State of the art
Despite the implementation of National TB Programs (NTP) in the southern Mediterranean region and, although some progress has been made in certain countries, the disease burden is still significant and there is a potential risk for regression of the epidemiological parameters. The expansion of virulent and host-adapted strains able to escape BCG immunization, along with the emergence of drug resistance and epidemics, accounts amongst the most significant threats in this regard. Fortunately, unlike the African Sub-Saharan region where HIV co-infection represents the most prominent factor tackling TB control, HIV prevalence in North African countries is relatively low and the hope to efficiently reduce the disease burden could still be realistically achievable. For the NTPs to be more effective in their strategy, they should rely, aside from demographical and clinical parameters, on epidemiological data that integrate Mycobacterium tuberculosis genetic diversity, drug resistance, and its interplay with the host immune system. Strengthening the capacity of these countries to implement research activities related to these issues will be critical to tighten TB surveillance and control.
In the three targeted North African countries, TB prevalence obeys to a decreasing gradient; being the highest in Morocco (82.1 new cases/100 000 inhabitants), intermediate in Algeria (63 new cases/100 000 inhabitants), and the lowest in Tunisia (24 new cases/100 000 inhabitants). This overall relatively high prevalence of TB has been merely attributed to socio-demographic parameters, insufficient control measures, and suboptimal vaccination/DOTS coverage rates. Since many decades, improving control efforts have been mainly focused on these parameters, which may not prove sufficient to bring down TB prevalence to levels comparable to those reached by developed countries. Indeed, factors associated with the population structure of circulating M. tuberculosis and their interactions with the host have been poorly investigated. Addressing these issues might be the missing key element to guide an effective approach for TB control and surveillance. This could be achieved by implementing large scale molecular studies to evaluate the genetic diversity and the transmission dynamics of M. tuberculosis. Indeed, it has been clearly demonstrated that results gained from population-based molecular studies of M. tuberculosis, help to better control the overall NTP action and define appropriate intervention strategies to prevent outbreaks and identify weakness of the NTPs (Drobniewski et al., 2003; Barnes and Cave, 2003).
TB is a health threat common to both sides of the Mediterranean See that could be amplified through both legal and illegal population migration flows. Large North African communities have been established in Europe since many decades ago, a situation that has certainly impacted the epidemiology of TB therein. In France, for instance, according to data gathered in 2006 (Bulletin d’Epidemiologie et d’Hygiène, March 2008), 45% of TB patients were born in a foreign country. The TB incidence in foreigners is 149 versus 8.5 for French patients. The analysis of MDR-TB cases showed that 81% of these cases are foreign born, 10% from the Maghreb and 30% from sub-Saharan African countries. On another hand, Spain has one of the highest incidences of TB in Western Europe; nevertheless, the incidence of MDR-TB is considered low (WHO report 2008). There are not official rates for immigration related TB, but there was a progressive increase in the number of MDR-TB isolates from foreign patients during the last 10 years, from 5% to 7% in 2007. One of the major groups of immigrants comes from Morocco. In this context, a EU FP6 funded project involving partner 6 (INH) with a Spanish institution (Instituto de Salud Carlos III, Madrid) is addressing the impact of immigration on HIV and TB epidemiology in the Mediterranean region.
Previous studies have shown that M. tuberculosis lineages were stably associated with host populations that can be delineated according to their ethnic origin (Hirsh et al., 2004). Such stability has been linked to the adaptation by specific M. tuberculosis lineages to the genetic, cultural, or environmental characteristics of particular host populations. Interestingly, it has been shown that in urban cosmopolitan area, the mycobacterium lineages were much more likely to spread in sympatric (within communities of the same genetic background) than in allopatric (between communities of different genetic background) patient populations (Gagneux et al., 2006). Such a restrictive, community-specific transmission pattern, may not apply for the North African communities living in Europe as M. tuberculosis lineages prevalent therein are identical to those predominating in North Africa. That is, predominant clones identified in North Africa may not remain restricted to the North African communities established in Europe. Hence, a better control of the disease in MPC would contribute to better reduce the disease burden in Europe. EUMEDNET-TB will thus provide us with a better insight into TB dynamics in North Africa and to better understand its potential consequences for Europe.