Molecular epidemiological studies suggest that particular Mycobacterium tuberculosis strains have an enhanced capacity to spread within a community. One strain, the Beijing genotype, has been associated with outbreaks in a number of communities throughout the world. IS6110 restriction fragment length polymorphism (RFLP) analysis was performed on M. tuberculosis isolates from 566 of the 721 patients (78.5%) diagnosed with tuberculosis (TB) on Gran Canaria Island from 1993 to 1996, as well as 35% of isolates from 1991-1992 (85 strains). RFLP identification of the family of strains of the Beijing genotype was confirmed by spoligotyping. Medical records of all patients were reviewed and epidemiological links were identified. Of 566 M. tuberculosis isolates from 1993 to 1996 with RFLP available, 72% belonged to clusters. The largest contained 75 cases and was caused by a strain of the Beijing genotype that was introduced to the island in 1993. It was found in 10 patients in 1993 (5.5%), 12 in 1994 (8.1%), 18 in 1995 (16.4%), and 35 in 1996 (27.1%). Epidemiological linkage was confirmed for 68% of cases. This study has demonstrated rapid dissemination of this strain of the Beijing genotype. This genotype might play an important role in the future of the worldwide tuberculosis epidemic.

The frequency and determinants of exogenous reinfection and of endogenous reactivation of tuberculosis in patients previously treated are poorly understood. In Gran Canaria Island, Spain, between 1991 and 1996, 962 tuberculosis cases were confirmed by culture. Drug susceptibility testing was performed on available bacterial isolates and IS6110-based RFLP genotyping was carried out. Twenty-three patients (2.4%) had two positive cultures separated by at least 12 mo, 18 of whom had bacterial DNA available for genotypic analysis. The initial and final isolates from eight (44%) were different genotypes, indicating exogenous reinfection. Six of them were retreated after cure and two retreated after default. Six were HIV seronegative and two were HIV seropositive. Endogenous reactivation was seen in the remaining 10 patients of whom eight were retreated after default and two after cure. Three of the eight (38%) being retreated after default developed multidrug resistance. One genotype was responsible for a second episode of tuberculosis in five cases, three exogenous reinfections and two endogenous reactivations. In the context of a moderate incidence of tuberculosis, exogenous reinfection is an important cause of TB recurrence, even in HIV-seronegative patients.