Virulence ofFrancisellaspp. in Chicken Embryos

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Abstract
We examined the utility of infecting chicken embryos as a means of evaluating the virulence of differentFrancisellasp. strains and mutants. Infection of 7-day-old chicken embryos with a low dose ofF. novicidaorF. tularensissubsp.holarcticalive vaccine strain (LVS) resulted in sustained growth for 6 days. Different doses of these two organisms were used to inoculate chicken embryos to determine the time to death. These experiments showed that wild-typeF. novicidawas at least 10,000-fold more virulent than the LVS strain. We also examined the virulence of several attenuated mutants ofF. novicida, and they were found to have a wide range of virulence in chicken embryos. Fluorescent microscopic examination of infected chicken embryo organs revealed thatF. tularensisgrew in scattered foci of infections, and in all cases theF. tularensisappeared to be growing intracellularly. These results demonstrate that infection of 7-day-old chicken embryos can be used to evaluate the virulence of attenuatedF. tularensisstrains.