Subpopulations of Francisella tularensis ssp. tularensis and holarctica: identification and associated epidemiology.

Tularemia is primarily caused by two subspecies of Francisella tularensis worldwide, ssp. tularensis (type A) and ssp. holarctica (type B), which were originally delineated by phenotypic differences. Application of molecular typing methods to investigate population structure of F. tularensis has confirmed that categorizing the two subspecies via phenotypic characteristics corresponds with genotypic differentiation. In addition, genotyping methods have demonstrated that both subspecies, type A and type B, can be further distinguished into subpopulations and, in some cases, biological relevance has been ascribed to these identified subpopulations. Genetic variation among both type A and type B subpopulations has been shown to correlate with differences in geographic distribution and has also been coupled to distinct ecological niches, animal hosts and replication foci. Among type A subpopulations, strain variation is linked to differing clinical manifestations in humans and virulence in mice. This article will highlight our current understanding of F. tularensis subpopulations, including methods for their detection, their observed epidemiologic differences, implications for public health and basic research programs, as well as future challenges yet to be solved.