This study provides novel characterization of the heterogeneity and determinants of infectiousness of HIV-positive TB patients by applying molecular strain characterization to track airborne TB transmission to guinea pigs. This research has for the first time (to our knowledge) demonstrated that amongst HIV-positive patients TB infectiousness is extremely variable, that a few HIV-positive patients were highly infectious, and that inadequately treated MDR TB patients accounted for the great majority of TB transmission. In contrast to seminal studies of TB transmission using a similar guinea-pig method of detection half a century ago, this study was conducted in a real-life busy ward in a low-resource setting with unselected patients, composed of a heterogeneous mix of new and established diagnoses of drug-susceptible and drug-resistant TB, with varying treatment regimens. These results therefore have important implications for TB infection control, especially in the era of increasingly integrated TB and HIV care and the emergence of XDR TB strains.
Average patient infectiousness over the whole study period for these HIV-positive TB patients with high rates of MDR TB was up to six times greater than that calculated for the heterogeneous mix of patients in the 1950s studies (q = 1.25) [4
]. However, this average masks considerable variability in infectiousness between patients. Three highly infectious patients were observed, all with MDR TB, with q-values of 40, 52, and 226. It should be noted that these q-values reflect TB transmission from humans to guinea pigs. The infectious dose of M. tuberculosis
for humans is unknown; hence the concept of infectious quanta used in airborne infection models [27
]. For fully virulent M. tuberculosis
strains just one droplet nucleus can establish infection and disease in guinea pigs, but for strains of reduced virulence for guinea pigs, up to four aerosolised colony-forming units may be required to establish a single pulmonary primary focus [30
]. Thus some caution is needed in comparing the infectiousness of patients in this study with published q-values calculated for human-to-human transmission, such as q = 13 for an untreated office worker who infected 27 coworkers over 4 wk prior to diagnosis [31
], and q = 250 for an outbreak associated with intubation and bronchoscopy of a TB patient [11
]. However, comparisons can be drawn with q-values calculated for Riley's study: q = 1.25 average for all patients; q = 60 for the most infectious case, with laryngeal TB [27
]. Direct comparisons should, however, be made cautiously owing to methodological differences between the studies such as the type of guinea pig used and the cutoff for a positive skin test [32
]. This model for identifying infectious patients required TB strains to be not only transmissible to guinea pigs, but also sufficiently virulent to cause disease from which a strain could be recovered with corresponding spoligotype. Animals with positive PPD skin test conversions but culture negative for TB were observed throughout the study. Due to the continuous exposure to ward air, mixed infections might be expected in the animals, but in fact were not observed despite the culture of separately dissected lung foci. Because they are relatively uncommon, mixed infections were not specifically sought in patients [33
The relative infectiousness of patients in this study was highly variable, and a patient with MDR TB was found to be highly infectious, producing 226 infectious quanta/h. The TB strain responsible, not a Beijing strain, was also seen in a second highly infectious MDR TB patient, producing 52 infectious quanta/h. This observation suggests a potentially strain-related factor involved in transmissibility, perhaps an enhanced ability to survive aerosolisation and the physical stresses of being airborne. An alternative explanation might be an effect on disease phenotype. It is interesting to note that both patients had fever and cough and produced large volumes of sputum, between 10 and 90 ml daily. Neither patient had cavitation on chest X-ray. Although no formal ear-nose-throat assessment was documented, neither was diagnosed with laryngeal TB. Both patients also spent time on the ward untreated. The first was untreated for the first 11 d of a 32 d admission before second-line drugs were commenced, due to difficulties in access to medications. The second patient had recently been commenced on suboptimal treatment (standard first-line therapy plus streptomycin) and this was suspended for ten of 26 ward days because of adverse effects.
The finding that sputum smear positivity was associated with TB transmission concurs with previous studies [5
]. The effect of treatment on the infectiousness of TB patients is also well known [2
], with numbers of viable bacteria falling precipitously following initiation of effective chemotherapy [34
]. Whilst some data suggest that apparent cure of MDR TB may be achieved with first-line drugs [36
], other studies have shown poor outcomes for such patients [38
]. The current study shows how suboptimal treatment of MDR TB patients is likely to facilitate ongoing TB transmission. There is conflicting evidence concerning the relative transmissibility of MDR versus drug-susceptible TB strains [14
]. In this study, patients with MDR TB were significantly more likely than those without MDR TB to transmit TB to guinea pigs. However, this finding should be interpreted with caution because of colinearity with suboptimal treatment (5 of 6 identified infectious MDR TB patients were on suboptimal regimens, and the other had treatment initiation delayed for 11 days whilst suitable medications were acquired). Regardless, the high relative infectiousness of inadequately treated MDR TB patients demonstrated in this study underscores the importance of prompt specific treatment guided by rapid drug-susceptibility testing, rather than restricting MDR TB testing and specific therapy to patients who survive failing empiric first-line therapy, as currently happens in most low-resource settings. It also has important implications for hospital policies that allow suboptimally treated MDR TB cases to be cared for in multi-bedded rooms.
