin. Unfortunately, many people with XDR-TB will die because it is virtually impossible to formulate an effective treatment before the resistance pattern of the M. tuberculosis strain has been identified. Bacteriological culture is considered the diagnostic gold standard and can identify mycobacteria in over 80% of TB cases, with a specificity of over 98%. However, culturing the mycobacteria takes 4 to 6 weeks and makes diagnosis and treatment a prolonged process. In this study 60 patients suspected of TB disease, from the Anti-retroviral (ARV) clinic at the Tshwane District Hospital (TDH) were collected from October 2008 to April 2009. This study evaluated the use of the QuantiFERON-TB GOLD ELISA assay in a high burden setting. Tshwane District Hospital, South Africa. The sensitivity and specificity of the QFT assay in the clinic were 30% (9/30) and 63% (19/30) respectively when compared to the gold standard culture results. Analysis suggested that the sensitivity of the QuantiFERON assay is determined by a limiting patient CD4 value of between 150 and 200. Real-time PCR assays were used for rapid identification of Mycobacterium spp and to determine the presence of isoniazid and rifampicin resistant genes of M. tuberculosis strains. The real-time PCR assay identified 28% (17/60) M. tuberculosis, 2% (1/60) M. kansasii and 70% (42/60) of the isolates Mycobacterium spp negative. No M. avium were detected. The 17 M. tuberculosis positive specimens were further analysed for the presence of INH and RIF resistance genes. All 17 specimens had either no mutation or one or more mutations at the specific gene targets (rpo1, rpo2, katG and inhA). This study showed several possibilities for the use of both an immunological assay as well as molecular methods for the diagnosis of TB. This study suggested that in terms of routine diagnosis of TB in high HIV prevalence settings the QFT test should be used with caution. Realtime PCR for both detection and identification showed useful results and can be used together with culture results to improve turnaround times for TB diagnosis. Copyright