The evolution of extensively drug resistant tuberculosis (XDR-TB): history, status and issues for global control.

Tuberculosis (TB) is a devastating disease caused by Mycobacterium tuberculosis that killed an estimated 4000-5000 person each day during 2005. Although infections with drug sensitive strains can be effectively cured with a 6 to 9 month regimen of multiple antibiotics, the inability to deliver and complete appropriate courses of therapy on a global level has led to the selection of resistant strains over the past 50 years. The selection and spread of multiple drug resistant M. tuberculosis continued for decades leading to two operationally distinct forms of the disease, multiple drug resistant (MDR-TB) and extensively drug resistant (XDR-TB). The estimate for MDR-TB and XDR-TB cases for 2005 were 424,000 and 27,000 respectively, and the situation is worst in areas with high incidences of HIV infection. The outcome was predictable based on basic microbiological principles, and the resultant and future epidemic effectively modeled mathematically. This situation was brought to the forefront when an outbreak of XDR-TB occurred in Tugela Ferry, KwaZulu-Natal, South Africa, in 2005 and began to spread. Unfortunately, we do not know the true extent of XDR-TB globally. However, all signs point to an emerging epidemic of TB infections that will be difficult, if not impossible to cure. A few new drugs are in clinical trials, but it is too early to know the final outcome; some may fail, and none will be available for several years. The situation will continue to worsen unless more resources are made available to discover and deliver better treatment options.