After an initial screen for co-prevalent disease
, we actively followed over 2000 household contacts of sputum smear positive TB cases for a total of 4312 person years. Twenty-six secondary cases were identified, giving an incidence rate of 603/100,000 person years. HIV positivity, despite a low prevalence in this population, was the one significant risk factor for progression to TB disease. We were able to calculate the incidence rates of secondary disease in relation to initial Mantoux and ELISPOT results and to show how many secondary cases were ‘captured’ by a positive test. Only 15 (71%) of the 21 secondary cases with recruitment results were positive on either test. Furthermore 4 Mantoux positive secondary cases were ELISPOT negative and 4 ELISPOT positive secondary cases were Mantoux negative. The results of this study have implications for the design of trials of new interventions, particularly vaccines, in Africa, and the revision of current guidelines for TB control.
The 33 (1.5%) co-prevalent cases and the 26 secondary cases identified in this study population can be compared with the number identified in other TB case contact studies, although few have presented incidence rates of secondary disease - which can only be calculated when the total person time of follow-up is known. Guwatudde et al 
conducted a case contact study in Uganda, following 1206 contacts of 423 TB cases for 2 years. As in our study, over half of the TB cases in these African TB contacts were found at an initial screen - they identified 51 (4.2%) co-prevalent cases and 25 secondary cases. The median age of the secondary cases was 31 years and 13 (52%) were HIV positive. The higher rate of HIV positivity compared to our study explains a large part of the higher prevalence of cases at screening and the larger relative number of secondary cases at follow-up. Furthermore, a higher proportion (66% vs 38%) of the Ugandan contacts were Mantoux positive at recruitment, suggesting a higher background rate of M. tuberculosis
infection. Egsmose et al 
, as part of a study in rural Kenya, found that 17 (4.3%) of 392 TB case contacts became ‘tubercle bacilli excretor’ TB cases within one year, after exclusion of those diagnosed with TB before enrolment. Puffer et al 
followed 1358 household contacts of 298 ‘sputum positive’ American TB cases in the 1940s for 8754 person years and identified 30 cases of clinical TB: the overall attack rate of clinical TB was 690/100,000 person years. Ferebee et al 
found 479 (1.9%) cases of TB in 25,512 American household contacts of TB cases (not limited to sputum smear positive cases) in the 1950s. Of the remaining contacts 12,594 were assigned not to receive isoniazid: in this group, 107 (0.8%) secondary cases were identified over one year of follow-up; 62 of these had pulmonary disease.
It is possible that we have underestimated the true number of secondary cases in this study. Some of those who died may have had TB, although this appeared unlikely in most from the verbal autopsies. We also excluded some of those who had undergone TB treatment, because they did not meet our criteria for diagnosis. Some of these individuals may well have had TB disease. We have also shown that some cases preferentially access the government clinic for diagnosis in The Gambia, even with free access to the MRC study clinic. While we are confident that we have captured those that were diagnosed at the government clinic, we suspect strongly that stigma is a significant impediment to the acknowledgement of both symptoms and to seeking a TB diagnosis and treatment in The Gambia and many other African settings. Furthermore, as identified by Guwatudde et al
, the limitations of current diagnostic tools make an underestimation of the true number of secondary cases possible, especially in relation to the difficult diagnosis of TB in children. Under-diagnosis of secondary TB in children would lead to a selection bias, if present, and has implications for the interpretation of age as a predictor of progression to disease.
This study sheds new light on the appropriateness of major TB guidelines, in particular the NICE guidelines – the results do not support the two-step appraoch that is advocated,
at least in settings such as The Gambia. Using such an approach, only seven of the 21 progressors in our study, that were initially tested by both tests, would have been eligible for preventive treatment. This compares to 15 who were positive on one or other test. These results were consistent with those obtained when considering only the initial results of those six contacts with an isolate concordant by genotyping with that of their respective index case. Few data from other studies are available to shed more light on this issue. In a retrospective study of practice in 120 child TB suspects in Newcastle, Taylor et al
identified 5 TB cases: 4 had a positive Mantoux test at the beginning of the investigations, 3 had a positive Quantiferon T cell assay test. This, together with the results presented here, suggest caution in the application of the NICE guidelines.
