Efficiency ofLeptotrombidiumChiggers (Acari: Trombiculidae) at TransmittingOrientia tsutsugamushito Laboratory Mice

Thirteen different laboratory colonies of Leptotrombidim chiggers [L. chiangraiensis Tanskul & Linthicum, L. deliense Walch and L. imphalum (Vercammen-Grandjean &Langston)] were evaluated for their ability to transmit Orientia tsutsugamushi (Hyashi) to mice. Of 4,372 transmission attempts using individual chiggers from all 13 colonies, 75% (n = 3,275) successfully infected mice. Transmission rates for the individual chigger colonies ranged from 7 to 80%. Increasing the number of chiggers that fed on a given mouse generally increased transmission rates. Transmission of O. tsutsugamushi to mice by different generations (F1-F11) of certain chigger colonies was stable; however, transmission rates varied greatly in other colonies. Transmission rates (both vertical and horizontal) of several L. changraiensis colonies and the L. deliense colony were the highest, suggesting that these colonies may be useful for the development of a chigger-challenge model that can be used to evaluate the efficacy of candidate scrub typhus vaccines or therapeutic agents in laboratory mice.