The highly infectious nature of some of the HIV-positive MDR TB patients identified in this study has important implications for TB infection control. Administrative control measures that facilitate the rapid diagnosis, isolation, and prompt treatment of such patients are paramount. With increasing congregation of infectious and susceptible individuals not only in hospitals but also in such settings as antiretroviral therapy roll-out, HIV antenatal care, and voluntary counselling and testing facilities [39
], environmental control measures are also of great importance. As the infectiousness of a TB source increases, the relative protection provided by dilutional room ventilation decreases [31
] and may become inadequate at the relatively low levels of air exchange usually provided by mechanical ventilation. High rates of ventilation would be required to provide protection from the extremely infectious newly diagnosed MDR TB case observed in this study. Achieving this through mechanical means is an expensive solution for much of the world where TB is most prevalent. In contrast, well-designed natural ventilation [40
] provides high ventilation rates for little cost, and furthermore is highly applicable to areas such as crowded waiting rooms where infectious, untreated TB patients are most likely to be encountered. TB infection control must be a priority in the current roll-out of enhanced HIV care, and should be carefully considered in the design and construction of any new infrastructure for such programmes.
The need for strengthened TB infection control is also highlighted by the recent outbreak of XDR TB amongst HIV-coinfected patients in South Africa; this outbreak was predominantly nosocomial and resulted in extremely high mortality [41
]. The variability of infectiousness of patients demonstrated in this study highlights the usefulness of a potential test for TB infectiousness that would allow targeted isolation of the most infectious patients in the settings where isolation facilities are sparse, as is unfortunately the case in much of the world where TB is most prevalent. One patient in our study, with MDR TB, infected over half of the guinea pig colony. The development of tests that allow early identification and isolation of such patients in a clinical setting is a research priority.
There are some limitations to this study. The first is the incomplete set of spoligotyping data, with results in only 49 of 118 pulmonary TB admissions. Despite this deficiency, ten of at least 12 infectious patients were identified. It is possible that transmission occurred from other patients for whom spoligotyping was unavailable, but certain factors suggest that this is not the case. Most patients without spoligotype results had negative sputum cultures, and whilst smear-negative TB transmission occurs [8
], smear-positive patients account for the majority [5
]. In this study, 16 culture-positive patients, of whom four were smear positive, had no spoligotype result. Fortuitously, these patients were either temporally or phenotypically (drug susceptibility pattern) unrelated to the ten guinea pig clusters linked with infectious patients, excluding them as coinfectors. Indeed the two clusters of MDR TB guinea pigs with unidentified infectious sources became infected at times corresponding to the ward residency of smear-positive MDR TB patients without spoligotype results. We cannot exclude the possibility that guinea pig infections occurred from staff or visitors with TB, however all staff and visitors wore particulate respirators. It is possible that the large monoclonal outbreak observed in the guinea pigs was in fact made up of more than one strain. However, the concordant drug susceptibility data and epidemiological match with a patient on the ward at an appropriate time prior to the infections obviates the need for secondary typing of strains. The Wells-Riley model has inherent limitations [42
], but these do not influence evaluations of relative infectiousness, and it allows comparison with published values of TB infectiousness calculated using the same model. The design of this study did not permit determination of the duration of patient infectiousness because of the interval of one month between skin tests, and the variability in the period required for these guinea pigs to become PPD positive following TB infection. It is possible that values for patient infectiousness are underestimates, because the entire period of a patient's hospital admission was used for the exposure duration variable in calculations, and it would normally be expected for patient infectiousness to tail off once treatment was initiated, although this would not be the case with suboptimal treatment. However, in univariate analyses patient days on the ward was not significantly associated with TB transmission. The relatively narrow age range and the small number of women amongst the patients is a further limitation of this study, because both young age and male sex have been associated with TB transmission to contacts [15
]. Because all patients were HIV positive, our study was unable to yield evidence concerning the infectiousness of HIV-positive versus HIV-negative MDR TB patients, and this could be a future area of study using our airborne infection facility.
In conclusion, this study has demonstrated the potential of HIV-positive patients with MDR TB to be highly infectious. With the great majority of TB transmission in this study occurring from inadequately treated MDR TB patients, these results identify the importance of early drug susceptibility testing and initiation of effective chemotherapy for drug-resistant TB to prevent ongoing transmission and facilitate TB control. Furthermore, this study highlights the importance of environmental control measures to prevent airborne TB transmission in crowded health care settings, especially in areas with a high prevalence of HIV infection and drug-resistant TB, and in today's era of emerging XDR TB. HIV-positive patients with unrecognised or inadequately treated MDR TB coinfection may be highly infectious, and effective TB infection control measures are essential to prevent health care facilities from disseminating drug-resistant TB.