One might consider, as an alternative strategy, preventive treatment for those who are positive on either test. A possible approach would be to treat all those who are Mantoux positive, then perform a T cell based test on those who are Mantoux negative, treating those with a positive result. Thus, instead of testing all Mantoux positive individuals with a T cell assay as is recommended, one would test all Mantoux negative individuals. These issues can be considered in terms of numbers needed to treat to prevent one TB case. From our results, 60 Mantoux positive contacts need to be treated to prevent a case, compared to 59 ELISPOT positive contacts and 90 contacts overall (ie. regardless of test result). Taking those positive by skin test plus those skin test negative contacts that are ELISPOT positve, 56 would need to be treated to prevent one TB case. These calculations assume 100% efficacy of prophylactic treatment, which would be expected to be less than 90% in reality 
It would be important to consider repeating each test in those initially negative as we have previously shown that some who later become cases, converted to a positive test 3 months after the initial screen 
. Furthermore, it is of note that even those TB case contacts that were both Mantoux and ELISPOT negative had an incidence rate of secondary disease of 400/100,000 person years. This compares to the incidence rate of all cases of TB of 242/100,000 per year for the whole Gambian population (http://www.who.int/globalatlas/predefinedreports/tb/PDF_Files/gmb.pdf
), regardless of Mantoux and/or ELISPOT status prior to progression to disease. One could therefore make a case, in making policy for prophylactic treatment, for treating all those who have a known contact with a sputum smear positive TB case in The Gambia and similar settings. International guidelines presently suggest such a strategy only in Mantoux negative individuals with a particular susceptibility to develop TB disease
. Of relevance to The Gambia and other African countries in particular, HIV positive individuals cleary fall into this category.
The results of the molecular genotyping provide insights into the TB case contact model as a platform for TB research in The Gambia: six (67%) of the nine pairs of index cases and secondary cases with isolates available were concordant. Therefore, the other three secondary cases acquired their M. tuberculosis
from someone other than their respective index case, most probably from outside their household. The mix of intra- and extra- household transmission likely varies according to the degree of TB endemnicity in the general population. In highly endemic suburbs of Capetown, South Africa, for example, isolates match on molecular subtyping in less than half (46%) of households with two TB patients, and the authors estimated that only 19% of M. tuberculosis
transmission occurs within households 
. Therefore, one should be cautious when interpreting findings from TB case contact studies, taking into account the level of community transmission outside households and molecular genotyping on index case and secondary case isolates should be performed when possible.
This results of this study have implications for research, clinical and public health practice. The incidence rate that we have documented here can form a guide for those choosing to use a TB case contact study platform for research. While approximately 5 to 6% of Mantoux or ELISPOT positive TB case contacts develop TB within 2 years in The Gambia, the majority are already cases at initial screening and it is clear that multi-site studies, with a higher overall case yield, may be needed to answer certain research questions. Such consortia are already being established. For example a Gates Grand Challenge project across multiple African sites utilises a TB case contact study model
. Two key conclusions can be drawn from the ELISPOT and Mantoux test results. Firstly, it seems that certain individuals respond preferentially to one or the other test, at least in the early stages of M. tuberculosis
infection. This may be due to varying incubation periods, as we have indicated previously
, or there may be a more fundamental immunological basis. Thus, both tests may be best used together at an initial screen – perhaps to test first with Mantoux and all those that are negative with ELISPOT-this takes advantage of the likelihood that ELISPOT is probably not subject to the booster phenomenon
. Secondly, where contact with an index case continues throughout treatment and conversion of both the Mantoux and ELISPOT tests occur over time
, repeated testing should be considered. At a wider public health policy level, these results do not support replacement of the Mantoux test with a T cell based test for the diagnosis of TB infection, at least in settings such as The Gambia.