Transstadial transmission of Francisella tularensis by Ixodes ricinus ticks infected during the nymphal stage

The degree of infection of nymphs of Ixodes ricinus which finished their feeding on experimentally infected mice on the day of their death from tularaemia represented more than 10(7) cells of Francisella tularensis per tick after feeding. The high degree of positivity was preserved in ticks also during one month of metamorphosis. Transstadial transmission of the agent from the stage of nymph to the stage of imago was confirmed in adults either by cultivation or by experiments attempting to transmit the infection to white mice, even after more than a year after infection of nymphs or more than a year after metamorphosis. Some of the ticks which did not transmit the infection while feeding were found to be positive subsequently. Results of quantitative examination of the degree of infection indicate also the possibility of reproduction of the agent in imagines after feeding on hosts. Incompletely engorged nymphs which fed repeatedly on mice after being collected from their host dead from tularaemia, were able to transmit the infection during interrupted feeding. Further possibilities of transmission were indicated by findings of consumption of infected ticks by hosts or by elimination of F. tularensis in faeces of infected nymphs and adults.