Mycobacterium tuberculosis Strains Potentially Involved TB Epidemic in Sweden a Century Ago

A hundred years ago the prevalence of tuberculosis (TB) in Sweden was one of the highest in the world. In this study we conducted a population-based search for distinct strains of Mycobacterium tuberculosis complex isolated from patients born in Sweden before 1945. Many of these isolates represent the M. tuberculosis complex population that fueled the TB epidemic in Sweden during the first half of the 20th century. Methods: Genetic relationships between strains that caused the epidemic and present day strains were studied by spoligotyping and restriction fragment length polymorphism. Results: The majority of the isolates from the elderly population were evolutionary recent Principal Genetic Group (PGG)2/3 strains (363/409 or 88.8%), and only a low proportion were ancient PGG1 strains (24/409 or 5.9%). Twenty-two were undefined. The isolates demonstrated a population where the Euro-American superlineage dominated; in particular with Haarlem (41.1%) and T (37.7%) spoligotypes and only 21.2% belonged to other spoligotype families. Isolates from the elderly population clustered much less frequently than did isolates from a young control group population. Conclusions: A closely knit pool of PGG2/3 strains restricted to Sweden and its immediate neighbours appears to have played a role in the epidemic, while PGG1 strains are usually linked to migrants in todays Sweden. Further studies of these outbreak strains may give indications of why the epidemic waned.