Drug resistance arises through either acquired resistance or transmission. As discussed in Chapter 3, there is evidence that transmission plays an important role in the development of MDR TB. Of the 511,000 individuals estimated by the World Health Organization (WHO) to have MDR TB in 2007, 57 percent had never previously been treated for TB and thus could not have acquired resistance from prior treatment. The remaining 43 percent were retreatment cases; however, it is known that reinfection through transmission also occurs in this population. This chapter reviews what is known about the transmissibility of TB from person to person, as well as experience with infection control programs to reduce or eliminate transmission. Overall, many presenters emphasized that a number of fundamental questions about transmission remain unanswered and that, if infection control is to be successful, much more research and evaluation of these issues will be necessary.
According to Mphahlele, experiments done for more than half a century have demonstrated great variability in the infectivity of patients with TB. Guinea pigs have often been used in these studies because they are highly susceptible to infection by human M.tb., more so than mice and rabbits and perhaps as much as AIDS patients; in addition, their immunological response is similar to that of humans.
Beginning in the 1950s, Riley and colleagues exposed guinea pigs to untreated TB patients to measure the infectiousness of M.tb. Of the 156 guinea pigs, 71 were found to be infected at the end of the 2-year study, demonstrating the ease of transmission in the absence of direct contact (Riley et al., 1959). Recent studies conducted at the Airborne Infection Research (AIR) facility in Witbank, Mpumalanga Province, South Africa, examined both the infectivity of MDR TB and the question of whether transmission can be interrupted. The infectivity experiment, which involved 360 guinea pigs exposed to 26 MDR TB patients, revealed that some infecting MDR and XDR TB strains may be immunogenic but not sufficiently virulent to cause progressive disease in guinea pigs and that guinea pigs may be clearing TB infection. In that study, 75 percent of the guinea pigs became infected, and of those strains recovered from the guinea pigs, only two matched the strains of patients. Despite large positive skin test results, however, many of the guinea pigs showed no histological evidence of residual TB infection, and the burden of TB infection was higher among recent large skin test converters than among more remote converters. Many of the guinea pigs