To our knowledge, this study is the first longitudinal nationwide survey of MDR and XDR-TB, covering a 15-year period (1994–2008), simultaneously integrating bacteriological and molecular data on the isolates, as well as epidemiological investigation, clinical aspects of the patients and treatment outcomes. The country studied has an epidemiological situation characteristic of that of many other low TB incidence countries 
. In a context of an overall slowly decreasing TB incidence, the MDR-TB rates have been relatively stable over the last 15 years, at around 1.8% of the TB population tested for DST, but the resistance patterns of the isolates have become more severe. The study shows a majority of foreign-born patients (81%) and a significant increase over time both in resistance to ethambutol, amikacin and ofloxacin (p<0.05) and in the number of antibiotics the first isolates are resistant to (p<0.001), resulting in an upward trend of pre-XDR and XDR cases. Importantly, this trend has continued in the years after the end point of the present study. During the period 2009–2011, a total of 51 new MDR-TB strains were identified at first isolation: 36% presented a pre-XDR resistance pattern and 10 (20%) were XDR. Out of these 10, five were “panresistant”.
In-country transmission has played little to no role in fueling these trends, as indicated by the molecular-guided epidemiological investigations of the various patient strain clusters: 9 links only, in 8 clusters, detected or confirmed events of TB transmission. None of these involved XDR cases but one was a pre-XDR nosocomial transmission. In the other clusters, it was impossible to find any link between patients except that most cases in a same cluster were foreigners of the same geographic origin. The lack of proven links could reflect the difficulty of conducting retrospective field investigations in this specific population and/or the fact that many of these patients were already infected by a strain circulating in their country before they arrived in Belgium. The latter hypothesis is supported by data from the systematic screening of all asylum seekers for TB upon their arrival, and by the large diversity of strains with a unique genotype isolated in 67% of the MDR-TB patients.
Of note, no attempt has been made to systematically correlate the results of the cluster analysis to a possible classification of the cases as primary versus acquired resistance because the available information had, in most cases, been provided by the patients themselves without any documented proof.
Along the same lines, in contrast to what can be expected for high-TB incidence settings such as Uzbekistan 
, our genotyping results indicate that none of the 24 cases who acquired additional resistance to second-line drugs during treatment was due to an exogenous re-infection.
The increasing frequency of pre-XDR and XDR cases in Belgium appears to be driven mainly by the increased proportion of MDR-TB patients originating from Asia (p<0.001) and Central and Eastern Europe (p<0.001). Interestingly, this trend seems to be accompanied by a shift in the bacterial strain population structure in these patient groups, as seen by the increased proportions over time of the Beijing profiles isolated from these two world regions (p
0.064 and p
0.051, respectively). These findings are consistent with reports indicating both an increase in MDR- and XDR-TB rates, and an apparent selective expansion of some specific MDR- and pre-XDR-TB clones of M. tuberculosis
Beijing in these world regions 
. In accordance with the latter point, 22 (44.9%) of the 49 Beijing isolates identified in Belgium were caused by just two MIRU-VNTR genotypes: 12 by genotype 100–32, and 10 by genotype 94–32 (coded according to MIRU-VNTRPlus nomenclature, 
Overall, a successful treatment outcome was obtained for 67.8% of the MDR-TB cases, above the 56.3% cure rate of MDR-TB patients observed in Europe in 2010 
. This rate in Belgium increased from 63.0% to 75.8% after the creation in 2005 of the BELTA-TBnet project, which more particularly ensured access to a number of antibiotics that have not been marketed as anti-tuberculosis drugs but may potentially prove to be useful additions to second-line treatment regimens, especially when treating pre-XDR and XDR cases, such as linezolid 
, meropenem-clavulanic acid 
and thioridazine 
. XDR-TB however remains difficult to treat. Only 2 patients were cured among the 5 cases diagnosed as XDR in their first specimen tested in Belgium. Nevertheless, this observation is in line with a systematic review of 13 studies 
showing a successful treatment outcome in up to 65% of XDR-TB patients, particularly if not co-infected with HIV. In addition to BELTA-TBnet, other factors have contributed to the successful treatment outcome, such as the lower mean age of the MDR population but also the long hospitalization with controlled antibiotic administration and the special attention by qualified health care workers during the follow-up of these patients 
, in line with the national guidelines for the treatment of MDR-TB, updated in 2010 
according to the WHO recommendations 
In conclusion, while the annual MDR-TB case notification rate in Belgium has remained low, the severity of the drug resistance patterns has been increasing year by year, leading to more pre-XDR, XDR and “panresistant” TB strains. This trend mostly reflects the degrading situation of drug-resistant TB in Asia and Central and Eastern Europe, from which originate increasing numbers of MDR-TB patients in the country. Therapeutic outcomes have nevertheless been satisfactory and improved over the study period. This can be attributed to rapid diagnosis using early molecular detection of genotypic resistance, to molecular-guided cluster analysis confirming or disproving transmission and ensuring appropriate public health actions, as well as to enlarged therapeutic options and adapted structures for the patients’ long term treatment. In the face of the still growing proportions of pre-XDR, XDR and “panresistant” cases, these efforts must be maintained and intensified.