Relation between the level of infection in Ixodes ricinus with Francisella tularensis and the level of bacteremia in the host

The dynamics of bacteremia on white mice subcutaneously infected with an inoculum of 100 and 10 cells of F. tularensis (strain 273) was compared in two parallel experiments. The rise of bacteremia was relatively uniform, about 3 logarithms a day in both groups of animals, so that it reached values of 10(9)-10(10) cells per 1 ml of blood ante finem. Larvae of Ixodes ricinus were fed on white mice in different stages of bacteremia, so that groups of ticks with different degrees of infection were obtained. Our results of quantitative examination show an evident correlation between the degree of bacteremia of the host and the degree of infection of ticks. The highest values of positivity 10(6)-10(7) cells were recorded in larvae, which finished their feeding on the day of the host's death, during the time when bacteremia was reaching the highest degree 10(10) cells per ml of the blood. Persistence of the agent in the organism of infected larvae could be observed up to hatching, under given experimental conditions one month after infection. In nymphs developed from larvae with the highest degree of infection this ability was preserved in about one third of individuals still after 5 months of starvation. In ticks with low degrees of infection it disappeared during the experiments.