Susceptibility of various mouse strains to systemically- or aerosol-initiated tularemia by virulent type A Francisella tularensis before and after immunization with the attenuated live vaccine strain of the pathogen

The following study determined whether commonly available mouse strains could be used to reveal vaccines capable of protecting against aerosol exposure to virulent type A strains of Francisella tularensis. The attenuated live vaccine strain (LVS) of the pathogen, F. tularensis LVS, was used as the model vaccine for these studies. The results showed that most mouse strains survived intradermal immunization with 10 5 cfu of LVS, except for A/J mice the majority of which died following exposure to 10 4 cfu (colony forming units). Most LVS-immunized mouse strains survived subsequent i.d. challenge with >50 LD 50 of a virulent type A strain of the pathogen. However, all mouse strains tested remained susceptible to lethal infection by a low dose (∼20 cfu) aerosol challenge with virulent F. tularensis even after vaccination with LVS, though immunized C3H/HeN mice appeared to be slightly more resistant than the rest to such an assault.