Among the approximately 1.3 million new cases of TB in China each year, 35.2 percent are drug-resistant, 5.7 percent are MDR TB, and about 0.5 percent are XDR TB, according to a 2007–2008 nationwide survey. The most important question to ask about this drug resistance, said Gao, is where it came from. Was it primary, in that a drug-resistant strain was transmitted from one person to another? Or was it acquired, in that a strain developed drug resistance during treatment because of noncompliance with a treatment regimen, poor-quality drugs, or treatment with a single drug? The significance of the distinction is that primary drug resistance indicates a need for better TB control to interrupt transmission, whereas acquired drug resistance indicates a need for better patient management to prevent the evolution of resistance.
A simple way to make this distinction is by referring to the history of care. If a patient has not been treated before, drug resistance is often assumed to be primary and the result of transmission. If a patient has been treated previously, drug resistance is often assumed to be acquired. WHO data indicate that 6−10 percent of new TB cases and 13–60 percent of cases in previously treated patients are MDR. One possible conclusion that could be drawn from these data, said Gao, is that previously treated patients should be the focus of improved patient care. In most parts of the world, however, new cases make up more than half of MDR TB incidence. In Shanghai, for example, from 2002 to 2006, 59 percent of the MDR TB cases were among new TB patients, indicating the transmission of drug resistance. The same observation can be made for XDR TB patients, although the total numbers are much smaller. In Shanghai from 2004 to 2007, 6 of 11 XDR TB patients represented new cases.
Another way to track transmission is by genotyping different strains. If two people have strains with the same genotype, transmission can be assumed. A search of the biomedical literature reveals several cases in which drug-resistant strains were responsible for clusters of MDR and XDR TB. In a 2009 study conducted in Samara, Russia, for example, 63 of 189 patients with XDR TB fell into two large clusters of similar genotypes. Gao observed that this means one of the patients could transmit, on average, to 31 or 32 patients.
In an ongoing study in China’s Shandong, Shanghai, and Sichuan provinces, Gao and his colleagues found different clustering rates (i.e., clusters of TB with similar genotypes) in different geographic areas. According to preliminary results, 39 percent of TB patients in Shandong Province fell into clusters of similar genotypes, indicating primary transmission from
1 This section is based on the presentation of Qian Gao, Fudan University.