Susceptibility of selected murine and microtine species to infection by a wild strain of Francisella tularensis subsp. holoarctica

The purpose of this study was to compare susceptibility of BALB/c mice, common voles (Microtus arvalis) and yellow-necked mice (Apodemus flavicollis) to infection by a virulent Francisella tularensis subsp. holarctica strain. Median survival in these three species following experimental infection with 320 colony forming units of F. tularensis (both intraperitoneally and subcutaneously) amounted to 4.5, 7 and 4 days, respectively. Survival curves of BALB/c and yellow-necked mice were very similar and were significantly different from that of common voles. LD50 was 0.5 and 37.9 colony forming units in BALB/c mice and common voles, respectively. The bacterial burden in the spleen, liver, lung, kidney and blood of common voles started to develop later post exposure and amounted to lower levels (except in kidneys) than in BALB/c mice. The results demonstrate that yellow-necked mice are even more susceptible to infection by F. tularensis than BALB/c mice and that the common vole is a small mammalian host with a susceptibility which is two-orders-of-magnitude lower.