A previously undescribed malformed half-engorged female Dermacentor nuttalli was noticed in our tick specimen collection. This tick exhibited abnormal legs compared with normal D. nuttalli ticks. The first leg on the right side and the third leg on the left side were much smaller than normal ticks, and reduced to a small and degenerated appendix, respectively. Both of their pulvillus and claws were absent.

Tick-borne tularemia was first described in 1924. Nearly 100 years later, questions remain about the tick vector(s) that pose(s) the greatest risk for transmitting Francisella tularensis (Ft), the causative agent of tularemia. Additionally, few studies have identified genes/proteins required for Ft to infect, persist, and replicate in ticks. To answer questions about vector competence and Ft transmission by ticks, we infected Dermacentor variabilis (Dv), Amblyomma americanum (Aa), and Haemaphysalis longicornis (Hl; invasive species from Asia) ticks with Ft, finding that although Aa ticks initially become infected with 1-log higher Ft, Ft replicated more robustly in Dv ticks, and did not persist in Hl ticks. In transmission studies, both Dv and Aa ticks efficiently infected naïve mice, causing disease in 57% and 46% of those mice, respectively. We identified a putative Ft chitinase, FTL1793, generated a FTL1793 mutant, and found that FTL1793 was deficient in tick infection, persistence, and replication in ticks. Recombinant FTL1793 exhibited chitinase activity in vitro, suggesting that this chitinase may provide an alternative energy source for Ft in ticks. Taken together, Dv ticks appear to pose a greater risk for harboring and transmitting tularemia and FTL1793 plays a major role in promoting tick infections by Ft.