Integrative transnational analysis to dissect tuberculosis transmission events along the migratory route from Africa to Europe

Background: Growing international migration has increased the complexity of tuberculosis transmission patterns. Italy's decision to close its borders in 2018 made of Spain the new European porte entree for migration from the Horn of Africa (HA). In one of the first rescues of migrants from this region at the end of 2018, tuberculosis was diagnosed in eight subjects, mainly unaccompanied minors. Methods: Mycobacterium tuberculosis isolates from these recently arrived migrants were analysed by Mycobacterial Interspersed Repetitive-Unit/Variable-Number of Tandem Repeat (MIRU-VNTR) and subsequent whole genome sequencing (WGS) analysis. Data were compared with those from collections from other European countries receiving migrants from the HA and a strain-specific PCR was applied for a fast searching of common strains. Infections in a cellular model were performed to assess strain virulence. Results: MIRU-VNTR analysis allowed identifying an epidemiological cluster involving three of the eight cases from Somalia (0 single-nucleotide polymorphisms between isolates, HA cluster). Following detailed interviews revealed that two of these cases had shared the same migratory route in most of the trip and had spent a long time at a detention camp in Libya. To confirm potential en route transmission for the three cases, we searched the same strain in collections from other European countries receiving migrants from the HA. MIRU-VNTR, WGS and a strainspecific PCR for the HA strain were applied. The same strain was identified in 12 cases from Eritrea diagnosed soon after their arrival in 2018 to the Netherlands, Belgium and Italy. Intracellular replication rate of the strain did not reveal abnormal virulence. Conclusions: Our study suggests a potential en route transmission of a pan-susceptible strain, which caused at least 15 tuberculosis cases in Somalian and Eritrean migrants diagnosed in four different European countries.